HomeMy WebLinkAbout4434ORDINANCE NO. 4434
AN ORDINANCE of the City Council of the
City of Kent, Washington, repealing Section 2 of
Ordinance No. 4234 that had adopted Kent's 20L7
Surface Water Design Manual, and adopting Kent's
new 2O22 Surface Water Design Manual.
RECITALS
A. The city of Kent's existing surface water Design Manual
("KSWDM") was last updated approximately five years ago, in January of
2OL7, through the adoption of Ordinance No. 4234'
B, The public health, safety, and welfare of the residents of Kent
require the proper design, construction, and maintenance of surface water
and stormwater control systems. The KSWDM presents the City's strategy
to reduce public hazards due to floods, erosion, and landslides; enhance
public health, safety, and welfare; better protects the City's environmental
resources, specifically our native salmon; improves regulatory review by
introducing more certainty in the requirements; assists developers by
providing more detailed information about how to analyze and design
acceptable stormwater control systems; provides the City with the latest
technology available for designing and constructing stormwater controls;
and meets state and federal regulatory requirements for local municipalities
under the Endangered Species Act and Clean Water Acts.
Surface Water Design Manual
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C. Due to the requirements of federal and state law, and updates
King County made recently to its Surface Water Design Manual, it is
appropriate for Kent to update its KSWDM. During preparation of the current
KSWDM, City staff met with the development community and the manual
was subject to review under the State Environmental Policy Act ("SEPA")'
The City's SEPA responsible official has determined that the proposed
amendments to the KSWDM are procedural in nature and further SEPA
analysis is not required for the adoption of the KSWDM.
D. A draft version of this ordinance and the KSWDM was submitted
to the Washington State Department of Commerce for expedited review on
4/2L/2022, which was granted on 4/25/2022. The ordinance and the
KSWDM were discussed as informational items before the Economic and
Community Development Committee and the City's Land Use and Planning
Board on 5/9/2022. They were considered by the City's Land Use and
Planning Board after a duly noticed public hearing on 5/23/2022 and by the
public Works Committee on 6/6/2022, both of which recommended Council
adoption. Council has considered this ordinance, the 2O22 edition of the
KSWDM, together with all public comment, and has determined that
adoption is appropriate.
NOW, THEREFORE, THE CIry COUNCIL OF THE CITY OF KENT,
WASHINGTON, DOES HEREBY ORDAIN AS FOLLOWS:
ORDINANCE
SECTION 7. - Repealer - 2077 edition of Kent Surface Water Design
Manual. Section 2 of Ordinance No. 4234, which adopted the 2017 edition
of the Kent Surface Water Design Manual is hereby repealed in its entirety.
SECTION 2. - Adoption - 2022 edition of Kent Surface Water Design
Manual. Kent's 2022 Surface Water Design Manual, attached and
incorporated as Exhibit A, is hereby adopted for application and enforcement
Surface Water Design Manual
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within the city of Kent. To the extent not inconsistent with any express
provision of the Surface Water Design Manual, the Directors of the City's
Public Works Department and the Economic and Community Development
Department may amend, revise, supplement, and incorporate minor
changes to the Surface Water Design Manual as they become necessary.
Major, substantive changes shall include an opportunity for public review
and comment before Kent City Council takes any action to adopt any such
major, substantive changes.
SECTION 3. - Severability. If any one or more section, subsection,
or sentence of this ordinance is held to be unconstitutional or invalid, such
decision shall not affect the validity of the remaining portion of this ordinance
and the same shall remain in full force and effect.
SECTION 4. - Savings. The 2OL7 edition of the Surface Water Design
Manual, which is repealed and replaced by this ordinance, shall remain in
full force and effect until the effective date of this ordinance.
SECTION 5. - Corrections by City Clerk or Code Reviser. Upon
approval of the city attorney, the city clerk and the code reviser are
authorized to make necessary corrections to this ordinance, including the
correction of clerical errors; ordinance, section, or subsection numbering; or
references to other local, state, or federal laws, codes, rules, or regulations.
' SECTION 6, - Effective Date. This ordinance shall take effect and be
in force 30 days from and after its passage, as provided by law'
DANA RALPH, MA
June 2L,2022
Date Approved
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ATTEST:
KIMBERLEY A.oTo, clw c
AP D AS TO FORM:
WHITE, ACTING CITY ATTORNEY
June 2t,2O22
Date Adopted
June 24.2022
Date Published
Surface Water Design Manual
Repeal 2077 Ed, A AdoPt 2022 Ed'
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CITY OF KENT
SURFACE WATER DESIGN MANUAL
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ACKNOWLEDGEMENTS
City Of Kent Surface Water Design Manual htly 2022
ACKNOWLEDGEMENTS
ACKNOWLEDGEMENTS
This Manual is based on the 2021King County Surface Water Design Manual
Individuals involved in the preparation, review, or approval of this Kent Surface Water Design
Manual are as follows:
Mayor
Dana Ralph
Kent City Council
Toni Troutner
Bill Boyce
Brenda Fincher
Zandria Michaud
Interim Chief Administrative Officer
Pat Fitzpatrick
Public Works Director
Chad Bieren, P.E.
Deputy Public Works Director, Engineering
Kelly Peterson
Environmental Engineering Manager
Mike Mactutis, P.E.
Project Manager
Laura Haren
Technical Review
Mike Mactutis, P.E.
Dave Brock, P.E.
Brennan Taylor, P.E.
Shawn Gilbertson
Stephen Lincoln, P.E.
Erin George
GIS Mapping
Jim Cordova
Sarah Bums
Document Production
Nancy Yoshitake
Satwinder Kaur
Marli Larimer
Les Thomas
City of Kent Surlace Water Design Manual Jtly 2022
City Of Kent Surface Water Design Manual luly 2022
PREFACE
HOW TO USE THIS DOCUMENT
KENT
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City of Kent Surface Water Design Manual Iuly 2022
City of Kent Surface Water Design Manual Iuly 2022
HOW TO USE THIS DOCUMENT
The city of Kent has adopted the202l King County Surface Water Design Manual (KCSWDM) as amended by this
document. This document includes all changes, additions, and deletions to the KCSWDM and is to be used for all
development, as applicable, within the city of Kent.
In the event a conflict exists between the King County Surface Water Design Manual and the Kent Surface Water
Design Manual, Kent City Code, or Kent Design and Construction Standards, Kent standards and code will generally
control. The applicant must notifu city review staff if conflicts are discovered.
CITY EQUIVALENTS FOR COUNTY AGENCIES AND
ORDINANCES
For proposals located within the City of Kent, all references in the KCSWDM to the following King County
departments are to be replaced by reference to the Kent Economic and Community Development Department
o DDES (Department of Development and Environmental Services)
o DNR (Department of Natural Resources)
o SWM (Surface Water Management)
o WLR (Water and Land Resources)
For proposals in the City of Kent, all references in the KCSWDM to the King County Sensitive Areas
Ordinance (SAO) are to be replaced by reference to the Kent City Code, particularly Chapters ll,14, and l5'
COUNTY DESIGNATIONS THAT DO NOT APPLY IN THE CITY
The following designations are used inthe202l KCSWDM but are not cunently used in the City of Kent; any
reference in the KCSWDM to the existence of areas with these designations or thresholds or requirements for
such areas is to be disregarded for development applications within the City of Kent:
. Coal Mine Hazard Area
. Critical Drainage Area
r Rural Residential Development
. Shared Facility
MANUAL UPDATES
With a publication of this complexity there may be errors that must be corrected and clarifications that are
needed- There will also be new information and technological updates. The City of Kent Department of
Public Works intends to publish corrections, updates and new technical information on our Departmental
Home Page at http://www.kentwa.gov/stormwater/. The web site will not be used to make revisions in key
policy areas - such as the thresholds and minimum requirements in Chapter l. We encourage you to visit this
web iite periodically and incorporate updates and corrections into your copies of the manual' You can also
visit this web site for updates and additional information about other Kent Public Works surface water
activities.
City of Kent Surface Water Design Manual luly 2022
ADDENDUM ORGANIZATION
The information presented in this document is organized as follows:
Preface-How to Use This Document: This preface provides instructions for using the City of Kent's
SWDM. It also defines terms in the King County manual that are used differently for the City of KenU City
departments that are equivalent to county departments referred to in the KCSWDM; and designations from the
King County manual that do not apply to proposals in the City of Kent'
Chapter l-Drainage Review Requirements: A completely revised Chapter I has been provided in this
document. It is to be used instead of Chapter I in the 2021 KCSWDM for all proposals in the City of Kent'
This chapter sets forth the thresholds and requirements for drainage review, describes the three types of
drainage review, and summarizes the nine Core and five Special Requirements.
Chapter 2-Drainage Plan Submittal: A completely revised Chapter 2 has been provided in this document.
It is to be used instead of Chapter 2 in the 2021 KCSWDM for all proposals in the City of Kent' This chapter
describes the required format and components of submittals for the three types of drainage review'
Chapter 3-Hydrologic Analysis and Design: The City of Kent has made no changes to Chapter 3 of the
2021 KCSWDM. The King County version of Chapter 3 applies for proposals in the City of Kent.
Chapter 4-Conveyance System Analysis and Design: The City of Kent has made several minor changes to
Chapter 4 of the 2021 KCSWDM. This chapter provides replacement text for the sections that are changed'
Apart from these changes, the King County version of Chapter 4 applies for proposals in the City of Kent.
Chapter S-Flow Control Design: The City of Kent has made minor changes to Chapter 5 of rhe202l
KCSWDM. This chapter provides replacement text for the section that is changed. Apart from these changes,
the King County version of Chapter 5 applies to proposals in the City of Kent.
Chapter 6-Water Quality Design: The City of Kent has made a few minor changes to Chapter 6 of the
202i KCSWDM. This chapter provides replacement text for the section that is changed. Apart from this
change, the King County version of Chapter 6 applies to proposals in the City of Kent.
Appendices - Appendix A presents the City of Kent Maintenance Requirements for Privately Maintained
niainage Facilities (reprinted from Appendix D of the Kent Construction Standards), there is no Appendix B
(King County includes Master Drainage Plan requirements here - does not apply in Kent), Appendices C and
D are the Small Site Drainage Requirements and Erosion and Sediment Control Standards, respectively. These
are included as separately bound documents included with the King County Manual'
References - The City of Kent accepts certain reference materials as written in the 2021 King County Surface
Water Design Manual. Other references are not applicable in Kent. Some reference materials have been
revised to reflect Kent-specific forms or development procedures. References from the King County Manual
are identified below along with how they are to be utilized in Kent (in bold).
a
a
a
a
a
a
City of Kent Surlace Water Design Manual IuIy 2022
TABLE OF CONTENTS
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City of Kent Surface Water Design Manual luly 2022
TABLE OF CONTENTS
TABLE OF CONTENTS
KEY TERMS AND DEFINITIONS.
CHAPTER I
Drainage Review and Requirements
I .l Drainage Review
I .l.l Projects Requiring Drainage Review.........
1.1 .2 Drainage Review Types and Requirements ......
L 1.3 Drainage Design Beyond Minimum Compliance......'...
I .2 Core Requirements .......
1.2.1 Core Requirement #1: Discharge at The Natural Location '.'..
1.2.2 Core Requirement#2: Offsite Analysis
1.2.3 Core Requirement #3: Flow Control Facilities
1.2.4 Core Requirement #4: Conveyance System....'.....
L25 Core Requirement #5: Erosion and Sediment Control
1.2.6 Core Requirement #6: Maintenance and Operations
L2.7 Core Requirement#7: Financial Guarantees and Liability
1.2.8 Core Requirement #8: Water Quality
1.2.9 Core Requirement #9: Flow Control BMPs
1.3 SpecialRequirements.
1.3.1 Special Requirement #1: Other Adopted Area-Specific Requirements
Special Requirement #2: Flood Hazard Area Deliniation
Special Requirement #3: Flood Protection Facilities......
Special Requirement #4: Source Controls
t.3.2
t.3.3
1.3.4
. 1-1
. 1-l
. l-3
.1-4
l-11
t-13
I -13
t-14
-25
-39
-43
-49
-51
-52
-62
-76
-76
-77
-79
-80
-82
-84
1.3.5 Special Requirement #5: Oil Control
1.4 AdjustmentProcess...............
1.4.1 AdjustmentAuthority
1.4.2 Criteria for Granting Adjustments
|.4.3 AdjustmentApplication Process.........
1.4.4 AdjustmentReviewProcess..........
I .4.5 Request For Reconsideration Procedure
CHAPTER 2
Drainage Plan Submittal
2.1 Plans for Permits and Drainage Review
2.1 .l Plans Required for Initial Permit Submittal
2.1 .2 Plans Required for Drainage Review
2.2 Plans Required with Initial Permit Submittal......
2.2.1 Subdivisions, Short Plats and Binding Site Plans
2.2.2 Commercial Site Development....................
2.2.3 Single-familyResidential....
2.2.4 Other Permits
2.3 Plans Required for Drainage Review.......
2.3.1 EngineeringPlan Specifications.............
2.3.2 Projects in Targeted Drainage Review....
2.4 Plans Required After Drainage Review
2.4.1 Plan Changes After Permit Issuance
2.4.2 Final Corrected Plan Submittal
2.4.3 Final Plat, Short Plat, and Binding Site Plan Submittals.....
CHAPTER3
Hydrologic Analysis and Design
CHAPTER 4
Conveyance System Analysis and Design
4.1 Route Design Easment Requirements
4.1.2 Easment And Setback Requirements
4.2 Pipes, Outfalls, and Pumps
4.2.1 Pipe Systems
4.2.2 OutfallSystems........
4.3 Culverts And Bridges.
City of Kent Surface Water Design Manual Iuly 2022
4.3.1 Culverts........
CHAPTER 5
Flow Control Design....
5.I DetentionFacilities
5.I .l Detention Ponds
5.1 .2 Detention Tanks
5.1.5 Parking Lot Detention......
5.1.6 Roof Detention
CHAPTER 6
Water Quality Design
6.2 General Requirements For WQ Facilities
6.2.3 Slopes, And Embankmetns
6.7 Proprietary Facility Designs ......
APPENDIX
APPENDIX A........................
Appendix A - Maintenance Requirements For Flow Control, Conveyance, And Facilities
APPENDIX B
Appendix B - Master Drainage Plan Objectives, Criteria, Components, And Review Process.......'..........
APPENDIX C..........................
Appendix C - Simplified Drainage Requirements
APPENDIX D..........................
Appendix D - Construction Stormwater Pollution Prevention....
D.l General CSWPP Requirements
REFERENCES
MAPS
Erosion Hazard Areas ............
Flow Control Applications..
Landslide Hazard / Landslide Hazard Drainage Areas..."..'.'.....
Soils Wellhead Areas
Water Quality Applications
Wetlands
Lower Mill Creek Regulatory Area......
4-8
5-1
5-l
5-l
5-2
5-3
5-3
.......6-1
.......6-1
.......6-1
.......A-l
6-1
.A-2
.A-4
.A-6
.A-8
.A-9
R-l
M-1
M-l
M-1
M-l
M-l
M-l
.A-t I
.A-13
.....A-13
....M-l
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KEY TERMS AND DEFINITIONS
KENT
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KF,Y TERMS AND DEFINITIONS
KEY TERMS AND DEFINITIONS
A complete list of definitions follows Chapter 6.
Proper application of the drainage review and requirements in this chapter requires an understanding of the
following key terms and their definitions. Other key terms may be defined in subsequent chapters. All such key
terms are highlighted in bold italic throughout the manual. Other important terms Ihat are not key terms are
defined in the text when they are first introduced. These are highlighted in italic when they are first introduced but
are not highlighted throughout the manual. All terms defined in this chapter are also found in the "Definitions"
section of this manual as are other important terms defined throughout the Manual.
Acceptable discharge point: An enclosed drainage system (i.e., pipe system, culvert, or tightline) or open
drainage feature (e.g., ditch, channel, swale, stream, river, pond, lake, or wetland) where concentrated runoff
can be discharged without creating a significant adverse impact'
Agricultural project: any project located on, and proposing improvements consistent with, the permitted uses
of land zoned for Agriculture.
Arterial: A high traffic-volume road or street primarily for through traffic. The term generally includes roads or
streets considered collectors. It does not include local access roads which are generally limited to providing
access to abutting proPertY.
Base flood: The flood having a one percent chance of being equaled or exceeded in any given year. Also
referred to as the 001Q0-year flood". The base flood is determined for future flow conditions, except in areas
where the FEMA Flood Insurance Study includes detailed base flood calculations. In that case, the FEMA
data shall apply.
Bioretention A flow control best management practice consisting of a shallow landscaped depression designed
to temporarily store and promote infiltration of stormwater runoff. Standards for bioretention design, including
soil mix, plants, storage volume and feasibility criteria, are specified in Appendix C.
Certffied Erosion and Sediment Control Leud (CESCL).' an individual who has cunent certification through
an approved erosion and sediment control training program that meets the minimum training standards
estabiished by the Washington Department of Ecology Department (Ecology). The CESCL must have the skills
to assess srle conditions and construction activities that could impact the quality of stormwater and, the
effectiveness of erosion and sediment control measures used to control the quality of stormwater discharges.
Certification is obtained through an Ecology approved erosion and sediment control course.
City Review Staff: the City of Kent employee(s) responsible for the conditioning, review, inspection, and
approval of development permits. This is generally staff from the Development Review section of the
Economic and Community Development Department but may include other city employees such as Public
Works staff.
Civil engineer.. a person licensed by the state of Washington as a professional engineer in civil engineering.
Construct or modify: to install a new drainage pipe or ditch or make improvements to an existing drainage pipe
or ditch, for purposes other than maintenance, which either serves to concentrate previously unconcentrated
surface water or stormwater runoff or serves to increase, decrease or redirect the conveyance of surface water
or stormwater runoff. "Construct or modiff" does not include installation or maintenance of a driveway culveft
installed as part of a single-family residential building permit.
Conveyance system nuisance problem: a flooding or erosion problem that does not constitute a severe Jlooding
problem ot severe erosion problem and that results from the overflow ofa constructed conveyance system for
iunoffevents less than or equal to a 10-year event. Examples include inundation ofa shoulder or lane ofa
roadway, overflows collecting in yards or pastures, shallow flows across driveways, minor flooding of crawl
spaces or unheated garages/outbuildings, and minor erosion.
Critical aqufer recharge areu: the critical area designation, defined and regulated in KCC I 1.06, which is
applied to areas where extra protection of groundwater quantity and quality is needed because of known
City of Kent Surface Water Design Manual Iuty 2022
KEY TERMS AND DEFINITIONS
susceptibility to contamination and impoftance to drinking water supply. Such areas are delineated onlhe King
County Critical Aquifer Recharge Area Map available at DPER or on the County's Geographic Information
System (GIS). See the "Definitions" section for more details.
Development: any activity that requires a permit or approval, including, but not limited to, a building permit,
grading permit, shoreline substantial development permit, conditional use permit, special use permit, zoning
variance or reclassification, subdivision, short subdivision, urban planned development, binding site plan, site
development permit, or right-of-way use permit. "Development" does not include a Class I, II or III, forest
practice conducted in accordance with Chapter 76.09 RCW and Title 222WAC.
Director: the Economic and Community Development Director or the Public Works Director as appropriate
for the permit condition or other issue under consideration.
Drainagefacilities: aconstructed or engineered feature that collects, conveys, stores, treats, or otherwise
*unagei surface water or stormwater runoff. "Drainage facility" includes, but is not limited to, a constructed
or engineered sheam, lake, wetland, or closed depression, or a pipe, channel, ditch, gutter, flow control facility,
flow control BMP, water quality facility, erosion and sediment control facility, and any other structure and
appurtenance that provides for drainage.
Effective impervious surface: impervious surfaces that are connected via sheet flow or discrete conveyance to
a drainage system. Impervious surfaces are considered ineffective if: I ) the runoff is fully dispersed as
described inAppendix C of this manual;2) residential roof runoff is infiltrated in accordance with the full
infiltration BMP described in Appendix C of this manual; or 3) approved continuous runoff modeling methods
indicate that the entire runoff file is infiltrated.
Erodible or leachable materials, walstes, or chemicals.' materials or substances that, when exposed to rainfall,
measurably alter the physical or chemical characteristics of the rainfall runoff (Examples include but are not
limited to erodible soil, uncovered process wastes, manure, fertilizers, oily substances, ashes, kiln dust, garbage
dumpster leakage, commercial-scale vehicle and animal wash waste, galvanized structural, architectural,
cabinet, and utility steel, architectural copper, bronze, brass, and lead, treated lumber, etc.).
Erosion hazard area: the critical area designation, defined and regulated in KCC 2lA,that is applied to areas
underlain by soils that are subject to severe erosion when disturbed. See the "Definitions" section for more
details.
Existing Jtooding: Flooding over all lanes of the roadway or driveway has occurred in the past and can be
verified by City records, City personnel, photographs, or other physical evidence.
Existing site conrlitions.. shall mean the native vegetation and soils that existed at the site prior to the influence
of Euro-American settlement. Existing site conditions outside of the Lower Mill Creek, Lower Springbrook
Creek, Lower Garrison Creek or within the Green River Natural Resource Area as shown on the Flow Control
Application Map shall be assumed to be forested land cover unless reasonable, historic information is provided
that indicated that the site was prairie prior to settlement.
Exposed: subject to direct or blown-in precipitation and/or direct or blown in runoff. Notfully covered.
Exposed area or exposed msterial: not covered sufficiently to shield from rainfall and stormwater runoff. At a
minimum, full coverage to not be considered exposed requires a roof with enough overhang in conjunction with
walls of sufficient height to prevent rainfall blow-in; and the walls must extend into the ground or to a berm or
footing to prevent runofffrom being blown in or from running onto the covered area.
Finished area: Any enclosed area of abuilding that is designed to be served by the building's permanent
heating or cooling system.
Frequently Flooded urea.' the critical area designation, defined and regulated in KCC I1.06, which is applied
to areas subject to inundation by a 1 OO-year flood event or areas at risk from channel migration. Flood hazard
areas generally include, but are not limited to, aquatic areas (e.g., streams or lakes), wetlands, or closed
depressions. See the "Definitions" section for more details.
of the adjoining floodplains which areFloodway:
reasonably
the channel ofthe river or stream and those portions
required to carry and discharge the base flood flow.ining floodplains that
City of Kent Surface Water Design Manual ll
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KEY TERMS AND DEFINITIONS
are considered to be'oreasonable required" are defined by the City flood hazard regulations as stated in
Chapter 14 of the Kent City Code.
Flow control BMP: asmall scale drainage facility or feature that is part of a development site strategy to use
processes such as infiltration, dispersion, storage, evaporation, transpiration, forest retention, and reduced
impervious surface footprint to mimic pre-developed hydrology and minimize stormwater runoff.
FIow duratioz.. The aggregate time that peak flows are at or above a particular flow rate of interest (e.g., the
amount of time over the last 40 years that peak flows were at or above the 2-year flow rate).
Fult build-out conditions: The tributary area is developed to its full zoning potential except where there are
existing streams, lakes, wetlands, closed depressions, geologic hazard areas, or open space tracts.
Fully covered: covered sufficiently to shield from rainfall and stormwater runoff. At a minimum, full coverage
r"qui.es a roof with enough overhang in conjunction with walls of sufficient height to prevent rainfall blow-in;
and the walls must extend into the ground or to a berm or footing to prevent runoff from being blown in or
from running onto the covered area. Not exposed'
Fully dispersed the runoff from an impervious surface or non-native pervious surface has dispersed per the
criteria for fully dispersed surface in Section 1.2.3.2.
Geologic Hazard Aress.. Include Seismic HazardAreas, Erosion Hazard Areas, and LandslideHazatd Areas'
Groundwater prolection aress: include critical aquifer recharge sreqs as defined in KCC I I '06, sole source
aquifer areas as designated by the federal Environmental Protection Agency, and wellhead protection areas as
mapped by the Washington State Department of Health.
High-use site: thatarea within a commercial or industrial site that typically generates or is subject to runoff
"ontuining
high concentrations ofoil due to high traffic turnover, on-site vehicle or heavy or stationary
equipment use, or the frequent transfer ofliquid petroleum or coal derivative products.
High-use sites include:
1. That area of a commercial or industrial site that:
a) has an expected average daily traffic (ADT) count equal to or greater than 100 vehicles per 1,000
square feet ofgross building area; or
b) is subject to petroleum storage or transfer in excess of 1,500 gallons per year, not including delivered
heating oil at the end-user point ofdelivery; or
c) is subject to use, storage, or maintenance of a fleet of 25 or more diesel or jet fuel (aviation turbine
fuel) vehicles that are over l0 tons net weight (trucks, buses, trains, airplanes, tugs, mobile and fuel-
driven or hydraulic stationary heavy equipment, etc'); or
2. The interior ofany road intersection and that portion oflanes leading into the intersection subject to
braking, tuming, or stopping, with a measured ADT count of 25,000 vehicles or more on the main roadway
and 15,000 vehicles or more on any intersecting roadway. Projects proposing primarily pedestrian or
bicycle use improvements are excluded.
Hisloric site conditions.. those that existed onthe site prior to any development in the Puget Sound region. For
lands not currently submerged (i.e., outside the ordinary high water mark of a lake, wetland, or stream), historic
site conditions shall be assumed to be forest cover unless reasonable, historic, sife-specific information is
provided to demonstrate a different vegetation cover. In Kent this area is designated by the Lower Mill Creek
Regulatory Map. In some stream basins, as allowed per Section 1.2.3.1.8, historic site conditions for lands not
currently submerged may be assumed tobeT5o/o forest, l5%o grass, and l0% impervious surface.
Impaired waterbody or impaired receiving wuter: where the receiving waterbody is either:
(1) listed as impaired according to Ecology's Water Quality Assessment categories 2, 4, or 5 for water or
sediment, as documented in the state's Water Quality Assessment 303(d)/305(b) Integrated Report and is
displayed in WA Ecology's electronic database and map viewer of these waterbodies, or
City of Kent Surface Water Design Manual Iuly 2022
KEY TERMS AND DEFINITIONS
(2) is currently designated by the City as polluted based on credible data indicating exceedance or concern for
exceedance of the state's numeric water quality standard as documented in the latest published list of King
County-Identified WQ Problems (Reference Section l0) posted at
manual.aspx, or
(3) where subject to any other local, state, or federal cleanup plan or contaminated site designation.
Impervious surfoce: a hard surface area that either prevents or retards the entry of water into the soil mantle as
under natural conditions before development; or that causes water to run offthe surface in greater quantities or
at an increased rate offlow compared to the flow present under natural conditions prior to development (see
also "new impervious surface"). Common impervious surfaces include, but are not limited to, roof; walkways,
patios, driveways, parking lots, or storage areas, areas that are paved, graveled or made ofpacked or oiled
earthen materials or other surfaces that similarly impede the natural infiltration of surface water or stormwater.
For the purposes of applying the impervious surface thresholds and exemptions contained in this manual,
p"r-"uble pavement, vegetated roofs, and pervious surfaces with underdrains designed to collect stormwater
iunoff are considered impervious surface while an open uncovered flow control or water quality facility is not.
However, for the purposes of computing runoff, uncovered flow control or water quality facilities shall be
modeled as impervious surfaces as specified in Chapter 3.
Land disturbing activity: any activity that results in a change in the existing soil cover, both vegetative and
non-vegetative, or the existing soil topography. Land disturbing activities include, but are not limited to
demoliiion, construction, clearing, grading, filling, excavation, and compaction. Land disturbing activity does
not include tilling conducted as part ofagricultural practices, landscape maintenance, or gardening.
Landscape msnogement plan: a City of Kent approved plan for defining the layout and long-term maintenance
of landscaping features to minimize the use of pesticides and fertilizers to reduce their discharge, and to reduce
the discharge ofsuspended solids and other pollutants.
Landslide hazard area: the critical area designation, defined and regulated in KCC I 1.06, which is applied to
areas subject to severe risk oflandslide due to topography, soil conditions, and geology.
Landslide huzurd drsinsge areo: an area mapped by the City of Kent where it has been determined that
overland flows from a project will pose a significant threat to health and safety because of its close proximity to
a lundslide hqzard ureathat is on a slope steeper than l5Yo. Such areas are delineated on the Landslide Hazard
Drainage Areas map adopted with this manual.
Low Impact Development (LID): A stormwater and land use management strategy that strives to mimic pre-
disturbance hydrologic processes ofinfiltration, filtration, storage, evaporation and transpiration by
emphasizing conservation, use of on-site natural features, site planning, and distributed stormwater
management practices that are integrated into a project design.
LID Best Management Practices: Distributed stomwater management practices, integrated into a project
design, that emphasize pre-disturbance hydrologic processes of infiltration, filtration, storage, evaporation and
trans-piration. LID BMPs are referred to as flow control BMPs in this manual and include, but are not limited
to, biroretention, permeable pavements, limited infiltration systems, roof downspout controls, dispersion, soil
quality and depth, and minimal excavation foundations.
LID Principles: Land use management strategies that emphasize conservation, use of on-site natural features,
and site planning to minimize impervious surfaces, native vegetation loss, and stormwater runoff'
Mqintenunce.. those usual activities taken to prevent a decline, lapse, or cessation in the use of currently
serviceable structures, facilities, BMPs, equipment, or systems if there is no expansion of any of these, and
there are no significant hydrologic impacts. Maintenance includes the repair or replacement of non-functional
facilities and BMPs, and the replacement of existing structures with different types of structures, if the repair or
replacement is required to meet current engineering standards or is required by one or more environmental
permits and the functioning characteristics of the original facility or structure are not changed. For the
pu.por"t of applying this definition to the thresholds and requirements of this manual, city review staff will
ietirmine whether the functioning characteristics of the original facility, structure, or BMP will remain
sufficiently unchanged to consider replacement as maintenance. Drainage review is not required for projects
proposing only maintenance
City of Kent Surface Water Design Manual lv Iuly 2022
KEY TERMS AND DEFINITIONS
Note; The following pavement mqintenance practices qre not categorically exempt from drainage review:
removing and replacing a paved surface to base course or lower (i.e. "replaced impervious surfaces"),
extendiig the eige offavement or paving graveled shoulders, or resurfacing that meets the definition of "new
impervious surface" in this manual.
Nationul Potlutant Discharge Elimination Syslem (NPDES): the national program for issuing, modifring,
revoking, reissuing, terminating, monitoring, and enforcing permits; and imposing and enforcing pretreatment
requirements; under sections 307, 402,3 18, and 405 of the federal CWA for the discharge of pollutants to
ruifu"" waters of the state. These permits are referred to as NPDES permits, and in the state of Washington are
administered by the Washington State Department of Ecology.
Native vegetated surface: asurface in which the soil conditions, ground cover, and species ofvegetation are
like those of the original native condition for the site. More specifically, (l) the soil is either undisturbed or has
been treated according to the "native vegetated landscape" specifications in Appendix C, Section C.2.1.8; (2)
the ground is either naturally covered with vegetation litter or has been top-dressed between plants with 4
inches of mulch consistent with the native vegetated landscape specifications in Appendix C; and (3) the
vegetation is either (a) comprised predominantly of plant species, other than noxious weeds' that are indigenous
to lhe coastal region ofthe Pacific Northwest and that reasonably could have been expected to occur naturally
on the s/e or (!j comprised of plant species specified for a native vegetated landscape in Appendix C.
Examples of these plant species include trees such as Douglas fir, western hemlock, western red cedar, alder,
big- leaf maple and vine maple; shrubs such as willow, elderberry, salmonberry and salal; and herbaceous
plants such as sword fem, foam flower, and fireweed.
Natural discharge sres: an onsite area tributary to a single natursl discharge location.
Natural discharge locstion: the location where surface and storm water runoff leaves (or would leave if not
infiltrated or retained) the site or proiect sile under existing site conditions.
Nstural onsite drainage feature: A natural swale, channel, stream, closed depression, wetland, or lake'
New impervious surface: the addition of a hard or compacted surface like roofs, pavement, gravel, or dirt; or
the addition of a more compacted surface, like paving over pre-existing dirt or gravel. Permeable pavement and
vegetated roofs are considered new impervious surface for purposes of determining whether the thresholds for
application of minimum requirements are exceeded, as are lawns, landscaping, sports fields, golf courses, and
oihe. ut"us that have modified runoffcharacteristics resulting from the addition ofunderdrains designed to
collect stormwater runoff. OPen,
uncovered retention/detention facilities shall not be considered impervious surfaces for purposes ofdetermining
whether the thresholds for application of minimum requirements are exceeded. Open, uncovered
retention/detention facilities shall be considered impervious surfaces for purposes of runoff modeling.
New pervious surface: the conversion of a native vegetated surface or other native surface to a non- native
pervious surface (e.g., conversion of forest or meadow to pasture land, grass land, cultivated land, lawn,
iandscaping, bare soil, etc.), or any alteration ofexisting non-native pervious surface that significantly increases
surface and storm water runoff (e.g., conversion of pasture land, grass land, or cultivated land to lawn,
landscaping, or bare soil; or alteration ofsoil characteristics)'
New PGIS: new impervious sudace that is pollution-generating impervious surface or any alteration of
existing pollution-generating impervious surface that changes the type ofpollutants or results in increased
pollution loads and/or concentrations.
New PGPS: new pervious suffice thatis pollution-generaling pervious surfuce or any alteration of existing
pollution-generating pervious surface that changes the type ofpollutants or results in increased pollution loads
andl or concentrations.
Permeable psvement: Pervious concrete, porous asphalt, permeable pavers or other forms of pervious or
porous puuing material intended to allow passage of water through the pavement section. It often includes an
aggregate base that provides structural support and acts as a storrnwater reservoir.
City of Kent Surface Water Design Manual Iuly 2022
KEY TERMS AND DEFINITIONS
Pervious Surface: Any surface material that allows stormwater to infiltrate into the ground. Examples include
lawn, landscape, pasture, and native vegetation areas. Note for purposes of threshold determination and runoff
volume modeiing for detention and treatment, vegetated roofs and permeable pavements are to be considered
impervious surfaces along with lawns, landscaping, sports helds, golf courses, and other areas that have
modified runoff characteristics resulting from the addition of underdrains.
Pollution-generating impervious surface (PGI,S)I an impervious surface considered to be a significant source
of pollutants in stormwater runoff. Such surfaces include those that are subiect to vehiculsr zse, industrial
activities, or storage of erodible or leachable materials, lr'ostes, or chemicals, and that receive direct rainfall or
the run-on or blow-in of rainfall. A covered parking area would be included if runoff from uphill could
regularly run through it or if rainfall could regularly blow in and wet the pavement surface. PGIS includes
metal roofs unless ihey are coated with an ineft, non-leachable material (see Reference 1 l-E); or roofs that are
exposed to the venting of significant amounts of dusts, mists, grease laden vapors/FOG, or fumes from
manufacturing, commercial, or other indoor activities. PGIS includes vegetated roofs exposed to pesticides,
fertilizers, or los of soil. Other roofing types that may pose risk but are not cunently regulated are listed in
Reference I 1-E. Lawns, landscaping, sports fields, golf courses, and other areas that have modified runoff
characteristics resulting from the addition of underdrains that have the pollution generating characteristics
described under the "pollution-generating pervious surface" definition are also considered PG1S.
Pollulion-generating pervious surface (PGP,S); a non-impervious surface considered to be a significant source
ofpollutants in surface and storm water runoff. Such surfaces include those that are subiect to vehicular use'
industrial activities, storage of erodible or leachable materials, wsstes, or chemicals, and that receive direct
rainfall or the run-on or biow-in of rainfall; or subject to use of pesticides and fertilizers, or loss of soil' Such
surfaces include, but are not limited to, the lawn and landscaped areas of residential, commercial, and industrial
sites or land uses, golf courses, parks, sports fields (natural and artificial turf), cemeteries, and City approved
grassed modular grid pavement.
Project site: that portion of a site and any offsite areas subject to proposed project activities, alterations, and
improvements including those required by this manual.
Rain Garden A shallow, landscaped depression with compost-amended native soils and adapted plants. The
depression is designed to pond and temporarily store stormwater runoff from adjacent areas, and to allow
stormwater to pass through the amended soil profile.
Receiving woters: bodies of water, surface water systems, or groundwater receiving water from upstream man-
made or natural systems.
Redevelopment project: a project that proposes to add, replace, or modiff impervious surfaces for purposes
other than a residential subdivision or maintenan ce on a sile that is already substantially developed in a manner
consistent with its current zoningor with a legal non-conforming use, or has an existing impervious surface
coverage of 35Yo or more. The following examples illustrate the application of this definition.
A RedeY Proiect that Adds and
Replaces lmpervioue Surface
A Redevelopment Project thsl
Adds Hew lmpervioua $urfaoe
A Redewlopment Project that
Replaces Imperviou* Surfaae
Residenti'al Site
Replaced impervious surface: any existing impervious surface onthe ptoiect site lhat is proposed to be
removed and re-established as impervious surface, excluding impervious surface removed for the sole purpose
of installing utilities or performing maintenance on underground infrastructure. For structures, removed means
the removal of buildings down to the foundation. For other impervious surfaces , removed means the removal
Edsting
Parking
Commercial Site
Existing Eldg Existing
Existinq
Fervious
,Area
t65%l
lmperviou:
Area (351'6)
rEl
New
Parki
Eaisting
lmpervi
Area
tuisting
Parking
City of Kent Surface Water Design Manual vl Iuly 2022
KEY TERMS AND DEFINITIONS
down to base course or bare soil. For purposes ofthis definition, base course is the layer ofcrushed rock that
typically underlies an asphalt or concrete pavement. It does not include the removal of pavement material
tiriough grinding or other surface modification unless the entire layer of PCC or AC is removed. Replaced
impervious surface also includes impervious surface that is moved from one location to another on the project
site where the following two conditions are met:
(A) the area from which the impervious surface is moved from will be restored to the same or better runoff
discharge characteristics as the area being covered by the moved impervious surface, and
(B) impervious surface at the new location is either designated as non- pollution generating or the pollution
generating characteristics remain unchanged compared to that of the original location.
Replaced PGIS: replaced impervious surface thatis pollution-generating impervious surfuce.
Sensitive luke: adesignation applied to lakes that are particularly prone to eutrophication from development-
induced increases in phosphorus loading. The Water Quality Applications Map indicates the lakes within the
City of Kent that are affected by this problem.
Severe building flooding problem.. there is flooding of the finished Jloor areal of a habitable building,z or the
electricaVheating system ofa habitable building for runoffevents less than or equal to a 100-year event.
Examples include flooding of finished floors of homes and commercial or industrial buildings, or flooding of
electrical/heating system components in the crawl space or garage of a home.
Severe erosion problem: there is an open drainage feature with evidence of or potential for erosion/incision
sufficient to poie a sedimentati onhazatdto downstream conveyance systems or pose a landslide hazardby
undercutting adjacent slopes. Severe erosion problems do not include roadway shoulder drilling or minor ditch
erosion.
Severe Jlooding problem: a severe building Jlooding problem or a sevete roadway flooding prohlem.
Severe roadway flooding problem.. there is flooding over all lanes of a roadway,3 ot a sole access drivewaya is
severely impacted, for runoffevents less than or equal to the 100-year event. A severely impacted sole access
driveway is one in which flooding overtops a culverted section of the driveway, posing alhreat of washout or
unsafe ut..5 conditions due to indiscemible driveway edges, or flooding is deeper than 6 inches on the
driveway, posing a severe impediment to emergency access.
Single family residential project: any project that (a) constructs or modifies a single family dwelling unit, (b)
.uk.r i*p.ovements (e.g., driveways, roads, outbuildings, play courts, etc.) or clears native vegetation on a lot
that contains or will contain a single family dwelling unit, or (c) is a plat, short plat, or boundary line
adjustment that creates or adjusts lots that will contain single family dwelling units.
Site: asingle parcel; or, two or more contiguous parcels that are under common ownership or documented legal
control; oia portion ofa single parcel under documented legal control separate from the remaining parcel, used
as a single parcel for a proposed project for purposes ofapplying for authority from the city ofKent to carry out
I Finishedfoorarea,forthepurposesofdefiningseverebuitdingfloodingproblem,meansanyenclosedareaofabuildingthatisdesigned
to be served by the building's permanent heating or cooling system
2 Habirable building means any residential, commercial, or industrial building that is equipped with a permanent heating or cooling
system and an electrical system.
3 Roodtvay,forthepurposesofthisdefinition,meansthetraveledportionofanypublicorprivateroadorstreetclassifiedassuchCityof
Kent Road Design and Construction Standards.
4 Sole access driveway means there is no other unobstructed, flood-free route for emergency access to a habitable building.
City of Kent Surface Water Design Manual vll Iuly 2022
KEY TERMS AND DEFINITIONS
a proposed project. For projects located primarily within dedicated rights-of-way,the length of the project site
and the right-of-way boundaries define the site.
Steep slope hszurd areu: the critical area designation, defined and regulated in KCC I 1.06 that is applied to
areas on a slope of 40% or more within a vertical elevation change of at least l0 feet. See the "Definitions"
section for more details.
Subject to vehiculur use: the surface is regularly used by motor vehicles including but not limited to
motorcycles, cars, trucks, busses, aftcraft,tractors, and heavy equipment. The following surfaces are
considered regularly used by motor vehicles: roads, un-vegetated road shoulders, bike lanes within the traveled
lane of a roadway, driveways, parking lots, unrestricted access fire lanes, vehicular equipment storage yards,
and airport taxiways and runways. The following surfaces are not considered regularly used by motor vehicles
paved bicycle pathways separated from and not subject to drainage from roads for motor vehicles, fenced or
iestricted access fire lanes, and maintenance access roads with a recurring use of no more than one routine
vehicle access per week.
Threshold discharge ares: anonsite area draining to a single nataral discharge location, or multiple natural
discharge locations that combine within one-quarter-mile downstream (as determined by the shortest
flowpath). The examples below illustrate this definition. This term is used to clarify how the thresholds,
exemptions, and exceptions of this manual are applied to sites with multiple discharge locations'
Example of a Sit€ with a
Single Natural Discharge
and a Single Thresholtl
DischargeArea
Natural
Discharge
Location
Erampleof a Sitewith
Multiple ltlatural
Discharges and a Single
Thrcshold DischargE Area
Natural
Discharge
Location
Example of a site with
Multiple Natunl Discharges
and ffultiple Threshold
DischargeAreas
Natural
Discharge
Location
l
I
I
IY
I
I
I
I
I
I
I
t
I
I I,
I
I
I
a>;
I
I,
I
a
l
I
I
__---_____1_-_-__
I
I
I
I
t-v
% t\iile
Downstream
(sFp(eFt flow
path)
_ _r_
Iv Iv
Transportation redevelopment project: a stand-alone transportation improvement project that proposes to add,
replace, or modify imperuious surface, for purposes other than maintenance, within a length of dedicated public
or private road right-of-way that has an existing impervious surface coverage of thirty-five percent or more.
Road right-of-way improvements required as part of a subdivision, commercial, industrial or multifamily
project may not be defined as a separate transportation redevelopment project.
Treatment Chain: A combination of two or more treatment Best Management Practices connected in series
(i.e., the design water volume passes through each facility in turn).
Unsubmerged porlionz Any portion outside the ordinary high water line of streams, lakes, and wetlands.
ll/etlands: All areas in the city of Kent that are inundated or saturated by surface or groundwater at a
frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of
urgltution typically adapted for life in saturated soil conditions. Wetlands generally include swamps, marshes,
Ar€a
Natural
Area
"r.
Y.
Natural
Area
THRESHOLD
t/
Area
Natural
-Qiscllarge-
Area 2
AREA
-,,,Aae.a,z
City of Kent Surface Water Design Manual vlll Iuly 2022
KEY TERMS AND DEFINITIONS
and similar areas. Wetlands do not include those artihcial wetlands intentionally created from nonwetland
sites, including, but not limited to, irrigation and drainage ditches, grass-lined swales, canals, detention
facilities, wastewater treatment facilities farm ponds, and landscape amenities. The burden of proving an area
to be nonwetland rests with the applicant. Wetlands include artificial wetlands intentionally created from
nonwetland areas for the purpose of mitigating conversion of wetlands. The City relies on the methodology
contained inthe Wetlands Delineation Manual, U.S. Army Corps of Engineers Technical Report Y-87-l for
identifying and delineating wetlands within the city. Wetland classes are based on the classification system
described in the U.S. Fish and Wildlife Service's Classification of Wetlands and Deepwater Habitats of the
United States (Cowardin et al. 1979) (See Kent City Code Chapter I I .05).
Wetland Buffer or wetland buffer zone: An area that surrounds and protects a wetland from adverse impacts
to the functions and values of a wetland (See Kent City Code Chapter I 1.05
City of Kent Surface Water Design Manual lx IuJy 2022
City of Kent Surface Water Design Manual July 2022
CHAPTER 1
DRAINAGE REVIEW AND REQUIREMENTS
KENT
WasHrNcroN
CITY OF KENT
Surface Water Design Manual
2022
CHAPTER 1
Drainage Review and Requirements
.l
l.t
l.l
l.l
,2
t.2
Drainage Review
I Projects Requiring Drainage Review........
.2 Drainage Review Types and Requirements
.3 Drainage Design Beyond Minimum Compliance
Core Requirements
I Core Requirement #l: Discharge at The Natural Location
1.2.2 Core Requirement#2: Offsite Analysis
1.2.3 Core Requirement #3: Flow Control Facilities
1.2.4 Core Requirement #4: Conveyance System........'
|.2.5 Core Requirement #5: Erosion and Sediment Control.'.'..
2.6 Core Requirement #6: Maintenance and Operations
.2.7 Core Requirement#7: Financial Guarantees and Liability
.2.8 Core Requirement #8: Water Quality2.9 Core Requirement #9: Flow Control BMPs
Special Requirements
3.1 Special Requirement #l: Other Adopted Area-Specific Requirements.....'.......'...
3.2 Special Requirement #2: Flood HazardArea Deliniation....'.....'.'.'.
3.3 Special Requirement #3: Flood Protection Facilities....'.
3.4 Special Requirement #4: Source Controls
3.5 Special Requirement #5: Oil Control........
Adjustment Process
1.4.1 AdjustmentAuthority
1.4.2 Criteria for Granting Adjustments
1.4.3 AdjustmentApplication Process..
1.4.4 Adjustment Review Process
1.4.5 Request For Reconsideration Procedure
-1
-1
-J
-4
I
3
J
4
-82
-84
-84
-85
-81
-87
-89
-25
-39
I
I
I
I
r.3
I
I
I
I
I
1.4
-43
-49
-51
-52
-61
-76
-76
1a
-79
80
City of Kent Surlace Water Design Manual Iuly 2022
City of Kent Surface Water Design Manual htly 2022
CHAPTER I
DRAINAGE REVIEW AND REQUIREMENTS
I DRAINAGE REVIEW AND REQUIREMENTS
This chapter describes the drainage review procedures and types, the drainage requirements, and the
adjustment procedures necessary to implement surface water runoff policies codified in Chapters 7.05 and
7.06 of the Kent City Code. It also provides direction for implementing more detailed procedures and
design criteria found in subsequent chapters of this manual.
Chapter Organization
The information presented in Chapter I is organized into four main sections as follows:
r Section I .1, "Drainage Review"
r Section l.2,"Core Requirements"
r Section 1.3, "Special Requirements"
r Section 1.4, "Adjustment Process"
Formatting of Chapter Text
The text of Chapter I and subsequent chapters has been formatted using the following conventions to aid the
user in finding, understanding, and properly applying the thresholds, requirements, and procedures
contained in this manual:
. Italic is used to highlight the following: (a) terms when they are first introduced and defined within the
same paragraph; (b) special notes that supplement or clariff thresholds, requirements, and procedures; (c)
sentences considered important for purposes ofunderstanding thresholds, requirements, and procedures; and
(d) titles of publications.
. Bold italic is used to highlight terms considered key to understanding and applying drainage review
thresholds, requirements, and procedures. These are called "key terms" and are defined below. This
convention upptier after the key term is defined and does not necessarily apply to tables and figures.
. Bold is used to highlight words and phrases that are not key tetms but are considered important to
emphasize for purposes of finding and properly applying thresholds, requirements, and procedures.
I..I" DRAINAGE REVIEW
Drainage review is the evaluation by city review staff of a proposed project's compliance with the
drainage requirements of this manual. During drainage review, city review staff may also consider the
propot"d project's compliance with other Kent requirements (which are not covered in this manual), such
as those specified in the Kent Wetland Management Code, Kent Geologic Hazard Areas Code, and other
environmental plans and policies. If required, drainage review becomes an integral part of the overall
permit review process. This section describes when and what type of drainage review is required for
a proposed project and how to determine the drainage requirements that apply.
City of Kent Surface Water Design Manual l-l Iuly 2022
Guide to Using Section 1.1
The following steps are recommended for efficient use of Section I . I :
l. Determine whether your proposed project is subject to the requirements of this manual by seeing if it
meets any of the thresholds for drainage review specified in Section 1.1.1. Making this determination
requires an understanding of the key definitions listed below.
2. If drainage review is required per Section 1 .1 . l, use the flow chart in Figure LI .2.A to determine what
type of drainage review will be conducted by city review staff. The type of drainage review defines
the scope of drainage requirements that will apply to your project as summarized in Figure 1"I.2'4.
3. Check the more detailed threshold information in Section I .1 .2 to verify that you have determined the
correct type of drainage review.
4. After veriffing drainage review type, use the information in Section l.l .2 to determine the core
requirements (found in Section 1.2) and the special requirements (found in Section 1.3) that must be
evaluated for compliance by your project. To determine what actions are necessary to comply with
each applicable core and special requirement, see the more detailed information on these requirements
contained in Sections 1.2 and 1.3 of this chapter'
Note; For Steps 2 through 4, it is recommended that you qrrange a preapplication meeting with city
review staffto confirm the type of drainage review and scope of drainage requirements that apply to your
proposed project.
Cify of Kent Surface Water Design Manual t-2 Jrly 2022
1.1.1 PROJECTS REQUIRING DRAINAGE REVIEW
Drainage review is required for any proposed project (except those proposing only routine maintenance,
repair, or emergency modifications) that is subject to a Kent development proposal, permit, or approval
listed below, AND that meets any one of the following conditions:
1. The project adds or will result in 2,000 square feets or more of new impervious surface, replaced
impervious surface, or new plus replaced impervious surface, OR
2. Proposes to construct or modifr a drainage pipe/ditch that is l2 inches or more in size/depth, or
receives surface water or stormwater runoff from a drainage pipe/ditch that is 12 inches or more in
sizeldepth, OR
3. Contains or is adjacent to a flood hazard area, stream, lake, wetland, closed depression, erosion
hazard area or landslide hazard area, OR
4. Is located within a Landslide Hazard Drainage Area, OR
5. Is a redevelopment project proposing $ 1 00,000 6 or more of improvements to an existing high-use
site, OR
6, The project proposes 7,000 square feet or more of land disturbing activity.
If drainage review is required for the proposed project, the type of drainage review must be determined
basedonprojectandsitecharacteristicsasdescribedinSectionl.l.2. Thetypeofdrainagereviewdefines
the scope of drainage requirements that must be evaluated for project compliance with this manual.
5 Thethresholds fornewinpemioussurfoce,replacedimpemioussurface,andlanddisturbingactivityshallbeappliedbyprojectsiteand
in accordance with the definitions ofthese surfaces and activities.
6 This is the "Total Cost Estimate" as declared on the permit application submitted to the City of Kent or a valuation as determined by
the Engineer's Cost Estimate from the 2021 Kent Design and Construction Standards
T
H
R
E
s
H
o
L
o
Construction Permits
Conditional Use Permits
Grade and Fill Permits
Flood Control Zone Permit
Planned Unit Developments
Rezones
Shoreline Management Substantial Development Permits
Short Subdivision Developments
(Short PlaQ
Subdivision Developments (Plat)
Kent Permits and rovals
City of Kent Surface Water Design Manual l-3 IluJ'y 2022
1.1.2 DRAINAGE REVIEW TYPES AND REQUIREMENTS
For most projects adding 2,000 square feet or more of new plus replaced impervious surface, the full
range of core and special requirements contained in Sections 1 .2 and I .3 must be evaluated for compliance
through the drainage review process. However, for some types of projects the scope of requirements
applied is narrowed to allow more efficient, customized review. Each of the following three drainage
review types tailors the review process and application ofdrainage requirements to a project's size,
location, type of develepment, and anticipated impacts to the local and regional surface water system:
o Simplified Site Drainage Review, Section 1.1.2.1.
r Targeted Drainage Review, Section 1.1 .2.2.
o Full Drainage Review, Section 1.1.2.3.
Each project requires only one ofthe above drainage review types, with the single exception that a project
that qualifies for Simplified Site Drainage Review may also require Targeted Drainage Review. Figure
l.l.2.A can be used to determine the drainage review type that would be required. This may entail
consulting the more detailed thresholds for each review type specified in the above-referenced sections.
Table I .l .2.A can be used to quickly identify the requirements that are applied under each type of drainage
review. The applicant must evaluate those requirements that are checked off for a particular drainage
review type to determine what is necessary to meet compliance.
City of Kent Surface Water Design Manual t-4 Iuly 2022
SECTION I.1 DRAINAGE REVIEW
Yes
No
No
Yes No Yes
FULL DRATNAGE REVIEW
Section 1.1.2.3
Simplified stre
DRAINAGE
REVIEW
Section L1.2,1
Note: The project
rnay also be
subject to
Targeted
Drainage Review
as delernined
belov'.
Is the project asinglefamil.v residentiol or agricuhuril projeclthat results in >2,000
sf of new plw repluced impervious sutfoce or >7.000 sf of land disturbing activitl',
results in less than 5,000 squal'e leet olner.v plus replaced pollution genelating
impervious surface. results in less than % acle ofpollution generating pelvious
surfhces AND meets one of the fbllorving critelia?
. The ploject meets the Basic Exerrrption h'om florvcontlol in Cole Requirement#3.
lu'ote the Bosic Exemption thresholcls are applied by project site.
. ft'or projects inside the LJrban Growlh,'lrea on predorninalely till soils:
'I'he project results in no more than 7.947 square leet oltarget impervious sttrfhcesx as
deflned in Section l.l.2.l AND pr.oposed pervious area is equal to or less than 14.941 -
1.88 x (total target imperviorts surfaces)
. For projects itrsicle the LJrban Growth ilreo on predotninatelv ouhvash soils:
The pro.iect lesults in no iltore than 6.872 square lbet of target imperviotts sttrf-aces* as
clefined in Section l. l.2.l AND pr.oposed per.r,ious area is equal to or less than 20,343
2.96 x (total target impervious stttfhces)
. ls on agriatltural project that qualifes for the " lntperviotts Surfoce Percentoge
For " detailed in Core 3
Does thc project have the characteristics of one or more olthe fbllor'r'irlg
categories (see the morc detailed threshold language iu Sectiotr 1.1.2 2)'l
l. Projects tlrat contain or are adjaoent lo a flood" erosiott, or steep slope
hazord area:, or pro.jects r.r'ithin a Landslide Hazard Drainage Arca'
2. Proiects proposing to constfltct or nntlify adrainage pipe/ditch that is 12" or
larger or receives runoff frorn a 12" or larger drainage pipe/ditch.
3. Retlevelopntent proiects ploposlrlg > $100,000 in improvetrlerlts to all cxistirtg
high-use site.
Docs the pro.lect add > 2,000 sf ol
rreu plus repluced impervious
surfnce or >7 "000 sfof/arlrl
disturhing rctivitt?
TARGETED DRAINAGE REVIEW
Section L1.2.2
Rcassess rvhether
drainage levicrv is
required per Section
t.l.2.t
FIGURE I.I.2.A FLOW CHART FOR DETERMINING TYPE OF DRAINAGE REVIEW REQUIRED
City olKent Surlace Watcr Design Manual l-5 July 2022
SECTION I.1 DRAINAGE REVIEW
Simplified
Single-family residential projects that result in : 2,000 sf of new plus replaced impervious surface or >7 ,000 sf of land
disturbing activity but do not exceed the new plus replaced PGIS, new PGPS, and new pervious surface thresholds
specified in Sec. l l.2.l.
Simplified Site projects or other small projects that are not subject to Full Drainage Review as determined in Section 1.1'2.3,
AND which have the characteristics of one or more of the following categories of projects:
1. Projects containing or adjacent to aJlood, erosion,or steep slope hagrd area; projects within a Landslide Hazard
Drainage Area.
2. Projects proposing to consttuct ot modify a drainage pipe/ditch that is 12" or larger or receives runoff from a 12" or
larger drainage pipe/ditch.
3 . Redevelopment projects proposing > $ 100,000 in improvements to a high-use site'(r)
Targeted
All projectsthatresultin>2,000 sf of newplusreplacedimpervioussurfaceor>7,000sf of landdisturbingaclivitybulare
not subject to Simplified Site Drainage Review.Full
DRAINAGE REVIEW TYPE
FullTareeted
Catesorv 3Catesorv I Cateeory2
Simplificd
{SIMPLIFIED SITE REQUIREMENTS
,//ro''.s(z),/CORE REQUIREMENT #I
Discharse at Natural Local.ion
/,',.t-Q)1/rzty',0,CORE REQUIREMENT #2
Offsite Analysis
1/<z>1/<t>s-Q)CORE REQUIREMENT #3
Flow Control
///,0,.vQ)CORE REQUIREMENT #4
Conveyance Svstem
//{r+t /{CORE REQUIREMENT #5
Erosion & Sediment Control
/*(2)/,/1/totCORE REQUIREMENT #6
Maintenance & Operations
1/ttt /,,,1/ <t>s.Q)1/rztCORE REQUIREMENT #7
Financial Guarantees & Liabilitv
/,,,g(z)/,0,CORE REQUIREMENT #8
Water Oualitv
,/CORE REQUIREMENT #9
Flow Control BMP's
y'et
1/rzt{,0,/,',SPECIAL REQUIREMENT #I
Other Adopted Requirements
/,',/,',/,0,SPECIAL REQUIREMENT #2
Developments within Floodplain/Floodway
/,',{,,,/,0,1/<t>SPECIAL REQUIREMENT #3 Flood
Protection Facilities
{,,,{,,,/,,,1/ttt/,0,SPECIAL RBQUIREMENT #4
Source Control
{,',1/ttt1/<+>SPECIAL REQUIREMENT #5
Oil Control
2constructthatmodior2-inch are also3ectsoilcontrols Category projects.fy pipe/ditch()Category proJ installing
ofDocumentationstaff basedrevlew on or conditions.required.ificbesi/e-spec compliancebyappliedcityprojectMay(2)
tothatthresholdslimitortheirThesehaveorapplication project.specificprecludemayrequirementsexemptions)(3
lndetailedwithlesStheAtoReviewdrainagerequirementsthatDrainagecomplimplifiedproposedsubjectSimplifiedproject4)(
wothat uldSectionsln.2I Iand .3 thosetoallwithcoretheand requirementslsCrequirementsexceptpresumedcomplyspecialAppendix
2ASReviewln.2SectionthetoIitlstoTargetedDrainagespecifiedapplyprojectsubject
TABLE I.I.2.A REQUIREMENTS APPLIED UNDER EACH DRAINAGE REVIEW TYPE
City of Kent Surface Water Design Manual 1-6 July 2022
SECTION 1.I DRAINAGE REVIEW
t.l.2.t SIMPLIFIED SITE DRAINAGE REVIEW
Simplified Site Drainage Review is a simplified alternative to Full Drainage Review for small residential
projects, clearing projects or small agricullural projects that meet the threshold requirements below. The
core and special requirements applied under Full Drainage Review are replaced with simplified site
requirements that can be applied by a non-engineer. These requirements include flow control Best
Management Practices (BMPs) such as setting aside open space to limit future site clearing, and using
simple measures such as splash blocks and gravel trenches to disperse or infiltrate runoff from impervious
areas. Also included are simple BMPs for erosion and sediment control (ESC). Formal water quality
treatment is not necessary. This altemative to Full Drainage Review acknowledges that drainage impacts
for many small development proposals can be effectively mitigated without construction of costly flow
control and water quality facilities.
The Simplified Site Drainage Review process minimizes the time and effort required to design, submit,
review, and approve drainage facilities for these proposals. In most cases, the requirements can be met
with submittals prepared by contractors, architects, or homeowners without the involvement of a licensed
civil engineer.
Threshold
Simplified Site Drainage Review is allowed for any project that is subject to drainage review as
determined in Section 1.1.I and that meets all of the following criteria:
. The project is a single-family residential project, AND
o The project will result in 2,000 square feetT or more of new impervious surface, replaced impervious
surface, or new plus replaced impervious surface, or
r The project will result in 7,000 square feetT or more of lund disturbing activity,
AND that meets the following criteria:
The project will result in less than 5,000 square feet of new plus replaced pollution generating
impervious surface, will result in less than 3/+ acl:e of new pollution generating pervious surfaces, AND
meets one of the following four additional criteria:
l. The project meets the Basic Exemption from flow control in Core Requirement#3. Note the
Basic Exemption thresholds are applied by project site.
2. For nrojects inside the Urban Growth Area on predominately till soils:
The project results in no more than7,947 square feet of target impervious surfaces as defined below
AND proposed pervious area is equal to or less Ihan 14,941 - 1.88 x (total target impervious
surfaces).
3.:
The project results in no more than 6,872 square feet of target impervious surfaces as defined below
AND proposed pervious area is equal to or less than20,343 -2.96 x (total target impervious surfaces).
4.@:
The project is an agricultural project that qualifies for "lmpervious Surface Percentage Exemption For
Agricultural Projects" as cited in Core Requirement 3 (Flow Control Facilities).
7 The thresholds of2,000 and 7,000 square feet shall be applied by project site. All other thresholds specified in terms ofsquare feet of
impervious or pervious surface shall be applied by threshotd discharge area and in accordance with the definitions ofthese surfaces in
Sectionl.l. Note:thecalculationoftotalimpervioussurfaceaddedonafterJanuaryS,200lmayexcludeanysuchadfudimpervious
surface that is confrmed by Planning and Economic Developmenl engineering staf to be already mitigated by a County approved and
inspectedflow control facilily or B MP.
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SECTION 1.I DRAINAGE REVIEW
1.1.2.2
Note: Some projects qualifyingfor Simplified Site Drainage Review may also require Targeted Drainage
Reviau if they meet any of the threshold criteria in Section LL2'2.
Any potential Simplified site proposal may elect to go through Full Drainage Review described in Section
1.1 .2.3.
Scope of Requirements
IF Simplified Site Drainage Review is allowed, THEN the applicant may apply the simplified site
submittal and drainage design requirements detailed in Simplified Drainage Requirements adopted as
Appendix C. These requirements include simplified BMPs for flow control and erosion and sediment
control. Note: An open spqce tract or covenant may be required to preserve uncleared areas.
Exemption from Core and Special Requirements
The simplified drainage requirements applied under Simplified Site Drainage Review are considered
sufficient to meet the overall intent of the core and special requirements in Sections 1.2 and 1.3, except
under certain conditions when a proposed project has characteristics that trigger Targeted Drainage
Review (see the threshold for Targeted Drainage Review in Section I .1 .2.2) and may require the
involvement of a licensed civil engineer. Therefore, any proposed project that qualifies for Simplified Site
Drainage Review as determined above and complies with the simplified site drainage requirements detailed
in Appendix C is considered exempt from all core and special requirements in Sections 1.2 and 1.3 except
those which would apply to the project if it is subject to Targeted Drainage Review as specified in Section
1,1.2.2.
TARGETED DRAINAGE REVIEW
Targeted Drainage Review (TDR) is an abbreviated evaluation by city review staff of a proposed project's
compliance with selected core and special requirements. Projects subject to this type of drainage review
are typically small-site proposals or other small projects that have site-specific or project-specific drainage
concerns that must be addressed by a licensed civil engineer. Under Targeted Drainage Review,
engineering costs associated with drainage design and review are kept to a minimum because the review
includes only those requirements that would apply to the particular project.
Threshold
Targeted Drainage Review is required for those projects subject to drainage review as determined in
Section 1.1.1, AND that are not subject to Full Drainage Review as determined in Section L I .2.3, AND
that have the characteristics of one or more of the following project categories:
o TDR Project Category #1: Projects that contain or are adjacent to aflood hazard area, erosion
hazard area, or steep slope hazard srea as defined in KCC I 1.06; OR projects located within a
Landslide Hszsrd Drainage Area.
o TDR Project Category #2: Projects that propose to construct or modify a drainage pipe/ditch that is
12 inches or more in sizeldepth or receives surface and stormwater runoff from a drainage pipe/ditch
that is 12 inches or more in sizeldepth.
o TDR Project Category #3: Redevelopment projects that propose $ 100,000 or more of improvements
to an existing high-use site.
Scope of Requirements
IF Targeted Drainage Review is required, THEN the applicant must demonstrate that the proposed project
complies with the selected core and special requirements corresponding to the project category or
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SECTION 1.1 DRAINAGE REVIEW
categories that best match the proposed project. The project categories and applicable requirements for
each are described below and summarized in Table 1.1.2.4.
Note; If the proposed project has the chqracteristics of more than one project cqtegory, the requirements
ofeach applicable category shall apply.
Compliance with these requirements requires submittal of engineering plans and/or calculations stamped
by a licensed civil engineer registered in the state of Washington, unless deemed unnecessary by city
review staff. The engineer need only demonstrate compliance with those core and special requirements
that have been predetermined to be applicable based on specific project characteristics as detailed below
and summarized in Table 1.1.2.A. The procedures and requirements for submittal of engineering plans
and calculations can be found in Section 2.3.
In addition, city review staff may require the applicant to demonstrate compliance with any one or more
of the nine core requirements in Section 1.2 based on project or site-specific conditions. For example, if
a Category # 1 TDR Project contains or is adjacent to a flood hazqrd sres, erosion hazard area, or steep
slope hazard areu, OR located within a Landstide Hazard Drainage Area city review staff may require
compliance with "Core Requirement # I : Discharge at the Natural Location" (Section I .2. 1). This may in
tum require compliance with "Core Requirement #2: Offsite Analysis" (Section 1.2.2) if a tightline is
required by Core Requirement #l . If atightline is found to be unfeasible, city review staff may instead
require a flow control facility per "Core Requirement #3: Flow Control" (Section |.2.3). lf a tightline is
feasible, "Core Requirement #4: Conveyance System" (Section 1.2.4) would be required to ensure proper
size and design. Any required flow control facility or tightline system may also trigger compliance with
"Core Requirement #6: Maintenance and Operations" (Section 1.2.6),"Core Requirement #7: Financial
Guarantees and Liability" (Section 1.2.7), and possibly "Core Requirement #8, Water Quality" (Section
1.2.8) if runoff from pollution-generating impervious surfaces is collected'
The applicant may also have to address compliance with any applicable requirements in the Kent City
Code for flood hazard areil, erosion hazard areo, or steep slope hazard area, OR projects located within a
Lsndslide Hazard Drsinage Area as determined by city review staff'
TDR Project Category #1
This category includes projects that are too small to trigger application of most core requirements, but
may be subject to site-specific requirements pertaining to floodplains, streams, lakes, wetlands, closed
depressions, Landslide Hazard Drainage Areas, ErosionHazard Areas, or other area-specific drainage
requirements adopted by the City. Such projects primarily include single-family residential proiects
in Simplified Site Drainage Review.
IF the proposed project meets the characteristics of TDR Project Category #1, THEN the applicant
must demonstratethat the project complies with the following six requirements:
. Core Requirement #2: Offsite Analysis, Section 1.2.2
o Core Requirement #5: Erosion and Sediment Control, Section 1.2.5
. Special Requirement # 1 : Other Adopted Area-Specific Requirements, Section I .3. I
. Special Requirement #2: Floodplain/Floodway Delineation, Section 1.3.2
. Special Requirement #3: Flood Protection Facilities, Section 1.3.3
. Special Requirement #4: Source Control, Section 1'3.4
TDR Project Category #2
This category is intended to apply selected core and special requirements to those projects that
propose to constracl or modify a drainage system of specified size, but are not adding sufficient
impervious surface to trigger Full Drainage Review.
IF the proposed project meets the characteristics of TDR Project Category #2, THEN the applicant
must demonstrate that the proposed project complies with the following requirements:
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SECTION 1.I DRAINAGE REVIEW
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Core Requirement # I : Discharge at the Natural Location, Section I .2. 1
Core Requirement#2: Offsite Analysis, Section 1.2'2
Core Requirement#4: Conveyance System, Section 1.2.4
Core Requirement #5: Erosion and Sediment Control, Section 1.2.5
Core Requirement #6: Maintenance and Operations, Section 1.2'6
Core Requirement#7: Financial Guarantees and Liability, Section 1.2'7
Special Requirement #4: Source Control, Section 1'3.4
TDR Project Category #3
This category is intended to improve water quality by applying water quality, source control, and oil
control requirements to redevelopment projects located on the most intensively used srles developed
prior to current water quality requirements. These are referred to as high-use sites and are defined
below,
IF the proposed project meets the characteristics of TDR Project Category #3, THEN the applicant
must demonstrate that the proposed project complies with the following requirements:
. Core Requirement # I : Discharge at Natural Location, Section I '2. I
r Core Requirement #5: Erosion and Sediment Control, Section 1.2'5
r Core Requirement #6: Maintenance and Operations, Section 1.2'6
r Core Requirement #7: Financial Guarantees and Liability, Section 1.2.7
. Core Requirement #8: Water Quality, Section 1.2.8
. Special Requirement #4: Source Control, Section 1.3.4
r Special Requirement #5: Oil Control, Section 1.3.5
Note: In some cases, city review staffmay determine that application of these requirements does not
require submittal of engineering plans and calculations stamped by a licensed civil engineer. For
example, if catch basin inserts are proposed to meet oil control requirements, engineered plans and
calculations may not be necessary. A plot plan showing catch basin locations may sffice.
FULL DRAINAGE REVIEW
Full Drainage Review is the evaluation by city review staff of a proposed project's compliance with the full
range of core and special requirements in this chapter. This review addresses the impacts associated with
adding new impervious surface and changing land cover on typical siles'
Threshold
Full Drainage Review is required for any proposed new and redevelopment proiects that are subject to
drainage review as determined in Section 1.1,1, AND which meet one or more of the following criteria:
. Projects which will result in 2,000 square feets or more of new impervious surface, or new plus
replaced impervious surfuce but which do not qualify for Simplified Site Drainage Review as
specified in Section 1 . L2.1, OR
8 The thresholds of 2,000, 5,000, and 7,000 square feet shall be applied by project site.
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a 7,000 square feets or more of lund disturbing activity, but which do not qualify for Simplified Site
Drainage Review as specified in Section 1.1.2.1, OR
Projects located within a Landslide Hazard Drainage Area which add 2,000 square feet or more of
new impervious surface but which do not qualify for Simplified Site Drainage Review per Section
1.1.2.1, OR
Redevelopment projects proposing $500,000 or more of site improvements which create 5,000
square feet or more of contiguous pollution-generating impervious surfice through any
combination of new and/or replaced impervious surface.
Scope of Requirements
IF Full Drainage Review is required, THEN the applicant must demonstrate that the proposed project
complies with the following requirements:
o All nine core requirements in Section 1.2
. All five special requirements in Section 1.3
Engineering plans and calculations stamped by a licensed civil engineer registered in the state of
Washington must be submitted to demonstrate compliance with these requirements. The procedures and
requirements for submittal of engineering plans and calculations are as directed by Economic and
Community Development in the Drainage Review Process.
Drainage review for a proposed project's impact on surface and storm waters may be addressed by
processes or requirements apart from Kent's. Agencies such as those listed below may require some form
of drainage review and impose drainage requirements that are separate from and in addition to Kent's
drainage requirements. The applicant is responsible for coordinating with these agencies and resolving any
confl icts in drainage requirements.
DRAINAGE DESIGN BEYOND MINIMUM COMPLIANCE
This manual presents Kent's minimum standards for engineering and design of drainage facilities. While
the City believes these standards are appropriate for a wide range of development proposals' compliance
solely with these requirements does not relieve the professional engineer submitting designs of his or her
responsibility to ensure drainage facilities are engineered to provide adequate protection for natural
resources and public and private propefiy.
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Developer/Local Agency Agreement
Hydraulic Project Approval
Short Term Water Quality Modification Approval
Dam Safety permit
NPDES Stormwater permit
UIC Well Registration
Forest Practices Class IV permit
Sections 10, 401 , and 404 permits
Seattle/King County Department of Public Health
Washington State
Department of Transportation
Department of Fish and Wildlife
Deparlment of Ecology
Department of Natural Resources
United States Army Corps of Engineers
Permit/ApprovalAgency
City of Kent Surface Water Design Manual l-l I luly 2022
SECTION 1.I DRAINAGE REVIEW
Compliance with the standards in this manual does not necessarily mitigate all probable and significant
environmental impacts to aquatic biota. Fishery resources and other living components of aquatic systems
are affected by a complex set of factors. While employing a specific flow control standard may prevent
stream channel erosion or instability, other factors affecting fish and other biotic resources (such as
increases in stream flow velocities) are not directly addressed by this manual. Likewise, some wetlands,
including bogs, are adapted to a very constant hydrological regime. Even the most stringent flow control
standard employed by this manual does not prevent increases in runoff volume that can adversely affect
wetland plant communities by increasing the duration and magnitude of water level fluctuations. Thus,
compliance with this manual should not be construed as mitigating all probable and significant stormwater
impacts to aquatic biota in streams and wetlands, and additional mitigation may be required.
In addition, the requirements in this manual primarily target the types of impacts associated with the most
typical land development projects occurring in the City. Applying these requirements to vastly different
types of projects, such as rock quarries or dairy farms may result in poorer mitigation of impacts.
Therefore, different mitigation may be required.
Additional mitigation may also be required to compensate for loss of critical area habitat functions
associated with reducing standard buffer widths and clearing restrictions as allowed through the
approval of Rural Stewardship Plans and Farm Management Plans per KCC 11.06'
City of Kent Surface Water Design Manual t-12 Iuly 2022
SECTION I.2 CORE REQUIREMENTS
1.2.1
1.2 coRE REQUIREMENTS
This section details the following eight core requirements:
. Core Requirement #1 : Discharge at the Natural Location, Section I .2.1
o Core Requirement #2: Offsite Analysis, Section 1'2.2
o Core Requirement #3: Flow Control, Section 1.2.3
o Core Requirement #4: Conveyance System, Section 1.2.4
r Core Requirement #5: Erosion and Sediment Control, Section 1.2.5
. Core Requirement #6: Maintenance and Operations, Section 1.2.6
. Core Requirement #7: Financial Guarantees and Liability, Section 1.2.7
. Core Requirement #8: Water Quality, Section 1.2.8
. Core Requirement #9: Flow Control BMPs, Section 1.2.9
CORE REQUIREMENT #I: DISCHARGE AT THE NATURAL
LOCATION
All surface and stormwater runoff from a project must be discharged at the natural location so as not to be
diverted onto or away from downstream properties. The manner in which stormwater runoff and surface water
are discharged from the project site must not create a significant adverse impact to downhill properties or
drainage facilities (see "Discharge Requirements" below)
Note: Projects that do not discharge all project site runoffat the natural location will require an approved
adjustment of this requirement (see Section 1.4). City review staffmay waive this adjustment, however, for
piojects in which only a small portion of the project site does not discharge runoffat the natural location and the
runofffrom that portion is unconcentrated and poses no significant qdverse impact to downstream properties.
Intent: To prevent adverse impacts to downstream properties caused by diversion of flow from one flow path to
another, and to discharge in a manner that does not significantly impact downhill properties or drainage systems.
Diversions can cause greater impacts (due to greater runoff volumes) than would otherwise occur from new
development discharging runoffat the natural location. Diversions can also impact properties that rely on runoff
water to replenish wells and omamental or fish ponds . Projects that do not discharge at the natural location will
require an approved adjustment of this requirement (see Section I '4.
tr DISCHARGE REQTIIREMENTS
Proposed projects must comply with the following discharge requirements (7,2,and 3) as applicable:
1. Where no conveyance system exists at the abutting downstream property line and the natural (existing)
discharge is unconcentrated, any runoffconcentrated by the proposed project must be discharged as follows:
a) IF the 100-year peak dischargee is less than or equal to 0.2 cfs under existing conditions and will
remain less than or equal to 0.2 cfs under developed conditions, THEN the concentrated runoff may be
discharged onto a rock pad or to any other system that serves to disperse flows.
b) IF the 100-year peak discharge is less than or equal to 0.5 cfs under existing conditions and will remain
less than or equal to 0.5 cfs under developed conditions, THEN the concentrated runoff may be
e peak discharges for applying this requirement are determined using the approved runoff model with I S-minute time steps as detailed in Chapter
3.
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SECTION I.2 CORE REQUIREMENTS
1.2.2
discharged through a dispersal trench or other dispersal system provided the applicant can demonstrate
that there will be no significant adverse impact to downhill properties or drainage systems.
c) IF the 10O-year peak discharge is greater than 0.5 cfs for either existing or developed conditions, or ifa
significant adverse impact to downhill properties or drainage systems is likely, THEN a conveyance
system must be provided to convey the concentrated runoffacross the downstream properties to an
acceptable discharge point.r0 Drainage easements for this conveyance system must be secured from
downstream properfy owners and recorded prior to engineering plan approval
2. IF a proposed project or any nqturol discharge area within a project is located within a Landslide Hazard
Drainage Area and, in fact, ultimately drains over the erodible soils of a defined lsndslide hazard area
withslopessteeperthanT5Yo,THEN atightlinesystemmuslbeprovidedthroughthelandslidehazard
areuto an acceptable discharge point unless one ofthe following exceptions applies. The tightline system
must comply with the design requirements in Core Requirement #4 and in Section 4.2.2 unless otherwise
approved by city review staff. Drainage easements for this system must be secured from downstream
property owners and recorded prior to engineering plan approval.
Exceptions: A tightline is nol required for any nuturul discharge locution where city review staff
approves an alternative system based on a geotechnical evaluation/recommendation from a licensed
geotechnical engineer that considers cumulative impacts on the hazard area under built out conditions AND
one of the following conditions can be met:
a) Less than 2,000 square feet of new impervious surface will be added within the natural discharge
area,OR
b) The developed conditions runoff from the naturul discharge areais less than 0.1 cfs for the 100-
year runoff event and will be infiltrated for runoff events up to and including the 100-year event, OR
c) The developed conditions runoff volumett fromthe natursl discharge ares is less than 50% of the
existing conditions runoff volume from other areas draining to the location where runoff from the
nutural dischurge areu enters the landstide hazard area onto slopes steeper than l5oh, AND the
provisions of Discharge Requirement I are met, OR
CORE REQUIREMENT #2: OFFSITE ANALYSIS
Project proponents must submit an offsite analysis report that assesses potential offsite drainage and water
quality impacts associated with development of theproject site and that proposes appropriate mitigation
measures for those impacts. The initial permit submittal shall include, at minimum, a Level 1 downstream
analysis as described in Section 1.2.2.1 below. If impacts are identified, the proposed projects shall meet any
applicable problem-specific requirements specified in Section 1.2.2.2 for mitigation of impacts to drainage
problems and Section I .2.2.3 for mitigation of impacts to water quality problems.
Intent: To identify and evaluate offsite flooding, erosion, and water quality problems that may be created or
10 Acceprable discharge point means an enclosed drainage system (i.e., pipe system, culvert, or tightline) or open drainage feature (e.g., ditch,
channel, swale, stream, river, pond, lake, or wetland) where concentrated runoff can be discharged without creating a significant adverse impact
I I For the purposes of applying this exception, the developed conditions runoff volume is the average annual runoff vol ume as computed per
Chapter3. Theanalysisisperformedusingtheentrreperiodofrecord. Thetotalvolumeisdividedbythenumberoffullwateryearsbeing
analyzedto determine the annual average runoffvolume. Any areas assumed not to be cleared when computing the developed conditions
runoffvolume must be set asrde in an open space tract or covenant in order for the proposed project to qualifu for this exception.
Preservation ofexisting forested areas in Landslide Hazard Drainage Areas is encouraged.
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1.2.2.1
aggravated by the proposed project, and to ensure appropriate measures are provided for preventing creation or
aggravation of those problems. In addition, this requirement is intended to ensure appropriate provisions are made, as
n..a.a, to mitigate oiher identified impacts associated with the quantity and quality of surface and storm water runoff
fromthe project site (e.g., impacts to the hydrology of a wetland as may be identified by a "critical area report." The
primary component of an offsite analysis report is the downstream analysis. This examines the drainage system
within one-quarter mile downstream of the project site or farther as described in Section 1.2.2.1 below - It is intended
to identifu existing or potential/predictable downstream flooding, erosion, and water quality problems so that
appropriatemitigation,asspecifiedinSection l.2.2.2,andl.2.2.3,canbeprovidedtopreventaggravationofthese
pioblems. A secondary component ofthe offsite analysis report is an evaluation ofthe upstream drainage system
io verif,, and document that significant flooding and erosion impacts will not occur as a result of the proposed
project- The evaluation must extend upstream to a point where any backwater effects created by the project cease'
tr EXEMPTION FROM CORE REQTTIREMENT #2
With the exception of;
. Projects that trigger Core Requirement #3 (Flow Control Facilities) which must at minimum perform
offsite analysis sufficient to identifu and address "Downstream Drainage Problems Requiring Special
Attention (Section 1.2.2.1.|),Problem Type 4 (Potential Impacts to Wetland Hydrology problem),"
and
. Projects that trigger Core Requirement # 8 (Water Quality Facilities) which must at minimum perform
offsite analysis sufficient to identify and address "Downstream Water Quality Problems Requiring
Special Attention (Section I .2'2'l'2),"
A proposed project is exempt from Core Requirement #2 if any one of the following is true:
l. City review staff determines there is sufficient information to conclude that the project will not have a
significant adverse impact on the downstream and/or upstream drainage system, OR
2. The project adds less than 2,000 square feet of new impervious surface, AND less than3/q acre of new
p"rriou-t surface, AND does not construct or modify a drainage pipe/ditch that is 12 inches or more in
iizeldepth or that receives runoff from a drainage pipe/ditch that is l2 inches or more in sizeldepth, AND
does not contain or lie adjacentto alandstide, steep slope, or erosion hazard area,OR
3. The project does not change the rate, volume, duration, or location ofdischarges to and from the proiect
site (e.g.,where existing impervious surface is replaced with other impervious surface having similar
runoff-generating characteristics, or where pipe/ditch modifications do not change existing discharge
characteristics).
DOWNSTREAM ANALYSIS
The level ofdownstream analysis required depends on specific site and downstream conditions. Each project
submittal must include at least a Level I downstream analysis. Upon review of the Level 1 analysis, DPER
may require a Level 2 or Level 3 analysis. If conditions warrant, additional, more detailed analysis may be
required.
The Level I downstream analysis is a qualitative survey of each downstream system and is the first step in
identifuing flooding problems, or potential impacts to wetland hydrology problems as described below under
.,Downstream Drainage Problems Requiring Special Attention." Each Level I analysis is composed of four
tasks at a minimum:
. Task l: Define and map the study area
o Task 2: Review all available information on the study area
r Task 3: Field inspect the study area
o Task 4: Describe the drainage system, and its existing and predicted problems
City of Kent Surface Water Design Manual l-15 July 2022
sEcTroN 1.2 coRE REQUIREMENTS
Upon review of the Level I analysis, city review staff may require a Level 2 or 3 downstream analysis,
depending on the presence of existing or predicted flooding, erosion, or nuisance problems identified in the
Level I analysis.
Levels 2 and 3 downstream analyses quantify downstream flooding, erosion, or nuisance problems by
providing information on the severity and frequency of an existing problem or the likelihood of creating a
new problem. A Level 2 analysis is a rough quantitative analysis (non-survey field data, uniform flow
analysis). Level 3 is a more precise analysis (survey field data, backwater analysis) of significant problems.
If conditions warrant, additional, more detailed analysis may be required beyond Level 3. For Levels 2 and3
downstream analyses, an additional Task 5, addressing mitigation of existing and potential flooding, erosion,
or nuisance problems, will be required.
Bxtent of Downstream Analysis
The downstream analysis must consider the existing conveyance system(s) for a minimum flowpath
distance downstream of one-quarter mile and beyond that, as needed, to reach a point where the proiect
site areaconstitutes less than l5o/o of the tributary area. This minimum distance may be increased as
follows:
. Task 2 of a Level I downstream analysis (described in detail in Section 2.3.1.1) is a review of all
available information on the downstream area and is intended to identify existing drainage and water
quality problems. In all cases, this in-formation review shall extend one mile downstream qf the proiect
_fu. The existence of flooding or erosion problems further downstream may extend the one-quarter-
mile/|S|f,o minimum distance for other tasks to allow evaluation of impacts from the proposed
development upon the identified flooding or erosion problems. The existence of documented water
quality problems beyond the one-quarter-milelllYo distance may in some cases require additional
mitigation of impacts as determined necessary by city review staff based on the type and severity of
problem.
. lf aproject's impacts to flooding or erosion problems are mitigated by improvements to the
downstream conveyance system, the downstream analysis will extend a minimum of one-quarter mile
beyond the improvement. This is necessary because many such improvements result in a reduction of
stormwater storage or an increase in peak flows from the problem location.
. At their discretion, city review staff may extend the downstream analysis beyond the minimum
distance specified above on the reasonable expectation ofdrainage or water quality impacts.
A detailed description of offsite analysis scope and submittal requirements is provided in Section 2.3.1 .l
Hydrologic analysis methods and requirements for Levels 2 and 3 downstream analysis are contained in
Chapter 3; hydraulic analysis methods are contained in Chapter 4'
1.2.2.1.1 DOWNSTRBAM PROBLEMS REQUIRING SPECIAL ATTENTION
While the area-specific flow control facility requirement in Core Requirement #3 serves to minimize the
creation and aggravation of many types of downstream drainage problems, there are some types that are more
sensitive to creation/aggravation than others depending on the nature or severity of the problem and which flow
control facility standard is being applied. In particular, there are four types of downstream problems where the
City has determined that the nature and/or severity of the problem warrants additional attention through the
downstream analysis and possibly additional mitigation to ensure no creatiorVaggravation:
l. Conveyance system nuisance problem
2. Severe erosion problem
3. Severeflooding problem
4. Potential Impucts to lltetland Hydrologt problem.
These four types of downstream drainage problems are further described below and precisely defined at the
City of Kent Surface Water Design Manual r -16 IuJy 2022
SECTION I.2 CORE REQUIREMENTS
beginning of Chapter 1.
Conveyance System Nuisance Problems (Type 1)
Conveyance system nuisance problems are minor but chronic flooding or erosion problems that result from
the overflow ofa constructed conveyance system that is substandard or has become too small as a result of
upslresm development Such problems warrant additional affention because of their chronic nature and
because they result from the failure of a conveyance system to provide a minimum acceptable level of
protection.
lf a conveyance system nuisance probtem is identified or predicted downstream, the need for additional
mitigation must be evaluated as specified in Section I .2.2.2 under ooDrainage Problem-Specihc Mitigation
Requirements." This may entail additional onsite flow control or other measures as needed to prevent creation
or significant aggravation of the problem.
For any other nuisance problems which may be identified downstream, this manual does not require mitigation
beyond the area specific flow control facility requirement applied in Core Requirement #3. This is because
preventing aggravationof such problems (e.g., those caused by the elevated water surfaces of ponds, lakes,
wetlands, and closed depressions or those involving downstream erosion) can require two to three times as
much onsite detention volume, which is considered unwananted for addressing nuisance problems. However,
if under some unusual circumstance, the aggravation of such a problem is determined by city review staff to be
a significant adverse impact, additional mitigation may be required.
Severe Erosion Problems (Type 2)
Severe erosion problems can be caused by conveyance system overflows or the concenhation of runoff into
erosion-sensitive open drainage features. Severe erosion problems warranl additional attention because they
pose a significant threat either to health and safety or to public or private property.
If a severe erosion probtem is identified or predicted downstream, additional mitigation must be considered as
specified in Section L2.2.2 under ooDrainage Problem Impact Mitigation Requirements." This may entail
additional onsite flow control or other measures as needed to prevent creation or aggravation of the problem.
Severe Flooding Problems (Iype 3)
Severe flooding problems, (i.e., a severe building Jtooding problem or severe roadway Jlooding problem) can
be caused by conveyance system overflows or the elevated water surfaces ofponds, lakes, wetlands, or closed
depressions. Severe flootling problems warrant additional attention because they pose a significant threat either
to health and safety or to public or private property'
If a severe flootling problen is identified or predicted downstream, the need for additional mitigation must be
evaluated as specified in Section 1.2.2.2 under "Drainage Problem Impact Mitigation Requirements." This
may entail consideration ofadditional onsite flow control or other measures as needed to prevent creation or
significant aggravation of the problem.
Potential Impacts to Wetlands Hydrology Problem (Type 4)
Potential impacts to wetlands hydrology can be caused by changes in the rate, duration, and quantity of
stormwater discharged from the project site to a wetland.
Where wetlands are identified on the site, the applicant shall submit a critical area report at a level
determined by city review staff to adequately evaluate the proposal and probable impacts.
Where wetlands are identified off the site AND the project is not exempt from Core Requirement 3, the
applicant shall submit a critical area report at a level determined by city review staffto adequately evaluate
the proposal and probable impacts.
City of Kent Surface Water Design Manual 1 -t7 Juty 2022
SECTION 1,2 CORE REOUIR,EMENTS
1.2.2.1.2
Projects or threshold discharge areas within projects discharging to wetlands, unless exempt from providing
a flow control facility per Core Requirement 3, must demonstrate that the existing wetland hydroperiod is
maintained in accordance with the wetland hydrology protection guidelines in Reference Section 5.
Based upon the critical area report and, if applicable, the analysis of project compliance with the wetland
hydrology protection guidelines in Reference Section 5, city review staff will determine if changes in the rate,
duration, and/or quantity ofsurface and storm water runofffrom a proposed project or threshold discharge
area within a proposed project could significantly alter the hydrology of a wetland-- in which case, city
review staff will require (as described in Section 1.2.2.2 under "Drainage Problem Impact Mitigation
Requirements"), implementation of additional flow control or other measures to mitigate the adverse impacts
of this alteration in accordance with the wetland hydrology protection guidelines in Reference Section 5.
DOWNSTREAM WATER QUALITY PROBLEMS REQUIRING SPECIAL
ATTENTION
A water quality problem. for the purposes of impact mitigation in this manual, is a situation in which a
waterbody of the State is documented by the Federal Government, State, County, or City to be exceeding or
at concern of exceeding the State's numeric water quality standards, or is subject to a federal, state, or county
cleanup program or action. Water quality problems and associated water quality standards encompass
surface water, groundwater, and sediment quality. The goal of this manual is to prevent creation or
significant aggravation of such problems to the maximum extent practicable. While the area-specific water
quality facility requirement in Section I .2.8.1, the source controls required in Section 1 .3.4, and the oil
controls required in Section 1.3.5 all serve to minimize the creation and aggravation of many types of
downstream water quality problems, there are some types that are either not addressed by these requirements
(e.g., temperature problems) or warrant additional measures/considerations to minimize the proposed project's
impacts to the maximum extent practicable. In particular, there are currently 7 types of downstream water
quality problems for which the County has determined that additional attention needs to be given to
preventing or minimizing increases in the pollutant or pollutants of concem discharging from the.srfu. These
are as follows:
1. Bacteria Problem
2. Dissolved Oxygen Problem
3. Temperature Problem
4. Metals Problem
5. Phosphorus Problem
6. Turbidity Problem
7. High pH Problem
These problems are defined below and the mitigation of impacts to them is addressed in Section 1 .2.2.3
Bacteria Problem (Type 1)
A bacteria problem is defined as a stream reach, lake, or other waterbody ofthe state that is either (1)
currently designated by the state as a Category 51 4, or 2 Waterbody due to exceedance or concern for
exceedance of the state's numeric water quality standard for fecal coliform as documented in the state's
Water Quality Assessment 303(dy305(b) Integrated Report and as displayed in WA Ecology's electronic
City of Kent Surface Water Design Manual l-18 Iuly 2022
SECTION I.2 CORE REQUIREMENTS
database and map viewerr2 of these waterbodies, or (2) is currently designated by the City as a bacteria
problem based on credible data indicating exceedance or concern for exceedance ofthe state's numeric
water quality standard for fecal coliform.
Dissolved Oxygen (DO) Problem (Type 2)
A dissolved oxygen problem is defined as a stream reach, lake, or other waterbody of the state that is either (1)
currently designated by the state as a Category 514ror 2 Waterbody due to exceedance or concern for
exceedance of the state's numeric water quality standard for dissolved oxygen as documented in the state's
Water Quality Assessment 303(d)/305(b) Integrated Report and as displayed in WA Ecology's electronic
database and map viewerr2 of these waterbodies, or (2) is currently designated by the City as a DO problem
based on credible data indicating exceedance or concem for exceedance of the state's numeric water quality
standard for dissolved oxygen.
Temperature Problem (Type 3)
A temperature problem is defined as a stream reach, lake, or other waterbody of the state that is either (1)
"u.rently
designated by the state as a Category 51 4, or 2 Waterbody due to exceedance or concern for
exceedance of the state's numeric water quality standard for temperature as documented in the state's Water
Quality Assessment 303(dy305(b) Integrated Report and as displayed in WA Ecology's electronic database
and map viewer'' of these waterbodies, or (2) is currently designated by the City as a temperature
problem based on credible data indicating exceedance or concem for exceedance ofthe state's numeric
water quality standard for temperature.
Metals Problem (Type 4)
A metals problem is defined as a stream reach, lake, or other waterbody of the state that is either (1) currently
designated by the state as a Category 51 4, or 2 Waterbody due to exceedance or concern for exceedance
of the state's numeric water quality standards for metals (e.g., copper, zinc, lead, rl€rcur/r etc.) as
documented in the state's Water Quality Assessment 303(dy305(b) Integrated Repoft and as displayed in
WA Ecology's electronic database and map viewerr2 of these waterbodies, or (2) is currently designated by
the City as a metals problem based on credible data indicating exceedance or concem for exceedance of the
state's numeric water quality standards for metals (e.g., copper, zinc, lead, mercury' etc.), or (3) where
subject to any other local, state, or federal cleanup plan or contaminated site designation.
Phosphorus Problem (Type 5)
A phosphorus problem is defined as a stream reach, lake, or other waterbody of the state that is either (l)
currently designated by the state as a Category 51 4, or 2 Waterbody due to exceedance or concern for
exceedance of the state's numeric action standard for tofal phosphorus as documented in the state's Water
Quality Assessment 303(dy305(b) Integrated Report and as displayed in WA Ecology's electronic database
and map viewer13 of these waterbodies, or (2) is currently designated by the City as a nutrient problem
based on credible data indicating exceedance or concem for exceedance ofthe state's numeric action standard
for total phosphorus.
Turbidity Problem (Type 6)
l2ThelinktotheQueryToolis https://fbrtress.wa.gov/ecv/wats/aoplQvecbgarcb.alpx;selectallappropnatemediums.
The Map Tool is at https://ibrtress.wa.gov/ecv/wqamapviewer/default asp
rr The link to the Query Tool is https://fortress.wa.gov/ecy/r,vats/approvedsearch.asox ; select all appropriate mediums
The Map Tool is at https://tbrtress.wa.gov/ecv/rvqarnapviewer/detbult.aspx?res:1280x1024
City of Kent Surface Water Design Manual l-t 9 luly 2022
SECTION 1.2 CORE REQUIREMENTS
1.2.2.2
tr
A turbidity problem is defined as a stream reach, lake, or other waterbody of the state that is either (l)
currently designated by the state as a Category 5, 4, or 2 Waterbody due to exceedance or concern for
exceedance of the state's numeric water quality standard for turbidity as documented in the state's Water
Quality Assessment 303(dy305(b) Integrated Report and as displayed in WA Ecology's electronic database
and map viewerrr of these waterbodies.
High pH Problem (Type 7)
A High pH problem is defined as a stream reach, lake, or other waterbody of the state that is either (1)
currently designated by the state as a Category 51 4, or 2 Waterbody due to exceedance or concern for
exceedance of the state's numeric water quality standard for high pH as documented in the state's Water
Quality Assessment 303(dy305(b) Integrated Report and as displayed in WA Ecology's electronic
database and map viewert' of these waterbodies, or (2) is currently designated by the City as a pH
problem based on credible data indicating exceedance or concem for exceedance of the state's numeric
water quality standard for pH.
NOTE: The Washington State Department of Ecology regularly updates its water quality designations.
Additional lake, stream, or river reaches may be included on the list without notice. Please refer to the
Ecology's Water Quality Assessment and 303(D) List website at:
http ://www.ecy. wa. gov/programs/Wq/3 03 d/index.htrn I
DRAINAGE PROBLEM IMPACT MITIGATION
A proposed project must not significantly aggravate existing downstream problems or create new
p.obl"-r as a result of developingthe site. This manual does not require development proposals to fix or
otherwise reduce the severity of existing downstream drainage problems, although doing so may be an
acceptable mitigation.
PRINCIPLES OF IMPACT MITIGATION FOR DRAINAGE PROBLEMS
Aggravation of an existing downstream problem means increasing the frequency of occurrence and/or
severity of the problem. Increasing peak flows at the location of a problem caused by conveyance system
overflows can increase the frequency of the problem's occurrence. Increasing durations of flows at or above
the overflow return frequency can increase the severity ofthe problem by increasing the depth and duration of
flooding. Controlling peaks and durations through onsite detention can prevent aggravation of such problems
by releasing the increased volumes from development at return frequencies below the conveyance overflow
return frequency, which limits their effect to just causing the conveyance system to flow full for a longer period
of time.
When a problem is caused by high-water surface elevations of a volume-sensitive water body' such as a lake,
wetland, or closed depression, aggravation is the same as for problems caused by conveyance overflows'
Increasing the volume of flows to a volume-sensitive water body can increase the frequency of the problem's
occurrence. Increasing the duration offlows for arange ofretum frequencies both above and below the
problem return frequency can increase the severity of the problem; mitigating these impacts requires control of
flow durations for a range of return frequencies both above and below the problem return frequency. The net
effect of this duration control is to release the increased volumes due to development only atwater surface
elevations below that causing the problem, which in turn can cause an increase in these lower, but more
frequently occurring, water surface elevations. This underscores an unavoidable impact of development
upstream of volume-sensitive water bodies: the increased volumes generated by the development will cause
some range of increase in water surface elevations, no matter what detention standard is applied.
Creating a new problem means increasing peak flows and/or volumes such that after development, the
frequency of conveyance overflows or water surface elevations exceeds the thresholds for the various
problem types discussed in Section 1.2.2.1. For example, application of the Level 1 flow control standard
requires matching the existing site conditions 2- and lO-year peak flows. The I 0O-year peak flow is only
partially attenuated, and the flow increase may be enough to cause a severe Jlooding problem The potential for
causing a new problem is often identified during the Level I downstream analysis, where the observation of a
City of Kent Surface Water Design Manual r-20 July 2022
SECTION I.2 CORE REOUIREMENTS
reduction in downstream pipe sizes, for example, may be enough to predict creation of a new problem. A Level
2 or 3 analysis will typically be required to veriff the capacity of the system and determine whether 100-year
flows can be safely conveyed.
tr SIGI\'IF'ICAI{CE OF IMPACTS TO EXISTING DRAINAGE PROBLEMS
The determination of whether additional onsite mitigation or other measures are needed to address an existing
downstream problem depends on the significance of the proposed project's predicted impact on that problem.
For some identified problems, city review staff will make the determination as to whether the project's impact is
significant enough to require additional mitigation. For Type 7,2, and 3 downstream drainage problems
described in Section 1.2.2.1.1,this threshold of significant impact or aggravation is defined below. For aType 4,
"Potential Impscts to Wettand Hydrologt problem," city review staffwill make this determination based on
required critical area report findings, whether the project is in compliance with the wetland hydrology
protection guidelines found in Reference Section 5, the project's relative conhibution to the identified
wetland's hydrology, and the mitigation proposed in meeting other requirements (e.g. flow control facilities
and flow control BMPs).
For conveyance system nuisance problems, the problem is considered significantly aggravated if there is any
increase in the project's contribution to the frequency ofoccurrence and/or severity ofthe problem for runoff
events less than or equal to the 10-year event. Note; Increases in the project's contribution to this type of
problem are considered to be prevented if sfficient onsite flow control and/or ffiile improvements qre
provided as specified in Table 1.2.3.4.
For severe erosion problems, the problem is considered significantly aggravated if there is any increase in
project's existing contribution totheflow durationta of peak flows ranging from 50% of the2-year peak flow
up to the full SO-year peak flow at the eroded area. Note; Increases in the project's contribution to this type of
problem are considered to be prevented if Conservationflow control or offsite improvements are provided as
specified in Table 1.2.3.A.
For severe buildingJlooding problems, the problem is considered significantly aggravated ifthere is any
increase in the project's existing contributionrs to the frequency, depth, and/or duration ofthe problem for
runoffevents less than or equal to the 100-year event'
For severe roadway flooding problems, the problem is considered significantly aggravated if any of the
following thresholds are exceeded and there is any increase in the project's contributionre to the frequency,
depth, and/or duration ofthe problem for runoffevents less than or equal to the 100-year event:
r The existingfloodingr6 over all lanes of a roadway or overtopping the culverted section of a sole access
driveway is predicted to increase in depth more than a quarter-inch or 10Yo (whichever is greater) for the
10O-year runoffevent.
r The existing flooding over all lanes of a roadway or severely impacting a sole access driveway
is more than 6 inches deep or faster than 5 feet per second for runoffevents less than or equal to the 1O0-year
event. A severely impacted sole access driveway is one in which flooding overtops a culverted section
of the driveway, posing athreatof washout or unsafe access conditions due to indiscernible driveway
t4 Flowdurationmeanstheaggregatetimethatpeakflowsareatoraboveaparticularflowrateofinterest(e.g.,theamountoftimeoverthe
last 40 years that peak flows were at or above the 2-year flow rate).
ri Increasesintheprqect'scontributionareconsideredtobepreventedifsufficientonsiteflowcontroland/oroffsiteimprovementsareprovided
as specified for "severe flooding problems" in Table 1.2.3.A
t6 Existingfloodiing, for the purposes ofthis definition, means flooding over all lanes ofthe roadway or driveway has occurred in the past and can
be verified by City records, City personnel, photographs, or other physical evidence.
City of Kent Surface Water Design Manual t-2t h:ly 2022
SECTION 1.2 CORE REQUIREMENTS
edges, or flooding is deeper than 6 inches on the driveway, posing a severe impediment to emergency
access.
The existing flooding over all lanes of a sole access roadwayt7 is more than 3 inches deep or faster than 5
feet per second for runoffevents less than or equal to the 1OO-year event, or is at any depth for runoff
events less than or equal to the l0-year event.
tr DRAINAGE PROBLEM-SPECIF'IC MITIGATION REQT]IREMENTS
l. IF a proposed project or threshold discharge ureawithina project drains to one or more of the Type 1,
type Z, or Type 3 downstream drainage problems described in Section | .2.2.1 as identified through a
downstream analysis, THEN the applicant must do one of the following:
a) Submit a Level 2 or Level 3 downstream analysis per Section 2.3.1 demonshating that the proposed
project will not create or significantly aggravate the identified downstream problem(s), OR
b)Show that the nutural discharge srea or threshold discharge area draining to the identified
problem(s) qualifies for an exemption from Core Requirement #3: Flow Control (Section 1.2.3) or an
exception from the applicable area-specific flow control facility requirement per Section 1.2.3.1 OR
Document that the applicable area-specific flow control facility requirement specified in Core
Requirement #3 is adequate to prevent creation or significant aggravation of the identified downstream
drainage problem(s) as indicated in Table 1.2.3.A with the phrase,'No additional flow control
needed," OR
Provide additional onsite flow control necessary to prevent creation or significant aggravation ofthe
downstream problem(s) as specified in Table l.2.3.Aand further detailed in Section 3.3.5, OR
Provide offsite improvements necessary to prevent creation or significant aggravation of the identified
downstream drainage problem(s) as detailed in Chapter 3 unless identified as not necessary in Table
1.2.3.A, OR
Provide a combination of additional onsite flow control and offsite improvements sufficient to prevent
creation or significant aggravation of the downstream drainage problem(s) as demonstrated by a Level
2 or Level 3 downstream analysis.
c)
e)
IF it is identified that the manner of discharge from a proposed project may create a significant adverse
impact as described in Core Requirement #1, THEN city review staff may require the applicant to
implement additional measures or demonstrate the impact will not occur.
IF it is identified through a critical area review as described under 'oPotential Impacts to Wetlands
Hydrology Problem (Type 4)o', that changes in the rate, duration, and/or the quantity of surface and
storm water runoff from a proposed project or threshold discharge srea wilhin a proposed project could
significantly alter the hydrology of a wetland (Type 4 problem), THEN city review staff shall require the
applicant to implement additional flow control or other measures to mitigate the adverse impacts of this
alteration in accordance with the wetland hydrology protection guidelines in Reference Section 5.
Intent: To ensure provisions are made (ifnecessary) to prevent creation or significant aggravation ofthe four
types of downstream drainage problems requiring special attention by this manual, and to ensure compliance
with the discharge requirements of Core Requirement #1.
In addressing downstream problems per Drainage Problem-Specific Mitigation Requirement I above, the
additional onsite flow control will often be the easiest provision to implement. This involves designing the
required onsite flow control facility to meet an additional set of performance criteria targeted to prevent
l7 Soleaccessroadwaymeansthereisnootherflood-freerouteforemergencyaccesstooneormoredwellingunits.
d)
0
)
-'t
City of Kent Surface Water Design Manual l-22 Iuly 2022
SECTION I.2 CORE REQUIREMENTS
1.2.2.3
significant aggravation of specific downstream drainage problems. To save time and analysis, a set of
predetermined flow control performance criteria corresponding to each of the three types of downsheam
problems is provided in Table 1.2.3.A and described in more detail in chapter 3.
Note that in some cases, the area-specific flow control facility requirement applicable to the proposed project
per Section I .2.3 . 1 is already sufficient to prevent significant aggravation of many of the defined downstream
drainage problem types. Such situations are noted in Table l.2.3.Aas not needing additional onsite flow
control or offsite improvements. For example, if the project is located within a Conservation Flow Control
Area subject to the Level 2 flow control standard per Section 1.2.3 .1 .B and a conveysnce system nuisance
problem is identified through offsite analysis per Core Requirement #2, no additional onsite flow control is
needed, and no offsite improvements are necessary'
WATER QUALITY PROBLEM IMPACT MITIGATION
As stated in Section 1.2.2.1,the goal of this manual is to prevent creation and/or significant aggravation of
water quality problems to the maximum extent practicable. This is accomplished through a number of
mitigation requirements, including (l) the area-specific water quality facility requirement in Section
1.2.8.1 , (2) any mitigation required by other adopted area-specific requirements per Special Requirement
# 1 , Section L3. I , (3) the source controls required in Special Requirement #4, Section I .3 .4, (4) the oil
control required in Special Requirement #5, Section 1.3.5, and (5) the water quality problem-specific
mitigation requirements presented in this section. Note thqt this mqnual does not require development
proposals to fix or otherwise reduce the severity of existing downstream water quality problems, although
doing so may be an acceptable mitigation.
tr WATER QUALITY PROBLEM-SPECIFIC IVIITIGATION REQTIIREMENTS
IF a proposed project drains to one or more of the 7 types of downstream water quality problems defined in
Section 1.2.2.1as identified through a downstream analysis, THEN the applicant must comply with the
following problem-specif,rc mitigation requirements that apply. Note that city review staffmay require
additional measures if the opportunity exists to further mitigate the pollutants of concern qssociated with
these types of problems.
Bacteria Problem (Type 1)
IF the proposed project drains to a bacteria problem located within the quarter mile/l1Yo distance
downstream (or beyond as deemed necessary by city review staff), THEN the following requirements must
be met as applicable:
l. IF a water quality facility is required per Core Requirement #8, THEN filter cartridges or
stormwater wetland shall be used to meet the area-specific water quality facility requirement'
Cartridge filters are the preferred option. Other treatment options for meeting the area-specific
facility requirement may be used in lieu of a filter cartridges or stormwater wetland only if combined
with an emerging technology treatment method that provides equivalent removal of fecal coliform
as demonstrated through an experimental design adjustment per Section 1.4.
2. IF the proposed project is a residential subdivision, THEN signage shall be provided in the subdivision's
public areas (i.e., recreation/open space areas and right-of-way) requesting that pet waste be picked up in
order to protect downsheam water quality. The extent and location of this signage shall be reviewed and
approved by city review staff.
3. IF the proposed project is a multifamily development with a recreation/open area or is a park
improvement, THEN signage shall be provided requesting that pet waste be picked up in order to
protect downstream water quality. The extent and location of this signage shall be reviewed and
approved by city review staff.
Dissolved Oxygen (DO) Problem (Type 2)
IF the proposed project drains to a DO problem located within the quarter milell5o/o distance downstream
City of Kent Surface Water Design Manual t-23 Iuly 2022
SECTION I.2 CORE REQUIREMENTS
(or beyond as deemed necessary by city review staf|, THEN the following requirements must be met as
applicable:
l. IF the proposed project includes a wetpond or wetvault, THEN the wetpool depth shall not exceed 6
feet, AND the outflow system shall include a measure designed to promote aeration of the facility's
discharges for 2-year runoff events and smaller. One way to do this is to create a drop in flow elevation
within a manhole by placing the outlet invert of the incoming pipe a minimum of l2 inches above the
Z-year headwater elevation of the outgoing pipe. Alternatively, if the outflow system discharges to an
open channel, the same drop in flow elevation could be achieved by placing the outlet invert a
minimum of l2 inches above the 2-year tailwater elevation created by the channel. Other equivalent
approaches may be used as approved by city review staff'
2. IF the proposed project includes a wetvault, THEN the required ventilation area specified in Chapter
6 shall be doubled.
3. IF the DO problem is documented to be caused by excessive phosphorus and a water quality facility
is required per Core Requirement #8, THEN a water quality facility option from the Sensitive Lake
Protection menu shall be a component of the required treatment system'
Temperature Problem Oyp" 3)
IF the proposed project drains to a temperature problem located within the quafter milelllYo distance
downstream (or beyond as deemed necessary by city review staff), THEN the following requirements must
be met as applicable:
l. IF a water quality facility is required per Core Requirement #8, THEN use of a wetpond is prohibited
unless it will be at least 50% shaded at midday in the summer or its discharges will flow through 200
feet or more of open channel that is at least 50%o shaded at midday in the summer. City review staff
shall review and approve the extent and location ofthis shading.
2. IF the proposed project includes open drainage features, THEN vegetation or other means shall be
used where practicable to maximize shading of the drainage features, except bioswales and filter
strips. The extent and location of this shading shall be reviewed and approved by crff review staff.
Metals Problem (Type 4)
IF the proposed project drains to a metals problem located within the quarter milelllYo distance
downstream (or beyond as deemed necessary by city review staf|, THEN the following requirements must
be met as applicable:
l. IF a water quality facility is required per Core Requirement #8, THEN a water quality facility option
from the Enhanced Basic WQ menu shall be a component of the project's required treatment system.
2. IF the proposed project is a residential subdivision, THEN a covenant shall be recorded for each lot
and common areatractprohibiting use of leachable heavy metals (e.g., galvanized metals) that will
be exposed to the weather (use the covenant in Reference Section 8-Q).
3. IF the proposed project includes road right-of-way improvements, THEN use of leachable heavy
metals (e.g., galvanized metals) that will be exposed to the weather (e.g., guard rails, street lights,
etc.) shall be avoided.
Phosphorus Problem (Type 5)
IF the proposed project drains to a phosphorus problem located within the quarter mile/l1Yo distance
downstream (or beyond as deemed necessary by city review staff), THEN the following requirements must
be met as applicable:
l. IF a water quality facility is required per Core Requirement #8, THEN the project shall be assumed to
be located within a designated Sensitive Lake WQ Treatment Area for the putposes of applying the
area-specific water quality treatment requirement in Section 1.2'8' 1.
City of Kent Surface Water Design Manual 1-24 July 2022
SECTION I.2 CORE REOUIREMENTS
1.2.3
2.For the purposes of applying the Erosion and Sediment Control Standards in Appendix D, the project
shall be assumed to be located within a designated Sensitive Lake WQ Treatment Area.
Turbidity Problem (Type 6)
IF the proposed project drains to a turbidity problem located within the quarter milelll%o distance downstream
(or beyond as deemed necessary by city review staff) AND the downstream flow path from the proiect site to
the turbidity problem is through a landslide hazard area, steep slope hazard area, erosion hazard area or any
actively eroding area, THEN the project shall provide a tightline system through the area in accordance with
the same criteria and exceptions specified in Core Requirement # 1 , Discharge Requirement 2 for projects
located within a designated Landslide Hazard Drainage Area. Other means for safely conveyingptoiect site
discharges through the area ofconcem for erosion may be proposed subject to approval by city review staff.
High pH Problem (Type 7)
IF the proposed project drains to a pH problem located within the quarter mile/l5o/o distance downstream (or
beyond as deemed necessary by city review staff) AND the proposed project includes a concrete vault
structure for stormwater control purposes, THEN the vault's submerged surfaces shall be coated or otherwise
treated to prevent alteration of pH.
CORE REQUIREMENT #3: FLOW CONTROL FACILITIES
All proposed projects, including redevelopment projects, must provide onsite flow control facilities to
mitigate the impacts of storm and surface water runoff generated by new impervious surface, new pervioas
sudice, and replaced impervious surface targeted for flow mitigation as specified in the following sections'
Flow control facilities shall, at a minimum, meet the performance criteria for one of the area-specific flow
control standards described in Section 1.2.3.1 and be implemented according to the applicable flow control
facility implementation requirements in Section 1 .2.3.2.
Intent: To ensure the minimum level of control needed to protect downstream properties and resources from
increases in peak, duration, and volume ofrunoffgenerated by new development. The level ofcontrol varies
depending on location and downstream conditions identified under Core Requirement #2.
tr EXEMPTIONSNARIANCES FROM CORE REQUIREMENT #3
The following are exemptions and variances from the flow control provisions of Core Requirement #3 :
Basic Exemption
A proposed project is exempt if it meets the following criteria:
a) Less than 5,000 square feet of new plus replaced impervious surface will be created, AND
b) Less than To acres of new pervious surface will be added.
Green River Management Agreement and Detention Requirements
In accordance with the provisions of the Green River Management Agreement, if the proposed development
is located in an area that has a direct, gravity draining outlet to the Green River or is tributary to such a system
(other than via the Black River Pumping Station) then stormwater runoff can be released, after all the following
conditions are met:
1) Flood storage must be provided that is equivalent to the amount of rainfall falling on the entire site for a
1g0-year,7-day design storm, corresponding to 9.8 to 10.0 inches depending on the site's location as
shown on the 100-year,7-day Isopluvial Maps, AND
2) It must be demonstrated that the downstream conveyance system has the capasity to carry the resultant
flows without overflowing, AND
3) Appropriate water quality treatment is provided as required. AND
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City of Kent Surface Water Design Manual l-25 Iily 2022
SECTION 1.2 CORE REQUIREMENTS
1.2.3.1
4) Conservation flow control is provided
Additional detention and release rate requirements may be required by the Washington State Department of
Fish and Wildlife when a Hydraulic Project Approval (HPA) is required by the agency, or by the City to
mitigate for conveyance problems downstream of the development or for impacts to critical areas.
For the S 259thl3rd Avenue Regional Detention facility (refer to flow control map) the above conditions
shall apply with the exemption that only one-half of the Green River Management Agreement flood storage
volume shall be required.
Flow Control Requirements for Horseshoe Acres and Union Pacific Pump Stations
Developments within the service areas draining to the Horseshoe Acres and Union Pacific Pump Stations along
the Green River are required to provide on-site Conservation flow control and are required to provide on-site
water quality treatment.
Release rates and/or detention requirements may be imposed by the City when the Applicant's downstream
analysis indicates that the existing stormwater conveyance system from the development to the pump station
does not have the necessary capacity to carry the increased flows, OR the pump station does not have the
capacity to store and/or pump the additional runoff flows without local flooding.
Appropriate off-site mitigation measures to off-set identified conveyance or pumping constraints may be
proposed by the Applicant and accepted at the discretion of the Director.
Area-snecific Resional Facilities
Stormwater detention and treatment are not required for any development that discharges to the Valley
Regional Detention/Enhanced Wetland Facility also known as the Green River Natural Resource Area
lsee ttre flow control applications map for the relevant subbasin boundary). The facility has been sized to
provide detention and treatment for both existing and future developments'
Stormwater treatment and detention are required for any development that discharges to the 98th Avenue
South Regional Treatment/Detention Pond because this system is designed to handle existing erosion
problems and is not designed to provide adequate detention and treatment according to current standards for
existing or future developments.
Stormwater detention and treatment are also required for any development that discharges to the Meridian
Meadows (Springwood) Regional Detention Pond/Wetland Facility or the Upper or Lower Mill Creek
Regional Detention Facilities. These facilities were not designed to provide regional detention or treatment
according to current standards for existing or future developments.
Master Drainage Plans have been approved for Kentview and The Lakes development areas (shown on the
Flow Control Applications Map). Drainage standards applied within these areas shall be in accordance with
the provisions of the respective Master Drainage Plans.
AREA-SPECIFIC FLOW CONTROL FACILITY REQUIREMENT
Projects subject to Core Requirement #3 must, at a minimum, comply with the area-specific flow control
standards per the threshold information detailed in this section. These standards are described below.
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City of Kent Surface Water Design Manual t-26 h:ly 2022
SECTION I.2 CORE REQUIREMENTS
Guide to Applying the Area-Specific Flow Control Facility Requirement
The flow control facility requirement varies across the city landscape according to the flow control area within
which the project or a threshold discharge area ofthe project is located. Flow control areas are designated by the
city to target the level offlow control performance to the broad protection needs ofspecific basins or subbasins.
There are culently nine such flow control areas, which are depicted on the Flow Control Applications Map
adopted with this manual:
o Conservation Flow Control Areas
r Flood Problem Flow Control Areas
o Green River Natural Resource Area regional FC facility
. Green River Management Agreement FC Areas
r Riverview Basin Plan FC Area'
. Lakes Regional Detention FC Area
r Horseshoe Acres FC Area
o Union Pacific Pump Station Area
o S.259thl3"d Ave. S. Regional Detention Area
Each flow control area has an area-specific set of minimum flow control facility performance criteria, design
assumptions, surfaces that must be mitigated, and exceptions. These provisions all cornprise what is referred to as
the "area-specific flow control facility requirement."
Note that the minimum required performance of the facility as specified by this requirement may need to be
increased to ensure that downstream drainage problems are not created or significantly aggravated as set forth in
Section 1.2.2.2, "Drainage Problem-Specific Mitigation Requirements." Table 1.2.3.A provides a quick guide for
selecting the flow control performance criteria necessary to meet both the area-specific flow control facility
requirement and the problem-specific mitigation requirement. This is further explained in Step 4 below.
For efficient application of the flow control facility requirement, the following steps are recommended:
l. Use the Flow Control Applications Map to determine the flow control area in which your project is located.
2. Consult the detailed requirement and exception language for the identified flow control area to determine if
and how the flow control facility requirement applies to your project. This requirement and exception
language is detailed on subsequent pages fol each ofthe nine flow control areas depicted on the Flow
Control Applications Map. If a flow control facility is not applicable per the area-specific exceptions,
proceed to Step 6.
3. If downstream drainage problems were identified through offsite analysis per Core Requirement #2 and ate
proposed to be addressed through onsite flow control, use Table I .2.3 .A to determine if and what additional
flow control performance is necessary to mitigate impacts (i.e., to prevent creation or aggravation of the
identified problems).
4. Use Section 1.2.3.2 to identify the applicable requirements for irnplementing the flow control facility
requirement. These requirements cover facility siting, analysis and design, unusual situations, and other site-
specifi c considerations.
5. Use Core Requirement 9 to identify the flow control BMPs that must be applied to yout proiect site
regardless of whether a flow control facility is required.
City of Kent Surlace Water Design Manual t-27 Iily 2022
AREA.SPECIFIC STANDARD
Flood Problem FC AreasIdentified Problem Conservation FC Areas
Apply the acisting or historic site conditions Level2
flow control standard (whichever is appropriate
based on downstream flow control area) AND match
exislins sile conditions 1O0-vear peaks
Apply the historic site conditions Level 2 flow
control standard, which matches historic durations
for 50Vo of2-yr through 50-year peaks AND
matches historic 2- and 10- vear peaks
No Problem Identified
Apply the minimum area-specific
flow control perlormance criteria.
No additionalflow control or other mitigation is
needed
Additional Flow Control
No additional flow control or
other mitig.ation is needed
Type I Drainage Problem
Conveyance System
Nuisance Problem
No additional flow control is needed, but other
mitigation may be requireda)
No additional flow control is needed, but other
mitigation may be requireda\
Type 2 Drainage Problem
Severe Erosion Problem
Additional Flow Control
If flooding is from a closed depression, make design
adjustments as needed to meet the special provision fol
closed depressisn5 (3Xi)
Additional Flow Control
Apply the historic site conditions Level 3 flow
control standard.
If flooding is lrom a closed depression, make design
adjustments as needed to meet the special provision
for closed dePressions(3)(s)
Type 3 Drainage Problem
Severe Flooding Problem
Additional Flow Control
ECD may require design adjustments per the
wetland hydrology protection guide- lines in
Reference Section 5
Type 4 Potential Impact to
Wetland Hydrology as
Determined through a Critical
Area Review per KCC 11.06 or
Offsite Analvsis
Additional Flow Control
ECD may require design adjustments per the
wetland hydrology protection guidelines in
Reference Section 5
Notes:
(l) More than one set of problem-specific performance criteria may apply if two or more downstream problems are identified through offsite analysis per
CoreRequirement#2. lfthishappens,theperformancegoalsofeachapplicableproblem-specificcriteriamustbemet. Thiscanrequireextensive,
time-consuming analysis to implement multiple sets of outflow performance criteria if additional onsite flow control is the only viable option for
mitigating impacts to these problems. In these cases, it may be easier and more prudent to implement the historic site conditions Level 3 flow
control standard in place ofthe otherwise required area-specific standard. Use ofthe historic Level 3 flow control standard satisfies the specified
performance criteria for all the area-specific and problem-specific requirements except ifadjustments are required per the special provision for closed
depressions described below in Note 5.
(2) Overflow T, is the return period o f conveyance system overflow. To determine T, requires a minimum level 2 downstream analysis as detailed in
Section 2.3. 1.1 . To avoid this analysis, aT, of 2 years may be assumed.
(3) Offsite improvements may be implemented in lieu of or in combination with additional flow control as allowed in Section 1.2.2.2 and detailed in
Section 3.3.5.
(4) A tightline system may be required regardless ofthe flow control standard being applied ifneeded to meet the discharge requirements ofCore
Requirement #1 or the outfall requirements of Core Requirement#4, or is deemed necessary by Economic and Community Development where the
risk ofsevere damage is high.
(5) Special Provision for Closed Depressions with a Severe Flooding Problem:
IF the proposed project discharges by overland flow or conveyance system to a closed depression experiencing a severe flooding problem AND the
amount ofimpervious surface area proposed by the project is greater than or equal to 10% ofthe 100-year water surface area ofthe closed depression,
THEN use the point ofcompliance analysis technique described in Section 3-3.6 to verify that water surlace levels are not increasing for the retum
frequenciesatwhichfloodingoccurs,uptoandincludingthelO0-yearfrequency. Ifnecessary,iterativelyadjustonsiteflowcontrolperlormance to
preventincreases. Note: Thepointofcomplianceanalysisreliesoncertainfeldmeasurementstakendirectlyatthecloseddepression(e.g,soils
fes5,topography,etc.). Ifpermissiontoenterprivatepropertyforsuchmeasurementsisdenied,cityreviewstaffmqtwaivethisprovisionqndapply
the Level 3flow control standard with a mandalory 20% safetyfactor on the storage volume
TABLE I.2.3.A
STJMMARY OF FLOW CONTROL PERFORNIANCE CRITERIA ACCEPTABLE FOR IMPACT MIT'IGATION(I)
SECTION 1.2 CORE REQUIREMENTS
IMPERVIOUS SURFACE PERCENTAGE EXEMPTION FOR AGRICULTURAL
PROJECTS
For agricultural projects located within an Agricultural Production District (APD), Farmland Preservation
Program (FPP), or site zoned A, any onsite threshold discharge area is exempt from the flow control facility
requirement if it meets all of the following conditions:
a) The total (new, replaced, and existing) amount of impervious surface that is not fully dispersed per the
criteria on page l-46 must be no more lhan4Yo of the threshold discharge area, AND
City of Kent Surface Water Design Manual 1-28 Iuly 2022
SECTION I.2 CORE REOUIREMENTS
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t
b) New impervious surfaces and new pervious surfaces must not disturb, impact, or replace native vegetation,
AND
c) Flow control BMPs must be applied to new impervious surfaces as specified in Core Requirement 9, AND
d) All impervious surface area, except I 0,000 square feet of it, must be set back from its natural location of
discharge from the site at least 100 feet for every 10,000 square feet oftotal impervious surface and its
runoff must be discharged in an unconcentrated manner that promotes infiltration and evapotranspiration,
AND
e) Increased runofffrom the new impervious surface and new pervious surface must not significantly impact
a critical area, severe flooding problem, or severe erosion problem, AND
0 The manner in which runoff is discharged from the project site must not create a significant adverse impact
per core requirement # I .
BASIC F'LOW CONTROL AREAS
Basic flow control (Level l) is not permitted anywhere in Kent.
CONSERVATION FLOW CONTROL AREAS
Conservation Flow Control Areas cover most developable areas of Kent. Conservation Flow Control Areas are
the default designation unless otherwise indicated on the City of Kent Flow Control Applications Map adopted
with this manual.
Note; For projects located at or near the delineqted boundary of the Conservation Flow Control Area, site
specific topography or drainage information may be needed to verify that a project or any threshold discharge
irea' of a proiecl is within the Jlow control area. Any threshold discharge area is considered to be within the
Conservation Flow Control Area if the threshotd discharge area drains to a waterbody or drainage system
that is clearly within the mapped Conservation Flow Control Area.
Minimum Required Performance
Facilities in Conservation Flow Control Areas must comply with the following flow control performance
standard and assumptions unless modified by offsite analysis per Core Requirement #2 (see Table 1.2'3'A).
Level 2 Flow Control: Match developed discharge durations to predeveloped durations for the range of
predeveloped discharge rates from 50% ofthe 2-year peak flow up to the full 50-year peak flow.
Also match developed peak discharge rates to predeveloped peak discharge rates for the 2- and 10-
year return periods. Assume historic site conditions as the predeveloped condition.
Intent
The Level 2 flow control standard assuming historic site conditions is intended to limit the amount of time that
erosive flows are at work generating erosion and sedimentation within natural and constructed drainage
systems. Such control is effective in preventing development-induced increases in natural erosion rates and
riducing existing erosion rates where they may have been increased by past development of the.srle. This is
u""o-plith.d by maintaining at historic predevelopment levels the aggregate time that developed flows exceed
an erosion-causing threshol d (i.e., 50zo of the histori c 2-year peak flow). Maintaining natural erosion rates
within streams and their tributary areas is important for preventing increases in stream channel erosion and
sediment loading that are detrimental to salmonid habitat and production.
Effectiveness in Addrrcssing Downstream Problems
While the Level2 flow control standard assuming historic site conditions provides a reasonable level of
protection for preventing most development-induced problems, it does not necessarily prevent increases in
existing site cinditions 1OO-year peak flows that can aggravate severe flooding problems as described in Core
City of Kent Surface Water Design Manual l-29 Iuly 2022
SECTION I.2 CORE REQUIREMENTS
Requirement #2, nor does it necessarily prevent aggravation of all severe erosion ptoblems' Consequently, if
one-or more of these problems are identified through offsite analysis per Core Requirement #2, addilional
onsite flow control and/or offsite improvements will likely be required (see "Drainage Problem-Specific
Mitigation Requirements" in Section 1.2.2.2.
Target Surfaces
Facilities in Conservation Flow Control Areas must mitigate (either directly or in effect) the runoff from the
following target developed surfaces within the threshold discharge ares for which the facility is required:
l. New impervious surface that is notfully dispersed or not farmland dispersed as specified in Appendix C.
For individual lots within residential subdivision projects, the extent of new impervious surface shall be
assumed as specified in Chapter 3.
2. New pervious surface Ihatis notfutty dispersed or not farmland dispersed as specified in Appendix C. For
individual lots within residential subdivision projects, the extent of new pervious surface shall be assumed
to be the entire lot area, except the assumed impervious portion and any portion in which native conditions
are preserved by covenant, tract, or easement. In addition, the new pervious surfuce on individual lots shall
be assumed to be l00oZ grass.
3. Existing impervious surface added since January 8, 2001 that is not/n lly dispersed or not farmland
dispersed as specified in Appendix C, and not yet mitigated with a City-approved flow control facility or
flow control BMP. Note; January 8, 2001 is the effective date of the ESA 4(d) Rulefor Puget Sound
Chinook salmon.
Replaced impervious surface thatis notfully dispersed or not farmland dispersed as specihed in
Appendix Cbn a non-redevelopment project in which the total of new plus replaced impervious surface is
5,000 square feet or more, OR new pervious surface is 3/q acre or more.
Replaced impervious surfuce that is not/a lly dispersed on a transporlation redevelopment proiect in
which new impervious surface is 5,000 square feet or more and totals 50Vo or more of the existing
impervious surface within the project limits.
Replaced impervious surface thalis notfutly dispersed or not farmland dispersed as specified in
Appendix C, on a parcel redevelopment project in which the total of new plus replaced impervious
tui1o"" is 5,000 square feet or more and whose valuation of proposed improvements (including interior
improvements andexcluding required mitigation improvements) exceeds 50% of the assessed value of: (a)
the existing project site improvements on commercial or industrial projects or (b) the existing site
improvements on other projects.
Exceptions
The following exceptions apply only in Conservation Flow Control Areas:
l. The historic site conditions assumption for application of Level 2 flow control may be reduced through a
basin plan or study approved by the City and the Washington State Department of Ecology. One possible
reduction is to an assumption of 75o/o forest, 150% grass, and 10% impervious surface (75/l5ll0 conditions)
or existing site conditions, whichever generates the lowest 100-year peak flow. Another possible change
that could be made through a City and Ecology approved basin plan or study is to the lowest peak flow
(50% of the 2-year peak flow) above which discharge durations must be matched. This peak flow, known
as the geomorphic threshold"of bed load movement, may be changed based on the actual channel
conditions necessary to protect or allow for restoration ofwater body beneficial uses and habitat functions
essential to salmonids.
2. The facility requirement in Conservation Flow Control Areas is waived for any threshold discharge area in
which there is no more than a 0.15-cfs difference (when modeled using l5 minute time steps) or no more
than a 0.1-cfs difference (when modeled using t hour time steps) in the sum of developed 1OO-year peak
4
5
6
City of Kent Surface Water Design Manual I -30 Iuly 2022
SECTION 1.2 CORE REQUIREMENTS
flows for those target surfaces subject to this requirement and the sum of historic site conditions 10O-year
peak flows (modeled using same time step unit (e.g., hourly or l5 minute) used to calculate the developed
flow) for the same surface areas. Agricultural zoned projects in current agricultural use may use existing
site conditions as the predeveloped condition for purposes of this exception calculation. Note: for the
purposes of this calculation, target surfaces served by flow control BMPs per Appendix C may be modeled
in accordance with the flow control BMP facility sizing credits in Core Requirement 9, Table 1.2.9.4.
3. The facility requirement in Conservation Flow Control Areas may be reduced or waived for any threshold
discharge area where a plan or study approved by the City and Ecology shows that a lower standard (e.g.,
Level I flow control) is sufficient or no facility is necessary to protect or allow for restoration of water
body beneficial uses and habitat functions essential to salmonids.
4. The facility requirement in Conservation Flow Control Areas as applied to replaced impervious surface
may be waived if the City has adopted a plan and implementation schedule approved by state Department
of Ecology for fulfilling this requirement in regional facilities.
5. The facility requirement in Conservation Flow Control Areas as applied to replaced impervious surface
may be reduced by DPER using the procedures detailed in Sections 1.4.3 and 1.4.4 of the adjustment
process, if the cost of flow control facilities to mitigate all target surfaces exceeds that necessary to
mitigate only for new impervious surface plus new pervious surface and also exceeds 1/3 of the valuation
of proposed improvements (including interior improvements) or twice the cost of a facility to mitigate
equivalent surfaces on a new development site, whichever is less.
tr FLOOD PROBLEM FLOW CONTROL ARBAS
Flood Problem Flow Control Areas are designated by the City of Kent where it has been determined that a
higher average level offlow control is needed to prevent aggravation ofexisting documented flooding or
erosion problems. Such areas are delineated on the Flow Control Applications Map.
Note: For projects located at or near the delineated boundary of the Flood Problem Flow Control Area, site-
specific topography or drainage information may be needed to veriff that a project or any threshold discharge
area of a project is within the flow control area. Any threshold discharge area is considered to be within the
Flood Problem Flow Control Area if the threshold discharge area drains to a waterbody or drainage system
that is clearly within the mapped Flood Problem Flow Control Area.
Minimum Required Performance
Facilities in Flood Problem Flow Control Areas must comply with the following flow control perfotmance
standard and assumptions unless modified by offsite analysis per Core Requirement #2 (see Table L2.3.A).
Level 3 Flow Control: Apply the Level 2 flow control standard, AND match the developed 1OO-year peak
discharge rate to the predeveloped 100-year peak discharge rate. Ifthe Flood Problem Flow Control Area is
located within a Conservation Flow Control Area, then historic site conditions shall be assumed as the
predeveloped condition except for the purposes ofmatching 1O0-year peak discharge rates. For all other
situations and for the purposes of matching 100-year peak discharge rates, exisling site conditions may be
assumed.
Intent
The Level 3 flow control standard is intended to prevent significant increases in existing water surface levels
for2-year through 1OO-year retum frequencies. Such increases are expected to occur as the volume ofrunoff
discharging to the water body is increased by upstream development. Because inflow rates to these water
bodies are typically much higher than the outflow rates, increased runoff volumes from upstream development
are, in effect, stacked on top of existing volumes in the water body, resulting in higher water surface levels. The
duration-matching and 1OO-year peak-matching criteria of the Level 3 flow control standard counteract this
stacking effect by slowing the anival of additional runoff volumes. Because it can prevent significant
aggravation ofexisting flooding, the Level 3 standard is also applicable to other flow control areas where
severe flooding problems have been identified per Core Requirement #2'
City of Kent Surface Water Design Manual I -31 July 2022
ECTION I.2 CORE REOUIREMENTSS
2.
Effectiveness in Addressing Downstream Drainage Problems
If the Level 3 flow control standard is implemented onsite, no additional measures are required to prevent
aggravation of the three types of downstream drainage problems described in Core Requirement #2. The one
exception is for a wetland or lake that is a closed depression with a severe Jlooding problem, and the proposed
project is adding impervious surface area amounting to more than l\Yo of the 100-year water surface area of
the closed depression. In this case, additional onsite flow control or offsite improvements may be necessary as
determined by a "point of compliance analysis" (see "Special Provision for Closed Depressions" in Table
1.2.3.A and see Section 3.3.6, "Point of Compliance Analysis").
Target Surfaces
Facilities in Flood Problem Flow Control Areas must mitigate (either directly or in effect) the runoff from
the following target developed surfaces within the threshotd discharge area for which the facility is required:
1. The target surfaces are the same as those required for facilities in Conservation Flow Control Areas unless
otherwise allowed by the area-specific exceptions for Conservation Flow Control Areas.
Exceptions
The following exceptions apply only in Flood Problem Flow Control Areas:
l. Any required application of the Flood Problem Flow Control Areas facility requirement to replaced
impervious surfuce may be waived if the City has adopted a plan and implementation schedule approved
by the state Department of Ecology for fulfilling this requirement with regional facilities.
Any required application of the Flood Problem Flow Control Areas facility requirement to replaced
impervious surface may be reduced by the Economic and Community Development Department using the
procedures detailed in Sections 1.4.3 and 1.4.4 of the adjustment process, if the cost of flow control
facilities to mitigate all target surfaces exceeds that necessary to mitigate only for new impervious
surfoce plus new pervious surface and also exceeds r/: of the valuation of proposed improvements
(including interior improvements) or twice the cost of a facility to mitigate the same surfaces on a new
developmentsite, whichever is less. The amount of reduction allowed by this exception shall be limited
such that the cost of flow control facilities is at least equal to that necessary to mitigate only fot new
impervious surface plus new pervious surface, and beyond this amount, is no greater than r/: of the
vaiuation of proposed improvements (including interior improvements) or twice the cost of a facility
to mitigate equivalent surfaces on a new development site, whichever is less.
Any required application of the Flood Problem Flow Conhol Areas facility requirement to replaced
impervious surfuce may assume existing site conditions as the predeveloped condition for the purposes of
matching the developed 10O-year peak discharge rate to the predeveloped 100-year peak discharge rate.
FLOW CONTROL FACILITY IMPLEMENTATION REQUIREMENTS
Flow control facilities shall be designed and implemented in accordance with the following requirements,
allowances, and flexible compliance provisions:
Onsite vs. Offsite Implementation
All required flow control facilities must be implemented onsite except where the below requirements can be
met for direct discharge to a regional or shared facility constructed to provide flow control for the proposed
project. Regional facilities are typically constructed as part of a basin plan. Shared facilities may be
constructed under a City-developed shared facility drainage plan or under an agreement between two or more
private developers. These requirements apply to proposed new facilities. The relationship between onsite and
offsite implementation for existing regional facilities is described under "Exemptions from Core Requirement
3
1.2.3.2
City of Kent Surface Water Design Manual t-32 JuIy 2022
SECTION I.2 CORE REOUIREMENTS
1. The regional or shared facility must be of adequate size and design to meet the current flow control
requirements for the proposed project's increased surface and stormwater runoff. Note: the currentflow
control requirements are those specified by Core Requirement #3 of this manual unless superseded by other
adopted area-specificflow control requirements per Special Requirement #l (see Section,1.3.1). In some
cases where the current flow control requirements differ from those used to originally design the regional or
shared facility, additional analysis and possible retrofitting of the facility may be required to ensure adequate
size and design.
2. The regional or shared facility must be fully operational at the time the proposed project is constructed. In the
case of a shared facility, the proposed project must comply with the terms and conditions of all conhacts,
agreements, and permits associated with the shared facility.
3. The conveyance system between Ihe project site and the regional facility must be approved by city review
staffand meet all the following criteria:
a) The conveyance system between the project site and the ordinary high water line of the regional facility
shall be comprised of manmade conveyance elements (pipes, ditches, outfall protection, etc.) and shall be
within public right-of-way or a public or private drainage easement, AND
b) The conveyance system shall have adequate capacity per Core Requirement #4, Conveyance System,
for the entire contributing drainage area, assuming buitd-oul condilions to cunent zoning for the
equivalent urea portion (defined in Figure I .2.3.A below) and existing conditions for the remaining area,
AND
c) The conveyance system will be adequately stabilized to prevent erosion, assuming the same basin
conditions as assumed in Criterion (b), AND
d) The conveyance system will not divert flows from or increase flows to an existing wetland or stream
sufficient to cause a significant adverse impact.
In the case ofa shared facility, the criteria are the same, except the conveyance system need only have
adequate capacily and erosion protection for buildout ofthe participating porlion ofthe contributing drainage
area. The participating portion includes those properties that have agreements for use of the shared facility.
Methods of Analysis and Design
Flow control facilities must be analyzed and designed using a continuous flow simulation method such as
HSPF (Hydrologic Simulation Program-FORTRAN) or the simplified HSPF-based runoff files method'
Specifications for use of the runoff files method and associated computer program, Western Washington
Hydrologic Model, (WWHM3), are found in Chapter 3. Detailed design specifications for flow control
facilities are found in Chapter 5.
Flow control facilities to be constructed on the Valley floor (below elevation 35 feet) shall take into account
the anticipated Valley floor flood levels in the facility design. The facility is to be designed assuming a freely
draining outlet, but potential onsite flooding and facility performance should be evaluated considering
tailwater conditions that can exist due to Valley floor flooding for up to the 1OO-year flood. This evaluation
shall be completed by routing the 100-year flood through the site storm drain/detention pond system
considering high tailwater conditions (100-year flood). Potential onsite flooding needs to be identified and an
overflow is to be provided that is capable ofconveying the 1O0-year peak flood flow from the site in
accordance with these conditions:
1. The overflow shall be directed to and released at the natural location without causing risk or damage to
downstream properties, AND
2. No flooding of traffic lanes will occur, AND
3. No structural flooding will occur.
Developments sited on the Valley floor within the FEMA mapped floodplain will also be required to include
compensatory flood storage volumes. This volume shall be calculated as 100% of the flood storage volume
lost from the 10O-year floodplain when comparing pre- and post-development conditions. Compensatory flood
City of Kent Surface Water Design Manual l -33 July 2022
SECTION 1.2 CORE REOUIREMENTS
volumes can be added to the on-site flow control facility after the flow control volume has been determined.
Storage volumes required for flow control will not be credited toward compensatory flood volumes (see
Special Requirement #2, Section 1.3.2).
SIZING CRBDITS F'OR FULLY DISPERSED SURFACES
Afulty dispersed surface (either impervious or non-native pervious) is one that conforms to the BMP strategy
for "full dispersion" detailed in Appendix C, Section C.2.1. This strategy calls for minimizing the area of onsite
developed surface relative to nutive vegetated surface,together with the application of dispersion techniques
that utilize the natural retention/detention capacity of the native vegetaled surface to mitigate the runoff effects
of the developed surfaces. Developed surfaces conforming to this strategy are considered to have a negligible
impact downstream, and therefore, may be modeled as forest and are not subject to the area-specific flow
control facility requirement (Section 1.2.3.1) or the area-specific water quality facility requirement (Section
1.2.8.1). In order for developed surfaces to qualiff asfully dispersed, they must meet the basic criteria listed
below and further detailed in Appendix C, Section C.2.1.
Criteria for Fully Dispersed Surfaces
l. The total area of impervious surface beingfully dispersed must be no more than l5%o of the total area of
native vegetated surface being preserved by a clearing limit per KCC 16.82 or by recorded tract, easement,
or covenant within the same threshold discharge urea. The total area of impervious surface plus non-
native pervious surfaceztbeingfully dispersed must be no more than 35o/o of a threshold discharge areu.
2. The runoff fromafully dispersed surface must be discharged using one of the following dispersion
devices in accordance with the design specifications and maximum area of fully dispersed surface for each
device set forth in Appendix C, Section C.2.1 :
a) Splash blocks
b) Rock pads
c) Gravel filled trenches
d) Sheet flow
Note; The dispersion device must be situated so as to discharge within the same threshold discharge area
ofthe surface it serves.
3. A native vegetated flowpath segment of at least 100 feet in length (25 feet for sheet flow from a
nonnative pervious surface) must be available along the flowpath that runoff would follow upon discharge
from a dispersion device listed in Minimum Requirement 2 above. The native vegetated flowpath segment
must meet all of the following criteria:
a) The flowpath segment must be over native vegetated surface'
b) The flowpath segment must be onsite or an offsite tract or easement area reserved for such
dispersion.
c) The slope of the flowpath segment must be no steeper than l5o/o for any 20-foot reach of the
flowpath segment.
d) The flowpath segment must be located between the dispersion device and any downstream
drainage feature such as a pipe, ditch, stream, river, pond, lake, or wetland.
e) The flowpath segments for adjacent dispersion devices must comply with the minimum spacing
requirements in Appendix C, Section C.2.1 . These requirements do not allow overlap of flowpath
segments, except in the case where sheet flow from a non-native pervious surface overlaps with the
flowpath of any dispersion device listed in Minimum Requirement 2 above.In this case, the longer of
the two overlapping flowpath segments must be extended at least 1 foot for every 3 feet of distance
along the most representative path that runoff would travel from the upstream end to the discharge end
of the non-native pervious surface.
City of Kent Surlace Water Design Manual t-34 J:uly 2022
SECTION I.2 CORE REQUIREMENTS
4. On sr'/es with septic systems, the discharge of runoff from dispersion devices must not be upgradient of the
drainfield. This requirement may be waived by city review staff if site topography clearly prohibits flows
from intersecting the drainfield.
5. The dispersion of runoff must not create flooding or erosion impacts as determined by city review staff' If
runoff is proposed to be discharged toward a lsndslide hazard area, erosion hazard areu, or steep slope
hazard srea (i.e., slopes steeper than20%o), city review staff may require the applicant to have the proposal
evaluated by a geotechnical engineer or engineering geologist.
SIZING CREDITS FOR USE OF FLOW CONTROL BMPS
Projects that implement flow control BMPs as detailed in Core Requirement 9 and Appendix C, whether
required or optional, may use the flow control BMP modeling credits as described and allowed in Section
1.2.9.4 and Table 1.2.9.A.
MITIGATION OF TARGET SURFACES THAT BYPASS FACILITY
On some siles, topography may make it difficult or costly to collect all target surface runoff for discharge to the
onsite flow control facility. Therefore, some project runoff subject to flow control may bypass required onsite
flow control facilities provided that all of the following conditions are met:
l. The point ofconvergence for runoffdischarged from the bypassed target surfaces and from the project's
flow control facility must be within a quarter-mile downstreamzs of the facility's proiect site discharge
point, AND
2. The increase inthe existing site conditions 10O-year peak discharge from the area of bypassed target
surfaces must not exceed 0.4 cfs, AND
3. Runoff from the bypassed target surfaces must not create a significant adverse impact to downstream
drainage systems, salmonid habitat, or properties as determined by Economic and Community
Development, AND
4. Water quality requirements applicable to the bypassed target surfaces must be met, AND
5. Compensatory mitigation by a flow control facility must be provided so that the net effect at the point of
"onu".gen""
downstream is the same with or without the bypass. This mitigation may be waived if the
existing site conditions 100-year peak discharge from the area ofbypassed target surfaces is increased by
no more than 0.1 cfs (modeled using I hour time steps) or no more than 0.15 cfs (modeled using l5 minute
time steps) and flow control BMPs as detailed in Appendix C are applied to all impervious surfaces within
the area of bypassed target surfaces. One or combination of the following methods may be used to provide
compensatory mitigation by a flow control facility subject to permission/approvals from other parties as
deemed necessary by Economic and Community Development:
a) Design the project's flow control facility or retrofit an existing offsite flow control facility as needed to
achieve the desired effect at the point ofconvergence, OR
b) Design the project's flow control facility or provide/retrofit an offsite flow control facility to mitigate
an existing developed area (either onsite or offsite) that has runoffcharacteristics (i.e., peak flow and
volume) equivalent to those of the bypassed target surfaces but is currently not mitigated or required to
be mitigated to the same flow control performance requirement as the bypassed target surfaces.
BYPASS OF RUNOFF FROM NON-TARGET SURFACES
IF the existing 100-year peak flow rate from any upstream offsite area (not targeted for mitigation) is greater
than 50% of the 10O-year developed peak flow rate (undetained) for the area that must be mitigated, THEN the
City of Kent Surface Water Design Manual l -35 Iuly 2022
sEcrloN 1.2 coRE REQUIREMENTS
runoff from the offsite area must bypass onsite flow control facilities. The bypass of offsite runoff must be
designed so as to achieve all of the following:
1. Any existing contribution of flows to an onsite wetland must be maintained, AND
2. Offsite flows that are naturally attenuated by the project site under predeveloped conditions must remain
attenuated, either by natural means or by providing additional onsite detention so that peak flows do not
increase, AND
3. Offsite flows that are dispersed or unconcentrated on the project site under predeveloped conditions must
be discharged in a safe manner as described in Core Requirement #l under "Discharge Requirements."
MITIGATION TRADES
A project's flow control facility may be designed to mitigate an existing developed non-target surface area
leitner onsite or offsite) in trade for not mitigating paft or all of the project's target surface area, provided that
the arrangement is approved by the city and all of the following conditions are met:
1. The existing developed non-target surface area (i.e., anareaofexisting impervious surface and/or non-
native pervious surface) must have runoff discharge characteristics (i.e., peak flow and volume) equivalent
to those of the target surface area for which mitigation is being traded and must not be currently mitigated
to the same flow control performance requirement as the target surface area, AND
2, Runoff from both the target surface area being traded and the flow control facility must converge prior to
discharge ofthe runofffrom the target surface area being traded onto private property without an easement
or through any arca subject to erosion' AND
3. The net effect in terms of flow control at the point of convergence downstream must be the same with or
without the mitigation trade, AND
4. The undetained runofffrom the target surface area being traded must not create a significant adverse
impact to downstream drainage systems, salmonid habitat, or properties prior to convergence with runoff
from the flow control facility.
The existing non-targeted surface area that is mitigated for purposes of required flow control must be
documented and tracked by city review staff. Documentation should clarifu that future redevelopment of
this existing non-targeted area used for the mitigation trade will incur additional flow control mitigation
requiremenis if the redevelopment exceeds Core Requirement #3 thresholds. This additional flow control
mitigation must be met in addition to that previously required and provided for the mitigation trade.
Applicants may be advised to size flow control facilities sufficient for both the mitigation trade area and
future development of the existing non-targeted area
FACILITY REQUIREMENT IN LANDSLIDE HAZARD DRAINAGB AREAS
Proposed projects subject to Discharge Requirement 2 in Core Requirement #1 must provide a tightline system
except wh-ere city review staff approves an altemative system based on a geotechnical analysis that considers
"u-ulutiu.
impacts from the project and surrounding areas under full built- out conditions' AND one of the
following conditions can be met:
a) Less than 2,000 square feet of new impervious surface will be added within the natural discharge
area,OR
b) The developed conditions runofffrom the nstural discharge areais less than 0.1 cfs for the 100- year
runoff event and will be infiltrated for runoff events up to and including the 1OO-year event, OR
c) The developed conditions runoffvolume 29 from the natural discharge area is less than 50Yo of the
existing "onditions
runoff volume from other areas draining to the location where runoff from the
natural discharge srea enters the landslide huzard urea onto slopes steeper than 15o/o' AND the
provlslons of Discharge Requirement I are met, OR
5
City of K€nt Surface Water Design Manual l -36 Jrly 2022
SECTION I.2 CORE REQUIREMENTS
d) City review staff determines that a tightline system is not physically feasible or will create a significant
adverse impact based on a soils report by a geotechnical engineer.
Systems proposed as an alternative to the required tightline must meet all of the following requirements:
L Approval by the city shall be based on a geotechnical analysis that considers cumulative impacts from the
project and surrounding areas under full built-out conditions.
2. Proposed facilities, FCBMPs, and dispersal systems.must meet all applicable feasibility and setback
requirements contained in the SWDM.
3. Facility outflows must meet the discharge dispersal requirements specified in Discharge Requirement I of
Core Requirement #1.
4. The geotechnical analysis and proposed system design must address facility overflows and recommend
additional measures, factors ofsafety in facility design, etc. based on an evaluation ofrisk ofslope
instability or failure and potential impacts to life, structures, and property.
5. For projects adjacent to or containing a landslide, steep slope, or erosion hazurd areu as defined in KCC
1 1.06, the applicant must demonstrate that onsite drainage facilities and/or flow control BMPs will not
create asignificant adverse impact to downhill properties or drainage systems.
Manifold Detention Facilities
A maniftld detentionfacilifi is a single detention facility designed to take the place of two or more otherwise
required detention facilities. It combines the runoff from two or more onsite drainage areas having separate
naiural discharge points, and redistributes the runoffback to the natural discharge points following detention.
Because manifold detention facilities divert flows from one natural discharge point to another and then back,
they are not allowed except by an approved adjustment (see Section 1.4, "Adjustment Process").
Use of Underground Detention Facilities
Open detention ponds are preferred over underground detention facilities (vaults or tanks) because open
vegetated ponds provide additional stormwater treatment in addition to quantity controls'
Underground vaults or tanks shall only be permitted to meet detention requirements with the following
conditions fully implemented:
l. All landscaping on the development site, including individual lots and open spaces, must strictly
comply with soil depth and amendment requirements outlined in Kent City Code Chapter 15.07'
2. All vaults must be thoroughly cleaned, and all filters changed after construction is complete and prior
to the release ofany financial guarantees'
3. Vaults are not allowed in the City right of way'
4. Vaults shall be "Infiltration Vaults" as per Chapter 5, section 5.2.4 of the King County SWDM unless
site or project conditions warrant otherwise as determined by a geotechnical engineering report to be
reviewed and approved by the development engineering reviewer. The applicant shall submit a
geotechnical report that addresses the viability of a bottomless infiltration vault. Special consideration
should be given to the effect of groundwater on vault functionality and capacity.
5. Bottomless vault bottom must be at or above the seasonal high groundwater level for Infiltration
Vaults used for detention purposes.
6. Vaults are allowed when the water draining to them is from private property and the vault is privately
owned and maintained. For cases where the facility receives runoff from public Right-of-Way, vaults
(and associated water quality facilities) will be maintained by the city and the associated property
City of Kent Surlace Water Design Manual t-37 Iuly 2022
SECTION I.2 CORE REQUIREMENTS
owners shall reimburse the city for the maintenance. The responsibility to reimburse the city for
required maintenance shall be memorialized in a recorded Stormwater Facility Maintenance Covenant
recorded against the property. For subdivisions, the homeowner's responsibility to reimburse the city
for maintenance shall be included on the face of the recorded subdivision and be included in the
subdivision's Covenants, Conditions & Restrictions, (CC&R's). The CC&R's shall be recorded in
conjunction with the Final Subdivision.
No credit will be permitted toward required detention volumes for any element of the conveyance system'
Aquifer Recharge Areas
The City of Kent implements a wellhead protection program to protect and preserve regional groundwater
resources. This program is critical to the City to ensure that the water supply obtained from groundwater is
maintained at the highest quantity and quality levels possible. Protection of quantity and quality of
groundwater is also important to the City and regional interests due to the linkage between groundwater and
surface water. Baseflow represents a high percentage of streamflow in the dry summer months in the Puget
Sound Region.
To work towards protection of the groundwater resource, the City of Kent, Covington Water District, and
Water District I 1l have designated and mapped "Wellhead Protection Areas". These areas are shown on the
Soils/Wellhead Area Map included with this manual in the cover pocket. The City of Kent requires that any
project located within Wellhead Protection Areas and subject to the requirements of this manual
enhance or maintain groundwater recharge quantity and qualify to the maximum extent possible' This
shall predominantly be achieved through the construction of infiltration facilities as described in Chapter 5 -
Flow Control Design.
City of Kent Surface Water Design Manual I -38 IuJy 2022
SECTION I.2 CORE REQUIREMENTS
R
E
1.2.4
1.2.4.1
CORE REQUIREMENT #4: CONVEYANCE SYSTEM
All engineered conveyance system elements for proposed projects must be analyzed, designed, and constructed
to provide a minimum level of protection against overtopping, flooding, erosion, and structural failure as
specified in the following groups of requirements:
o ooConveyance Requirements for New Systems," Section 1.2.4.1
r ooConveyance Requirements for Existing Systems," Section I '2.4'2
r "Conveyance System Implementation Requirements," Section 1'2.4'3
Intent: To ensure proper design and construction ofengineered conveyance system elements. Conveyance
systems are natural and engineered drainage facilities that provide for the collection and transport of surface
water or stormwater runoff. This core requirement applies to the engineered elements of conveyance systems-
primarily pipes, culverts, and ditches/channels.
CONVEYANCE REQUIREMENTS FOR NEW SYSTEMS
All new conveyance system elements,ts both onsite and offsite, shall be analyzed, designed, and constructed
according to the following requirements. All analyses shall examine the full range of anticipated tailwater
conditions. Also see Section 4.1 for route design and easement requirements.
Pipe Systems
1. New pipe systems shall be designed with sufficient capacity to convey and contain (at minimum) the 25-
year peak flow, assuming developed conditions for onsite tributary areas and existing conditions for any
offsite tributary areas.
2. Pipe system structures may overtop for runoff events that exceed the25-year design capacily, provided the
overflow from a I O0-year runoff event does not create or aggravate a severe flooding problem or sevete
erosion problem as defined in Core Requirement #2, Section 1.2.2. Any overflow occurring onsite for
runoffevents up to and including the 10O-year event must discharge at the natural location for the proiect
site. ln residential subdivisions, such overflow must be contained within an onsite drainage easement,
tract, covenant, or public right-of-way.
3. The upstream end ofa pipe system that receives runofffrom an open drainage feature (pond, ditch, etc')
shall be analyzed and sized as a culvert as described below'
Culverts
l. New culverts shall be designed with sufficient capacity to meet the headwater requirements in Section
4.3.1 andconvey (at minimum) the 25-year peak flow, assuming developed conditions for onsite tributary
areas and existing conditions for any offsite tributary arezu.
2. New culverts must also convey as much of the I 00-year peak flow as is necessary to preclude creating or
aggravating a severe flooding problem or severe erosion problem as defined in Core Requirement #2,
Section I .2.2. Any overflow occuning onsite for runoff events up to and including the 100-year event
must discharge at the natural location for the project site. ln residential subdivisions, such overflow must
be contained within an onsite drainage easement, tract, covenant, or public right-of-way.
3. New culverts proposed in streams with salmonids shall be designed to provide for hsh passage based on
current Washington State fish-passage laws and regulations.
18 Ne.tv conveyance system elements are those that are proposed to be constructed where there are no existing constructed conveyance elements.
City of Kent Surface Water Design Manual 1-39 July 2022
SECTION I.2 CORE REQUIREMENTS
1.2.4.2
Ditches/Channels
l. New ditches/channels shall be designed with sufficient capacity to convey and contain, at minimum, the
25-year peak flow, assuming developed conditions for onsite tributary areas and existing conditions for any
offsite tributary areas.
2. New ditches/channels must also convey as much of the 1O0-year peak flow as is necessary to preclude
creating or aggravatin g a severe flooding probtem or sevete erosion problem as defined in Core
Requirement 2, Section 1.2.2. Any overflow occuning onsite for runoff events up to and including the
1gQ-year event must discharge at the natural location for the project site. In residential subdivisions, such
overflow must be contained within an onsite drainage easement, tract, covenant, or public right-of-way'
Tightline Systems Traversing Steep Slopes
New tightline conveyance systems traversing slopes that are steeper than l5o/o and greater than 20 feet in
height, or are within asteep slope hazard urea shall be designed with sufficient capacity to convey and contain
(at minimum) the 1OO-year peak flow, assumingfull build-out conditionslg for all tributary areas, both onsite
and offsite. Tightline systems shall be designed as detailed in Section 4'2.2.
Bridges
New bridges shall be designed to pass the 100-year peak flow with clearance as specified in Section 4.3.3 and in
accordance with the floodplain development standards in KCC 14.09.
CONVEYANCE REQUIREMENTS FOR EXISTING SYSTEMS
The following conveyance requirements for existing systems are less rigorous than those for new systems to
allow some salvaging of existing systems that are in useable condition. Existing systems may be utilized if
they are capable of providing a minimum level of protection as-is or with minor modifications.
Existing Onsite Conveyance Systems
No Change in Flow Characteristics: Existing onsite conveyance systems that will not experience a change in
flow characteristics (e.g., peak flows or volume of flows) as a result of the proposed project need not be
analyzed for conveyance capacity.
Change in Flow Characteristics: Existing onsite conveyance systems that will experience a change in flow
characteristics as a result of the proposed project must comply with the following conveyance requirements:
1. The existing system must be analyzed and shown to have sufficient capacity to convey and contain (at
minimum) the l0-year peak flow assuming developed conditions for onsite tributary areas and existing
conditions for any offsite tributary areas.
2. The applicant must demonstrate that the 10O-year peak flow to the existing system will not create or
aggravate a severe flooding problem or severe erosion problem as defined in Core Requirement #2,
Section 1.2.2.
3. Minor modifications may be made to the conveyance system to achieve the required capacity stated above.
Examples of minor modifications include raising a catch basin rim, replacing or relaying a section of pipe
to match the capacity of other pipes in the system, improving a pipe inlet, or enlarging a short, constricted
reach ofditch or channel.
4. Modifications to an existing conveyance system or element that act to attenuate peak flows due to the
presence ofupstream detention storage shall be made in a manner that does not significantly increase peak
te Full build-out conditionsmeans the tributary area is developed to rts full zoning potential except where there are existing environmentally
sensitive areas, open space tracts, and/or native growth protection easements/covenants
City of Kent Surlace Water Design Manual l -40 Jr:ly 2022
SECTION 1.2 CORE REQUIREMENTS
1.2.4.3
flows downstream. For example, if water is detained in a pond upstream of a restrictive road culvert, then
installing an overflow system for the culvert should prevent overtopping of the road without significantly
reducing existing detention storage.
Existing Offsite Conveyance Systems
L Existing offsite conveyance systems need not be analyzed for conveyance capacity except as required by
Core Requirement#2, or if offsite improvements or direct discharge are proposed per Core Requirement
#3.
2. Improvements made to existing offsite conveyance systems to address the problem-specific mitigation
requirements in Section 1.2.2.2 need only change existing conveyance capacity sufficiently to prevent
aggravation ofthe drainage problem(s) being addressed.
CONVEYANCE SYSTEM IMPLEMENTATION REQUIREMENTS
Conveyance systems shall be designed and implemented in accordance with the following requirements,
allowances, and flexible compliance provisions:
Methods of Analysis and Design
Properly-sized conveyance elements provide sufficient hydraulic capacity to convey peak flows of the return
frequencies indicated in Sections 1.2.4.1 and 1.2.4.2. Conveyance capacity shall be demonstrated using the
methods of analysis detailed in Chapter 4. Design flows for sizing conveyance systems shall be determined
using the appropriate runoff computation method specified in Section 3.2-
Spill Control Provisions
Projects proposing to construct or replace onsite conveyance system elements that receive runofffrom non-
roof-top poltution-generating impervious sarface must provide a spill control device as detailed in Section
4.2.1 prior to discharge from the project sr/e or into a natural onsite drainagefeature.zo More specifically, this
requirement applies whenever a proposed project does either of the following:
o Constructs a new onsite conveyance system that receives runofffrom non-roof-top pollution-generaling
impervious surface,OR
r Removes and replaces an existing onsite conveyance system element that receives runoff from 5,000
square feet or more of non- roof-top pollution-generating impervious surfuce onsite.
The intent of this device is to temporarily detain oil or other floatable pollutants before they enter the
downstream drainage system in the event of an accidental spill or illegal dumping. It may consist of a tee
section in a manhole or catch basin, or another alternative as specified in Section 4.2.1 . The spill control
device should be installed upstream of any onsite water quality or flow control facility. Note that in addition to
this spill control requirement to protect offsite and natural drainage systems, there are other spill control
requiiements in this manual for discharges to certain water quality facilities and all infiltration facilities (see the
deiign criteria for water quality facilities in Chapter 6 and the general requirements for infiltration facilities in
Section 5.2). The application of these requirements must be such that all stated intents are satisfied. If no
facilities are present, then the spill control device must be installed upstream from the final discharge point to
the downstream drainage system.
10 Natural onsile drainage feature means a natural swale, channel, stream, closed depression, wetland, or lake.
City of Kent Surlace Water Design Manual l-41 Iuly 2022
SECTION I.2 CORE REQUIREMENTS
Composition
Where feasible, conveyance systems shall be constructed of vegetation-lined channels, as opposed to pipe
systems. Vegetative channels shall generally be considered feasible if all of these conditions are present:
1. The channel gradient generally does not exceed five percent, AND
2. No modifications to currently adopted standard roadway cross-sections in the Kent Design and
Construction Standards are necessitated by the channel, AND
3. The channel will be accessible for maintenance (see Section 1.2.6)' AND
4. The channel will not be subject to erosion.
Exceptions: The following are exceptions to the requirement for vegetative channels.'
r Conveyance systems proposed under roadways, driveways, or parking areas
o Conveyance systems proposed between houses in urban-zoned plats and short plats
o Conveyance systems conveying roof runoff only.
Outfalls
An outfall is defined as a point where collected and concentrated surface and stormwater runoff is discharged
from a pipe system or culvert.
Energy Dissipation: At a minimum, rock erosion protection is required at outfalls from all drainage systems
and elements except where city review staff determines that erosion protection is being provided by other
means or is not needed. Details on outfall structures are included in Section 4.2.2.
New Point Discharges Over Steep Slopes / Landslide Hazard Areas: Proposed outfalls that will discharge
runoffin a location where the natural (existing) discharge is unconcentrated over a slope steeper than l5o/o and
greater than 20 feet in height, or discharge runoff through a designate d steep slope haznrd area mvst meet the
following criteria:
l. A tightline conveyance system must be constructed to convey the runoffto the bottom ofthe slope
unless other measures are approved by city review staffbased on an evaluation/report by a licensed
geotechnical engineer.
2. The geotechnical analysis must consider cumulative impacts from the project and surrounding areas
under full built-out conditions.
3. Tightline systems must be designed so that existing baseflow conditions are not significantly changed
and adequate energy dissipation is provided at the bottom ofthe slope'
4. Where alternative measures (e.g., dispersal trench) to the tightline system are approved upstream of a
tandslide huzard urea or steep slope hazard area,they may be placed no closer than 50 feet from the
top ofthe hazard area slope based on an evaluation/report by a licensed geotechnical engineer'
Outfalls to the Green River
New stormwater outfalls or modifications to existing stormwater outfalls discharging to the Green River are
allowed only through the adjustment process. These outfalls must comply with requirements of the Green
River Pump Operations Procedure Plan, which establishes storage volumes and release rate criteria for
developments proposing to constract or modify outfalls. Copies of the plan are available from the city of Kent.
Interflow and Interception
Interflow is near-surface groundwater that moves laterally through the soil horizon following the hydraulic
gradient of underlying relatively impermeable soils. When interflow is expressed on the surface, it is termed a
ipring or seepqge. Any significant springs or seepage areas that impact a roadway or structure proposed by the
project must be intercepted and directed into a conveyance system. Where roadways may impede the passage
of interflow to downstream wetlands or streams, provision for passage of unconcentrated flows must be made.
City olKent Surface Water Design Manual t-42 Iuly 2022
SECTION I.2 CORE REOUIREMENTS
1.2.5
Pump Systems
Pump systems may be used to convey water from one location or elevation to another within the proiect site
prouid"d they meet the design criteria specified for such systems in Section 4.2.3 and will be privately owned
and maintained.
Pump systems that discharge flows fromthe project site thalwould not have discharged by gravity flow under
exisiing site conditions will require an approved adjustment to Core Requirement #l (see Section 1.4,
"Adjustment Process"). These pump systems will be considered only when they are necessary to prevent
creation or aggravation of a flooding or erosion problem as specified in Section 1.2.2. Pump systems
discharging to th" Gr""n River must also comply with the Green River Pump Operations Procedure Plan.
CORE REQUIREMENT #5: EROSION AND SEDIMENT COI\TROL
All proposed projects that will clear, grade, or otherwise disturb the site must provide erosion and sediment
controls to prevent, to the maximum extent possible, the transport of sediment from the proiect site to downstream
drainage facilities, water resources, and adjacent properties. All proposed projects that will conduct construction
activities onsite or offsite must provide stormwater pollution prevention and spill controls to prevent, reduce,
or eliminate the discharge of pollutants to onsite or adjacent stormwater systems or watercourses. To prevent
sediment transport, Erosion and Sediment Control (ESC) measures and Stormwater Pollution Prevention
and Spill Control (SWPPS) measures that are appropriate to the proiect site must be applied through a
comprehensive Construction Stormwater Pollution Prevention (CSWPP) plan as described in Sections
1.2.5.1 and 1.2.5.3 and shall performas described in Section 1.2.5.2. In addition, these measures, both
temporary and permanent, shall be implemented consistent with the requirements in Section 1.2-5.3 that
apply to the proposed project.
Intent:
. To prevent the transport of sediment and other impacts, like increased runoff, related to land disturbing
activities. Erosion of disturbed areas on construction sites can result in excessive sediment transport to
adjacent properties and to surface waters. This sediment can result in major adverse impacts, such as
flooding from obstructed drainage ways, smothering of salmonid spawning beds, algal blooms in lakes'
and exceedances of state water quality standards for turbidity. These impacts can also result from the
increased runoffgenerated by land disturbing activities on construction sites.
. To prevent, reduce, or eliminate the discharge of pollutants to onsite or adjacent stormwater systems or
watercourses from construction-related activities such as materials delivery and storage, onsite
equipment fueling and maintenance, demolition of existing buildings and disposition of demolition
materials and other waste, and concrete handling, washout and disposal.
CSWPP MEASURES
Construction Stormwater Pollution Prevention (CSWPP) measures include Erosion and Sediment Control
(ESC) measures and Stormwater Pollution Prevention and Spill (SWPPS) measures'
The following ESC measures shall be provided as specified below and as further detailed inthe Erosion and
Sediment Control (ESC) Standards, adopted as Appendix D:
l. Clearing Limits: Prior to any site clearing or grading, areas to remain undisturbed during project
construction shall be delineated. At a minimum, clearing limit delineation flagging shall be provided
at the edges of all stream, lake, or wetland buffers.
Z. Cover Measures: Temporary and permanent cover measures shall be provided when necessary to
protect disturbed areas. Temporary cover shall be installed if an area is to remain unworked for more
than seven days during the dry season (May I to September 30) or for more than two days during the
wet season (October I to April 30), unless otherwise determined by the City. Any area to remain
unworked for more than 30 days shall be seeded or sodded, unless the City determines that winter
weather makes vegetation establishment unfeasible. During the wet season' slopes and stockpiles
3H:1V or steeper with more than l0 feet of vertical relief shall be covered if they are to remain
1.2.5.1
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City of Kent Surlace Water Design Manual t-43 Iuly 2022
SECTION I.2 CORE REQUIREMENTS
unworked for more thzn 12 hours. The intent of these measures is to prevent erosion by having as
much area as possible covered during any period of precipitation.
3. Perimeter Protection: When necessary, perimeter protection to filter sediment from sheet flow shall
be provided downstream of all disturbed areas. Perimeter protection includes the use of vegetated
strips, as well as more conventional constructed measures such as silt fences. Such protection shall be
installed prior to upstream grading.
4. Traflic Area Stabilizationz Unsurfaced entrances, roads, and parking areas used by construction
traffic shall be stabilized to minimize erosion and tracking of sediment offsite.
5. Sediment Retention: Surface water collected from disturbed areas of the site shall be routed through
a sediment pond or trap prior to release from the sile. This does not apply to areas at the perimeter of
the site small enough to be treated solely with perimeter protection. Sediment retention facilities shall
be installed prior to grading any contributing area'
6. Surface Water Controls: Surface water controls shall be installed to intercept all surface water from
disturbed areas, convey it to a sediment pond or trap, and discharge it downstream ofany disturbed
areas. However, areas at the perimeter of the s/e small enough to be treated solely with perimeter
protection do not require surface water controls. Significant sources ofupstream surface water that
drain onto disturbed areas shall be intercepted and conveyed to a stabilized discharge point
downstream of the disturbed areas. Surface water controls shall be installed concurrently with or
immediately following rough grading.
7 . Dust Control: Preventative measures to minimize wind transport of soil shall be implemented when a
traffichazardmay be created or when sediment transported by wind is likely to be deposited in water
resources.
8. Dewatering Control: The water resulting from construction site de-watering activities must be treated
prior to discharge or disposed ofas specified.
g. Flow Control: Surface water from disturbed areas must be routed through the project's onsite flow
control facility or other provisions must be made to prevent increases in the existing site conditions 2-
year and l0-year runoffpeaks discharging from the project site during construction (flow control BMP
areas (existing or proposed) shall not be used for this purpose)'
10. Control Pollutants: Stormwater pollution prevention (SWPPS) measures are required to prevent,
reduce, or eliminate the discharge of pollutants to onsite or adjacent stormwater systems or
watercourses from construction-related activities such as materials delivery and storage, onsite
equipment fueling and maintenance, demolition of existing buildings and disposition of demolition
materials and other waste, and concrete handling, washout and disposal. Section D.2.2 desctibes BMPs
specific to this purpose; additionally, several of the ESC BMPs described herein are applicable.
11. Protect Existing and Proposed Flow Control BMPs: Sedimentation and soil compaction reduce the
infiltration capacily of native and engineered soils. Protection measures shall be applied/installed and
maintained so as to prevent adverse impacts to existing flow control BMPs and areas of proposed flow
control BMPs for the project. Adverse impacts can prompt the requirement to restore or replace
affected BMPs.
12. Maintain BMPs: Protection measures shall be maintained to assure continued performance oftheir
intended function, to prevent adverse impacts to existing flow control BMPs and areas of proposed flow
control BMPs, and protect other disturbed areas of the project.
13. Manage the Project: Coordination and timing of site development activities relative to ESC concerns,
and timely inspection, maintenance and update of protective measures are necessary to effectively
manage the project and assure the success of protective ESC and SWPPS design and implementation.
SWPPS MEASURES
City of, Kent Surface Water Design Manual t-44 Iuly 2022
SECTION I.2 CORE REQUIREMENTS
1.2.5.2
Each of the following categories of SWPPS measures must be considered for applicatio n to the proiect site as
detailed inthe Stormwater Pollution Prevention and Spill Control (SWPPS) Standards. The SWPPS
standards are located inthe Construction Stormwater Pollution Prevention Standards adopted as Appendix D
of this manual:
. Follow effective pollutant handling and disposal procedures.
. Provide cover and containment for materials, fuel and other pollutants.
. Manage the project site to maximize pollutant control and minimize pollutant sources.
. Protect from spills and drips ofpetroleum products and other pollutants.
. Avoid over application or untimely application of chemicals and fertilizers.
. Prevent or treat contamination of stormwater runoff by pH modifying sources.
CSWPP PERFORMANCE AND COMPLIANCE PROVISIONS
The changing conditions typical of construction sites call for frequent field adjustments of existing ESC and
SWPPS measures or additional ESC and SWPPS measures in order to meet required performance. In some
cases, strict adherence to specified measures may not be necessary or practicable based on sl'/e conditions or
project type. In other cases, immediate action may be needed to avoid severe impacts. Therefore, careful
attention must be paid to ESC and SWPPS performance and compliance in accordance with the following
provisions:
A. CSWPP Supervisor
For projects in Targeted or Full Project Drainage Review, or projects in Directed Drainage Review as
determined by city review staft the applicant must designate a CSWPP supervisor who shall be
responsible for the performance, maintenance, and review of ESC and SWPPS measures and for
compliance with all permit conditions relating to CSWPP as described inthe CSWPP Standards. Fot
projects that disturb one acre or more of land, the CSWPP supervisor must be a Certified Professional
in Erosion and Sediment Control (see www.cpesc.net for more information) or a Certified Erosion
and Sediment Control Lead whose certification is recognized by the City. The City may also require a
certified ESC professional for sites smaller than one acre of disturbance if the City determines that onsite
ESC measures are inadequately installed, located, or maintained'
For larger, more sensitive sites, the City may require a cerlified ESC professional with several years of
experience in construction supervisiorVinspection and a background in geology, soil science, or
agronomy (See Appendix D, Section D.2'3'l for more information).
B. Monitoring Performance
The CSWPP supervisor shall have a turbidity meter onsite and shall use it to monitor surface and storm
water discharges from the project site and into onsite wetlands, streams, or lakes whenever runoff occurs
from onsite activities and during storm events. lf the project sile is subject to a NPDES general permit
for construction issued by the Washington State Department of Ecology (Ecologt), then the project must
comply with the monitoring requirements of that permit.
The CSWPP supervisor shall also use the specific SWPPS control BMP procedures for monitoring
surface and stormwater discharge for pollutants and acceptable discharge levels. The CSWPP supervisor
shall keep logs as required by the procedures of all measurements taken onsite and make them available
to the City on request.
C. ESC Performance
City of Kent Surface Water Design Manual l -45 Iuly 2022
SECTION I.2 CORE REQUIREMENTS
The above ESC measures shall be applied/installed and maintained so as to prevent, to the maximum
extent possible, the transport of sediment from the project site to downstream drainage systems or surface
waters or into onsite wetlands, streams, or lakes. This performance is intended to be achieved through
proper selection, installation, and operation of the above ESC measures as detailed inlhe CSWPP
Standards (detached Appendix D) and approved by the City. However, the City may determine atany
time during construction that such approved measures are not sufficient and additional action is required
based on one of the following criteria:
1 . IF a turbidity test of surface and storm water discharges leaving the proiect site is greater than the
benchmark value of 25 NTU (nephelometric turbidity units) set by the Washington State Department
of Ecology, but less than250 NTU, the CSWPP Supervisor shall do all of the following:
a) Review the ESC plan for compliance and make appropriate revisions within 7 days of
the discharge that exceeded the benchmark of 25 NTU, AND
b) Fully implement and maintain appropriate ESC measures as soon as possible, AND
c) Document ESC implementation and maintenance in the s#e log book.
2. IF a turbidity test of surface or storm water discharging to wetlands, streams, or lakes indicates a
turbidity level greater than 5 NTU above background when the background turbidity is 50 NTU
or less, or l0%o above background when the background turbidity is greater than 50 NTU, then
corrective actions and/or additional measures beyond those specified in Section L2.5.1 shall be
implemented as deemed necessary by the City inspector or onsite CSWPP supervisor.
3. IF discharge turbidity is 250 NTU or greater, the CSWPP Supervisor shall do all of the following:
a) Notifu the City by telephone, AND
Notiff the WA State Department of Ecology as required by NPDES Construction Stormwater
General Permit, as applicable, AND
b) Review the ESC plan for compliance and make appropriate revisions within 7 days of
the discharge that exceeded the benchmark of 25 NTU, AND
c) Fully implement and maintain appropriate ESC measures as soon as possible but no later than
10 days after the discharge that exceeded the benchmark, AND
d) Document ESC implementation and maintenance in the sile log book. AND
e) Continue to sample discharges until turbidity is 25 NTU or lower, or the turbidity is no more than
l0o% over background turbidity.
4. IF the City determines that the condition of the construction site poses a hazard to adjacent
property or may adversely impact drainage facilities or water resources, THEN additional
measures beyond those specified in Section I .2.5.l may be required by the City'
D. SWPPS Performance
SWPPS measures shall be applied/installed and maintained so as to prevent, reduce, or eliminate the
discharge of pollutants to onsite or adjacent stormwater systems or watercourses or onto adjacent
properties. This performance is intended to be achieved through proper selection, installation, and
operation of the above SWPPS measures as detailed in the CSWPP Standards (detached Appendix D)
and approved by the City. However, the CSWPP supervisor designated per Section L25.2.A or the City
may determin e at any time during construction that such approved measures are not sufficient and
additional action is required based on the criteria described in the specific SWPPS BMP standard and/or
conditions of an approved adjustment:
City of Kent Surface Water Design Manual t-46 July 2022
SECTION I.2 CORE REQUIREMENTS
1.2.5.3
E. Flexible Compliance
Some projects may meet the intent of Core Requirement #5 while varying from specific CSWPP
requirements contained here and in the CSWPP Standards . If a project is designed and constructed such
that it meets the intent of this core requirement, the City may determine that strict adherence to a specific
ESC requirement is unnecessary; an approved adjustment (see Section 1.4) is not required in these
circumstances. Certain types of projects are particularly suited to warrant this greater level of flexibility;
for instance, projects on relatively flat, well drained soils, projects that are constructed in closed
depressions, or projects that only disturb a small percentage of a forested site may meet the intent of this
requirement with very few ESC measures. However, SWPPS requirements may actually be emphasized on
well-drained soils, particularly in groundwater or well-protection protection areas, or in close proximity to
water bodies. More information on intent and general ESC principles is contained in the ESC Standards'
F. Roads and Utilities
Road and utility projects often pose difficult erosion control challenges because they frequently cross
surface waters and are long and narrow with limited area available to treat and store sediment-laden water
Because of these factors, road and utility projects are allowed greater flexibility in meeting the intent of
Core Requirement #5 as described in lhe CSWPP Standards. Projects that pose a very low risk of erosion
or sediment transport due to site conditions or project scope may also warrant greater flexibility.
G. Alternative and Experimental Measures
All measures proposed for erosion and sediment control shall conform to the details and specifications
inthe CSWPP Standards unless an alternative is approved by the City, and if the altemative is a new
technology, it must also be approved through Ecology's CTAPE program (see "Alternative and
Experimental Measures" inthe CSI(PP Standards, detached Appendix D).
CSWPP IMPLEMENTATION REQUIREMENTS
Proposed projects must identify, install, and maintain required erosion and sediment control and
stormwater pollution prevention and spill control measures consistent with the following requirements:
A. CSWPP Plan
As specified in Chapter 2, all proposed projects must submit a CSWPP plan for implementing CSWPP
measures. The CSWPP plan is comprised of the ESC plan and the SWPPS plan. The ESC plan must
show the location and details of all ESC measures as specified in Chapter 2 wrdthe CSWPP Standards
and shall include a CSWPP report, which contains additional directions and supporting information like
a detailed construction sequence as proposed by the design engineer and any calculations or information
necessary to size ESC measures and demonstrate compliance with Core Requirement #5. The CSWPP
plan shall also contain plan notes that outline specific permit conditions as outlined in Appendix D
Section D.4.2 Standard ESC and SWPPS Plan Notes. The City may require large, complex projects to
phase construction and to submit multiple ESC plans for the different stages of construction. New
CSWPP plans are not required for changes that are necessary during construction, unless required by the
City inspector,
B. Wet Season Construction
During the wet season (October I to April 30), any site with exposed soils shall be subject to the "Wet
Season Requirements" contained in the ESC Standards. In addition to the ESC cover measures; these
provisions include covering any newly seeded areas with mulch and seeding as much disturbed area as
possible during the first week of October to provide grass cover for the wet season. Other ESC
measures such as portable detention tanks and portable sand filters may be required for use during the
wet season.
City of Kent Surface Water Design Manual 1-47 luly 2022
SECTION I.2 CORE REQUIREMENTS
C. Construction within Streams, Lakes, or Wetlands and Buffers
Any construction that will result in disturbed areas on or within a stream or associated buffer, within a
wetland or associated buffer, or within 50 feet of a lake shall be subject to the Kent city code section 1 I .06
"Critical Areas" as well as the "Critical Area Restrictions" contained in the CSWPP Standqrds. These
provisions include phasing the project whenever possible so that construction in these areas is limited to
the dry season.
D. Maintenance
All ESC and SWPPS measures shall be maintained and reviewed on a regular basis as prescribed in the
CSWPP Standards.
E. Final Stabilization
Prior to obtaining final construction approval, the site shall be stabilized, structural ESC and SWPPS
measures (such as silt fences, sediment traps, and concrete waste collection pits) shall be removed, and
drainage facilities shall be cleaned as specified inthe CSWPP Standards. A separate ESC plan
describing final stabilization may be required by the City prior to implementation.
F. Consideration of Other Required Permits
Consideration should be given to the requirements and conditions that may be applied by other agencies as
part of other permits required for land-disturbing activities. In particular, the following permits may be
required and should be considered when implementing CSWPP measures:
o A NPDES General Permit for Construction (pursuant to the Washington State Department of
Ecology's Construction General Permit for Stormwater) is required for projects that will disturb one or
more acres for purposes of constructing or allowing for construction of a development, or projects
disturbing less than one acre thatare part of a larger common plan of sale2rthat will ultimately disturb
one or more acres.
zt Common plan of development or sale means a site where multiple separate and distinct construction activities may take place at different
times or on different schedules, but still under a single plan. Examples include: I ) phased projects and projects with multiple filings or lots,
even ifthe separate phases or filings/lots will be constructed under separate contract or by separate owners (e.g. a development where lots
are sold to separate builders); 2) a development plan that may be phased over multiple years, but is still under a consistent plan for long-term
development; and 3) projects in a contiguous area that may be unrelated but still under the same contract, such as construction ola building
extension and a new parking lot at the same facility.
City of Kent Surface Water Design Manual I -48 Iuly 2022
SECTION 1.2 CORE REQUIREMENTS
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1.2.6 CORE REQUIREMENT #6: MAINTENANCE AND OPERATIONS
Maintenance and operation of all drainage facilities is the responsibility of the applicant or property owner,
except those facilities for which Kent is granted an easement, tract, or right-of-way and officially assumes
maintenance and operation as described below.
Intent: To ensure that the maintenance responsibility for drainage facilities is clearly assigned and that these
facilities will be properly maintained and operated in perpetuity.
Drainage Facilities to be Maintained by the City of Kent
Kent will assume maintenance and operation22 of conveyance systems within improved public road rights-of-
way and flow control and water quality facilities if these systems/facilities are associated with a residential
subdivision with ten or more lots served by a public street, or a non-residential short plat or long plat
subdivision (i.e., commercial, industrial, etc.). Kent will not assume maintenance and operation of stormwater
facilities associated with residential subdivisions with 9 or less lots served by a private street or stormwater
facilities associated with commercial or industrial developments located on single parcels.
Kent will not assume maintenance responsibility for underground detention vaults except as described in
section 1.2.3, Core Requirement #3.
Kent wilt assume maintenance and operation of these facilities two years after final construction approval
by the Public Works and Economic and Community Development departments and an inspection by the City to
ensure the facilities have been properly maintained and are operating as designed.
Flow control and water quality facilities and flow control BMP devices to be maintained and operated by
Kent must be located in a tract or right-of-way dedicated to Kent. Required vegetated flow paths for full
dispersion and basic dispersion BMPs require a recorded declaration of covenant that stipulates restrictions
on use AND shall be located in an easement that includes provisions for access and maintenance. City of
Kent maintenance of these vegetated flow paths will be limited to their FCBMP functionality. All other
maintenance shall remain the responsibility of the owner(s). Access roads serving these facilities must also be
located in the tract or right-of-way and must be connected to an improved public road right-of-way.
Conveyance systems to be maintained and operated by Kent must be located in a drainage easement, tract, or
right-of-way granted to Kent. Note: Kent does not normqlly assume maintenance responsibility for conveyance
systems which are outside of improved public road right-of-way.
Exceptions to the above will be made on a case-by-case basis as determined by the Director.
Drainage Facilities to be Maintained by Private Parties
All privately maintained drainage facilities must be maintained as specified in Appendix A, ooMaintenance
Requirements for Flow Control, Conveyance, and WQ Facilities," and as further prescribed in Chapter 6 for
water quality facilities unless otherwise approved by city review staff. A copy of the Operation and
Maintenance Manual submitted as part of the permit application (see Section 2.3.1) shall be retained on site
and shall be transferred with the property to the new owner. A log of maintenance activity indicating when
cleaning occuned and where waste was disposed of shall also be kept by the owner and be available for
inspection by the City.
All privately maintained flow control BMPs must be maintained as specified inthe sitellot's declaration of
covenant and grant ofeasement per Section 1.2.9.
22 Kent does not assume maintenance of lot dratnage systems or drainage stub-outs serving single-family residential lot downspout, footing, or
yard drains, nor does Kent assume maintenance of those water quality facilities installed and integrated into site landscaping.
City of Kent Surface Water Design Manual t-49 Iuly 2022
SECTION I,2 CORE
A "Declaration of stormwater Facility Maintenance covenant,, must be completed for all privatedevelopments' Stormwater covenant io be obtained "" Co[-;;;;"
". from pran ,.uL* r'turr.. The timing ofcompletion of this agreement varies depending on trr" typ" ora-#iopr"nt. For stormwater facilities servingmore than one single family residential'unit iria short pd;r;;"j;; ;ss than four rots, rhe agreement sha, becompleted prior to plat recording' no. uil otier develoimeno o," ,?o"n"ropments (incrudinj commerciar,industrial' multifamily d"uetop'ient or..j"u"lop.n"nio;Gl"" aiir, residentiar buirdini), the agreementshall be completed prior to *gint"ti"g pru" approval. trtn"[ is a shared drainage system invorved with thedevelopment' the covenant must provil"
"t"ailangu"g" ;ilr" |,,;nership and maintenance responsibirities ofeach componant of the drainage tv"rnl. irr. responsibilities detailed in tt " oocum""in1", u" assumed by ahomeowners association o. ort".i"gui-o.gunirution as approved by the city of Kent.
whenever a flow control or water quality facility or flow control BMp is proposed to be located on a parcelseparate from the parcel or parcels
"ontining the target tG""r -iirgated by the facility or BMp,provisions must be made to ensu* th"t;;';;ner or owners of the target surfaces have a perpetuar right tooperate and maintain the facility' This may be done eitrrerby recJin*^un
"*"*ent granting this right to the,1.::Tf].?l;':?f[:l#f:::l "' uv 'oo*ving the land '" *ni"r' ,r'" raciriry sits (oian inte'rest therein) to
Kent may inspect all privately maintained dra111r; f-acilities.for compliance with these requirements. Ifproperty owner(s) fail to maintain their facilities to acceptabre standards, the city may issue a written noticespecifying the required remedial u"tlont-una'requiring a ,"n"art"-ro, timely compretion of the actions. If theseactions are not performed in a timelf rna"""i
11"
c6;; ;;m-e property to perform the actions neededni H|Hffi:Hff :ff :9 mi;11n**ff ffi #ill| u ;*^; 0," o "; il;;i ex i sts,,h e c i ty
If the proposed project is a residential subdivision deveropment, ail privately maintained conveyance systemsor other drainage facilities, which convey-n"*r ,rr.*sr, nriu#ffi.It , must be rocated in a drainageeasement dedicated to convey surface and stormwater. Individuul orn"., ofthe properties containing sucheasements must maintain the drainage facilities through th"t;;;^p;;. rn. t"gut inrt ur"nt
"."ating drainageeasements on private property must contain languageihar r"qui.", u irivate property owner to obtain writtenapproval from Kent prior to removing vegetation (except uy ioutin" lowing) from any drainage easement;J3"#:?r#1STnH"g"',:H:"f f *{ni"*n:####lt*;;*:11*liil"isamp,ecopyor
City of Kent Surface Water Design Manual I -50
Iuly 2022
SECTION I.2 CORE UIREMENTS1.2.7 CORE REQUTREMENT
LIABILITY #7: FINANCIAL GUARANTEES ANI)
All drainage facilities constructed or modified fo"rprojects (except downspout infiltration and dispersioniJ,ilil*;lllanv work performed i" th*;;;-"f-wav, musi r",nprv r"i', ciry of Kent,, nnan"iar suarantee
Intent: To ensure financial guarantees are posted to sufficiently cover the cost of conecting, if necessary,;Tffi;:Jffi,;H1fl:iJ::iil***;;",".tion *o,k, anJ,o.*,..*,ror one y"u,ti"satisra*ory
maintenance uno op",uiion a;;i";;r;ilJ;;;?'H:::i,lifl::T,3:;tl::f"?:ffi,ilrg HlLr",ffl[:T:",:1;[ffi:iffJi;nff":#lli! du-ag",,eru;;;;;. construction or maintenance of
surface water and Drainage F,acirities construction Bond
illf ffffi:il:il.x'.1ili:ffltj;"#'** r: required to consrruct drainage facirities pursuant to thisConstru'ion e;"ii,;" Reierence 8_;i ili;'rTi.:#".;i:,T:'iJ;#":i:ff".Hfi*if1i,jii:I*ffi"project-related work performed "" "r-;tf*;;.' Th"
"*u"t "n'""",-"rrt e bond shail be documented in theapproved engineers estimate' u"t"' city-,"ri), ,roy*or r"ir" in"' ,)quirement of this guarantee on projectsproposing only minor modificarions
"r;r;;;;"rr:ents to the drainage system 1e.g , catcibasin inserts, spitt;11:::l::..e'
pipe '"ptol"^"nr,, "i, ti -i,iha,,,",, in,,"r",i,r,iJ )oy u combined w,h o,ther required
ffi:Tfflilrle released in two stages as follows. Generauv, 80% of the bond is rereased upon compretion
L Payment of all required fees.
2. Construction of the drainage facilities.
Receipt of final construction approval from city review staff.
il".,ffi:*T;*,it-'"ilts meeting the requirements of rhe ciry of Kent construcrion Standards
The drainage facilities shall be maintained during a period of two years according to the city of Kent,s#ffi l;:,xl;':"ffiT:,?:;*:,",y,r;fitrt?r#:"ff
i:Til;,l,,heendof rhevea*he
l. For plats, record the final plat.
2' For tracts containing drainage facilities to.te maintained by Kent and not located within the finar plat, deedthe tract to Kent anJ set pd"ry ;;;r"in .onfo*u*" *irilrr" surveying standards.3 ' For easements containing drainage facilities- to be maintained by Kent and not located within the finar prat,provide easement docum-ents "d;"; t-.;orary survey marke.s to delineate the easement location.4'
}rt".""ffi;ITj#3J:to*'ion to ensure the drainage facitities have been properry maintained and are
5. Correct any defects noted in the final inspection.
6' Submit a letter to the city forma'y requesting a rerease of the bond.Following completion of these steps' the city will assume maintenance and operation of the drainage facirities.
3.
4.
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City of Kent Surface Water Design Manual
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Iuly 2022
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7.2.8
other rmporfant rnformation about core Requirement #g
;:l:I:tfi:Tf;ffi$"*tHliy"fiIfi:tTioran overa, water quarity protection srrategy required by this'
9-"F,1ff:ffitJlil1qJ:T;::iJ"'i:;j,1T,:;il:T*:visions, Section , 2 4-rhis provisionrunoff frompottutlo7-sen"*ini ni")riour rrr7o""r. rn" orori::I"v-:" svstem elements rhat receive
;Tffi i:il,::m$t jr.ltffi ,o'i"""iJ?""';",ilJi::1:,HlTtr;;;ffiffi*l;,""
o core Requirement #4: Conveyance system, Groundwater protection, section 1.2.4
-This provisionrequires that ditches/crtunn"rtf" lini"r
"*a.a a ."0*. ,rr, .irt orgroundwater contamination whenllur::l runofrfromp;ir;;";;r?i,,or ,*o",rirrir"rt *rthat comes into direciconra* with anr Special Requirement #4: Source Control, Section 1.3.4_This reccontrols from the King County sror.r'it, p"llution prevention ,luirement applies water quality sourcecommercial, industrial, and rnultiramii/i.";".u. [anuql to those projects proposing to. Special Requirement #5: Oil Control, Section 1.3.5_This requirerprojects proposing to develop or;;;i;;'" high-use site nent applies special oil controls to those
tr EXEMpTToNs F'ROM coRE REQUIREMENT #8
ffilffi"?r1possible
exemptions from the requirement to provide a formar water quarify facirity per core
1. Surface Area Exemption
A proposed non-redevelopment
of the following criteria:
pro.;ect or any threshold discharge area within the sile of such a
pro.ject is exempt if it meets all
a)Less than 5,000 square feet of new plus replaced pGIS
core Requirement #8 requires-that wQ treatment.facilities be provided.to remove pollutants from runoffS:',",1il%:r1i: the prolect site i" "";";;;;i with waterq;;i,v ;:;rty requirements menu round in
I|'"#:3XX are a group of facility options designed to provide revers of trearment targeted to resource
Guide to Applyin g Core Requirement #g
wQ implemen tation ureq trements are the lnI nrmum forfacilitiesulrementsreqtoachieve andintendedanalyzingperformanceand designing wQotherprotectiongoals
City ofKent Surface Water Design Manual t-52
will be created, AND
Iuly 2022
SECTION I.2 COREb) Less than lq arl;e of new pGpSwill be added.
2' surface Exemption for Transportation Redeveropment projects
a)
ffJil:i,::x.:#;:x{,g surfacewithin the project limits is less rhan 50%o of theexisting
b) Less than 5,000 square feet of new pGlSwill be added, ANDc) Less than Vq acre of new pGp^S will be added.
3. Cost Exemption for parcel Redevelopment projects
A proposed redevelopment ptoiect onu
"lF,tTl lnulti.nle^qarc el site .or any threshold discharge areawirhin the s/e of suc-h
"
pr"j;;iir ;"",np, iri, meets ari of ih" r"il*i"g criteria:a) The total valuation of the projects nr31os11 improvements (incruding interior improvements andexcluding required mitigatiln itoto'ul."nts) is iess dran ioi".orthe a-ssessed uur,i" lrlu; ,rr"
'ffi:Hfl?,"JH:1i;*:].,""l:,Tffi*"erciar
or inausiriar projects, o, (u) tr,e
"*isting site
b) Less than 5,000 square feet of newpGl,S will be added, ANDc) Less than lq acrc of newpGp,S will be added.
4. Soil Treatment Exemption
A proposed project or any drainage area within a project is exempt:. If the runoff fror
innrtrated i"; r:"!,n|i:::I::,i::,r,fi,#!:?:";:,tr:"i:;{:;T:;::{::,;,r,(:;i:f,:i({;i:;,*
depth' and infiltration Lte
"riterla *i""r i" s""ir"" !.ili:il;r";"rotion is not arowed for areasthatare infiltrated (l),:']1i",";.";;a# mite oru r"nriti* ,Joir., or (2) within one quarrer m'e offresh water with existing or designated aquatic rir" ur" *horJ h11.*. ','ouro o'r"r*irl' trigger
;fl :#:'n :i ffiil ?,ff TJ * til |::t"':'"lffi 3:; iJ (3 ) wi,h ; ;. q,"n". ,,'
"
; r "
,.,*l.l,l: Lake is a designation apptied by the Cirv to Iincreases in prrospnoruJioading. such laies are io";i3l"t
that are particularlv prone to eutrophication from devetopment_inducedrtred on rhe water euality Apprications fiai;il;,;fi this manuat.
City of Kent Surface Water Design Manual
I -53
Iuly 2022
SECTION 1.2 CORE REQUIREMENTS
I.2.8.1 AREA-SPECIFIC WATER QUALITY FACILITY REQUIREMENT
Projects subject to Core Requirement #8 must provide a water quality facility selected from a menu of
water quality facility options identified in the area-specific facility requirements and exceptions for the WQ
treatment area in which the proposed project or threshold discharge areq ofthe proposed project is
located. These WQ treatment areas are listed below, and their requirements and exceptions are detailed in
the following subsections:
A. Basic WQ Treatment Areas
B. Enhanced Basic WQ Treatment Areas
C. Sensitive Lake WQ Treatment Areas
Intent: The City of Kent contains numerous sensitive and significant water resources. The City has
determined that the minimum level of treatment adequate to prevent further degradation of water quality,
and to maintain the aquatic health of cunent fisheries, is that provided by the Enhanced Basic Water
Quality Treatment Areas Menu. The Director has the option to require additional treatment, or treatment of
existing impervious areas, when necessary to meet water quality standards and goals (see Reference 8-H)'
tr BASIC WQ TREATMENT AREAS
The Basic WQ Treatment Areas menu is applied where a general, cost-effective level of treatment is
adequate and where more intensive, targeted pollutant removal is not needed to protect receiving bodies'
In the City of Kent, the only acceptable use of the Basic WQ Treatment Areas menu is fot redevelopment
projects that are subject to Core Requirement #8 and where approved on a project-specific basis by the
Director. Under no circumstances shall a level of treatment less than what currently exists on a
redevelopment site be permitted. For example, if a site already has an enhanced-basic WQ facility, it
cannot be replaced by a basic WQ facility.
Treatment Goals and Options
The treatment goal for facility options in the Basic WQ Treatment Areas menu is 800/o removal of total
suspended solids (TSS) for flows or volumes up to and including the WQ design flow or volume for a
typical rainfall year, assuming typical pollutant concentrations in urban runofPa. TSS is the general
performance indicator for basic water quality protection because it is the most obvious pollutant of
concem. TSS is not a single pollutant - it is a general term for a highly variable mixture of solid pollutants
with variable particle size and particle density distributions, and to one degree or another containing a
variety of sorbed dissolvable pollutants. The basic WQ Treatment Areas menu includes facilities such as
wetponds, combined detention/wetponds, biofiltration swales, vegetated filter strips and sand filters. See
Chapter 6 for specific facility options and designs.
Intent
The Basic WQ menu is intended to be applied to both stormwater discharges that drain to surface waters
and those that infiltrate into soils that do not provide adequate groundwater protection (see Exemptions 3
and 4 from Core Requirement #8).
24Theinfluentconcentrationrangefordemonstratedpollutantremovalisl00to200mg/L. Forinfluentconcentrationslowerthanl00mg/l
the effluent goal is equal to or less than 20 mg/I. For influent concentrations greater than 200 mgll, the goal is greater than 80% TSS
removal.
City of Kent Surlace Water Design Manual 1 -55 Iuly 2022
SECTION 1.2 CORE REOUIREMENTS
tr ENHANCED BASIC WQ TREATMENT AREAS
The treatment goal for facility options in the Enhanced Basic WQ menu is to accomplish better
removal of heavy metals and potentially other toxic materials than can be achieved by basic treatment,
while still meeting the basic treatment goal of 80% TSS removal. The specific target performance is >
30% reduction of dissolved copper and> 60Yo removal of dissolved zinc. Dissolved copper and zinc are
indicators of a wider range of metals typically found in urban runoff that are potentially toxic to fish and
other aquatic life. The Enhanced Basic WQ menu includes options for use of a basic-sized stormwater
wetland, alarge sand filter, or a combination of two facilities in series. See Chapter 6 for specific facility
options and designs. Additional facility designs may appeat in Reference 14 in the future.
Intent
Facility options in the Enhanced Basic WQ menu are intended to remove more metals than expected from
those in the Basic WQ menu. Lower metal concentrations reduce the risk to fish from exposure to both
chronic and acute toxic concentrations ofmetals such as copper andzinc, and very low concentration
copper deleterious olfactory effects. As the toxicity of metals depends on their concentration, this standard
is most effective for project sites with a larger proportion of pollution-generaling impervious surface like
roadways and medium to high density subdivisions. The Enhanced Basic WQ menu is intended to apply to
all such areas in Kent unless otherwise denoted on the Water Quality Applications Map.
Target Surfaces
Facilities in Enhanced Basic WQ Treatment Areas must treat (either directly or in effect) the
runoff from the following target surfaces within the threshold discharge area for which the
facility is required:
l. New PGIS thatis notfully dispersed per the Criteria for Fully Dispersed Surfaces in Core
Requirement #3, or notfarmland dispersed as specified in Appendix C. For individual lots within
residential subdivision projects, the extent of new PG/S shall be assumed based on expected
driveway size as approved by crty review staff.
2. New PGPS thatis notfutly dispersed and from which there will be a concentrated surface discharge
in a natural channel or man-made conveyance system from the site, or notfarmland disperced as
specified in Appendix C. For individual lots within residential subdivision projects, the extent of
new pervious surface shall be assumed to be the entire lot area, except the assumed impervious
porlion as specified in Chapter 3 and any portion in which native conditions are preserved by
covenant, tract, or easement.
3. Existing impervious surface added since January 8, 2001 that is notfully dispersed' ot not
farmland dispersed as specified in Appendix C, and not yet mitigated with a City-approved water
quality facility or flow control BMP. Note: January 8, 2001 is the effective date of the ESA 4(d)
Rulefor Puget Sound Chinook salmon.
4. Reptaced PGIS thatis notfutty dispersed or notfarmland dispersed as specified in Appendix C on
a non-redevelopment proj ect.
5. Replaced PGIS that is notp lly dispersed onatansportation redevelopment proiect in which
new impervious surface is 5,000 square feet or more and totals 50Yo or more of the existing
impervious surface within the project limits.
6. Reptaced PGIS that is notfulty dispersed, or notfarmland dispersed as specified in Appendix C,
on a parcel redevelopment project in which the total of new plus replaced impervious surface is
5,000 square feet or more and whose valuation of proposed improvements (including interior
improvements and excluding required mitigation improvements) exceeds 50% of the assessed value
of: (a) the existing site improvements on commercial or industrial projects or (b) the existing site
improvements on other projects, whichever applies.
City of Kent Surface Water Design Manual l -56 July 2022
SECTION I.2 CORE REQUIREMENTS
Exceptions
The following exceptions apply only in Enhanced Basic WQ Treatment Areas:
l. The facility requirement in Enhanced Basic WQ Treatment Areas as applied to target PGPS
may be waived for projects where city review staff approves a landscape management plan
(LMP) that controls solids, pesticides, fertilizers, and other erodible or leachable materials leaving
the site.
LMP requirements can be found in Reference Section 4-C. LMP submittal requirements are given in
Section 2.3.1.5.
2. The Enhanced Basic WQ menu may be reduced to the Basic WQ menu for treatment of any
runoff that is infiltrated per the standards of Section 5.2. This exception is not allowed where
infiltrating into soils that do not meet the groundwater protection standards described in Section
5.2. 1, if within one-quarter-mile of a fresh water designated for aquatic life use or that has an
existing aquatic life use.
tr SENSITIVE LAKE PROTECTION MENU
Sensitive Lake WQ Treatment Areas are designated by Kent in the watersheds of lakes that have a
combination of water quality characteristics and watershed development potential that makes them
particularly prone to eutrophication induced by development. Such areas are delineated on the WQ
Applications Map adopted with this manual.
The usage of cartridge filter facilities shall be restricted to sites where the water quality facility will be
maintained by the private property owner; cartridge filters will not be allowed on sites where the City will
be maintaining the water quality facility.
Note: For projects located at or near the delineated boundary of the Sensitive Lake WQ Treatment Area,
site-specific topography or drainage information may be needed to verifi that the project or any threshold
discharge area of the project is within the WQ treatment area. Any threshold discharge area is
considered to be within the Sensitive Lake WQ Treatment Area if the threshold discharge area drains to
the sensitive lake itself or to any waterbody or drainage system that is clearly within the mapped Sensitive
Lake WQ Treatment Area.
Required Treatment Menu
Within Sensitive Lake WQ Treatment Areas, a water quality facility option from the Sensitive Lake
Protection menu shall be used to treat runoff from the surfaces listed under "Target Surfaces" below,
except where such treatment is waived or reduced by the area-specific exceptions at the end of this
subsection and except where the Enhanced Basic WQ menu is applicable as follows. lf 50% or more of the
runoff that drains to any proposed water quality facility is from one or more of the following land uses,
then a water quality facility option corunon to both the Sensitive Lake Protection menu and Enhanced
Basic WQ menu shall be used for the design of this facility, except if such treatment is waived or reduced
by the area-specific exceptions at the end ofthis subsection:
1. Residential subdivision development in which the actual density of single family units is equal to or
greater than 8 units per acre ofdeveloped area.
2. Commercial, industrial, or multifamily land use.
3. A road with an expected average daily traffic (ADT) count of 2,000 or more vehicles or expected to
serve 200 or more homes.
City of Kent Surface Water Design Manual t-57 Iuly 2022
Treatment Goal and Options SECTION I.2 CORE REQU IREMENTS
The treatment goal for facility options in the sensitive Lake protection menu is 50yo annuaraverage totalphosphorus (Tp) removat ur*-ing tpi."ip"ir**t *;;;;;'in r.uun runoff.25 This goar waschosen as a realistic and cost-effe"i""'l*"r
"i.phosphorus ."*ouul. The sensitive Lake proiection menuincludes options for using either B_asic wq'a"iliri"J "ii;r*,i",";i#uinutions of two facilities inseries'26 or a single facilitv in
"omuination-*iit', tuna ,t";;;;;;Li#"no that reduce phosphorus. SeeChapter 6 for specific facility options una a"rign details.
on some developments or portions thereof that have surface uses that generate the highest concentrationsof metals in stormwater runoff,.the tr""*i", glli ililffiffir#;-d. > 3'yoreduction of dissorvedcopper and> 60vo removal of dissolved ,i*. ?r'rir "^p;"@;;;;;Ji* ur" of a water quarity faciriryoption that is common to both dt" s"nsitiu"iak" p.oie"tion,i"nu #Jih" Enhun".d Basic menu.
A^project discharging runoffvia surface flow contributes phosphorus loading to a sensitive rake regardlessot drstance from the lake' If discharge is via infiltratiol ,hr*d';;;re soirs, it is arso possibre thatphosphorus would be transported thiough ih. g.ouna for som! distance without attenuation. Thisgroundwater transport distance it "on,ii"."Jto be typicall/r" -"'['ilrn one-quarter mile. Therefore,onsite treatment using the Sensitive i"r." ir"r""{:t m"",f i, r"q"i"o prior to infirtration within one-quarter mile of a sensitive rake. Infirtration through fi"*;";r;j;;;;"rr"o ,o provide significantattenuation of TP' so the general groundwater protection criteria rpi"in"o in Section 5.j.r areconsideredilff'tr',|".l:1ffJhn"jffii;;f:li|m the s"n,itiue ia["-protection menu to the elsic we
where the treatment ooal. is expanded to include > 3lyoreduction of dissolved copper and> 60%onemovalof dissolved zinc' the facility options c"rr"" ," both the s""riii"" Lite protection menu and the
"tff;ffi"t"Hi"Yfrffil#::rd 'n;;;';our u, *"ii",;;l;i;;",ection goar orloz.,."l,,ouur or
Target Surfaces
Facilities in Sensitive Lake wQ Treatment o"uu: Tyrl- *itigate (either directry or in effect) the runofffrom the following target surface s witnin lie iiresnord dischtaT[i )r", rrrwhich the facirity is required:
l ' New PGIS that is notfutly dispersed per the criteria_for Fully Dispersed surfaces in coreRequirement #3' or notfirmlina axiircea yro.:t*g 1" oiol"lt^ c. For individuar rots within
ff:*"ffi1r'Jl$"0'i'il,f;:*ts, th" "it"niit o"rPGl,s shar'be assumed based on expected driveway
Intent
2. New PGp,Sthat is notfully dispersed and from which there will be a concentrated surface di scharge ina natural channel or man-made conveyance system from the site,or notfarmlandspecified in Appendix C. For dispersed ASindividual lots within residential subdivision projects, the extent of newpervious surface shall be assumed to be the entire lot area, except the assumed impervious portion asspecified in Chapter 3 and any portion in which native conditions are preserved by covenant, tract, oreasement. Note: where the runoffrom torget PGPS is separated.from the runofffrom target pGIS, theBasic WQ menu may be used in place of the Sensitive Lake Protection menufor treatment ofrunolf"from the target PGpS (see the area-specific at the end of this subsection)exceptions
25 Phosphorus concentrations of between 0 10 and 0.50 mgll- are considered typical of Seaftle area runoff(Table l, ,,water
QualityThresholds Decision paper," King county Surface water Management Division, April 1994).
26 In series means that the entire treatment water vorume flows from one facirity to the other in tum.
City of Kent Surface Water Design Manual l -58
Iuly 2022
SECTION I.2 CORE
3. Existing impervious surface added since January 8,2001 that is not fully dispersed, or not farmlanddispersedas specified in Appendix C, and not yet mitigated with City-approved water qualityafacility or flow control BMp. Note January B, 2001 is the effective date of the ESA 4(d) RuteforPuget Sound Chinook salmon.
5
6.
4
Exceptions
:7::::J:"""i:r#:ki:,j:{i.tty dispersed, or notfarmtond dispersed as specined in Appendix c, on
Replaced PGIS that is notfully dispersed on a trunsportarion redeveropmenr project in which new';",;#:":i;#ffi;j:;??t_*";;'r;;;;;e and totars sov"-ii,,i*"orrhe exisring impervious
Replaced PGIS thatis notfully dispersed, or notfarmland dispersed as specified in Appendix c,on a parcel redevelopment proiect-inwrriin tne total of new pir, ,"pto""d impervious sarfuce is5,000 square feet or 'no." und whose uuruuiion orproposed ilp;;;;;;"ts (incruding interiortmprovements and excluding required mitigation i.npiou",n"ntii;;;;, 50%o of themsessed varueofi (a) the existing site improvements on commerciai o. inaurt iuiffi"u or (b) the existing sitetmprovements on other projects, whichever applies.
2
The following exceptions apply only in Sensitive Lake we Treatment Areas:
l ' The Basic wQ menu jn]Y be used in place of the Sensitive Lake protection menu for treatment ofrunoffthat is infiltrated according tottre stanoaras in section ii. rrri, exception is not alowedwhere infiltrating into soils that oo"not r.*,rr" gr;il;;;. p.oi"""ti", standards described in Sectioni;3;l;#**one-quarter-mil" "f " th;;;ho.ou, sensitive receiving water or a tributary to that
' Application of the Enhanced Basic wQ me-nu as specified above for certain land uses may bewaived for treatment gf unv.unonhuti nnr"".J "".";a*;";: standards in Section 5.2 (Afacility from the sensitive Lut" p.otJiJr, -"nu i, ,titt requii";;;r, that requirement has beenreduced to the Basic wQ menu by;;;h"t exception) Thi;;;;il;.is not anowed where infirtratinginto soils that do not meet the groundwater frotection ,tunauoa, i"slribeo in Section 5.2.1, if withinone-quarter-mile of a fresh water designated ro. uqrutlciir"
"r. J."ilr", has an existing aquatic life use.
Application of the Enhanced Basic We menu as specified above fowaived fortreatmentor.a.nv*""iiirr"ti oir.r,u.g"d, "t;;;;_fi;h-"#*::jX*ff:"Til ll,
",
the way to the ordinary higi *"t* ;;;[;iu ,o"u.n *itt, u meun unnuar fr& of t,000 ,rr,or ror" 1utthe discharge o"'* "r-:Ti:'"t;;;;lj o, u rur. rh"r i;30t;;;es or rarger (A facliry rlom theSensitive Lake Protection m3nu.rs-stirr r"q"i..a unr"r, tt',ut r"il##r, has been reduced to the BasicwQ menu by another exception)' rrri, "*l"ption is not "pprirlii. *lrere the receiving water isimpaired for metals oer section 't i.zi,.\' o"i"ro"ur Anaiysis, ana l-..2.2.1.2: Downstream waterQuality problems Requiring Sp.;i;i il;iio", ,"r",, problem (Type a).
The Enhanced Basic wQmenu as specified above for heating runoff from a commercial land usemay be waived (A facility from the sJ"riti"" i"te protection'ri"""ir'rrilr required unress that::fl:ff}T|!T been reduced t" th" B;;;wa menu bv """ir,".-"*"ption) ifal0f the forrowing
a)
il:#"*i::#Till,!",f;"?,ljlTKj metars) are currentrv used or proposed to be used in areas or
J
4
City of Kent Surface Water Design Manual I -59
IuIy 2022
b) A covenant is recorded that prohibits future such use of leachable metals on the site exposed to
SECTION 1.2 CORE TS
the weather (use the covenant in Reference Section 8-Q), AND
1.2.8.2
c) Less than 50%o of the runoff draining^to the proposed water quality facirity is from any area of thesile comprised of one or both of the"fotowi"g l""a *"rl'-^
-
' ,?lT"1il1lT*:" with an expected ADr of 100 or more vehicles per 1,000 square feet ofo commercial rand use invorved with vehicle repair, maintenance, or sales.
5' The Basic wQ menu may be used for treatment of any runoff from target pGps that is treatedseparately from the runoff from target pGIS.
6' The facility requirement as applied to target PGPS maybe waived altogether for wr agriculturalproiect if there is a farm management pLn for ugri"uitural ,s"s p"r xcc 2lA.z4uno rcc 16.g2, orfor other land uses if citY ii"itt l t1in"oo.:y:r a landscape *'oiog"*"ot ptsn (LMp)that controrssolids' pesticides, fertilizers, and other erodibl" o, t"u"t uill. ,n",".i"r, leaving the site.
7 ' The facility requirement as applied to replace.d PGISmay be waived if the city has adopted a planand implementation schedure ior furfiiliig tt i, ,"quir"mJn*rin!
""g,onar facirities.
Note" If a lake management plan has been prepared and adopted by Kent, additional treatment and/or";:;;#::"7tr?:'!::;;,,:3:;:ov b" ,"qui,Jo, specified ii tni pioi"d pursuant to speciat
WATER QUALITY IMPLEMENTATION REQUIREMENTS
ffiffi;i:Y#}i:':'filj::,'njj::sffiXit impremented in accordance with the rorowing requirements,
A. Methods of Analysis and Design
water quality treatment facilities shall be analyzedand designed as detailed in chapter 6.B. Siting of Treatment Facilities
Required water quality facilities shall be located so as to treat the runofffrom all target surfaces, exceptas alrowed below under "Treatment Trades" and ,,Untreated nirlr,u.g"r.,,
Any other onsite or offsite runoff draining to a proposed treatment water quality facility must betreated whether it is,from a target pollution-generating surface or not and regardless of whetherthe runoff has already been treaiediy another-facili,y. fio rJiif -rrt be sized for arlflows/volumes entering the facilitv. irri. ir u""u*;;;;"#;rflrr,u"n"r, is determined in parr bythe total volume of runoff enterin! the facility.
C. Treatment of peryious Surfaces
Pollution-generating pervious surfaces subje^ct.19 core Requirement #g need only be treated using theBasic wQ menu' lt is also possible for the facility requiremeni tJ u" *uiu"a if there is a good faithagreement with city review staff to approve a lunds"ap" ,n"nug",n*t plan that controls ,olidr,pesticides, and fertilizers leaving the'site.
D. Treatment Trades
The runofffrom target pollution-generating surfaces may be released untreated ifan existing non-targeted pollution-generating surface of equiialent tir. unl o"t;t characteristics lying within thesame watershed or stream reach tributary area is treated ;;;;';;".t site. Such substitution issubject to all of the following restrictions:
City of Kent Surface Water Design Manual I -60
Iuly 2022
l. The existing non-targeted poll surface is not c urrently being treated by any phase
SECTION I.2 CORE REQUIREMENTS
of the proposed project, is not subject to NpDES or other permit requirements, and is not under a
2' 2' The existing.non-targeted pollution- generating surfa.ce that is treated for purposes of thetreatment trade must be documented ;dtr;;t"? iv'.iav review staff. Documentationshould clarify that future redevelopment of the u*irting non-targeted, treated area used forthe treatment trade will incur additional *r* q*i,,nutreatment requirements if theredevelopment exceeds core Requirement g thr;;fu;. Any additional water quaritytreatment tri8gered by redevelopm.ent of the non-arrg"a"o, treated area must be achievedby implementing an additional ileatment traAe.3. The proposal is reviewed and approved by cityreview staff.E. UntreatedDischarges
If site topogtaphic constraints are such that.runoff.loT a target pollution-generating surface mustbe pumped to be treated by the requiredwaler quality facility]then city review staffmay allow thearea's runoff to be released untreated' provided that
"rr "iir,l'r"irowing conditions are met:I
ffiTfi'rJl;;*':T:T1ffi"Ri:T::Jx?,oo*"iiJion, ora rinearsand nrter is not feasibre,
2' The untreated target surface is less than 5,000 square feet of new plus reptaced pGIS.3' Any target PGPS within the area to be released untreated shall be addressed with a landscapemanogement plan (LMP), which must be submitted;;;;J;pp.""ed by city review sraff. TheLMP applies to the entire site and all drailage;";;il;o the site within one or morecontiguous parcels under the same ownerst ip o, ao"un.,"niJd regal control.f,'. Use of Experimental Water euality F,acilities
water quality facilities other than those identified in-chapter 6 are ailowed on an experimentar basis ifit can be demonstrated they are lik;i; ; ;."t the poilutair r"."""r goar foq the appiicabre receivingwater. Use of such facirities .equires an experimen;i;;;j;;;;jurtm"nt to be approved by Kentaccording to Section 1'4' "Adjustment Process'", experime-ntaiiia prop.i"tary eMes may be arowedto provide pretreatment for filter BMPsor as tt "-re"lnJ *qHlliv in the Two-Facility i.reatmentTrain option' Apprications *irr u" i"ulr"ed on a;;y;;;urir. ao. an experimentar o.E:""ru;lttX$ l".j;.:ll'""4' lt musl be approved for use through the washington Department of
G. Owner Responsibility for Water euality
Regardless of the means by which a property owner chooses to meet the water quarity requirements ofthis manual-whether a-lvater quality facility, atrain s7 f^riliri"., u treatment trade, or anexperimental water qualitv acifity;t is uttimatery rh" .;;;;;ri;riity ortne properry owner ro ensurethat runoff from their srb do"' ni t."u* i*. quality prolr",,.,to, degrade downstream beneficial,i:?rff.iliJ*?,T'Ji"llffi 'jiH?:';?o";:hfi ;"Uffi .,",'"ensuretharthedischargerrom
compliance order or other regulatory action,
City of Kent Surface Water Design Manual l -61
luly 2022
1.2.9 CORE REQ UIREMENT #92 FLOW CO NTROL
SECTION I.2 CORE
BMPS
All proposed projects'^including redevelopment projects, must provide onsite flow contror BMps to
ilfii:iliil:iffi :.i::"fl ;H"'J;:*x;,x;*;xr**=y,-f .1"r"u,,u.a*,newpervi.us
in the roilowing sections. prow contrJ"rr, must besere;;;;;xTrl". *:##r,:il:nasspecified
;,H"ffi:JJ,"Jlitl],and provisions Jetailed in this sectionl* rn" design specifications ror each
Flow control BMPs are methods and designs for dispersing, infiltrating, or otherwise reducing orpreventing develonment-related i""*"rJ"runoff at ".;il;'r;urces of those increases. FIow controrBMPs include' but are not limiteJ to' p."r"r""r*
^ia-"i" "i ririi"*rror"o rurfaces tofury disperse;il'"#Jil,,l1,fiH:',"#ilH;rf:ffi;',t;;x**::irumij:[Tfi
l]*,permeabiepuu"-"n,,;Intent: To provide tt*.1!:." of hvdro.logic
ilo,::i, that is not possible/practicar to mitigate with a flowcontrol facility' Such impacts include in"?*r". in runoff voru"i",
""a flashiness and deJreases ingroundwater recharse' tn"reu'"d t"noif;;"il".""a nusr,in"rs'i""ir'a higher and more variabre streamvelocities at low floi's and more-freque;;;;., r"u"r nurrrurionrii"ro.u.ns and wetrands. This causeswash-out and stranding of aquaticffi"r,
"r*"r ,""".;;;;;;r^oio.gur,i" maner, Ioss ofvegetationdiversirv and habitat "iurirv,'unJi"i5",ou""""tr*:: d:;,#;, ;; harchins, and migrarion.Decreased groundwater
'""hu'g" '"auJ", *ui"r suppry for hum# ui-"'una summer uur"-no*, in streams,
ilijiilili' lff: :x x,,il:iJ*ffi*dx'ff To "*' "i,.'"i ;; *" ffi , and th e hab i tat q uur i,v o rBMPs seek to r.eou""'.unofrvolumes una nur6llllawniS, rearing' and flood r"ra..-ni"i,
"'ontrorimperviousne"""Jl"1i"s,;ff ";;fr i*l':ii:iix";:,,1:::ffff"#ffi Jiilfi ffitfl "
and retention of stormwater onsite' rrr"r' ii" g*r is to appry no*.*tro.t BMps to new impervioussurfaces, new pervious y,yy;r,lri"i"i"r*oi_,rr, *r/b;;;;;"",*,n, impervious surfaces addedsince Januarv 8,2001 (e/fec1i|,e aoi" of rn" nrs),ral o"it, iii*i)una chinook sarmon) tothemaxrmum extent feasibre without .uuring-nolirng or erosion impacts.
EXEMPTION FROM CORE REQUIREMENT #9
There is a single exemption from the flow control BMp provisions of core Requirement #9:L Basic Exempfion
1.2.9.1
A proposed project is exempt if it meets the following criteria:a) Less than 2,000 square feet of new prus repraced impervious surfacewil be created, ANDb) Less than 7,000 square feet of land disturbing activitywill occur.
FLow coNTRoL BMp REe'TREMENT' ovERvr'w
Projects that are subiect 1o core Require ment #9myst apply flow contror BMps to either supprement theflow mitigation p'ouid"d uv t"quittil"r ""*-t a"ilirrir'o. i.""i.i" n"* mitigation where.flowcontrol facilities are not requirei' arr r".rr rio* contror nnap, u." o.iaired in Appendix c of thissT,xi];li!5.;#;"' BMps must u" i'pr".*"ted per,r'" ."q"i."r"it, una approach deta'ed in
ii'ff"Tffrfr'J:"t":'iJi!3:r"#Jl3:;";ions
or road improvement projects, respectivery. As
req u i rement: ( r ) app r icati oi "i n-v r, ;##ff ff; ##il::[] ; tlj;,ri il1[: ff ?ytproject locarion , size. and. i,np"ruiousi;;;;, or.(2) using ";;;;;;r, runofrmodel rof;:il'o"" compriance witrrt'he ffi;ff?;"u"rop**t?r.r;;;;;;rr""ce
standard, described
City of Kent S urface Water Design Manual 1-62
Iuly 2022
SECTION I.2 CORE
Target surfaces
Target surfaces for application of core Requirement #g (FCBMpsl-11","g" new impervious surfbces, new
pervious surfaces' t"p?:"a itp;;;;;'ri,".fu""r, and any."iriingi,,,,p"rvious surfaces added on or afterIanuarv 8,2001(the effecrive ;;;;;f ,h" E"d"";;;"; s;",?""'oi i,ruu" prohibirion,, irru"a uy the federarfi:ffiffi:i'i;tiiil" Puget Sound Orinoot
'"ril""i".ii"r'"""1'v'mitigatea with an approved FCBMp or
Projects that trigger core Require ment #9 by disturbing 7,000 square feet or more of land, but wherenew plus replaced impervious it r"tt ii"r z,boo rqr"r"i.#*tlt"r,der basic oispersion a, an equurchoice for treatinp the target imp".uious s";t-, il;;rt*'tlii'#tration, Iimited lnfirtration,
i'ffi fi i1ifr :t|"ffi **:ffi *n',:,"*""y*l#li#j:{1ii"ii::i:_*..s;i+;;;minimumRequiremenrs," (Requirem"", +l "" u"in'irul. uno ur" no,-r"qrir;;;::o,ilTrr;il:;f_";H:iyi#6. Anv impervious surface '..""J;;;;'#T"Uli;ffi'd,.d#. in accordance with the flowcontrol facility requirement (Section'l 'i..zri,the facility i,npl",,'?"mon requirements (Sectionl'?i3];il*i1""ff'''" criteria r"' i"nll'",iJ" a"iri,i., is#;;;;,, exempr rrom the now contror
Any impervious or nervious surface served by the farmland dispersion BMp detailed in Appendix c,Section c'2'5' is "^itpt n"*ll" n"*'""iil' aira, *q;ir"rJ"i. ,i*" *u, new pervious areas rhat are,?fr:1il:[ffi1ifi;'i[i:l#;;; ;;d *ru, rcdl] ;; ffi i'D and (G) as required io protect
Projects or threshold d!3harsl.ar9as of projects
^91.{?1"*
as exempt from the flow contror fac'ityrequirement using the nirectbiscffi ir"J.niri"n in accordance wlth section 1.2.3.r donot have toachieve the Low t'*",,-?:r"';;#i!6'l"rror,"nun_"" ;;;;;j &scribed berow), nor considerbioretention' p"tt"ubl" pavement, and full dispersion. no*.*,;;Jl-oit ,oirtur" hording capacify ofnew pervious surfaces on those pt"-i*u i"r oi,tion, orprol-""-t; ffi be protected in accordance with
F:t;'95,t#1r3:":lt:l 'i;;f," q95".*r,". is.oi e,ir'inir*,ion.* deraired in Appendix C,
si,"1"aiff;,ffi ,;;:3:;;:13:;iiffi fi i:ff Lli:;;*:t;m*#;l*g#ifr
...ionper Appendix c, Section c.z.+ must b" -P,".n"*;o r",. "irro'in,;;ui;r, surfaces, if feasibre.A. Low Impact Development performance Standard
The LID performance Standard is defined as follows
For the target surfaces
1",1"r'y"i711*";;::::;:;:L",';:i"i,2i:i#T)!];j;trw;::;^:;;:::lX!,:;:;hai,,r,*g"f ;:,{,:,:,'i:,"t!:":,1::,:;X:::;Xi;t"r;;";:;;';i;":;7*'Assumehi,torisite-.
Projects that are either required or opt to demonstrate compriance with the LID performanceStandard using a continuous runoff moder must protect the so' ;"1;" capacityof newpervious in accordance with rent crJii, ffi** and Kent city coJe chapter 15.07.Projects that are required or opt to model compliance with the LID performance Standard are stirlilffi:4"ff;i?ffjfj'jir.l"rT"" 'p""in;;;; contror ."qui,",n"nt,-u, a"t",,in"d in core
City of Kent
IuIy 2022
requirements in Section I .2.9.4 SECTION I.2 CORE
B. Implernentation
3.
lTil:I;H:,.",::::1". t'*'FcBMp requirement are described in rhis section as forows;irpr.;;;;;;ijl9l nt"i"cts making improvements on an individuar sire/ror.
1"^cri*r*ir;;""'rt#;j;o"trement
shall be in u".ordun"" ;d;" ,,rndividuar
Lor BMp1.2.9.2, which soe.ci$, the selection of Btf
-.]*t-:oytl"o imprementarion of flow """11^:T.lte
extent orrhe
and provisions mu
it"ffi'#fi :ffi*'::p fjl:{}i'{rilr'#i-nx;r*i:111""i-qfr .rj:ryxr:'
;1'nnT[?]jl{;ijil['r,;*li];:rffiH,1:{ifr *gg**pi:t#J'; j;:,"
v c o n rro r ;*;;;il;ll il fl iff ;ig'."j ;;if "
*,2' For subdivision proje-cts, imprementation of'flow control BMps ftt n I ra.structure i mpro vemen rs (e. g., rouo r,, iao#llf. i:*? i- ? YPs for assoc iared p laroccur concurrentlv;
ti,:l]:*ti**rot'|'#iiq*iUl:#i,l ''*i,:""""
may be used to ."ar", ii""l,--^"::'^ '"t tne application of flow "nn- oir"" _ -iS.
allowedq,tur,t,6"1;t,;;l['axffi:J:;;H1trJiilttrHt*".1,'"yho".,lo]J'lil;o
reduce the target surl
rh es e c re d i rs, n o *, o]l: :,, y.bj :., ;ffi # ll3 ;["J,:',"# ,o;t*{ ff ;l:'T:.:,i:'
y'3'1 1''."1'',, ;;: #!o,.?T:;T;;'.Hf; fl "'"nt"a ^ p"n
"i,'r,,."
i r itv req u i rements
"
ro use
subdivision proiects rrots, provision, ,rr,'i1ooo'i"e t
';1" "r#J'X"-"::"
il;";;;'if ProRoseo project and
:r"-*0il&"r*l;,i#tffi flf fffi ;1*:;*r::o11lor BMPs per rabne
l.2.e.Amay be ,,J,:?;'03:3,i.3 "l;::gF'ror
tne appricarion ornow
f,iX'?#,*!!,'$;:::il 'u*," q'1u"ri'v
wu ro reduce the size ora re
racthtyrequiremenrs. ;.i o,.eouce'rtre'i1.r?i'riY;o'"'i" i"' " n"ri
tcQurred flow control
;x:fi,*i::gr"i*,""i,ffi ,,3:::";',ln"T;i:"*:y!qt1ff L";:f;i'fffi"Jii,"J"'rde ro*r,.i, n *,.,,,;;ijffilifl"Jli1frfiii,
The information presented in this section is organized as follows:' Section 1.2.9.2,,,1r
.,smar, ._ ",il:i:1"ffllH:;.,:l:,"Large Lot BMP Requirements,,, Section 1.2.9.2.2,.Implementatior
, ".Hk
a;","il: "1],fi :J#il," #: :_T l #;l; 1';1",,
1 .2.s.3 . t smar r ffi :ri:|;::H- Projects BMP Req uirements, " S ection
RequirementS,,
"*" "trpruvurlsllt and urban Road Improvement projects BMp
Section 1.2.g.3.2
Section 1.2.g.4,,.Requirements for Use of BMP Credits',
"Use of Credits by Subdivision Projects,', Section 1.2.9.4.1"Use of Credits by projects within Rights-of-\ry4' ,, Section 1.2.gCityofKent S urface Water Design Manual
t-64
Iuly 2022
For projects on individual siteslh
,Tj,T,fff rot eve r.ii;;ffii:"1t' flow control BMPs musr be^sere*ed and appried according ro rhe
subjec' j,,:::I"_o,;;il;*bl:iti#,1i,1iJ{:i::ifr
fi fl l,ffi [iT[ii*:il:-",**"fJTfJil;l****";f,:'ff:.:l|ffi :'"1'"'"resu'1ngr"'
Large Lot BMp Requirements (fors#esllots 42,000square feet)
ifffi::'J:l BMPs must u" uppli",1jl,1n"
::0., orRreferelle and ro rhe extenr specified ror rhe caresorvi::"';::ni::',,:,n ';:::;:]i!j':,?2':':tt'" *offiirojectu, a",.,il"a-lnii'*,o,,o*,n* e J
or- w ay,)'-d u i'; ; ;;;' ;;i^", : : il; ;:' "::
tf:i:
;: ! :' o 1 *, t r,r, n
"l "'
)
a recorded::me cases'
'n"
,vp" oi uur, required for o rull,Yilhin ";;r;';;;;requiremen*
mav have
n",",,,,;n ;;;;:::";;i;i{;j!" sec,ion', ;;:;;,";:::,;#:;:;:;;i;:;::rf ,::;::;ii{;"r,,;
t.2.9.2.1 SMALL LoT BMp REQUTREMENTS
IF the proposed project is on asitelloti:t !; apptiSd u, ,p""inJ t; il;"" smaller than 22,000lauarg feet, THEN flow control BMps
ilil1l,1?"#1ffi:i"r'ffii*f"::,il#"t:'#"?i Te
project
',,i."'""",lrate compriance
r".g., no,i",,'outbuirdings, *") 0,,
"llmarr
rotsaili,'#;r#i,iit;lJil:i,il,iffi**;";*
' q,r,ilt{ff:x#JfrH,1li;Tyffiru;;I^"fiffi**.,
mus,
ff ,H';,T::'::,:'il;i*ffi;TH[liij; 'i,?li;?f *'"opri*t,..,1,,ii,'.0"",,on
must
oovenant,
onlY in subdivisions *i".;;;;;i".1ffi'l^'"T:l1l:j full dispersion wirr be
2. where r,,o,.",,r,.,i"i.,, ffi:.,:l;:;,:ffi::l ffi fi:*xrJr:ffl H;T;;:"% r,, .fiooding ".lti^:it]:"
of target imp
A ppe ndix c" :::,i I T:1; :;; ftf.''o; p.i1 iffi H J,il fiil?i ffi1;lT :I";ililj1 if ",.
;$n'",,,ruijir"iii;ifi'Tffi "HHl;riiif ,:,#:##*i::*,{*x,y,:::m:",i1J,,,
3'
fiififfi'#i11'"i'^"'9ces not
ifieat"a bv Requirem"nT, r T.g 2 above,must be mitigated ro
'. '"0i";; ;;Ty^:?i1':t11ff;'Jffl. o. 'o.. eilp; n",;',".1"r19*,", ',,i,."1,"'ora
given BMp
3ft::lfl:flJo be impremenr";*fi:'3^;;i ?[|Jffi:i:} ff"l"'*'l.d '" "oo."o'-"Ll
peasibre nvps
implemenred:limitations and design ro;"ja;;',"#;:".:#? be located anvwhere onthe situltotas pafl of the propos'eJ f;jjf"t"s for each sNap. ft,.." rira. ,nri, o"
I.2 CORE RE
1.2.9.2 INDIVIDUAL LOT BMP REQUTREMENTS
Section 5.2, whichever is applicable
llows:
based on 0
In till soils, provide b ioretention volume based on
o
0.6 inches ofequivalent storage depth; in outwash soils provide bioretention volumeequivalent storage depth,
ume
City of Kenr Surface Water Design Manual
I inches of
I -65
In till soils,provide bioretention vol
luly 2022
SECTION I.2 CORE REQUIREMENTS
based on 0.8 inches ofequivalent storage depth; in outwash soils, provide
bioretention volume based on 0.4 inches of equivalent storage depth,
Permeable Pavement per Appendix C, Section C.2.7
4. All target impervious surfaces not mitigated by Requirements 1, 2 and 3 above, must be mitigated to
the maximum extent feasible using the Basic Dispersion BMP described below. Use of Basic
Dispersion is subject to evaluation of its feasibility and applicability as detailed in Appendix C.
Feasible BMPs are required to be implemented. Basic Dispersion BMPs may be located anywhere
on the sile/lot subject to the limitations and design specifications cited in Appendix C. The BMP
must be implemented as part of the proposed project.
. Basic Dispersion per Appendix C, Section C.2.4
5. BMPs must be implemented, at minimum, for an impervious area equal to at least l0% of the sitellot
for sitellotsizes up to 110000 square feet and at least 20Yoof the sitellotfor site[lot sizes between
11,000 and22,000 square feet. OR one or more BMPs from the following list are required to be
implemented to achieve compliance. These BMPs must be implemented as part of the proposed
project.
. Reduced Impervious Surface Credit per Appendix C, Section C.2.9
. Native Growth Retention Credit per Appendix C, Section C.2.10
6. The soil moisture holding capacity of new pervious surfaces must be protected in accordance with
Kent Grading Standards and Kent City Code Chapter 15.07. Kent Grading Standards require that the
duff layer or native topsoil be retained to the maximum extent practicable. KCC 15.07 requires soil
amendment to mitigate for lost moisture holding capacity where compaction or removal of some or
all ofthe dufflayer or underlying topsoil has occurred. The specifications for compost for soil
amendment can be found in Reference I l-C.
7 . Any proposed connection of roof downspouts to the local drainage system must be via a
perforated pipe connection as detailed in Appendix C, Section C.2.ll.
1.2.9.2.2 LARGE LOT BMP REQUIREMENTS
IF the proposed project is on a s#e/lot that is 22,000 square feet or largero THEN flow control BMPs
must be applied as specified in the requirements below OR the project must demonstrate compliance
with the LID Performance Standard (described in Section 1.2.9.1.8,p.1- 62) using an approved
continuous runoff model.
I . The feasibility and applicability of fult dispersion as detailed in Appendix C, Section C.2. I must be
evaluated for alltarget impervious surfaces. Iffeasible and applicable for any such surface, then
full dispersion must be applied to that surface and implemented as part of the proposed project.
Typically, full dispersion will be applicable only on the largest stTesllots where there may be enough
forest area available within a threshold discharge ureu to meet the l1Yo ruIio of fully dispersed
impervious area to native vegetated surface.
2. Where full dispersion of target impervious roof areas is not feasible or applicable, or will cause
flooding or erosion impacts, the feasibility and applicability of full infiltration of roof runoff must
be evaluated in accordance with Appendix C, Section C.2.2, or Section 5.2, whichever is applicable
based on the type of project.2T If feasible and applicable, full infiltration of roof runoff must be
27 ForprojectssubjecttoSimplifiedDrainageReview,andforanysinglefamilyresidentialprojectsubjectloFull DrainageReview,the
design requirements and specifications in Appendix C, Section C.2.2 may be used for evaluation and design of full infiltration on
City of Kent Surface Water Design Manual 1-66 IuIy 2022
SECTION 1.2 CORE REQUIREMENTS
implemented as part of the proposed project.
3. All target impervious surfaces not mitigated by Requirements I and 2 above, must be mitigated to
the maximum extent feasible using one or more BMPs from the following list. Use of a given BMP
is subject to evaluation of its feasibility and applicability as detailed in Appendix C. Feasible BMPs
are required to be implemented. The BMPs listed below may be located anywhere on the site/lot
subject to the limitations and design specifications for each BMP. These BMPs must be
implemented as part of the proposed project.
Limited Infiltration per Appendix C, Section C.2.3
Bioretention per Appendix C, Section C.2.6, sized as follows:
o Rainfall resion Tac 1.0 and less: In till soils , provide bioretention volume
based on 0.6 inches of equivalent storage depth; in outwash soils provide
bioretention volume based on 0.1 inches of equivalent storage depth
o Rainfall reeions greater than SeaTac 1.0: In till soils, provide bioretention
volume based on 0.8 inches of equivalent storage depth; in outwash soils,
provide bioretention volume based on 0.4 inches of equivalent storage depth,
Permeable Pavement per Appendix C, Section C.2.7
4. All target impervious surfaces not mitigated by Requirements 1,2 and 3 above, must be mitigated to
the maximum extent feasible using the Basic Dispersion BMP described below. Use of Basic
Dispersion is subject to evaluation of its feasibility and applicability as detailed in Appendix C.
Feasible BMPs are required to be implemented. Basic Dispersion BMPs may be located anyrvhere
onthe sitellot subject the limitations and design specifications cited in Appendix C. The BMP must
be implemented as part of the proposed project.
Basic Dispersion per Appendix C, Section C.2.4
5. BMPs must be implemented, at minimum, for impervious area amounts defined as follows. For
projects that will result in an impervious surface coverage on the buildable portion of the site/lot
of
less than 45Yo, fTow control BMPs must be applied to 50%o of target impervious surfaces. For
projects that will result in an impervious surface coverage 45-65yo on the buildable portion of the
site/lot, flow control BMPs must be applied to 50Yo of target impervious surfaces reduced by 15%
for each I oZ of impervious surface coverage above 45%io (e.g. impervious coverage of 55Yo results in
a requirement of FCBMPs applied to 35%io of target impervious surfaces). For projects that will
result in an impervious surface coverage greater than65Yo on the buildable portion ofthe site/lot,
flow control BMPs must be applied to 20%o of the target impervious surfaces or to an impervious
area equal to at least l0% of the site/lot, whichever is less. The buildable portion of the site/lot is
the total area of the site/lot minus
any critical areas and minus 200 ft. buffer areas from a steep slope hazard, landslide hazard area, or
erosion hazard area. If these minimum areas are not mitigated using feasible BMPs from
Requirements 1,2,3, and 4 above, one or more BMPs from the following list are required to be
implemented to achieve compliance. These BMPs must be implemented as part of the proposed
project.
individual lots. For all otherprqects, full infiltration must be evaluated and designed in accordance with the infiltration facility
standards in Section 5.2.
City of Kent Surface Water Design Manual 1 -67 July 2022
SECTION I.2 CORE REQUIREMENTS
Reduced Impervious Surface Credit per Appendix C, Section C.2.9
Native Growth Retention Credit per Appendix C, Section C.2.10
6. The soil moisture holding capacity of new pervious surfaces must be protected in accordance with
Kent Grading Standards and Kent City Code Chapter 15.07. Kent Grading Standards require that the
duff layer or native topsoil be retained to the maximum extent practicable. KCC 15.07 requires soil
amendment to mitigate for lost moisture holding capacity where compaction or removal of some or
all of the duff layer or underlying topsoil has occurred. The amendment must take place between
May I and October 1. The specifications for compost for soil amendment can be found in Reference
I l-c.
7. Any proposed connection of roof downspouts to the drainage system must be via a perforated
pipe connection as detailed in Appendix C, Section C.2.11.
1.2.9.2.3 IMPLEMENTATION REQUIREMENTS FOR INDIVIDUAL LOT BMPS
The flow control BMPs required in Sections 1.2.9.2 above must be implemented in accordance with
the following requirements :
L Implementation Responsibility. All flow control BMPs required for the sitellot must be
implemented (installed) by the applicant as part of the proposed project unless they have
already implemented as part of a subdivision project that created the lot per Section I .2.9.4 .
2. Maintenance Responsibility. Maintenance of all required flow control BMPs is the responsibility
of the owner of the sitellot served by these BMPs. The responsibility for such maintenance must be
clearly assigned to the current and future owners of the sitellot through a "declaration of stormwater
facility maintenance covenant" as described in Requirement 3 below.
2 Declaration of Stormwater Facility Maintenance Covenant. To ensure future maintenance of
flow control BMPs and allow for City inspection of BMPs, a declaration of stormwater facility
maintenance covenant must be recorded for each sitellot that contains flow control BMPs. A draft
of the proposed covenant must be reviewed and approved by city review staffprior to recording. All
required covenants must be recorded prior to final construction approval for the proposed project.
The stormwater facility maintenance covenant can be obtained on the COK website or from plan
review staff., and is designed to achieve the following:
a) Provide notice to future owners of the presence of flow control BMPs on the lot and the
responsibility of the owner to retain, uphold, and protect the flow control BMP devices,
features, pathways, limits, and reshictions.
b) Include as an exhibit, arecordable version2s of the following drainage plan information:
. The flow control BMP site plan showing all developed surfaces (impervious and
pervious) and the location and dimensions of flow control BMP devices, features,
flowpaths (if applicable), and limits of native growth retention areas (if applicable). This
plan(s) must be to scale and include site topography in accordance with the specifications
28 RecordableversionmeansonethatmeetsKent's"StandardFormattingRequirementsforRecordingDocuments"pursuanttoRCW
36.18.010 and 65.04.045, available online at http://vour.kingcountv.gov/recelec/records/docs/fonnatting requirements.pdfor from the
King County Recordefs Office. These requirements include specifications for such things as page size(8-12" x 14" or smaller), font
size (at least 8-point), and margin width (1 " on all sides ofevery page ifthere is a standard cover sheet).
City of Kent Surface Water Design Manual I -68 Jr;Jy 2022
4
5
' The flow control BMP design and maintenance defails for each flow control BMp perAppendix c, Section c.+.:. "rms i""r;d*;;;.#i?r"oo,,"abre) of each flow controlBMP device or feature and written maintenanceinJ oie.ation instructions and restrictionsfor each device, feature, flowpalh (i6il"-""" fi;: growrh retention area (if
tlit;:::,t?.ffi ilffi:';;Ttrffi ","r"""#:"r*ooi'""0t").se"n"re,.;;ili",prepared
Assure the exhibits are correctlv cross-referenced in the declaration ofcovenant (the
ij:o'"" is rvpicallv Exhibit A and the d"risril;;;;ce detairs are typicaily Exhibit
c)
*?:::rrli:JT,Xrr,"r:ffi:::ryl.* operated and mainrained at the owner,s expense
d) Grant the city of Kent or its successor the right to enter the property at reasonable times forpurposes of inspe*ing the flow contror BMp"s and a;;;fb;;;y corrective maintenance,repair' restoration, or mitigation *ort on the flow ";t.ffi#, that has not been performed bythe properrv owner withiria ."utonuut" ,ime. ser by;; ;j;;;Krrt, und to charge the properry
Sff"J#:n: cost of any maintenance, repair, restoruti*,
". -irrgation work performed by the
The right to enter typically applies to the entire property, but occasionally exempts areas on
:T,fl:"ff#,;iT:3J[:: ?n
in. ci'v . ie excruded ril; ";.;,;. Such areas u.i ,o u" ,io*n
e) Prohibit any modification or removal of flow contror BMps without written approval from thecity of Kent. In cases where the modification o, ,",nouut i, lon. uno". a city of Kentdevelopment permit, the approvar -;;;; obtained *; ",rr;;]ew sraff and a covenant mustbe recorded to reflect ttt"
"ttung"r. in uriotrr". *."^., d;Pi.o"uur ,,-,urt be obtained from cityreview staff and a covenant must be recorded to ."n""t it
"'Jt Jng"r. app.oval wi, be grantedontv if equivarenr protection in termsof;il;i;il;;a.r""1'J-,, provided by other,ri"unr.
Timing of Implementation' All required flow control BMps must be installed prior to finalinspection approvar of constructedil;;;;"-"nts.. For BMps that rery on vegetation, thevegetarion must be pranted and starring ;;;; prior to find ";;;;;ion approvar.
Acceptance standards' Flow control BMPs may be inspected during and/orfoilowing construction.Approvar orthe constructed BMps wirb;l;"d;;;;f;;;il#; ihe materiars and pracementappear to meet the snecificarions and that the er4p, upp"* ll ru;;;; ^ designed. onsiteobservations mav be used to "..it;;;",irr41 *9;, .o";rfi;ffimateriar receipts checked.Performance mav be evaruared bi";;^;iJ'*nit" i, r;;;";;itesting with a bucket of water orgarden hose to check pavement permeability or proper connection to g^ap devices/features, etc.Drainage concerns' If city review staff determines that there.is a potential for drainage impacts toa neighboring properrv, then addition;l;;;;;;, may be ,"qui."a. -so-re
flow controf BMps mayili,t"'rHill|:t;:"Jl#X"* situations' ";;;ti not u" urro*"J1,
",il'."ui"* sraffwhere they may
6.
7. Geotechnical concerns. A geotechnical englneer, engineering logis! or city review staffcontracted geologist must evaluate and
geo
approve flow control BMPs that are proposed:(A) on slopessteeperthan 15%; (B)within a setback from the top of equal to the total vertical height of theslopeslopearea that is steeper than lsyo; or (C) within 200 feet of asteep slope huzurd oreo, erosionhazard area, or landslide hazard area.In addition, city review staff may require review by ageotechnical engineer or engineering geologist of any Itrates, disperses, orCity of Kent Surface Water Design Manual t-69
proposed BMp that infi
Iuly 2022
SECTION I.2 CORE REQUIREMENTS
directs overflow adjacent to or towards a steep slope hazurd area, erosion hazard area, or landslide
huzurd areu. City review staff may also require some projects to route flows down or around such
slopes using non-perforated pipes. Some flow control BMPs may not be appropriate for these
locations, and will not be allowed by city review staffwhere flows may cause erosion problems'
8. Sewage system concerns. If city review staff determines that there is a potential conflict between
onsite sewage systems and flow control BMPs, additional measures may be required. Some
projects may need to route flows past onsite sewage systems using non-perforated pipes. Also,
some flow control BMPs may not be appropriate for these si/es, and will not be allowed where
sewage systems may be impacted.
1.2.9.3 SUBDIVISION AND ROAD IMPROVEMENT PROJECTS BMP
REQUIREMENTS
For subdivision and road improvement projects, flow control BMPs must be selected and applied
according to the subdivision and road improvement projects BMP requirements in this section. For
purposes of applying flow control BMPs to these projects, two categories of requirements have been
established based on the size of sitellot subject to improvements and by the project. These categories
of requirements are as follows:
. Subdivision Projects BMP Requirements
. Road Improvement Projects BMP Requirements
Flow control BMPs must be applied in the order of preference and to the extent specified for the
category of requirements applicable to the proposed project as described in the following subsections.
1.2.9.3.1 SUBDIVISION PROJECTS BMP REQUIREMENTS
IF the proposed project is a subdivision project, THEN Flow control BMPs for plat infrastructure
improvements (e.g., road and sidewalk etc.) of these projects shall meet the requirements described in
Section 1.2.9.3.2 below for "Road Improvement Project BMP Requirements." Implementation of flow
control BMPs required for/on the individual lots of the subdivision may be defened until a permit is
obtained for construction on each lot and is therefore optional. However, if the applicant wishes to
implement or make provision for implementation of BMPs for the lot improvements as part of the
subdivision project for purposes of receiving BMP modeling credits, the individual lot BMP requirements
described in Section 1.2.9.2 and implementation requirements for subdivision projects described Section
1.2.9.4.1must be mel
1.2.9.3.2 ROAD IMPROVEMENT PROJECTS BMP REQUTREMENTS
IF the proposed project is a road improvement project, THEN flow control BMPs must be applied as
specified in the requirements below.
I . The feasibility and applicability of full dispersion as detailed in Appendix C, Section C.2. I must
be evaluated for all target impervious surfaces. If feasible and applicable, full dispersion must be
implemented as part of the proposed project. Typically, small lot full dispersion will be
applicable only in subdivisions where enough forest was preserved by tract, easement, or
covenant to meet the minimum requirements for full dispersion in Appendix C, Section C.2.1 .1.
2. All target impervious surfaces not mitigated by Requirement I above, must be mitigated to the
maximum extent feasible using one or more BMPs from the following list. Use of a given BMP is
subject to evaluation of its feasibility and applicability as detailed in Appendix C. Infeasible BMPs
are not required to be implemented. The BMPs listed below may be located anywhere on the
sitellot subject to the limitations and design specifications for each BMP. These BMPs must be
implemented as part of the proposed project.
City of Kent Surface Water Design Manual l-70 luJy 2022
Full In filtration per Section c.2.2,or per Section 5.2,
SECTION I.2 CORE REQUI REMENTS
Limited Infiltration per Appendix C, Section C.2.3
Biorefention per Appendix c, Section c.2.6, sized. as follows:
IS applicable
o
. Permeable pavement per Appendix C, Section C.2.7
AII target impervious
lurfaces not mitigated by Requirements I and 2 a.love,must be mitigated to[i4u]1":;j::rf H:',#{ffi **iilffiilq,3,y:#,",ib"d;;,;;-r,",i.,,",,.Inreasible BMPs Lre nor required to o" in,,ltJllly,' :"0
applicab.ilitv as detailJ i" ooo*ox c.anywhere on ,h; iiittotsubjecr to the limitjlemented' ,
Basic Disp-ersion BMps may Te to"uteorhe BMPs must be i'npt.'"nt.J;ou,t;'ffi:fiffi:i
l,T;9.'o*tocations
.ir"o i"i#ndix c.. Basic Dispersion per Appendix C, Section C.2.4
The soil moisture hold.rentcraffi ffi ,,;.ff;fr*i,t::,iry;,'ilff :.H,:r.ru,:ff"i;1i,:T1ilil*the duff layer or native topsoil u" t"tuinJio"ti",.,,,u*irur
"^r"n, p'rulri"abre. KcC ts.bz requiresil*:TfffiT;fi:"#'r"te for rost ,n"r*." r'"rai"e
""p""r,, *r,.i"1'".0""r,on or removar of
*ru;*liffi rHi:;::il:li:rffi :;lrjffii*i:,1ffi.1in:,"tJ*gJ#*:"
REQUTREMENTS FOR USE OF BMP CREDITS
Projects that implemellfl:y control BMPs. whelhe.r required or optional, may use the flow contror
BMP credits o"""o"tll.*l{;l;::r#""","," *" irpi"r.","iri'l ilorr"r"nrs in Secrion t.2.e.2.48l il$::liif'Pd"""",ff ; t.;.;:;'.i ;:tl"'* r.. r"ci,"-." i;.jilll,
""0 Section 1.2.e.4.2 betow
Two kinds of credits are available' First, any impervious surface served_by a flow control BMp thatff 3lftiilT'ffi i::ffi l'#J,#l* ; ;}tr; :* ff ffI,o;I",ei ed as i "i;."# ii' iuu, "l ' To compute post-development runoff time series when sizing required flow conhol facilities.2' To compute post-deveropment^r''-year peak.fl^ows when assessing any of the peak flowexceptions from the area-specific no*
"Jnt or a"rrrry ,"q jr"o'""ir,"
Sections r .2.3.r .A,8, and c.3' To compute post-deveropment runoff t,r" ,^?.1:: when sizing required flow rate based waterlJflY"f;i:X'T"i:,-*',;il;,;,*ilii;:;:il:::;5hfi:fll,ed rand,,o", *-r,""',ir"*
HJi::fflf"HHil l#Hrr;x[firj;, as described above is rimited_to BMps that are treating
Second, unrt.no".u,.r- :" *"r.: :l**t
to a required water quality facitltl''-
v !r'sr @'ir
ai,p".,ioit,ffi ';",:ilil::,Tj:f ;jHiiFfJlr;:;"I,,I!;l::,",(:";:::#jio"
ft x','"TL13::',',Tfi :;[:,T;;i*;;'il'3.r)or,r'"".*-",0""1n"warerquariryracility
-t
based on 0.8 inches of equivalent storagevolume based on 0.4 inches of equivalent
In till soils, provide bioretention volumedepth ; in. outwash soi ls, provide bioretentionstorage depth,
ll soils, provide bioretention
outwash soils
depth,
volume based onprovide bioretention vol ume
4.
1.2.9.4
City of Kent Surface Water Design Manual l-71
luly 2022
SECTION I .2 CORE
TABLE .2.9.A wFLO CONTROL BMP FACILITY SIZINC CREDITS(tF'I ow Control BMP Type
Full dispersion
Fa cility Sizing Credit
Full in
Model fully dispersed surface as
Limited infiltration
Subtract rmpcrvious area that ls infifully Itrated
Basic dispersion
Model tributary rmpervious surface as 90%0%impervious,gfassModeldispersedrmpervioussurfaceas90%impervious,0%grassFarmlanddispersion
Bioretention
Dispersed areas are cons idered non-targeted for flow controlDispersedonareasslteswithfarmmanagementareplanslorconsideredwater nonqualitytreatment.
Model tributary impervious surlace 90%impervious,0%grassPermeablepavement(unlined with nounderdrain)
Grassed modular grid pavement
Model permeable pavement areaas 50%o rmpervious, 50o% grass
Model mod ular grid pavement as al grassRainwaterharvesting
Restricted footprint
adjustment
itCred al loonly wed vta and AS specified ln approvedls drainageofconditionsuse that
Whee strip driveways
Model footprint as restricted
Minimum disturbance foundation
Model credited area as 50%0%5impervious,grass
Open grid overdecking areapervlous
Model foundation asarea 50%impervio s0%us,grass
Nati ve wthgro retention credit
Model deck asafea 0%5 s0%rmpervious,grass
Perlorated plpe connection
mModel itigated impervious asafea 50o/o rmpervious,50o/o grassNone
(3)
City of Kenr Surface Water Design Manual t-72
Iuly 2022
1.2.9.4.
B.
2t For purposes olapplyins flo,o' binding site piln"'r "i6 rruw control BMPs' the term subdivision or subdivision prolecl refers to any project that is a short prat, plat,
City of Kent Surface Water Design Manual
t-73
Iuly 2022
3
b)The FCBMp
proposed
D' Subdivision Fufure Imprementation of BMps on Individuar LotsThese are flow contr
r"t, tv ui."];;#"lttPs stipulated to be installed.on some or a, of a subdivision,s proposed
'"ooi"iri"" p;;ffi"rT:[J,T:Til3"f ,f::""r' 'u.r, i"i." r" ."J"'* credits ror rhese-BMps, thel. Demonstrate throuph ;;^:^,--
rollowrng requirements:
1"1 "* i"^ii;;'iii:;fii.:ffi'::"3.::*"nt.that rhe now conjr.ol BMps sripurated ror eachl '2'9'2 and the BMp design ,p."in"uri.ng
to the individual lot, FIVrp .*"i."#;;"il 's."rion
be incruded in the TIR submifred *irn Jnr.in
Appendix C. This lot_specific assessment mustshall include an,
properry
"ppry Bil;:];;;;." Ji,;i'*J;":l*"il:l',"t,'*rubd;isi;;. in" *i'i,,,'"",ltlon necessary to select and2'
ff::'itiffffi:?i:1 or covenant and grant or easernenr ror ,
Requirement 3 "rtgii:.ry':d^!'credit'
ihis "'";;;;;;rr ii i1'l bt:tipulating the tvpe orsection 1.2'9'2.4, exceptas fbllows: as specified in Implementation
4. If the BMps installed within a dedicated tract satisfy some or all of the BMp requirements
for individ ual lots per Section I .2.9.2, then a note must be placed on the recordeddocuments for the subdivision indicating those lots for which BMps have been providedC. Subdivision Implemenfation of BMps on Individual LotsThese are flow control BMPs installed on AS ubdivision's proposed lots as part ofthe subdivision
project.For example,the subdivision developer may elect to pre-installcontrolBMPs required by the indi vidual lot some or all of the flowBMPrequirements in Secti on 1.2.9.2. To receive
credits for these BMps,the subdi vlsron project must meet all of the fol Iowing req ulrements:1. The flowcontrol BMPs must be installed and implemented rn accordance with the individual lot
BMP req ulrements in Section 1.2.9.2. This includes record tng a declarafion ofcovenant and
grant of easement for each lot with BMps as specified in Implementation Requirement 3 of
Section I .2.9.2.4. tf not all of the required BMPs are instal led on a Iot as part ofthe subdivision
pro..;ect, language must be included in the covenant noti$ing the future Iot owner ofrequired BMps.additional2. BMps to be install ed on individual lots as part of the subdivision pro3ect must be shown on
the site improvement plans submitted with the engineering plans for the proposed project as
specified in Section 2.3.1.2.
a)}.^Tgtyt site ptan(s) may be waived denrconceptual' showing onry the information "";;:*9.:"
the BMPs proposed or may be
fi*''""l'i!ff i.ffi ::ittrt"'#['i]:T::*#"[iiiruiil:JHfl fi "T#ffi **'segment" (see Appendix c, Sectioi-;;:;i,;',,il:':i:,tr
.ff,,ilJJi?fil"ri,ffl;li,il"
:il,ifffi ,:l[i:fi'i::, "t t:'"'; i'"0;;;1;,J",,**,.0,,.'*r'-#L
siruppu.ubre), andtr,::#;;:[1##,f}Hi':;:ki*".yi,';ff ;*-;;xi':iit'''g::",
devices,
design and maintena nce defails must include the dimensionsflowpaths, expressed of all
square foot ofCity of Kent Surface Water Design Manual
features, and
t-74
as unit amounts per
luly 2022
c)
3. If single family residential-lots are being credocuments for the su
"single famiry ,.r,o"lootut'ion
inai"utine";;ffifft1;te must be placed on the recorded
truaiuirion'niu;;;;;i""t and other improvements consrru*ed on rhe rors creared by thisthe a.a;nage ffi ;ffff :: H.l?ffiiffl i::y":"r#; ilffi ., (B M ps) stipurated incompliance wirh this stiputation ri;;il'r;tt"nt of easemenf recorded ro,.
"u"1, iJt."'q'Lvv 'tr
loj ff;it,e"
review when appricar'""-'r lri. Tsed in the small proiect gi"j:"sr;r;; submittedror a singte family residentiur UuTlain!';;;;il.4. ll":rr:.gjat tors are being creared, a nore rnror the subdivision r"di.;-d;;;Til;i,ilj: must be placed on the recorded documenrs
'rmprovemen" "o*'"ilffi*..i"t bv rhissubdivision musr imprement rhe flow
conrror besr manageme:lracrices irjttrirr,or,",ed in the oruinug"liun decraration of
;:*T:;:3i,''Jl'-*:fiTilil.:"i,!ffi#:ru:tl,ft:mr*ilr,J"TT;l:#:
s'
[i,il".1TrTffi'ffi:.?:fT-,:'":,:d, a note must be praced on the recorded documents
,, rmprovement
"
onrt.u" i'lnjnt:;"T': ::::_. r,con tro r best man agem'
"1 ::1i1"-:i 6lri'r *, #'ili; !i;'ru:T #Hiil?i;ff :l-,n.
o "*
"{Jj+:i:fifJT::ffffi?"Jilf,"Jffi1#;f:::l c",ori,""l"*iir, ir,i,,ripuration musr bepermit to make improvements to the lot.,, Iralnage review when applicati"t;;ffi;
1.2.9.4.2 USE OF CREDITS BY PROJECTS WITHIN RIGHT-OF.WAYIf a proposed p.o{:r is located primarily within an establisli mp lemen tat ion o r no * "";;; 6#ii;J Ji:;ffi :.::'::|;f 1d nu ur i" or pri vate righ t-o r-way,th ese e M ps,',
" p.o:."t ffi ; '#;:i Jl l;Jffi;lJ#:ilT"li; :. T;:i:;, f; c red i t iorI. The BMps must serve impervir2
fJf ,:-l:ty",{,, *,, .,-, i1;i;"ii,Iffi ;:l* ;:':,fflen!,he B Mpsnu",,n.J!l,lo,u,"oo.H,'li":,'#;tT"#r?""ffT", "rii.i"'" *,i;il;l'",iJ,L",*n
a road3',TjT;gi*f1ar wiil be privateryrnainrained, provisio
n.qui..,'"',rffi Ttffi :i*mn*n:*ildi"[":,rffi *i*#i:ifif,:""""*"'4' ili[T:%:iffi":iil1?fig maintenance orthe BMps, the.BMps must compry with the1.2. - -----^'r r'r4'r:ral guarantee and liability requirements i" c"* n"q"r",n"n, #7, section
City of Kent S urface Water Design Manual
t -75
IuIy 2022
SECTION I .3 SPECIAI
1.3
1.3.1
SPECIAL REQUIREMENTS
This section details the following five special ,project depending "'lo r*",io;";;;1;"J:;,iJ1ff.?ffuirements which may appry to the proposedr speciar Requirement #r: other Adopted Area-Specific Requiremenb, section r.3. rr Special Requirement #2: FroodHazardArea Derineation, Section L3.2o special Requirement #3: Flood protection Facirities, Section 1.3.3o Special Requirement #4: Source Control, Section 1.3.4r Special Requirement #5: Oil Control, Section 1.3.5
spECrAL REQ_UTREMENT
tl_. OTHER ADOPTEDAREA.SPE CTF,IC RE a ;iiilMENTS
Other adopted area_specific regulations mav Imore direct bearing "",1'",a*i""e";:'ff'off;:""J.j:3#l"j::ltadditionat requiremenb thathave a: ;il ;:ffiH ;;::n *"#;;;;;",,.ffi;::::"::":;resources and to ar
H:ffil:::";?Tft1##if:lll';i#frf,";1H; ;Jx;:x"::iiffiru"rff il;"*"T;"
o Master Drainage plans (MDps): MDps aplanned developments (ubDst; orir"l',"J'"
comprehensive drainage.plans prepared for urbancovered by a vrbe ,,,; '";;;;;ff;#A:?fffiffiff;jffi:3;i#"s.i'i;i:';5) projects
r Salm.on Conservation plans (SCps): Salmbased prans inrended,ro
'd""liv
*Tlil:,;:;.H:.r:;::,3"?Ht?;iryH*;J,,,iil,j:[,'mechanisms such asrrabir", ir;;;;;;L", ."gurutionr, ,rr#".r, ,Mps,.rand u"qrir-ir.;, and pubricri:ri;irlllffi ;f.ll]lt',ilru;h*f ;"11rff ffi l:r1"r,.i,*r,"",',ii,*awa,erseveral basins or subbasins. ov rne srate under wAC 0s_ioo_oioil;;;,"*
Flood Hazard Mana
:nil:x*f ffi H1?-",,",::3'ifi'fi #',i!J:tf ::;iil?.il"lgt:::rManagemen,p,anand
fit*ffi :it":iifl,,t;''1,:r*;:n:ilTl;;,#**it,,?fi ;i,lii:::ffj;
".r"li"r."o,;iiffi;#ffi::l::"1Jffi",fl"_vi,ion;it;;,#:vrtres;
(2) geographicauv based
warning i'piou.'lli;;:'j;:::?;11t maint.enance, relocatiJr ;;j.L#'';'^T;:::"10"tunap,ol""ri";ffi:;ffi 11,;:HJF;n?;;d{f l::i}];;itri#titi,tifi f h,ffi $h-;
Lake Management pl
"omp'"hensii";;;;;;;::?Xl,];Ji:fi%:il^1:ttrake managemenr prans to provide rorlmpacts from nutrient ftcon,rom,ougisp"";xlili:;:i"ff#:#i::lirff f#Hl;,,ln:?f*i#;ffi ,"",it covers' ---- -^srrrs6v 4rru source control requirements for proposed projects within the area
City of Kent S urface Water Design Manual
1-76
Iuly 2022
SECTION I.3 SPECIAL
Application of this Requirement
The drainage requireradditionffi;;#,HH;ft*?t'"?lT,Jpecific.regurations such as basin prans shal be appried inwhere confli"tt o""rr'b"r;;il;;;'r'i.'o,n?n'"' unless otherwise specified ;" trr"
"o"pt.o regurarion.shall supersede those in this manua uralro$e requirements of the adopie; ";;;";":;;c reguration
:T:.:;T:H_"",, r*::::i".
in other adopted area-specinc regurations incrude the ro'owing* More extensive water quality controls* Forest retention requirements
* Infiltrationrestrictions
* Groundwater recharge provisions* Discharge to a constructed regionar flow contror or conveyance facirity.
1.3.2
ffi??,ffiifir%Rur*EMENT #2: FLooD HA?ARD AREA
Flood hazard qreas a1FEMA;;d;#ffi T,':""Tli;:Hitr,,",",.Jli!!:;lr:il?L7;ff
:#'Jii;fi lilffi ,_,,,
tf$iiliffiffi::1" utto'a io'o'i;;;; ^,a"*.'i""i tr'il"'"i,," una co,,ni*iti 6.u"rop,,"nt,
::Jff ffi ;*iil*ffi ,r:fJ::;:,;:";r;n*:"r:ffi f ffi rffi ,?"t,!""0
o" r in"ut" J'l?l;#"' ;#f ff ' *'ffjiTT;jat i on d urins ex treme e \
l:"0'", o."iHff;"- deveropmenr. n"gul^1,"^Y";1'il;r';TJIJ*l];;5J"T,JiTri:,,1flains
are
r00-vear noodptain are round
" 6n#; ;:;;':?ffi ilXi:il"::ni..on..,nln-g a"""i"pr* wirhin aThe following requirem
requirements- -r*^''"'ents relate to mapping ofthe floodplain/floodway and compensatory storage
aproposed project is in a basin plan or lakenagement plan.,.
IF
ma
Requirement
Threshold
IF a proposed project
stream, lake, wetland.
other Kent regulations
hazards . .
contains or is adjacent to aor closed depression, or ifrequrre study offlood
Threshold
Requirement
City of Kent S urface Water Design Manual
l -77
Iuly 2022
SECTION I.3 SPECIAL
Application of this Requirement
i!:?:l#ii:XJ#X-"lo.studv
exists' then it mav be used as the basis ror derineating the floodprain andr<r"in"lih-".a.";ltlto"o the study was prepared in u.,unno Jo]rsistent with this manuar and otherp,"pu,"a"i*ffi;,i:;:,T;:hik[:ili0"#,0"^"0 h**o 'tuiii"il "", exisr, then one sharr be
i,'f :::ilT:'#,-#i j;::Til"T,';!fr ,'--fl ,,#fi :;ffi *?#r;tj*;,,'"'
the m ust provide offsettins
pre-development
theTHEN development
forstorage 00%f the
floodway fringe storage thatcapacity wasidedproonthethatlspropertytobefilled.
City of Kent Surface Water Design Manual l-78
Iuly 2022
SECTION I.3 SPECIAI,REQUI REMENTS
1.3.3 lTqqrAL REeUTREMENT #3: F,LooD pRorECTroNFACILITIES
Developing sites protectedby levees, revetments, or berms requires a high lever of confidence in theirshuctural inregritv and performan"..- rrop", *arysis, design,;;r;;;.*tion is necessary to protectagainst the potentiaty catastrophi" .onr"qu"*es if such racilities shourd fair.
Application of this Requirement
conformance with the requirements listed above shall be addressed in the Technical InformationReporr submitred with the proj"r,,
""gi".Jl"g plans (see S""ii*! j. , . f I
conformance also requires that certain easement requirements (outlined in Section 4.1) be met inorder to allow city access to the facility iirh; proposed project fontains an existing City of Kent floodprotection facility or proposes to rely on a ciry of t<ent noia-p.ot"Jiiil a"imr, the appricant sha'
fi:,",:|"oi: ;ffiil:j i:
the city or("nt **iltent with th" .i;;;;;;;;",ion .u,.,,nt requirements
IF a proposed project either:
r contains or is adjacent to a stream that has anexisting flood protection facility 1r""f, u, u-'levee, revetment, or berm), OR
r proposes to construct a new or to modi$, anexisting flood protection facility . . . ---' *"
THEN the applicant shall demonstrate
that thg flood protection facility, as
determined by a licensed professional
engineer, conforms with siting,
structural stability. environmental, and
all other relevant standards cited in thefollowing regulations and documents:
. Washingtonstate Integrated
S t r e am b ank p r o t e ct i oi Gu ide I i ne s,
. ?.!r of Engineers Manualfor
Design and Construction ojLevees(EM I I t0-2_1913),
. KCC 14.09 and
. Special Requirement #l (specifically
1,. King County Flood Hazard
Management plan)
AND, flood containment levees shall
meet or exceed the professional
engineering standards summarized inFEMA National Flood Insurance
mapping regularions (44 CFR,
subsection 65. I 0) or FEMA,s Analysis
and Mapping procedures for non_
Accredited Levee Systems.
Threshold
Requirement
City of Kent Surface Water Design Manual l-79
Iuly 2022
SECTION 1.3 SPECIAL REQUIREMENTS
1.3.4 SPECIAL REQUIREMENT #4: SOURCE CONTROLS
Water quality source controls prevent rainfall and runoff water from coming into contact with pollutants,
thereby reducing the likelihood that pollutants will enter public waterways and violate water quality
standards. King County prepared a Stormwater Pollution Control Manual for citizens, businesses, and
industries to identify and implement source controls for activities that often pollute water bodies. Kent
provides advice on source control implementation upon request. The City may, however, require
mandatory source controls at any time through formal code enforcement if complaints or studies reveal
water quality violations or problems.
Application of this Requirement
When applicable per the Stormwater Pollution Prevention Manual, structural source control measures,
such as car wash pads or dumpster area roofing, shall be applied to the entire st/e containing the proposed
project,notjusttheprojectsile. Iftheapplicantisatenantorlesseeforonlyaportionofthesile,city
review staff may limit the entire site application of structural source conhols to only that portion of the site
occupied or leased by the applicant. All applicable shuctural source conhol measures, such as covering
storage piles with plastic or isolating areas where pollutants are used or stored, are to be implemented after
occupancy and need not be addressed during the plan review process. All commercial and industrial
projects (irrespective of size) undergoing drainage review are required to implement applicable source
controls.
Activities That May Result In Structural Improvements
There are a number of activities that may require structures and/or specific drainage configurations in
order to protect stormwater and maintain compliance with city code. Roof structures, wheel washes,
cement pads, shutoff valves, containment berms and indoor mop sinks are all examples of things that
need to be in place prior to commencing the activity. These may require building permits and other
approvals prior to construction.
Below are some highlighted activities and the numbered BMP activity sheets in the Stormwater Pollution
Prevention Manual that provide more detail:
Commercial Comooslins
Structural improvements: paved composting and storage pads, leachate collection system, lined
collection ponds, wheel wash system
. A-24 Commercial Composting
Fuelinp of esuioment snd vehicles
Structural improvements: Portland cement pads, roofs, spill control devices, trench drains,
oil/water separators
. A-17 Fueling Operations
IF a proposed project is either:
. a commercial, industrial, or multifamily site
development, OR
. a redevelopment project proposing
improvements to an existing commercial,
industrial, multifamily site . . .
THEN the pro.ject must provide water quality
source controls applicable to the proposed project
in accordance with the King County Stormwater
P ol lut ion C onft ol Manual.
RequirementThreshold
City of Kent Surface Water Design Manual I -80 Ju1y 2022
SECTION I.3 SPECIAL REQUIREMENTS
. A-48 Older Fueling Operations
Horse stsbles
Structural improvements: Wash racks connected to sanitary sewer or separate infiltration area,
manure containment areas
o {- 35 Livestock
Processins snd Stockpilins of sand or sravel
Structural improvements: Wheel wash system and track-out control, catch basin inserts
. A-47 Wheel Wash System
Paintins. Finishins. & Coatins of Vehicles & Eauioment
Structural improvements: Permitted, enclosed paint booths
. A-22 Painting, Finishing, & Coating of Vehicles, Products' & Equipment
Restaurants and food trucks
Structural improvements: Indoor sinks for mat and rack washing and mop and wastewater disposal.
. A-8 Storage of Solid and Food Wastes
. A-12 Cleaning of Cooking Equipment
. A-39 Roof Vents and Fugitive Emissions
Outdoor storase of erodible materials. e.s. comnost. bark. sand. etc-
Structural improvements: Wheel wash system and track-out control, berms, containment areas,
covering, catch basin inserts
. A-41 Wheel Wash and Tire Bath Track-Out Control
Ouldoor slorase or orocessins of salvsniz.ed msterials
Structural improvements: Roofs or other covering, stormwater collection and treatment system
. A-27 Manufacturing and Post-Processing of Metal Products
Storsse of liauid materiqls
Structural improvements: Secondary containment, roofed structures, spill control devices
. A-2 Storage of Liquid Materials in Stationary Tanks
. A-3 Storage of Any Liquid Materials in Portable Containers
Utilitv Corridor Maintenance
Structural improvements: Road stabilization
. A-45 Maintenance of Public & Private Utility Corridors & Facilities
ll/ashins of cars. trucks and eouipment (not iusl commercial car washesl
Structural improvements: Dedicated wash pads, sewer connection, holding tanks, catch basin inserts
. A-13 Vehicle washing
ll/ood Treatment & Preservins
Structural improvements: Paved, contained and covered storage and processing areas
. A-23 Wood Treatment & Preserving
City of Kent Surface Water Design Manual l -81 July 2022
SECTION I.3 SPECIAL REQUIREMENTS
1.3.5 SPECIAL REQUIREMENT #5: OIL CONTROL
Projects proposing to develop or redevelop a high-use site must provide oil controls in addition to any
othlr water quality controls required by this manual. Such si/es typically generate high concentrations
of oil due to high traffic tumover, on-site vehicle or heavy or stationary equipment use, some business
operations, e.g., automotive recycling, or the frequent transfer of liquid petroleum or coal derivative
products..
A high-use site is any one of the following:
* A commercial or industrial site subject to an expected average daily traffic (ADT) count equal to or
greater than 100 vehicles per 1,000 square feet ofgross building area' OR
* A commercial or industrial site subject to petroleum storage and transfer in excess of 1,500 gallons per
year, not including routinely delivered heating oil, OR
* A commercial or industrial site subject to use, storage, or maintenance of a fleet of 25 or more diesel
vehicles that are over l0 tons gross weight (trucks, buses, trains, heavy equipment, etc.)' OR
* A road intersection with a measure ADT count of 25,000 vehicles or more on the main roadway and
15,000 vehicles or more on any intersection roadway, excluding project proposing primarily
pedestrian or bicycle use improvements.
The oil control requirement for high-use sites applies to all sites that generate high concentrations of oil,
regardless of whether the project creates new impervious surface or makes site improvements to an
existing high-use site. The traffic threshold identified focuses on vehicle tumover per square foot of
buildilg area (trip generation) rather than ADT alone. This is because oil leakage is greatest when engines
are idling or cooling. In general, all-day parking are€N are not intended to be captured by these thresholds
except for diesel vehicles, which tend to leak oil more than non-diesel vehicles. The petroleum storage
and transfer stipulation is intended to address regular transfer operations such as service stations, not
occasional filling ofheating oil tanks.
High-Use Menu
High-use oil control options are selected to capture and detain oil and associated pollutants. The goal of
treatment is to have no visible sheen in runoff leaving the facility, or to have less than 10 mg/L total
petroleum hydrocarbons (TPH), depending on the facility option used. Oil control options include
facilities that are small, handle only a limited tributary area, and require frequent maintenance, as well as
facilities thaltreatlarger areas and generally have less frequent maintenance needs. Facility choices
IF a proposed project either:
o develops a site that will have high-use site
characteristics (defined above), OR
r is a redevelopment project proposing
S 100,000 or more of improvements to an
existing high-use site.
r is a redevelopmentproject that results in new
plus replaced pollution generating impervious
surfaces of5,000 square feet or more or new
pollution generating pervious surfaces of%
acre or more improvements to an existing
high-use site...
THEN the project must treat runoff from the
high-use portion ofthe sire using oil control
treatment options from the High-Use menu
(described below and detailed in Chapter 6).
RequirementThreshold
City of Kent Surface Water Design Manual l-82 Iuly 2022
SECTION I.3 SP ECIAL
:?:"::: ;:f:ftT;:se'ts.' Iinear sand filters, and oit/water separators. See chapter 6 for specific racirity
Application of this Requirement
For high-use s'es located within a larger commercial center, only theimpervious surface associated withthe high-use portion of the site it tuffitolr"atment requiremenis. If common parking for multiprebusinesses is provided, treatment rt air ue apiiied to the ;;;;;;;;rking stars requirld for the high_usebusiness only' However' if the o""t''"", ,Jri"ction area utro ."""ru!l runoff from other areas, the waterquality facility must be sized to u"",
"iirv"i".'purring through it.
High-use roadway intersections shall treat the interse.ction itself, plus ranes where vehicles accumulateduring the signal cycle' including url run"r,"r.or tn" u"ginnin; oiirr" i"t-,*" pocket (see Figurel '3'5'A below)' If no left-turn p""ttt "^iti" ihe treatable u."irnurit"gin at a distance of 75 feet fiomthe stop line' If runoff from the i*".r*ii""'i."i* r" r"r"ii""'i*"
"",tection areas that do norcombine within the intersection, treatmenimay be rimited ," ";; ,;;;f rhe coilection areas. oircontrol facilities shall be designed rot urr no*J hibutary t" tn" ir
"""t"rol facirity including flow fromotherwise exempt areas that ui" not by;;r;;; ;"und the facility.
I?t:","":;LTTIJ"h"i'ffi;:"'.",$""1T*J$:ffJ:::Jfft: #J_::1Tii"oi".d by Kent, on,y
adjustment' catch basin inserts or" n)t ottor"aTo, oir contror. rwlse approved by an
Methods of Analysis
The traffic threshold for the High-use menu sha-ll be estimated using information from Trip Generation,published bv the Institul: :f tr.Ii:o"n"ri"" engi"eers, or fr"; ; t'.""ffi; study prepared by a professionarengineer or transpoftation speciarist *rtr,
""p"ri"nce in traffic estimation.
TREATABLE AREAS FOR HIGH-USE ROAD I
FIGURE I.3.5.A
NTERSECTIONS
I
I
I
High-use
area
City of Kent Surface Water Design Manual I -83
luly 2022
SECTION 1.4 ADJUSTMENT PROCESS
I.4 ADJUSTMENT PROCESS
For proposed projects subject to drainage review by city review stafl this process is provided for the
occasions when a project proponent desires to vary from one ofthe core or special requirements, or any
other specific requirement or standard contained in this manual. Proposed adjustments should be approved
prior to final permit approval, but they may be accepted up to the time Kent approves final construction or
accepts drainage facilities for maintenance.
Types of Adjustments
To facilitate the adjustment process and timely review of adjustment proposals, the following types of
adjustments are provided:
* Standard Adjustments: These are adjustments of the standards and requirements contained in the
following chapters and sections of this manual:
* Chapter l, "Drainage Review and Requirements"
* Chapter 2,"Drainage Plan Submittal"
* Chapter 3, "Hydrologic Analysis and Design"
* Chapter 4, "Conveyance System Analysis and Design"
* Chapter 5, "Flow Control Design"
* Chapter 6, "Water Quality Design"
* Appendix A, "Maintenance Requirements for Flow Control, Conveyance, and WQ Facilities"
* Appendix B, "Master Drainage Plans."
* Appendix C, "simplified Drainage Requirements" (detached)
* Appendix D, "Construction Stormwater Pollution Prevention Standards" (detached).
Requests for standard adjustments will be accepted only for permits pending approval or approved
permits that have not yet expired.
Experimental Design Adjustments: This type of adjustment is used for proposing new designs or
methods that are not covered in this manual, that are not uniquely sl'le specific, and that require
additional information to establish functional equivalence'
Blanket Adjustments: This type of adjustment may be established by the City based on approval of
any of the above-mentioned adjustments. Blanket adjustments are usually based on previously
approved adjustments that can be applied routinely or globally to all projects where appropriate'
Blanket adjustments are also used to effect minor changes or corrections to manual design
requirements or to add new designs and methodologies to this manual. There is no application process
for establishing blanket adjustments because they are initiated solely by the City.
1.4.1 ADJUSTMENT AUTHORITY
The Economic and Community Development Department shall have full authority to approve or deny all
types ofadjustments for any proposed project subject to drainage review by city review staff.
*
*
City of Kent Surface Water Design Manual l -84 August2022
SECTION 1.4 ADJUSTMENT PROCESS
1.4.2 CRITERIA FOR GRANTING ADJUSTMEI{TS
Adjustments to the requirements in this manual may be granted provided that granting the adjustment will
achieve the following:
1. Produce a compensating or comparable result that is in the public interest, AND
2. Meet the objectives of safety, function, appearance, environmental protection, and maintainability
based on sound engineering judgment.
Granting any adjustment that would be in conflict with the requirements of any other Kent department will
require review and concurrence with that department.
Criteria Exception
If it can be demonstrated that meeting the above criteria for producing a compensating or
comparable result will deny reasonable use of a property, approval of the adjustment will require an
adjustment criteria exception to be approved by the Economic and Community Development
Director (whoever is approving the adjustment). An adjustment that requires a criteria exception
may be granted following legal public notice of the adjustment request, the director's proposed
decision on the request, and a written finding of fact that documents the following:
l. There are special physical circumstances or conditions affecting the property such that strict
application of the criteria for producing a compensating or comparable result would deprive the
applicant ofall reasonable use ofthe parcel ofland in question, and every effort has been made to
find creative ways to meet the intent of the requirement for which the adjustment is sought, AND
2. Granting the adjustment for the individual property in question will not create a significant
adverse impact to public health, welfare, water quality, and properties downstream or nearby,
AND
3. The adjustment requires the best practicable alternative for achieving the spirit and intent of
the requirement in question.
In addition, the written finding of fact must include the following information:
. The current (pre-project) use ofthe s/e.
. How application of the requirement for which an adjustment is being requested denies reasonable
use of the srk compared to the restrictions that existed under the 2009 Surface Water Design
Manual.
. The possible remaining uses of the site if lhe criteria exception were not granted.
. The uses of the site thatwould have been allowed under the 2009 Surface Water Design Manual.
. A comparison of the estimated amount and percentage of value loss as a result of the requirements
of this manual versus the estimated amount and percentage of value loss as a result of requirements
that existed under the 2009 Surface Water Design Manual.
. The feasibility for the owner to alter the project to apply the requirements of this manual.
Experimental Design Adjustments
Experimental design adjustments may be required for departures from design specifications in flow
control facilities (Chapter 5), may be required for altematives to erosion and sediment control (ESC)
facilities in Appendix D; and are required for altematives to water quality facilities listed in Chapter 6
City of Kent Surface Water Design Manual l -85 August2022
SECTION I.4 ADJUSTMENT PROCESS
and Reference 14. Required water quality monitoring is in addition to any required by Ecology for their
approval. Kent does not pay for this monitoring. Monitoring costs are covered by the applicant and/or
the facility vendor or manufacturer according to their agreement.
Flow Control Facilitres
In most cases, a standard adjustment applies to altemative flow control designs. However, city review
staff may require an experimental design adjustment that includes monitoring when deemed warranted.
Erosion and Sediment Control Facilities
Erosion and sediment control (ESC) facilities that have Ecology CTAPE approval do not require an
experimental design adjustment. The City of Kent will consider an experimental design adjustment for
an ESC without CTAPE approval, but only for those that Ecology has approved as equivalent.
Water Quality Facilities
Experimental design adjustments for water quality facilities may be approved by crff review staff on a
limited basis if, upon evaluation, city review staff determines the following criteria are met:
. The facility has already been approved by Ecology's TAPE303I program, and has General Use
Level Designation (GULD). Facilities with conditional approval (CULD) may also be
considered if the manufacturer has applied to Ecology for GULD determination and will be
monitoring under the TAPE program. Approval by Ecology does not by itself constitute or
ensure approval by the City of Kent. For both GULD and CULD designations, Ecology's
approval documentation must indicate that maintenance is required no more frequently than
annually for flow control and water quality facilities excluding temporary construction ESC
facilities.
. Experimental design adjustments are not permitted for facilities that the city will be responsible to
maintain.
. City review staff determines that technical reports and data suggest facility performance could be
replicated. City review staff must have access to the full technical report(s) submitted to Ecology for
TAPE, and may require any other reports or data referred to but not provided.
. The Applicant agrees to monitoring as described in Reference 8-F; a monitoring quality assurance
project plan has been submitted to, reviewed and approved by city review staff; and the Consultant
providing the monitoring has provided the Applicant and city review staff with a cost analysis of the
monitoring program.
. For Presettling and Basic treatment water quality facilities only, if the facility has already been
approved by Ecology through Ecology's TAPE program, and has GULD, additional TSS percent
removal monitoring will not be required, but city review staff may require other monitoring, e.g.,
constructability, durability, and/or maintenance monitoring. City review staff may limit the number
ofinstallations until he/she is satisfied that the facility type is viable.
ro Technology Assessment Protocol - Ecology
City of Kent Surface Water Design Manual I -86 Atgust2022
SECTION I.4 ADJUSTMENT PROCESS
Additional Notes
If review of Ecology's basis for granting GULD is found to be flawed and city review staffthen
finds GULD should not have been issued, city review staffmay rescind its approval for new
installations of the facility in the City of Kent.
An experimental adjustment for ESC does not absolve the Applicant from meeting the requirements of
1.2.5.2. C, ESC Performance.
Conditions for approval of experimental design adjustments may include a requirement for setting
aside an extra area and posting a financial guarantee for construction ofa conventional facility
should the experimental facility fail to perform adequately. Once satisfactory durability, operation,
and performance of the experimental facility are verified, the set aside area could be developed and
the financial guarantee released.
The number of experimental installations of any one kind will normally be limited to two, until the
City of Kent has obtained sufficient evidence indicating performance meets criteria.
a
City of Kent Surface Water Design Manual l -87 August 2022
SECTION I.4 ADJUSTMENT PROCESS
r.4.3
1.4.4
ADJUSTMENT APPLICATION PROCESS
Adjustment Application tr'orm
The adjustment application form for standard and experimental design adjustments can be obtained from
plan review staff.
Standard Adjustments
The application process for standard adjustments is as follows:
+ Requests for standard adjustments will be accepted only for permits pending approval or approved
permits that have not yet expired.
* The completed adjustment request application forms must be submitted to city review staff along with
sufficient engineering information (described in Chapter 2) to evaluate the request. The application
shall note the specific requirement for which the adjustment is sought'
* If the adjustment request involves use of a previously unapproved construction material or
construction practice, the applicant should submit documentation that includes, but is not limited to, a
record of successful use by other agencies andlor evidence of meeting criteria for quality and
performance, such as that for the American Association of State Highway and Transportation Officials
(AASHTO) and the American Society of Testing and Materials (ASTM).
* If the adjustment requires a criteria exception, additional engineering or other information may be
required by city review staffto document that denial of reasonable use would occur, that every effort
was made to achieve compliance, and that the best practicable altemative will not cause significant
adverse impact.
* A fee reduction may be requested if it is demonstrated that the adjustment request requires little or no
engineering review.
Experimental Design Adjustments
The application process is the same as for standard adjustments except that requests will be accepted prior to
permit application. Justification supporting comparable performance of the proposed system to a
standard design is required.
Blanket Adjustments
There is no application process for establishing blanket adjustments because they are initiated and issued
solely by the City.
Applicants may apply to use SWDM web-posted blanket adjustments by submitting the adjustment
application form noted above to plan review staffassigned to the specific project proposal.
ADJUSTMENT REVIEW PROCESS
The general steps of the review process for specific types of adjustments are presented as follows:
Standard Adjustments
,'. City review staff will review the adjustment request application forms and documentation for
completeness and inform the applicant in writing as to whether additional information is required from
the applicant to complete the review. The applicant will also be informed if city review staff
City of Kent Surface Water Design Manual I -88 Atgust2022
SECTION I.4 ADJUSTMENT PROCESS
{.
*
determines that special technical support is required in cases where the adjustment involves a major
policy issue or potentially impacts a Public Works drainage facility.
The Economic and Community Development Director or designee will review and either approve or
deny the adjustment request following city review stafls determination that all necessary information
has been received from the applicant.
If a criteria exception is required for the adjustment, city review staff will issue a legal public notice
ofthe adjustment request that indicates the director's proposed decision on the request, including the
written finding of fact specified in Section 1.4.2. The public notice will include a l5-working-day
public comment period within which a request for reconsideration may be made to the Economic
and Community Development Director as described in Section 1.4.5. Absent a request for
reconsideration, the Director's decision becomes final after the two week public comment period.
Approvals of standard adjustments will expire upon expiration of the permit to which they apply.'lr
Experimental Design Adjustments
o The City will consider any flow control adjustment request, but will only consider experimental
design adjustments for water quality facilities that have been approved by Ecology's TAPE
program, and erosion and sediment control facilities that have been deemed by Ecology to be
equivalent to facilities they have approved through their CTAPE program.
o All information including but not limited to reports and data submitted to Ecology for their
TAPE approvals must be submitted to City plan review staff. This may require coordination
between the manufacturer and the applicant.
. City review staff will review the submitted material and any recommendations, and inform the
review staff will also give the applicant an estimate of the time needed to complete the review.
There is no guarantee that an experimental adjustment will be granted.
. If a criteria exception is required for the adjustment, city review staff will issue a legal public notice
ofthe adjustment request that indicates city review stafls proposed decision on the request,
including the written finding of fact specified in Section 1r4.2. The public notice will include a l5-
working-day public comment period within which a request for reconsideration may be made to the
Economic and Community Development Director as described in Section 1.4.5. Absent a request
for reconsideration, the Director's decision becomes final after the l5-working-day public comment
period.
o The Economic and Community Development Director or designee will review and either
approve or deny the adjustment request in writing, and this will in tum be communicated to the
Applicant by city review staffin writing.
Blanket Adjustments
Blanket adjustments will be established at the discretion of city review staff based on:
1. A previously approved standard or experimental design adjustment and supporting documentation,
AND
2. Experimental adjustment monitoring results in conjunction with any TAPE or CTAPE results AND
3. Information presenting the need for the blanket adjustment. Typically, blanket adjustments should
apply globally to design or procedural requirements and be independent of site conditions.
City of Kent Surface Water Design Manual l -89 August2022
SECTION I.4 ADJUSTMENTPROCESS
1.4.5 REQUEST FOR RECONSIDERATION PROCEDURE
The applicant may request reconsideration ofthe denial or conditions ofapproval ofan adjustment request
by submitting a formal letter to the Economic and Community Development Director within 15 working
days of the decision. This letter must include justification for review of the decision, along with a copy of
the adjustment request with the conditions (if applicable) and a listing of all previously submitted material.
The Economic and Community Development Director shall respond to the applicant in writing within l5
working days; this decision shall be final. A per-hour review fee will be charged to the applicant for City
review ofan appeal.
Criteria Exceptions
A criteria exception decision for an adjustment is a Process I land use decision pursuant to Chapter
12.01 KCC. Because the public is given an opportunity to comment on a criteria exception decision, the
applicant may request reconsideration of the decision by submitting a formal letter to the Director within
l5 working days of the legal public notice. This letter must include justification for reconsideration of
the decision, along with any supporting information/documentation. The Director shall respond to the
letter in writing within 15 working days. The Director's decision on the reconsideration request shall be
final.
City of Kent Surface Water Design Manual 1-90 August2022
CHAPTER2
DRAINAGE PLAN SUBMITTAL
KENT
WasHrNcroN
CITY OF KENT
Surface Water Design Manual
2021
CHAPTER2
Drainage Plan SubmittaI................
2.1 Plans for Permits and Drainage Review.....
2.1.1 Plans Required for initial permit Submittal
2.1.2 Plans Required for drainage review
2.2 Plans Required with Initialpermit submittal
2.2.1 Subdivisions, short plats and Binding site plans...'.
2.2.2 Commercial Site Development............
2.2.3 Single-family Residential ..................
2.2.4 OtherPermits..............
2.3 Plans Required for Drainage Review.
2.3.1 Engineering Plan Specifications..........
2,3.2 Proiects in Targeted Drainage Review
2.4 Plans Required After Drainage Review
2.4.1 Plan Changes After Permit Issuance......
2.4.2 Final Corrected Plan Submittal
2.4.3 Final Plat, Short Plat, and Binding Site Plan Submittals
..2-1
aa
aa
"ta
..2-4
..2-4
2-6
2-6
2-6
2-7
a1
2-34
2-34
2-34
2-34
City of Kent Surface Water Design Manual July 2022
City of Kent Surface Water Design Manual htly 2022
CHAPTER 2
DRAINAGE PLAN SUBMITTAL
2 DRAINAGE PLAN SUBMITTAL
A completelv revised Chapter 2 is nrovided below. It is to be used instead of Chanter 2 of the 2016
Kins Countv Surface Water Desisn Manual for all pronosals in the citv of Kent.
This chapter details the drainage related submittal requirements for engineering design plans as part of a
permit application to the Kent Economic and Community Development Department. The intent of these
requirements is to present consistent formats for design plans and the technical support data required to
develop the plans. These conventions are necessary to review engineering designs for compliance with
Kent ordinances and regulations, and to ensure the intent of the plan is easily understood and implemented
in the field. Properly drafted design plans and supporting information also facilitate the construction,
operation, and maintenance of the proposed system long after its review and approval. When plans
comply with the formats and specifications contained herein, they facilitate review and approval with a
minimum of time-consuming corrections and resubmittals.
Note that this chapter primarily describes how to submit drainage plans for review-what must be
submitted, in what formats, at what times, and to what offices. The basic drainage requirements that these
plans must address are contained in Chapter 1, "Drainage Review and Requirements." The specific design
methods and criteria to be used are contained in Chapters 3, 4' 5, and 6.
Several key forms used in the plan review process are reproduced in Reference Section 8, "Plan Review
Forms and Worksheets" accompanying Chapter 5 of the Kent Construction Standards.
Chapter Organization
The information presented in this chapter is organized into four main sections as follows:
o Section 2.7,"Plans for Permits and Drainage Review"
o Section 2.2,"Plans Required with Initial Permit Application"
r Section 2.3,'oPlans Required for Drainage Review"
o Section 2.4,"Plans Required After Drainage Review"
These sections begin on odd pages so that tabs can be inserted by the user if desired for quicker reference.
City of Kent Surface Water Design Manual 2-l July 2022
SECTION 2.1 PLANS FOR PERMITS AND DRAINAGE REVIEW
2,1
2.1,1
2.1.2
PLANS FOR PERMITS AND DRAINAGE REVIEW
The Economic and Community Development Department is responsible for the review of all engineering
aspects of private development proposals. Drainage review is a primary concern of engineering design.
This section provides an overview ofthe types ofengineered drainage plans required for engineering
review at various permit review stages. Detailed requirements are presented in Sections 2.2 and2.3.
PLANS REQUIRED FOR INITIAL PERMIT SUBMITTAL
Most projects require some degree of drainage plans or analysis to be submitted with the initial permit
application (see Table 2.2.1.A). At the City of Kent, subdivisions and short plats are reviewed in two
stages: preliminary and final. Subdivisions and short plats will require a s6Preliminary Plat/Short Plat
Drainage Package" with the initial permit application. The package will provide general information on
the proposal, including location of environmentally sensitive areas, road alignments and right-of-way, sile
topography, building locations, land use information, and lot dimensions. It will be used to determine the
appropriate drainage conditions and requirements to be applied to the proposal during the drainage review
process.
Single-family residential building permits require only a site plan with the initial permit application.
Commercial permits require full engineering plans (see below). Other permits may have project specific
drainage requirements determined by the Economic and Community Development Department.
PLANS REQUIRED FOR DRAINAGE REVIEW
For drainage review purposes, engineering plans consist of the following:
1. Site improvement plans, which include all plans, profiles, details, notes, and specifications necessary
to construct road, drainage, and off-street parking improvements (see Section 2.3.1.2).
2. A technical information report (TIR), which contains all the technical information and analysis
necessary to develop the site improvement plan and CSWPP plan. (see Section 2.3.1.1).
3. A construction stormwater pollution prevention (CSWPP)p/an, which identifies the measures
and BMP's required to prevent the discharge of sediment-laden water and other pollutants
associated with construction/land disturbing activities. The CSWPP plan includes two
component plans: an erosion and sediment control (ESC) plan (see Section 2.3.1.3)' which
addresses prevention of sediment-laden discharges; and stormwater pollution prevention and
spill (SWPPS) plan (see Section 2.3.1.4)rwhich addresses prevention of other pollutant
discharges.
Note: A landscape management plan is also included if applicable (see Section 2.3.L5)'
Projects under Targeted Drainage Review usually require engineering plans, except that only certain
sections of the technical information report are required to be completed and the site improvement plan
may have a limited scope depending upon the characteristics of the proposed project. The scope of these
plahs should be confirmed during the project pre-application meeting with the Economic and
bommunity Development Department. For other permits, such as single-family residential permits, the
scope of the targeted engineering analysis is usually determined during Economic and Community
Development Department engineering review.
Projects without major drainage improvements may be approved to submit amodified site
improvement plan. Major drainage improvements usually include water quality or flow control facilities,
conveyance systems, bridges, and road right-of-way improvements. For projects requiring engineering
plans for road construction, a modified site improvement plan is not allowed. See Section 2.3.1 .2 for
further information.
City of Kent Surface Water Design Manual 1A July 2022
Plans Required for Simplified Drainage Review
Simplified drainage plans are a simplified form of site improvement and CSWPP plans (without a TIR
or a separate SWPPS plan), that may be prepared by a non-engineer from a set of pre-engineered
design details. Simplified drainage plans are only allowed for single family or agricultural projects in
Simplified Drainage Review but may be required for individual lots created by a subdivision project to
show how required flow control BMPs, ESC, and SWPPS measures will be applied to future lot
construction.
For single-family residential permits, the level and scope of drainage plan requirements are determined by
the Economic and Community Development Department during drainage review. Some projects
qualifoing for Simplified Drainage Review may also require Targeted Drainage Review.
Type of Drainage Review(s)Plans Required for
Building Permit Drainage
Review
Type of Permit or Project Plans Required with Initial
Land Use Permit
Application
Full or Targeted Drainage
Review(2)
. Preliminary Plat/Short Plat
Drainage Package(r)
r Engineering Plans(3)
SUBDIVISIONS AND SHORT
PLATS*
Preliminary Plat(r)
/Short Plat Drainage Package:
- Conceptual Drainage
Plan
- Level I Downstream
Analysis
- Survey/Topographic
Information
Engineering Plans(3)Engineering Plans(2)'(3)Full ol Targeted Drainage ReviewCOMMERCIAL
Simplifi ed Drainage Plans(a)Simplified Drainage Review OR
Simplified Drainage Review AND
Targeted Drainage Review(2) OR
. Simplified Drainage Plans(a)
r Engineering Plans(3)
Full or largeted Drainage
Review(2)
Engineering Plans(3)
SINGt,E-FAMILY
RESIDENTIAL BUILDING
PERMITS OR PERMITS FOR
AGRICULTURAL
PROJECTS
Conceptual Drainage Plan(r)
Engineering Plans(3)Full or Targeted Dlainage
Review(2)
OTHER PROJECTS OR
PERMITS
Proj ect-specifi c (contact the
Economic and Community
Development Department)
Notes:
(r) Submittal specifications for these plans are found in the application packages.
(2) Submittal specifications for Targeted Drainage Review are found in Section 2.3.2'
(3) Submittal specifications for engineering plans are detailed in Section 2.3'1.
(a) Specifications for submittal of Simplilied drainage plans are found in Appendix C of the King County Manual, Simplified
Dr ainage Req uire me nts (detached).
(5) Refer to Chapter l, Table 1.1.2.A for definitions of the different drainage review types.
* Short plats meeting the thresholds defining small sites may go through the Simplified drainage review process.
TABLE 2.1.2.8 DRAINAGE PLAN SI.IBIVII-ITALS
City of Kent Surface Water Design Manual 2-3 Iuly 2022
SECTION 2.2 PLANS REQUIRED WITH INITIAL PERMIT SUBMITTAL
2.2
2.2.1
PLANS REQUIRED WITH INITIAL PERMIT SUBMITTAL
This section describes the submittal requirements for initial permit applications at the City of Kent. The timing
for submittal of engineering plans will vary depending on permit type. For subdivisions and short plats, this
submittal usually follows the City's approval of the Preliminary Plat/Short Plat Drainage Package. For
commercial building permits, engineering plans must be submitted as part of the initial permit application. For
other permit types the drainage plan requirements are determined during the permit review process'
Note: If engineering plans are required to be submitted with the initial permit application, they must be
accompanied by the appropriate supporting documents (e.g., required applicationforms, an environmental
checklist, etc.). For more details, see Reference Section 8'
Design PIan Certification
All preliminary plat/short plat drainage packages and engineering plans must be stamped by a licensed civil
engineer registered in the State of Washington.
All land boundary surveys and legal descriptions used for preliminary and engineering plans must be stamped
by a professional land surveyor registered in the State of Washington. Topographic survey data and mapping
prepared specifically for a proposed project may be performed by the licensed civil engineer stamping the
engineering plans as allowed by the Washington State Board of Regishation for Professional Engineers and
Land Surveyors.
SUBDIVTSIONS, SHORT PLATS AND BINDING SITE PLANS
Applications for preliminary subdivisions and short plats must include a preliminary plat/short plat drainage
package consisting of the following:
1. A Conceptual Drainage Plan prepared, stamped, and signed by a licensed civil engineer registered in the
State of Washington. This plan must show the following:
The level of detail of the plan should correspond to the complexity of the project.
Plans submitted for review shall contain the following information at a minimum:
a) A briefnarrative describing existing and proposed s/e conditions, including inventoried or delineated
wetlands, streams, ponds, lakes, rivers, steep or unstable slopes, erosion hazatd areas and landslide
hazard areas. Include a description ofthe existing use ofthe site and the proposed use ofthe site aftet
development.
b) Two simple drainage plans are required for aConceptual Drainage Plan: an existing condition
drainage plan and post-development drainage plan.
c) Both drainage plans must be drawn to an engineering scale (i.e. 1" :20', or I " : 30', for example, not
1/8" : l'), and must contain a north arrow. NOTE: North arrows shall either point to the top of the
page or to the right side ofthe page only.
d) Include a vicinity map, which clearly shows the location of the development parcel with respect to
public streets and other parcels and developments.
e) Drainage plans shall include property lines, including line lengths (bearings of property lines are
preferred, but not required).
f) All public and private roads, driveway accesses and road easements, with dimensions. All manmade or
natural features (streams, rivers, drainage ditches, railroadtracks, hills, depressions, structures ofall
kinds, steep slopes, ponds, lakes, etc.) and the existing direction of surface water flows shown by
arrows pointing in the direction of flow.
g) Setback dimensions from all properfy lines and from sensitive areas such as wetlands, streams, steep or
unstable slopes, and Native Growth Protection Tracts.
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SECTION 2.2 PLANS REQUIRED WITH INITIAL PERMIT SUBMITTAL
h) Existing and proposed building and landscape locations.
D Differentiate between developed portions ofa parcel, and undeveloped / natural areas ofa parcel, and
areas set aside for future development.
j) Outside storage areas and types ofsurfaces for storage areas'
k) The post-development drainage plan shall include a proposed conveyance system layout, and the
location of discharge points onto and off ofthe property; the total amount of impervious surface
created (including rooftops); the approximate building and parking lot / storage yard footprints; and all
proposed stormwater treatment, and proposed locations for stormwater management Best Management
Practices (detention ponds, biofiltration swales, oil / water separators, etc.).
D A legal description for the property and the Assessor's Tax Lot Number for the property.
m) The name, address, and telephone number of the person preparing the Site Plan'
n) The location and type of existing and proposed flow control facilities, water quality facilities and
conveyance systems clearly marked individually as public or private to designate ownership and
maintenance responsibilities in accordance with the plat agreement.
)A Level 1 Downstream Analysis as required in Core Requirement #2 and outlined under "TIR
Section 3, Offsite Analysis." The Level I Downstream Analysis is required for all short plats except
those meeting the exemptions outlined in Section 1.2.2 or qualifoing for Simplified Drainage Review
for the entire project. This offsite analysis shall be submitted to assess potential offsite drainage
impacts associated with development of the project, and to help propose appropriate mitigation of
those impacts. A higher level of offsite analysis may be requested by the Economic and Community
Development Department prior to preliminary approval, or as a condition of engineering plan
submittal. The offsite analysis must be prepared, stamped, and signed by a licensed civil engineer
registered in the State of Washington.
3. Survey/topographic information. The submitted site plan and conceptual drainage plan shall
include:
a) Field topographic base map to accompany application (aerial topography allowed with city of
Kent permission)
b) Name and address of surveyor and surveyor's seal and signature
c) Notation for field or aerial survey
d) Datum and benchmark/location and basis of elevation
e) Location of all streams, lakes, wetlands, closed depressions, or Hazard Areas (include any
corresponding King County or Kent designation number, or identifu as undesignated)
f) Contour intervals per the following chart:
2 feet at less than I 5% slope
5 feet at 15% slope or more
Densities ofdeveloped area ofover 2 DU per acre
5 feetDensities of developed areaof 2 DU or less per acre
Contour IntervalsZoning Designation
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SECTION 2.2 PLANS REQUIRED WITH INITIAL PERMIT SUBMITTAL
2.2.2
2.2.3
2.2.4
COMMERCIAL SITE DEVELOPMENT
Applications for commercial permits require that engineering plans be submitted as part of the initial permit
application. Most commercial projects will go through Full Drainage Review and require complete
"ngineering
plans. Projects which may qualify for limited scope engineering design should request Targeted
nrainage Review during the Pre-Application meeting with the Economic and Community Development
Department.
SINGLE-FAMILY RESIDENTIAL
Applications for single-family residential permits require a non-engineered site plan to be submitted. Refer to
the minimum requirements listed at the beginning of Section 2.3 (see detached Appendix C, Section C.5.1),
OTHER PERMITS
Other permit applications will require project-specific information. Initial submittal requirements can be
obtained by contacting the Economic and Community Development Department.
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SECTION 2.3 PLANS REQUIRED FOR DRAINAGE REVIEW
2.3
2.3.1
PLANS REQUIRED FOR DRAINAGE REVIEW
This section presents the specifications and contents required of plans to facilitate drainage review. Most
projects subject to Full Drainage Review will require engineering plans that include a technical
information report (TIR), site improvement plans, and a "construction stormwater pollution prevention
(CSWPP) plan," which includes an "erosion and sediment control (ESC) plan" and a "stormwater pollution
prevention and spill (SWPPS) plan." In addition, a landscape management plan may also be required to
comply with Core Requirement #8 (see Section 1.2.8). For more information on the types of projects
subject to Full Drainage Review, see Section 1 .l .2.3 .
Small projects with specific drainage concerns that are subject to Targeted Drainage Review also require
engineering plans that include the same elements, except that the TIR may be of limited scope. The site
improvement plans and ESC plans may also be of limited scope, but must meet all applicable
specifications. For more information on the types of projects subject to Targeted Drainage Review, see
Section 1.1.2.2.
Projects that qualifi for Simplified Drainage Review may be required to submit simplified drainage
plans. These are simplified drainage and erosion control plans that can be prepared by a non-engineer from
a set of pre-engineered design details, and do not require a TIR. The Simplified Drainage Requirements
booklet available at the King County Department of Permitting and Environmental Review and appended
to the King County Manual (detached Appendix C, Section C.5.1) contains the specifications for
simplified drainage plans and details on the Simplified Drainage Review process.
Note; Projects in Simplified Drainage Review may be required to submit engineering plans if they are
also subject to Targeted Drainage Review as determined in Section 1.1.2.2 and Appendix C of the King
County Mqnual. Also, short plats in Simplified Drainage Review will be required to submit engineering
plans if roadway construction is a condition ofpreliminary approval.
Design Plan Certification
All preliminary plat/short plat drainage packages and engineering plans must be stamped by a licensed
civil engineer registered in the State of Washington.
All land boundary surveys and legal descriptions used for preliminary and engineering plans must be
stamped by a piofessional land surveyor registered in the State of Washington. Topographic survey data
and mapping piepared specifically for a proposed project may be performed by the licensed civil engineer
stamping the englneering plans as allowed by the Washington State Board of Registration for Professional
Engineers and Land Surveyors.
ENGTNEERTNG PLAN SPECIFICATIONS
For drainage review purposes, engineering plans must consist of:
L A technical information report (TIR) as detailed in Section 2'3. I . I , AND
2. Site improvement plans as detailed in Section, 2.3.1.2 AND
3. A CSWPP plan, which includes an ESC plan as detailed in Section 2.3.1.3 and a SWPPS plan as
detailed in Section 2.3.1.4.
Also, if applicable per Section 1.2.8,a landscape management plan, as detailed in Section 2.3.1.5, must
be included.
Projects in Targeted Drainage Review require a limited scope TIR with site improvement plans and a
CSWPP plan, as detailed in Section 2.3.2. The Economic and Community Development Department may
allow a modified site improvement plan for some projects in Targeted Drainage Review (see Section
2.3.2) or where major improvements (e.g., detention facilities, conveyance systems, bridges, road right-of-
way improvements, etc.) are not proposed.
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SECTION 2,3 PLANS REQUIRED FOR DRAINAGE REVIEW
2.3.1.1
tr
TECHNICAL INFORMATION REPORT (TIR)
The full TIR should be a comprehensive supplemental report containing all technical information and
analysis necessary to develop the site improvement plan. This report should contain all calculations,
conieptual design analysis, reports, and studies required and used to construct a complete site improvement
plan based on sound engineering practices and careful geotechnical and hydrological design' The TIR must
be stamped and dated by a licensed civil engineer registered in the State of Washington.
The TIR shall contain the following ten sections, preceded by a table of contents:
1. Project Overview
2. Conditions and Requirements Summary
3. Offsite Analysis
4. Flow Control and Water Quality Facility Analysis and Design
5. Conveyance System Analysis and Design
6. Special Reports and Studies
7. Other Permits
8. CSWPP Analysis and Design
9. Bond Quantities, Facility Summaries, and Declaration of Covenant
10. Operations and Maintenance Manual
Every TIR must contain each of these sections; however, if a section does not apply, the applicant may
simply mark "N/A" with a brief explanation shall be provided. This standardized format allows a quicker,
more efficient review of information required to supplement the site improvement plan.
The table ofcontents should include a list ofthe ten section headings and their respective page numbers, a
list of tables with page numbers, and a list of numbered references, attachments, and appendices.
When the TIR package requires revisions, the revisions must be submitted in a complete TIR package.
TIR SECTION 1
PROJECT OVERVIEW
The project overview must provide a general description ofthe proposal, predeveloped and developed
conditions of the si/e, site areaand size of the improvements, and the disposition of stormwater runoff
before and after development. The overview shall identify and discuss difficult si/e parameters, the natural
drainage system, and drainage to and from adjacent propelfi, including bypass flows'
The following figures are required:
Figure 1. TIR Worksheet
Include a copy of the TIR Worksheet (see Reference Section 8-A).
Figure 2. Site Location
Provide a map that shows the general location of the site. Identify all roads that border Ihe site and all
significant geographic featureJ including all streams, lakes, wetlands, closed depressions, andHazard
Areas.
Figure 3. Drainage Basins, Subbasins, and Site Characteristics
This figure shall display:
l. Acreage of all subbasins
2. All site characteristics
3. Existing discharge points to and from the site
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SECTION 2.3 PLANS REQUIRED FOR DRAINAGE REVIEW
4. Routes of existing, construction, and future flows at all discharge points and downstream hydraulic
structures
5. A minimum City of Kent Storm Sewer Facility Map, l": 300' as a base for the figure (available at
the Kent Permit Center)
6. The length oftravel (also cite) from the farthest upstream end of a proposed storm system in the
development to any proposed flow control facility
Figure 4. Soils
Show the soils within the following areas:
l. The project site
2. The area draining to the sile
3. The drainage system downstream of the si/e for the distance of the downstream analysis
(see Section L2.2)
Copies of King County Soil Survey maps may be used; however, if the maps do not accurately represent
the soils for a proposed project (including offsite areas of concem), it is the design engineer's
responsibility to ensure that the actual soil types are properly mapped. Soil classification symbols that
conform to the SCS So il Survey for King County shall be used, and the equivalent KCRTS soil type (till,
outwash, or wetlands) shall be indicated (see Table 3.2.2.8).
All plats and short plats (creating lots less than22,00} squ,Ire feet) must evaluate onsite soils for suitability
of the full infiltration and other low impact flow control BMPs as specified in Core Requirement 9. This
soils report, as well as geotechnical investigations necessary for proposed infiltration facilities, shall be
referenced in the TIR Overview and submitted under Special Reports and Studies, TIR Section VI. A
figure in the required geotechnical report that meets the above requirements may be referenced to satisff l,
2, and3 above.
Projects located in outwash soils may need to provide a low-permeability liner or a treatment liner
for water quality facilities and upsheam conveyance ditches, consistent with the specifications for such
liners in Section 6.2.4.
tr TIR SECTION 2
coi\[DTTIoNS Ai\[D REQUIRBMENTS SUMMARY
The intent of this section is to ensure all preliminary approval conditions and applicable requirements
pertaining to sr'k engineering issues have been addressed in the site improvement plan. All conditions and
requirements for the proposed project should be included.
In addition to the core requirements of this manual, adopted basin plans and other plans as listed in Special
Requirement #1 should be reviewed and applicable requirements noted. Sensitive area requirements32,
conditions of plat approval, and conditions associated with development requirements (e.g., conditional
use permits, rezones, variances and adjustments, SEPA mitigations, etc.) should also be included.
tr TIRSECTION3
OFFSITE ANALYSIS
All projects in engineering review shall complete, at aminimum, an Offsite Analysis, except for projects
meeting the exemptions outlined in Section 1.2.2. The Offsite Analysis is usually completed as part of the
12 Any specific regulations related to floodplains, streams, lakes, wetlands, closed depressions, or geologic hazard areas.
City of Kent Surface Water Design Manual 2-9 Iulye 2022
SECTION 2.3 PLANS REOUIRED FOR DRAINAGE REVIEW
initial permit application and review process
have been altered since the initial submittal,
and is to be included
a new offsite analysis
in the TIR. Note If offiite conditions
may be required.
The primary component of the offsite analysis is the downstream analysis describedin detail below.
Upstieam *"as ai" included in this component to the extent they are expected to be affected by backwater
efiects from the proposed project. Other components ofthe offsite analysis could include, butare not
limited to, evaluation of impaits to fish habitat, groundwater levels, groundwater quality' or other
environmental features expicted to be significantly affected by the proposed project due to its size or
proximity to such features.
Levels of Analysis
The offsite analysis report requirements vary depending on the specific sile and downstream conditions.
Each project submittal shall include at least a Level I downstream analysis. Upon review of the Level I
analysis, the Economic and Community Development Department may require a Level 2 or Level 3
analysis. If conditions warrant, additional, more detailed analysis may be required. Note: Potential
impacts upstream of the proposal shall also be evaluated.
Level 1 Analysis
The Level I analysis is a qualitative survey of each downstream system leaving a sr/e. This analysis is
required for all proposed projects and shall be submitted with the initial permit application.
Depending on the findingi ofttr" Level I analysis, a Level 2 or 3 analysis may need to be completed
ot udditional informatiorimay be required. If further analysis is required, the applicant may schedule
a meeting with city review staff.
Level2or3Analysis
If drainage problems are identified in the Level I analysis, a Level 2 (rough quantitative)-analysis or a
Level 3 fmore precise quantitative) analysis may be required to further evaluate proposed mitigation
for the pioblem. Ciff rbview staff will determine whether a Level 2 or 3 analysis is required based on
the evidence of existing or potential problems identified in the Level I analysis and on the proposed
design of onsite drainage facilities. The Level 3 analysis is required when results need to be as
accu-rate as possible: f6r example, if the site is flat; if the system is affected by downstream controls;
if minor changes in the drainage system could flood roads or buildings; or if the proposed project will
contribute moie than 15 percent of the total peak flow to the drainage problem location' The Level 2
or 3 analysis may not be required if city review staff determines from the Level I analysis that
adequate mitigation can be developed'
Additional Analysis
Additional, more detailed hydrologic analysis may be required if city review staff determines that the
downstream analysis has not beeniufficient to accurately determine the impacts of a proposed project on
an existing or poiential drainage problem. This more detailed analysis may include a point of compliance
analysis as detailed in Section 3.3.6.
Scope of Analysis
Regardless of the level of downstream analysis required, the applicant shall define and map the study area
(Ta"sk l), review resources (Task 2), inspect the study area (Task 3), describe the drainage system and
problems (Task 4), and propose mitigation measures (Task 5) as described below.
Task 1. Study Area Definition and Maps
For the purposes of Task 2 below, the study area shall extend downstream one mile (minimum flow
path distince) from the proposed project discharge location and shall extend upstream as necessary to
Lnro-pu.t the offsite drainage area tributary to the propos ed proiect site. For the purposes of Tasks
3, 4, and 5, the study area shill extend downstream to a point on the drainage system where the
pioposed prsisgl srle constitutes a minimum of l5 percent of the total tributary drainage area, but not
i.5 thun ins-quarter mile (minimum flow path distance). The study area shall also extend upstream
ofthe project rir" u dirtun"e sufficient to preclude any back water effects from the proposed project.
The offsite analysis shall include: (l) a site map showing property lines, and (2) the best available
City of Kent Surface Water Design Manual 2-10 |ulye2022
SECTION 2,3 PLANS REQUIRED FOR DRAINAGE REVIEW
topographical map (e.g., at a minimum use the 1" : 300' City of Kent Storm Sewer Facility Maps
available at the Kent Permit Center) with the study area boundaries, sile boundaries, downstream flow
path, and potential/existing problems (Task 4) shown. Other maps, diagrams, and photographs such
as aerial photographs may be helpful in describing the study area.
Task 2. Resource Review
To assist the design engineer in preparing an offsite analysis, the resources described below shall be
reviewed as applicable for existing/potential problems in the study area (upstream and one mile
downstream of the project s/e). Unless otherwise indicated, ask city of Kent review staff about the
availability and applicability of these resources:
r Adopted basin plans
r Finalized drainage studies
. Basin Reconnaissance Summary Reports and I "-400' scale problem summary maps (available at
King County Department of Development and Environmental Services, King County Department
of Natural Resources and the library)
r Floodplain/floodway (FEMA) maps
r Other offsite analysis reports in the same subbasin, if available
. Critical and Sensitive Area maps
. U.S. Department of Agriculture, King County Soils Survey (available at King County Department
of Development and Environmental Services and the library)
r Wetlands Inventory maps (City-wide map included with this manual, detailed maps available at
the Economic and Community Development Department)
. City of Kent Erosion Hazard Area Map and Landslide Hazard Area Maps (City-wide map
included with this manual, detailed maps available at the Economic and Community Development
Department)
o Washington State Department of Ecology's latest published Clean Water Act Section 303d list of
polluted waters posted at the following website:
http ://www.ecy.wa. gov/proglams/wq/3 03 d/currentassessmt.html.
. King County designated water quality problems listed and documented in the latest version of
Reference Section l0 posted on King County's Surface Water Design Manual website. See also
"stormwater" on the iMap website: http://www.kingcounty.gov/operations/gis/Maps/iMAP.aspx.
Potential/existing problems identified in the above documents shall be documented in the Drainage
System Table (see Reference Section 8-B) and described in the text of the Level I Downstream
Analysis Report. If a document is not available for the site, note in the report that the information
was not available as of a particular date. If necessary, additional resources are available from Kent,
King County, the Washington State Department of Fisheries and Wildlife (WDFW), the State
Department of Ecology (Ecology), and the United States Army Corps of Engineers (Corps).
Task 3. Field Inspection
The design engineer shall physically inspect the existing onsite and offsite drainage systems of the
study area for each discharge location. Specifically, he/she shall investigate any evidence ofthe
following existing or potential problems and drainage features:
Level l Inspection:
1. Investigate any problems reported or observed during the resource review'
Locate all existing/potential constrictions or lack of capacity in the existing drainage system.
Identify all existing/potential drainage problems as defined in Section 1.2.2.1'
IdentiS existing/potential overtopping, scouring, bank sloughing, or sedimentation'
Identifu significant destruction of aquatic habitat or organisms (e.g., severe siltation, bank erosion,
2.
4.
5.
City of Kent Surface Water Design Manual 2-l I Iulye 2022
SECTION 2,3 PLANS REQUIRED FOR DRAINAGE REVIEW
or incision in a stream).
6. Collect qualitative data on features such as land use, impervious surfaces, topography, and soil
types.
7. Collect information on pipe sizes, channel characteristics, drainage structures, and relevant critical
areas (e.g., wetlands, streams, steep slopes).
8. Verify tributary basins delineated in Task l.
g. Contact neighboring property owners or residents in the area about past or existing drainage
problems, and describe these in the report (optional).
10. Note the date and weather conditions at the time of the inspection'
Level2 or 3 Inspection:
l. Perform a Level I Inspection.
2. Document existing site conditions (approved drainage systems or pre-1979 aerial photographs) as
defined in Core Requirement #3.
3. Collect quantitative field data. For Level 2, collect non-survey field data using hand tapes, hand
reel, and rods; for Level 3, collect field survey profile and cross-section topographic data prepared
by a licensed land surveyor.
Task 4. Drainage System Description and Problem Descriptions
Each drainage system component and problem shall be addressed in the offsite analysis report in three
places: on Jmap (Task 1), in the narrative (Task 4),andinthe Offsite Analysis Drainage System
Table (see Reference Section 8-B).
Drainage System Descriptions: The following information about drainage system components such
as pipeq cuiverts, bridges, outfalls, ponds, tanks, and vaults shall be included in the report:
L Location (corresponding map label and distance downstream/upstream from site discharge)
2. Physical description (type, size, length, slope, vegetation, and land cover)
3. Problems including copies of any relevant drainage complaints
4. Field observations
Problem Descriptions: All existing or potential problems (e.g., ponding water, high/low flows,
siltation, erosion, listed water bodies, etc.) identified in the resource review or field inspection shall
be described in the offsite analysis. These descriptions will help in determining if such problems are
one ofthree defined problem types that require special attention per Core Requirement #2 (see Section
1.2.2.1). Special attention may include more analysis, additional flow control, or other onsite or
offsite mitigation measures as specified by the problem-specific mitigation requirements set forth in
Section 1.2.2.2.
The following information shall be provided for each existing or potential drainage problem:
l. Description of the problem (ponding water, high or low flows, siltation, erosion, slides, etc.).
2. Magnitude of or damage caused by the problem (siltation of ponds, dried-up ornamental ponds,
road inundation, flooded property, flooded building, flooded septic system, significant destruction
of aquatic habitat or organisms).
3. General frequency and duration of problem (dates and times the problem occurred, if available)'
4. Retum frequency of storm or flow (cfs) of the water when the problem occurs (optional for Level
1 and required for Levels 2 and 3). Note: A Level 2 or 3 analysis may be required to accurately
identifi the returnfrequency of a particular downstream problem; see Section 3.3.3.
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SECTION 2,3 PLANS REQUIRED FOR DRAINAGE REVIEW
5. Water surface elevation when the problem occurs (e'g., elevation of building foundation, crest of
roadway, elevation of septic drainfields, or wetland/stream high water mark).
6. Names and concems of involved parties (optional for all levels of analysis).
7. Current mitigation of the drainage problem.
8. Possible cause of the drainage problem'
Whether the proposed project is likely to aggravate (increase the frequency or severity of) the
existing problem or create a new one based on the above information. For example, an existing
erosion problem should not be aggravated if Level 2 flow control is already required in the region
for the design of onsite flow control facilities. Conversely, a downstream flooding problem
inundating a home every 2 to 5 years will likely be aggravated if only Level 1 flow control is being
applied in the region. See Section 1.2.3.1 for more details on the effectiveness of flow control
standards in addressing downstream problems.
The following information shall be provided for each existing or potential water quality problem:
l. Description of the problem as documented by the State or City in the problem's listing' This
should include the pollutant or pollutants of concem, the nature or category of the listing, and
any other background information provided in the listing.
2. Flow path distance downstream of the project site and percentage of area draining to the
problem thatthe proiecl si/e occupies.
3. Possible or probable cause of the water quality problem.
4. Any current mitigation of the water quality problem.
Task 5. Mitigation of Existing or Potential Problems
For any existing or potential offsite drainage problem determined to be one of the three defined
problem types in Section 1.2.2.1, the design engineer must demonstrate that the proposed project
neither aggravates (if existing) nor creates the problem as specified in the drainage problem-specific
mitigatio-n-requirements set forth in Section 1.2.2.2. The engineer must review each relevant drainage
"o-pluint found and include a narrative explaining how each complaint problems is addressed or
mitigated. Actual copies of the relevant complaints must be included in the Analysis. To meet these
requlrements, the proposed project may need to provide additional onsite flow control as specified in
tabte t.Z.:.4 (see also Section3.3.5), or other onsite or offsite mitigation measures as described in
Section 3.3.5.
For any existing or potential water quatity problem determined to be one of the seven defined
water quality problem types in Section l.2.2.l,the design engineer must document how the
applicable water quality problem-specific mitigation requirement in Section 1.2.2.3 will be met.
U TIR SECTION 4
FLOW CONTROL, LOW IMPACT DEVELOPMENT (LID) AND WATER QUALITY
F'ACILITY ANALYSIS AND DESIGN
Existing Site Hydrology (Part A)
This section of the TIR shall include a discussion of assumptions and site parameters used in analyzing the
existing s/e hydrology. The definition of "existing site conditions" presented in Section I ' I shall be
applied for this section.
The acreage, soil types, and land covers used to determine existing flow characteristics, along with basin
-upr, grufhics, and exhibits for each subbasin affected by the development, should be included.
The following information must be provided on a topographical map:
l. delineation and acreage ofareas contributing runoffto the site
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SECTION 2.3 PLANS REQUI RED FOR DRAINAGE REVIEW
2. flow control facility and BMP location(s)
3. outfall location and description
4. overflow route(s)
The scale of the map and the contour intervals must be sufficient to determine the basin and subbasin
boundaries accurateiy. The direction of flow, the acreage of areas contributing drainage, and the limits of
development should all be indicated on the map.
Each subbasin contained within or flowing through the site shall be individually labeled and KCRTS
parameters for the approved stormwater model referenced to that subbasin.
All natural streams and drainage features, including wetlands and depressions, must be shown. Rivers,
closed depressions, streams, lJkes, and wetlands must have the 100-year floodplain (and floodway where
applicable) delineated as required in Special Requirement #2 (see Section 1.3.2) and by the critical areas
requirements in KCC I 1.06.
Developed Site Hydrology (Part B)
This section shall provide narrative, mathematical, and graphical presentations of parameters selected and
values used for the developed sile conditions, including acreage, soil types, land covers, roadway layouts,
and all constructed drainage facilities and any required flow control BMPs.
Developed subbasin areas and flows shall be clearly depicted on a map and cross-referenced to computer
printouis or calculation sheets. Relevant portions ofthe calculations shall be highlighted and tabulated in a
listing of all developed subbasin flows.
All maps, exhibits, graphics, and references used to determine developed sile hydrology must.be included,
maintaining the sarne subbasin labeling as used for the existing sile hydrology whenever possible. If the
boundaries ofthe subbasin have been modified under the developed condition, the labeling should be
modified accordingly (e.g., Subbasin "Am" is a modified version of existing Subbasin "A").
Performance Standards (Part C)
The design engineer shall include brief discussions of the following:
o The applicable area-specific flow control facility standard determined from the Flow Control
Applications Map.
o Modifications to the standard to address onsite or offsite drainage conditions.
. Applicable flow control BMP requirements determined from Sectionl.2.3.3 and Core Requirement
9.
. The applicable conveyance system capacity standards per Section 1.2.4'
. The applicable area-specific water quality treatment menu determined from the Water Quality
Applications Map per Section 1.2.8.1, and any applicable special requirements for source control
or oil control determined from Sections l'3.4 and 1'3.5'
Flow Control System (Part D)
This section requires:
An illustrative sketch of the flow control facility (or facilities), required flow control BMPs, and
appurtenances. The facility sketch (or sketches) must show basic measurements necessary to
calculate the storage volumes available from zero to the maximum head, all orifice/restrictor sizes
and head relationships, control structure/restrictor orientation to the facility, and facility orientation
on the si/e. The flow control BMP sketch (or sketches) must show basic measurements and
dimensions, orientation on the sr7e, flowpath lengths, etc.
The applicant shall include all supporting documentation such as computer printouts,
calculations, equations, references, storage/volume tables, graphs, soils data, geotechnical reports
and any other aides necessary to clearly show results and methodology used to determine the
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SECTION 2.3 PLANS REQUIRED FOR DRAINAGE REVIEW
storage facility volumes and flow control BMP applications.
Facility documentation files, flow duration comparison files, peaks files, retum frequency or
duration curves, etc., developed with the approved model shall be included to verifu the facility
meets the performance standards indicated in Part C.
The volumetric safety factor used in the design shall be clearly identified, as well as the
reasoning used by the design engineer in selecting the safety factor for this project.
If flow control BMP credits are used as allowed in Core Requirement 9, documentation must be
provided, explaining how the credits will be used and how the criteria for use of credits will be
met.
If the flow control system is an infiltration facility, the soils data, groundwater mounding analysis,
and other calculations used to determine the design infiltration rate shall be provided.
Flow control BMP infeasibility discussion and supporting documentation shall also be included in
Part D.
Water Quality System @art E)
This section provides an illustrative sketch of the proposed water quality facility (or facilities), source
controls, oil iontrols, and appurtenances. This sketch (or sketches) ofthe facility, source controls, and oil
controls must show basic measurements and dimensions, orientation on the site, location of inflow, bypass,
and discharge systems, etc.
The applicant shall also include all supporting documentation such as computer printouts, calculations,
equations, references, and graphs necessary to show the facility was designed and sized in accordance with the
specifications and requirements in Chapter 6. If the water quality credit option is used as allowed in
Section 6.1.3, documentation must be provided, identiffing the actions that will be taken to acquire the
requisite credits.
O TIR SECTION 5
COIWEYAIICE SYSTEM ANALYSIS AND DESIGN
This section shall present a detailed analysis of any existing conveyance systems, and the analysis and
design of the proposed stormwater collection and conveyance system for the development. This section
also-includes anyanalysis required for the design of bridges to convey flows and pass sediments and debris
per Section 4.4.3. Analysis information should be presented in a clear, concise manner that can be easily
iollowed, checked, and verified. All pipes, culverts, catch basins, channels, swales, and other stormwater
conveyance appurtenances must be clearly labeled and correspond directly to the engineering plans.
The minimum information included shall be pipe flow tables, flow profile computation tables,
nomographs, charts, graphs, detail drawings, and other tabular or graphic aides used to design and confirm
performance of the conveyance system'
Verification of capacity and performance must be provided for each element of the conveyance system.
The analysis must show design velocities and flows for all drainage facilities within the development, as
well as those offsite, which are affected by the development. If the final design results are on a computer
printout, a separate summary tabulation of conveyance system performance shall also be provided'
Q TIR SECTION 6
SPECIAL REPORTS AND STTJDMS
Some site characteristics, such as creeks, closed depressions, lakes, wetlands, or Hazard Areas, pose
unique road and drainage design problems that are particularly sensitive to stormwater runoff. As a result,
Keni may require the pieparation of special reports and studies to address further the site characteristics,
the potential
-for
impaits issociated with the development, and the measures that would be implemented to
mitigate impacts. Special reports shall be prepared by people with expertise in the particular area of
City of Kent Surface Water Design Manual 2-15 lulye 2022
SECTION 2.3 PLANS REQUIRED FOR DRAINAGE REVIEW
analysis. Topics of special reports may include any of the following:
r Geotechnical/soils (soils documentation supporting flow control BMP design, infiltration rate
determination and infeasibility conclusions may also be located in TIR Section 6)
. Critical areas analysis and delineation Floodplain delineation in accordance with Section 1.3.2
. Flood protection facility conformance in accordance with Section 1.3.3
. Slopeprotectior/stability
. Groundwater, including groundwater mounding analyses required for infiltration design
r Fluvialgeomorphology
r Erosion and deposition
r Anadromous fisheries impacts
o Structural design
. Geology/Geologic Hazard Areas
. Hydrology
o Water quality
r Structural fill
tr TIRSECTIONT
OTIIERPERMITS
Construction of road and drainage facilities may require additional permits from other agencies for some
projects. These additional permits may contain more restrictive drainage plan requirements. This section
bf the fIn should provide the titles of any other permits, the agencies requiring the other permits, and the
permit requirements that affect the drainage plan. Examples of other permits are listed in Section 1 . I .3.
tr TIR SECTION 8
ESC ANALYSIS AND DESIGN
This section of the TIR should include the analysis and design information used to prepare the required
construction stormwater pollution prevention (CSWPP) plan. This information should be
presented in two parts associated with the CSWPP plan's two component plans, the erosion sediment
control (ESC) plan (Part A) and the stormwater pollution prevention and spill control (SWPPS) plan
(Part B). See Sections 2.3.1.3 and2.3.1.4 for plan specifications and contents. This CSWPP plan is
intended to be equivalent to and may be more stringent than that required for the NPDES Stormwater
Construction Permit issued by Ecology.
ESC Plan Analysis and Design (Part A)
This section must include all hydrologic and hydraulic information used to analyze and design the
erosion and sediment control measures, including final site stabilization measures. The TIR shall
explain how proposed ESC measures comply with the Erosion and Sediment Control Standards in
detached Appendix D and show compliance with the implementation requirements of Core Requirement
#5, Section 1.2.5.
Part A must include the following:
1. Provide sufficient information to justify the overall ESC plan and the choice of individual erosion
control measures. At a minimum, there shall be a discussion of each measure specified in Section
City of Kent Surface Water Design Manual 2-16 Iulye2022
SECTION 2.3 PLANS REQUIRED FOR DRAINAGE REVIEW
1.2.5 and its applicability to the proposed project.
Include all hydrologic and hydraulic information used to analyze and size the ESC facilities shown
in the engineering plans. Describe the methodology, and aIlach any graphics or sketches used to size
the facilities.
Identify areas with a particularly high susceptibility to erosion because of slopes or soils as well as
areas to be protected for existing and proposed flow control BMPs. Discuss any special measures
taken to protect these areas as well as any special measures proposed to protect water resources on or
near the site.
Identify any ESC recommendations in any of the special reports prepared for the project. In the
projecigeoiechnical report supporting flow control BMP design, provide recommendations to address
mitigation of flow control BMP areas impacted by erosion andlor sedimentation during construction. If
these special reports' recommendations are not included in the ESC plan, provide justification.
If proposing exceptions or modifications to the standards detailed inthe Erosion and Sediment
Contiol Stindardi (Appendix D of the King County Manual), clearly present the rationale. If
proposing techniques or products different from those detailed in the ESC Standards, provide
supporting documentation so the City can determine if the proposed alternatives provide similar
protection.
SWPPS Plan Design (Part B)
The stormwater pollution prevention and spill control plan must identifu all activities that could
contribute pollutants to surface and storm water during construction. This section of the TIR must
provide sufficient information to justiff the selection of specific stormwater pollution prevention
(SWPPS) BMPs proposed to be applied to the pollution-generating activities that will occur with
construction of the proposed project. BMPs applicable to such activities are found inthe Construction
Stormwater Pollution Prevention and Spill Control (CSWPP) Standards (detached Appendix D) and the
King County Stormwater Pollution Prevention Manual (viewable at
http://www.kingcountv.gov/environment/water-and-land/stormwater/documents/pollution-prevention-
rnanual.aspx ).
At a minimum, there shatl be a discussion of each anticipated pollution-generating activity and the
pollution prevention BMPs selected to address ir. If there arc any calculations required for the
selected BMP, include those in the discussion. If an alternative BMP or major modification to one
of the City's standard BMPs will be used, a written request must be submitted for review and
approval, detailing how the alternative will work. An "Altemative BMP Request Form" is available in
the Stormwater Pollution Prevention Manual.
Updates or revisions to the SWPPS plan may be requested by the City of Kent at any time during
project construction ifthe City determines that pollutants generated on the construction site have the
potential to contaminate surface, storm, or ground water'
The SWPPS plan shall also discuss the receiving waters, especially if the receiving water body is listed
on the 303d list. Information must be provided that shows the plan meets TMDL requirements.
Discuss the 303(d) listed pollutant generated or used onsite and any special handling requirements or
BMPs.
tr TIR SECTION 9
BOI\D QUAI{TITMS, FACILITY SUMMARIES, AND DECLARATION OF COVENAI\T
Bond Quantities Worksheet
Each plan submittal requires a construction quantity summary to establish appropriate bond amounts.
Usingthe Bond Quantities l4/orlrsheet (Contact the City of Kent Economic and Community Development
2.
J
4
5
City of Kent Surlace Water Design Manual 2-17 Iulye 2022
SECTION 2.3 PLANS REQUIRED FOR DRAINAGE REVIEW
Department), the design engineer shall separate existing right-of-way and erosion control quantities from
other onsite improvements.In addition, the engineer shall total the amounts based on the unit prices listed
on the form.
Drainage facilities for single-family residential building permits shall be constructed and approved prior to
granting the certificate of occupancy.
Flow Control and Water Quality Facility Summary Sheet and Sketch
Following approval of the plans, a Flow Control and Water Quality Facility Summary Sheet and Sketch (see
Reference 8-C) shall be submitted along with an 8t /2" x I I " plan sketch for each facility proposed for
construction. The plan shall show a north arrow, the tract, the facility access road, the extent of the facility,
and the control structure location. The approximate street address shall be noted. At project completion, the
Summary Sheet and Sketch shall be updated in the Final Corrected TIR to reflect the completed project
(see Section 2.4.2).
Declaration of Covenant for Privately Maintained FIow Control and WQ Facilities
Any declaration of covenant and grant of easement required for proposed flow control and water quality
facilities per Section 1.2.6 must be included here for review and approval before recording. The
necessary covenant exhibits, and maintenance instructions associated with the facility type shall be
included with the declaration of covenant. After approval by city review staff, the declaration of
covenant and grant of easement must be signed and recorded with the City of Kent before any permit is
approved. A copy ofthe recorded document shall be included in the Final Corected TIR (see Section
2.4.2).
Declaration of Covenant for Privately Maintained Flow Control BMPs
Any declarations of covenant and grant of easement required for proposed flow control BMPs per Core
Requirementg must be included here for review and approval before recording. The necessary
covenant exhibits, and maintenance instructions associated with the flow control BMP type (see
Reference 5), shall be included with the declaration of covenant. After approval by city review staff,
all such documents must be signed and recorded with the City of Kent before any permit is approved. A
copy of the recorded document shall be included in the Final Corrected TIR (see Section 2.4.2) ot
otherwise provided to the County if no TIR was required.
tr TIR SECTION 10
OPERATIONS AND MAINTENANCE MAIIUAL
For each flow control and water quality facility and/or BMP that is to be privately maintained, and for
those that have special nonstandard features, the design engineer shall prepare an operations and
maintenance manual. The manual should be simply written and should contain a brief description of the
facility or BMP, what it does, and how it works. In addition, the manual shall include a copy of the
Stormtwqter Facility Maintenance Guide (seeAppendix A) and provide an outline of maintenance tasks
and the recommended frequency each task should be performed. This is especially important for flow
control BMPs and water quality facilities where proper maintenance is critical to facility performance'
For this reason, most of the flow control facility designs in Chapter 5 and the water quality facility
designs in Chapter 6 include "maintenance considerations" important to the performance of each
facility.
2.3.1.2 SITE IMPROVEMENT PLAN
Site improvement plans shall portray design concepts in a clear and concise manner. The plans must
present all the infoimatior n"iesrury for persons trained in engineering to review the plans, as well as
ihor" p..rons skilled in construction work to build the project according to the design engineer's intent.
City of Kent Surface Water Design Manual 2-18 Julye 2022
SECTION 2.3 PLANS REQUIRED FOR DRAINAGE REVIEW
Supporting documentation for the site improvement plans must also be presented in an orderly and concise
format that can be systematically reviewed and understood by others.
The vertical datum on which all engineering plans, plats, binding site plans, and short plats are to be
based must be North American Vertical Datum of 1988, (NAVD88) and the datum must be tied to at least
one City of Kent Survey Control Network benchmark. The benchmark(s) shall be shown or referenced on
the plans. Datum correlations can be found in Table 4.4.2.8 of the King County Manual.
Horizontal control for all plats, binding site plans, and short plats shall reference the North American
Datum of 1983/lggladjustment (NADS3/91) as the coordinate base and basis of bearings. All horizontal
control for these projects must be in U.S. Survey Foot and referenced to a minimum of two City of Kent
Survey Horizontil Control monuments. If two horizontal control monuments do not exist within one mile
of the project, an assumed or altemate coordinate base and basis of bearings may be used. Horizontal
controi monument and benchmark information is available from the Kent Public Works Department.
The site improvement plans consist of all the plans, profiles, details, notes, and specifications. necessary to
construct road, drainage structure, and off-street parking improvements. Site improvement plans include
the following:
r A base map (described in Table 2.3.1.A), and
r Site plan and profiles (see Section 2.3'1.1).
Note; Site improvement plans must also include grading plans if onsite grading extends beyond the
roadway.
Modified Site Improvement Plan
City review staff may allow a modified site improvement plan for some projects in Targeted Drainage
Review (see Section 2.3.2) or where major improvements (e.g., detention facilities, conveyance systems,
bridges, road right-of-way improvements, etc.) are not proposed. The modified site improvement plan
must:
1. Be drawn on a I I " x 17" or larger sheet,
2. Accurately locate structure(s) and access, showing observance of the setback requirements given in
this manual, or other applicable documents, and
3. Provide enough information (datum, topography, details, notes, etc.) to address issues as determined
by city review staff.
tr GENERALPLANFORMAT
Site improvement plans should use APILA Standard Map Symbols as appropriate, and must include
Standqid Ptan Noies (see Appendix A of the Kent Construction Standards). Each plan must follow the
general format detailed below:
1. Plan sheets and profile sheets, or combined plan and profile sheets, specifications, and detail sheets
as required shall be on 22 inch by 34 inch sheets. Right-of-way improvements must be on 22 inch by
34 inch. Original sheets shall be in PDF or DWG file format.
2. Drafting details shall generally conform to APWA Standard Map Symbols with lettering size (before
reduction) no smaller than Leroy 80 (Leroy 100 is prefened). Existing features shall be shown with
dashed lines or as half-toned (screened) to clearly distinguish existing features from proposed
improvements.
3. Each submittal shall contain a project information/cover sheet with the following:
a) Title: Project name and Kent Permit number
b) Table of contents (if more than three pages)
c) Vicinity map
City of Kent Surface Water Design Manual 2-19 Iulye 2022
SECTTON 2.3 PLANS REQUIRED FOR DRAINAGE REVIEW
d) Name and phone number of utility field contacts (e.g., water, sanitary sewer, gas, power,
telephone, and TV) and the One-Call number (l-800-424-5555)
e) Kent's preconstruction/inspectionnotificationrequirements
0 Name and phone number of the erosion control supervisor
g) Name and phone number of the surveyor
h) Name and phone number of the owner/agent
D Name and phone number of the applicant
j) Legal description
k) Plan approval signature block for the Development Review Engineer
D Name and phone number of the engineering firm preparing the plans (company logos acceptable)
m) Fire Marshal's approval stamp (if required)
n) Statement that mailbox locations have been designated or approved by the U.S. Postal Service
(where required)
o) List of conditions of preliminary approval and conditions of approved adjustments and variances
on all site improvements.
4. An overall site plan shall be included if more than three plan sheets are used. The overall plan shall
be indexed to the detail plan sheets and include the following:
a) The complete property area development
b) Right-otway information
c) Street names and road classification
d) All project phasing and proposed division boundaries
e) All natural and proposed drainage collection and conveyance systems with catch basin numbers
shown
5. Each sheet ofthe plan set shall be stamped, signed, and dated by a licensed civil engineer registered
in the State of Waihington. At least one sheet showing all boundary survey information must be
provided and stamped by a professional land surveyor licensed in the State of Washington.
6. Detail sheets shall provide sufficient information to construct complex elements of the plan. Details
may be provided on plan and profile sheets if space allows.
7. A title block shall be provided on each plan sheet. At a minimum, the title block shall list the
following:
a) Development title
b) Name, address, and phone number of the firm or individual preparing the plan
c) A revision block
d) Page (of pages) numbering
e) Sheet title (e.g., road and drainage, grading, erosion and sediment control, stormwater pollution
prevention and spill control)
8. The location and labet for each section or other detail shall be provided.
g. Critical areas, critical area buffers, and critical area building setbacks as required by Kent City Code
shall be delineated and labeled.
10. All match lines with matched sheet number shall be provided.
City of Kent Surface Water Design Manual 2-20 Iulye 2022
SECTION 2,3 PLANS REQUIRED FOR DRAINAGE REVIEW
1 1. All division or phase lines and the proposed limits of construction under the permit application shall
be indicated.
12. Reference all identified wetlands (sequentially if more than one).
I 3. The standard plan notes that apply to the project shall be provided on the plans (see Appendix A ofthe
Kent Construction Standards).
14. Commercial building permit applications shall include the designated zoning for all properties
adjacent to the development sile(s).
tr BASE MAP
A site improvement plan base map provides a common base and reference in the development and design
of any project. A base map helps ensure that the engineering plans, grading plans, and CSWPP plans are
all developed from the same background information. This base map shall include the information listed
in Table 2.3.1.4.
Feature Requirements
Ground Surface
Topography
Provide topography within the site and extending beyond the property lines. Contour
lines must be shown as described in "Plan View: Site Plan and Roadway Elements"
(see Section 2.3.1.2).
Surface Water
Discharge
Provide ground surface elevations for a reasonable "fan" around points ofdischarge
extending at least 50 feet downstream ofall point discharge outlets.
Hydrologic Features Provide spot elevations in addition to contour lines to aid in delineating the boundaries
and depth of all existing floodplains, wetlands, channels, swales, streams, storm
drainage systems and BMPs, roads (low spots), closed depressions, springs' seeps,
swales, ditches, pipes, groundwater, and seasonal standing water.
Other Natural
Features
Show the location and relative sizes ofother natural features such as rock outcroppings,
existing vegetation, and trees I 2 inches in diameter and greater that could be disturbed
by the project improvements and construction activities (within tree canopy), noting
species.
Provide arrows that indicate the direction ofsurface flow on all public and private
propefty and for all existing conveyance systems.
Flows
Show the floodplain/floodways as depicted on City of Kent maps or otherwise required
by the City of Kent.
Floodplains/
Floodways
General Background
Information
Show the location and limits of all existing:
r Property boundaries
o Structures
r Easements(includingdimensions)
e Total propefty (including dimensions)
o Roads and right-of-way
r Sanitary sewers and water utilities
r Common open space
o Public dedications
r Other manmade features affecting existing topography/proposed improvements.
Development
Limitations
Delineate limitations to the development that may occur as identified on the TIR
worksheet, Part l1 (see Reference 8-A).
TABLE 2.3.I.A BASE MAP REQTJIRENIENTS
City of Kent Surface Water Design Manual 2-21 Iulye2022
SECTION 2.3 PLANS REQUIRED FOR DRAINAGE REVIEW
tr SITE PLAII ANID PROFILES
The design engineer shall provide plans and profiles for all construction, including but not limited to the
following information.
Plan View: Site Plan and Roadway Elements
1. Provide property lineso right-of-way lines, and widths for proposed roads and intersecting roads.
Note: thi condition of all public right-of-way and the right to use it as proposed must be verified.
2. Provide all existing and proposed roadway features, such as centerlines, edges ofpavement and
shoulders, ditchlines, curbs, and sidewalks. In addition, show points of access to abutting properlies
and roadway continuations.
3. Show existing and proposed topography contours at2-foot intervals (5-foot intervals for slopes
greater than 1 5 perient, 1 O-foot intervals for slopes greater than 40 percent). Contours may be
extrapolated from USGS mapping, aerial photographs, or other topography map resources. However,
contours shall be field verified for roadway and stream centerlines, steep slopes, floodplains, drainage
tracts easements, and conveyance systems. Contours shall extend 50 feet beyond property lines to
resolve questions ofsetback, cut and fiIl slopes, drainage swales, ditches, and access or drainage to
adjacent property.
4. Show the location of all existing utilities and proposed utilities (except those designed by the utility
and not currently available) to the extent that these will be affected by the proposed project. Clearly
identifu all existing utility poles.
5. Identifu all roads and adjoining subdivisions.
6. Show right-of-way for all proposed roadways, using sufficient dimensioning to clearly show exact
locations on all sections of existing and proposed dedicated public roadway.
7. Clearly differentiate areas of existing pavement and areas of new pavement. If the project is a
redevelopment project, delineate areas of teplaced impervious surface.
8. For subdivision projects, use drawing scales of I ":50'. For commercial, multi-family, or other
projects, use scaies of l":20'. Show details for clarification, including those for intersections and
existing driveways, on a larger scale'
9. Identify all section, township and range information for the project area.
Plan View: Drainage Conveyance
L Sequentially number all catch basins and curb inlets starting with the structure farthest
downstream.
2. Represent existing storm drainage facilities and BMPs in dashed lines and label with "Existing."
3. Clearly label existing storm drainage facilities to be removed with "Existing to be removed."
4. Show the length, diameter, and material for all pipes, culverts, and stub-outs. Include the slope if not
provided on the profile view. Material may be noted in the plan notes. Where open conveyance is
provided, and a low-permeability liner or treatment liner is required per Section 6.2.4, indicate the
limits of such liner(s).
5. Clearly label catch basins as to size and type (or indicate in the plan notes).
6. Clearly label downspout and footing drain stub-out locations for those lots intending to connect to
the storm drainage flow control system. Locate all stub-outs to allow gravity flow from the lowest
comer of the lot to the connecting catch basin.
7. Show datum, benchmark locations, and elevations on each plan sheet.
8. Clearly label all stub-out locations for any future pipe connections'
g. Clearly show on the plans all drainage easementso tracts, access easements, Native Growth Retention
City of Kent Surface Water Design Manual 2-22 Iulye 2022
SECTION 2.3 PLANS REQUIRED FOR DRAINAGE REVIEW
Areas, Critical Area Tracts, Critical Area Setback Areas, and building setback lines.Show
dimensions, type of restriction, and use.
10. Using, arrows indicate drainage direction of hydraulic conveyance systems.
Plan View: Other
1. Show the location, identification, and dimensions of all buildings, property lines, streets, alleys, and
easements.
2, Verify the condition of all public right-of-way and the rights to use them as proposed.
3. Show the locations of structures on abutting properties within 50 feet of the proposed project site.
4. Show the location of all proposed drainage facility fencing, together with a typical section view of
each fencing type.
5. Provide section details of all retaining walls and rockeries, including sections through critical portions
of the rockeries or retaining walls.
6. Show all existing and proposed buildings with projections and overhangs.
7 . Show the location of all wells on site and within 100 feet of the site. Note wells to be abandoned.
8. Show structural BMPs required by the King County Stormwater Pollution Control Manual and any
subsequent revisions.
Profiles: Roadway and Drainage
l. Provide existing centerline ground profile at 50-foot stations and at significant ground breaks and
topographic features, with average accuracy to within 0.1 feet on unpaved surface and 0.02 feet on
paved surface.
2. For publicly maintained roadways, provide final road and storm drain profile with the same
stationing as the horizontal plan, reading from left to right, to show stationing ofpoints ofcurve,
tangent, ind intersection of vertical curves, with elevation of 0.01 feet. Include tie-in with intersecting
pipe runs.
3. On a grid of numbered lines, provide a continuous plot of vertical positioning against horizontal.
4. Show finished road grade and vertical curve data (road data measured at centerline or edge of
pavement). Include stopping sight distance.
5. Show all roadway drainage, including drainage facilities and BMPs, that are within the right-of-way
or easement.
6. On the profile, show slope, length, size, and type (in plan notes or on a detail sheet) for all pipes and
detention tanks in public right-of-way.
7. Indicatetheinvertsofallpipesandculvertsandtheelevationsofcatchbasingratesorlids. Itisalso
desirable, but not required, to show invert elevations and grate elevations on plan sheets.
8. For pipes thatare proposed to be within 2.0 feeLof finished grade, indicate the minimum cover
dimensions.
9. Indicate roadway stationing and offset for all catch basins.
10. Indicate vertical and horizontal scale.
I l. Clearly label all profiles with respective street names and plan sheet reference numbers, and indicate
all profile sheet reference numbers on plan sheets, ifdrawn on separate sheets.
72. Locate match points with existing pavements, and show elevations.
13. Show all property boundaries.
14. Label allmatch line locations.
15. Provide profiles for all l2-inch and larger nipes and for channels (that are not roadside ditches).
City of Kent Surface Water Design Manual 2-23 Iulye2022
SECTION 2.3 PLANS REQUIRED FOR DRAINAGE REVIEW
16. Show the location ofall existing and proposed (ifavailable or critical for clearance) gas, water, and
sanitary sewer crossings.
17. Show energy dissipater locations
18. Identifo datum used and all benchmarks (may be shown on plan view instead). Datum and
benchmarks must refer to established control when available'
19. Useaverticalscaleof l":5'. Asanexception,verticalscaleshall be 1":10'iftheoptional 1":100'
horizontal scale is used on projects with lots one acre or larger. Clarifying details, including those for
intersections and existing driveways, should use a larger scale.
20. Split sheets, with the profile aligned undemeath the plan view, are preferred but not required.
tr DETAILS
The design engineer shall provide details for all construction, including but not limited to the following'
Flow Control, Water Qualityo and Infiltration Facility Details
l. Provide a scaled drawing and supporting details of each detention pond or vault, flow control BMP
and water quality facility, including the tract boundaries.
2. Show predeveloped and finished grade contours at 2-foot intervals. Show and label maximum
design water elevation.
3. Dimension all berm widths.
4. Show and label at least two cross sections through a pond or water quality facility, or any BMP large
enough to require design elements of ponds and/or water quality facilities. One cross section must
include the restrictor when included in the design.
5. Specify soils and compaction requirements for pond construction and flow control BMP
construction. Specifu iow-permeability liners or treatment liners as required for ponds and ditches per
Section 6.2.4.
6. Show the location and detail of emergency overflows, spillways, and bypasses.
7. Specify rock protection/energy dissipation requirements and details.
8. Provide inverts of all pipes, grates, inlets, tanks, and vaults, and spot elevations of the pond bottom.
9. Show the location of access roads to control manholes and pond/forebay bottoms.
10. Provide plan and section views of all energy dissipaters, including rock splash pads. Specify the
size ofrock and thickness.
1 1. Show bollard locations on plans. Typically, bollards are located at the entrance to drainage facility
access roads.
12. Onthe pond or water quality facility detail, show the size, type (or in plan notes), slope, and length of
all pipes.
13. Show to scale the section and plan view ofrestrictor and control structures. The plan view must
show the location and orientation ofall inlet pipes, outlet pipes, and flow restrictors.
14. Draw details at one of the following scales: l":1" 1"-2" 1":4" 1":5" l":10" or 1":20'. Select a
scale that clearly shows required information.
Structural Plan Details
Any submittal that proposes a structure (e.g., bridge crossing, reinforced concrete footings, walls, or
uuultt; shall include plln sheets that include complete working drawings showing dimensions, steel
placement, and specihcations for construction. Structures may require a design prepared and stamped by a
professional structural engineer licensed in the State of Washington, and an application for a separate
commercial building permit.
City of Kent Surface Water Design Manual 2-24 Julye 2022
SECTION 2.3 PLANS REQUIRED FOR DRAINAGE REVIEW
2.3.1.3 EROSION AND SEDIMENT CONTROL (ESC) PLAN
This section details the specifications and contents for ESC plans. Note that an ESC plan includes the
plan's drawings plus an ESC report, which provides all supporting information and any additional
direction necessary for implementing ESC measures and meeting ESC implementation requirements. The
ESC plan may be simplified by the use of the symbols and codes provided for each ESC measure in the
Erosion and Sediment Control Standarab (detached Appendix D of the King County Manual). In
general, the ESC plan shall be submitted as a separate plan sheet(s). However, there may be some
relatively simple projects where providing separate grading and ESC plans is unnecessary'
General Specifications
The site improvement plan shall be used as the base of the ESC plan. Certain detailed information that is
not relevant (e.g., pipelcatch basin size, stub-out locations, etc.) may be omitted to make the ESC plan
easier to read. At a minimum, the ESC plan shall include all of the information required for the base map
(see Table 2.3.1.A),as well as existing and proposed roads, driveways, parking areas, buildings, drainage
facilities and BMPs, utility corridors not associated with roadways, relevant critical QreQS33 and critical
arearelevant critical areas'and critical area buffers, and proposed final topography. A smaller scale may
be used to provide better comprehension and understanding.
The ESC plan shall generally be designed for proposed topography, not existing topography, since rough
grading is usually the first step in site disturbance. The ESC plan shall address all phases ofconstruction
(e.g., clearing, grading, and installation of utilities, surfacing, and final stabilization). If construction is
being phased, separate ESC plans may need to be prepared to address the specific needs for each
construction phase.
The ESC plan shall be consistent with the information provided in Section 8 of the TIR and shall show the
following:
l. Identifu areas with a high susceptibility to erosion.
2. Provide all details necessary to clearly illustrate the intent of the ESC design.
3. Include ESC measures for all on- and offsite utility construction included in the project'
4. Specify the construction sequence. The construction sequence shall be specificallV_writt91 for the
pioposed project. An example construction sequence is provided in Appendix D of the King County
Manual.
5. Include ESC Stqndard PlanNotes.
6. Include an inspection and maintenance program for ESC measures, including designation of a
CSWPP supervisor who is a certified ESC professional and identification of phone numbers for
24-hour contact.
7. Include the basis and calculations for selection and sizing of ESC measures.
3i Relevantcriticalareas,forthepurposesoldrainagereview,includeaquaticareas,wetlands,./rlaodhazardareas,erosionhazard
areas, landslide hazard areas, steep slope haZard areos, and critical aquifer rechatge areas
City of Kent Surface Water Design Manual 2-25 Iulye 2022
SECTION 2.3 PLANS REQUIRED FOR DRAINAGE REVIEW
Clearing Limits
l. Delineate clearing limits.
2. Provide details sufficient to install and maintain the clearing limits.
Cover Measures
l. Specifu the type and location of temporary cover measures to be used onsite.
2. If more than one type of cover is to be used onsite, indicate the areas where the different measures
will be used, including steep cut and fill slopes.
3. If the type of cover measures to be used will vary depending on the time of year, soil type, gradient' or
some other factor, specifu the conditions that control the use of the different measures.
4. Specifu the nature and location of permanent cover measures. If a landscaping plan is prepared, this
may not be necessary.
5. Specify the approximate amount of cover measures necessary to cover all disturbed areas'
6. If netting or blankets, or plastic sheeting are specified, provide typical detail sufficient for
installation and maintenance.
7. Specifu the mulch types, seed mixes, fertilizers, and soil amendments to be used, as well as the
application rate for each item.
8. For surface roughening, describe methods, equipment and areas where surface roughening will be
use.
9. If PAM is used, show location(s) and describe application method'
10. When compost blankets are used, show location, application rates, and the name of the supplier
to document that compost meets quality specifications per Reference I 1-C'
Perimeter Protection
1. Speciff the location and type of perimeter protection to be used.
2. Provide fypical details sufficient to install and maintain the perimeter protection.
3. If silt fence is to be used, speciff the type of fabric to be used.
4. If compost berms or socks are used, documentation must be provided to assure the supplier meets
the criteria and compost meets quality standards per Reference I 1-C.
Traffic Area Stabilization
l. Locate the construction entrance(s).
2. Provide typical details sufficient to install and maintain the construction entrance.
3. Locate the construction roads and parking areas.
4. Speciff the measure(s) that will be used to create stabilized construction roads and parking areas.
Provide sufficient detail to install and maintain.
5. If a wheel wash or tire bath system will be installed, provide location, typical details for installation
and maintenance.
6. Provide a list of dust control products that will be used onsite and the location of potential application
areas.
Sediment Retention
l. Show the locations of all sediment ponds and traps.
2. Dimension pond berm widths and all inside and outside pond slopes.
3. Indicate the trap/pond storage required and the depth, length, and width dimensions.
City of Kent Surface Water Design Manual 2-26 Idye2022
SECTION 2.3 PLANS REQUI RED FOR DRAINAGE REVIEW
4. Provide typical section views through pond and outlet structures'
5. If chemical or electrocoagulation treatment of sediment-laden waters will be used, approval
documentation from Ecology must be included (see SWPPS plan requirements for chemical storage).
6. Provide details for disposal of contaminated or chemically treated waters (e.g., where Chitosan or CO2
have been used) (see SWPPS plan requirements for chemical storage).
7. Include appropriate approval documentation from local sewer districts if contaminated
or chemically treated water will be discharged to the sanitary sewer,
8. Provide typical details of the control structure and dewatering mechanism.
9. Detail stabilization techniques for outlet/inlet protection.
10. Provide details suff,rcient to install cell dividers.
I l. Speciff mulch or recommended cover of berms and slopes.
12. Indicate the required depth gage with a prominent mark at 1-foot depth for sediment removal.
13. Indicate catch basins that are to be protected.
14. Indicate existing and proposed flow control BMP areas that are to be protected.
15. Provide details of the catch basin and flow control BMP protection sufficient to install and
maintain.
Surface Water Control
1. Locate all pipes, ditches, interceptor ditches, and swales that will be used to convey stormwater.
2. Provide details sufficient to install and maintain all conveyances.
3. Indicate locations of outlet protection, and provide detail of protections.
4. Indicate locations and outlets of any possible dewatering systems. Provide details of alternative
discharge methods from dewatering systems if adequate infiltration rates cannot be achieved. Do not
route dewatering water, clean or untreated, through Stormwater sediment ponds.
5. Indicate the location of any level spreaders, and provide details sufficient to install and maintain.
6. Show all temporary pipe inverts.
7 . Provide location and specifications for the interception of runoff from disturbed areas and the
conveyance ofthe runoffto a non-erosive discharge point.
8. Provide locations and details of rock check dams.
9. Provide details, including front and side sections of typical rock check dams.
Protection of Existing and Proposed Flow Control BMP Areas
l. Provide perimeter protection at existing and proposed flow control BMP locations.
2. Provide cautionary plan notes emphasizing avoidance of negative impacts to receptor soils
and existing vegetation to remain.
BMP Maintenance
l. Provide adequate plan notes for guidance of BMP maintenance methods and schedules'
2. Include an inspection and maintenance program for ESC measures.
Management of the Project
1. Provide plan notes to clarifu and emphasize the management responsibilities for the project.
2. Include an inspection and maintenance program for ESC measures, including designation of a
CSWPP supervisor who is a certified ESC professional and identification of phone numbers for
24-hour contact.
City of Kent Surface Water Design Manual 2-27 Iulye2022
SECTION 2.3 PLANS REQUIRED FOR DRAINAGE REVIEW
Wet Season Requirements
l. Provide a list of all applicable wet season requirements'
2. Clearly identifr that from October I't through April 30'h, no soils shall be exposed for more than two
consecutive working days. Also note that this two-day requirement may be applied at other times
of the year if storm events warrant more conservative measures.
3. Clearly identifr that exposed soils shall be stabilized at the end of the workday prior to a
weekend, holiday, or predicted rain event.
Critical Areas Restrictions
1. Delineate and label the following critical areas, and any applicable buffers, that are on or adjacent
to the project site: aquatic areas, wetlands,flood hazard areas, erosion hazurd areas, landslide
hazsrd sreas, steep slope hszqrd areas, and critical aquifer recharge areas.
2. Ifconstruction creates disturbed areas within any ofthe above listed critical areas or associated
buffers, specifl the type, locations, and details ofany measures or other provisions necessary to
comply with the critical area restrictions in Appendix D and protect surface waters and steep
slopes.
2.3.t.4 STORMWATER POLLUTTON PREVENTTON AND SPILL (SWPPS) PLAN
This section details the specifications and contents for SWPPS plans, which together with ESC plans,
comprise the construction stormwater pollution prevention (CSWPP) plan that must be submitted as
part of the engineering plans required for drainage review. Additional guidance for developing the
SWPPS plan can be found inthe SWPPS Standards in the KCSWDM AppendixD, Construction
Stormwater Pollution Prevention Standards, in the King County Stormwater Pollution Prevention
Manual and in the Stormwater Management Manualfor Western Washington (SWMMWIry published
by the Washington State Department of Ecology (Ecology)'
The SWPPS plan must be kept on site during all phases of construction and shall address the
construction-related pollution-generating activities outlined in Subsection A below. The plan must
include a description of the methods the general contractor will use to ensure sub-contractors are aware
of the SWPPS plan. A form or record must be provided that states all sub-contractors have read and
agree to the SWPPS plan.
A SWPPS plan consists of the following three elements, which are further described in Subsections B,
C, and D below:
I . A site plan showing the location and description of BMPs required to prevent pollution and
control spills from construction activities and from chemicals and other materials used and stored
on the construction site. See Subsection B below for more specifics on the SWPPS site plan.
2. A pollution prevention report listing the potential sources of pollution and identifying the
operational, source control, and treatment BMPs necessary to prevent/mitigate pollution from
these sources. See Subsection C below for more specifics on the SWPPS pollution prevention
report.
3. A spill prevention and cleanup report describing the procedures and BMPs for spill prevention
and including provisions for cleanup ofspills should they occur. See Subsection D below for more
specifics on the SWPPS spill prevention and cleanup report.
A. ACTWTTY-SPECTFIC INFORMATION REQUTRED
City of Kent Surface Water Design Manual 2-28 lulye 2022
SECTION 2.3 PLANS REQUIRED FOR DRAINAGE REVIEW
At a minimum, the SWPPS plan shall address, if applicable, the following pollution-generating
activities typically associated with construction and include the information specified below for each
activity. Ifother pollution-generating activities associated with construction ofthe proposed project
are identified, the SWPPS plan must address those activities in a similar manner'
Storage and Handling of Liquids
1. Identify liquids that will be handled or stored onsite, including but not limited to petroleum
products, fuel, solvents, detergents, paint, pesticides, concrete admixtures, and form oils-
2. Specify types and sizes of containers of liquids that will be stored,handled onsite. Show
locations on the SWPPS site plan.
3. Describe secondary containment methods adequately sized to provide containment for all
liquids stored onsite. Show the locations of containment areas on the SWPPS site plan.
Storage and Stockpiling of Construction Materials and Wastes
l. Identify construction materials and wastes that may be generated or stockpiled onsite. Show the
locations where these materials and wastes will be generated and stockpiled on the SWPPS site
plan.
2. Specifi type of cover measures to be used to keep rainwater from contacting construction
materials and wastes that can contribute pollutants to storm, surface, and ground water.
3. If wastes are kept in containers, describe how rainwater will be kept out of the containers'
Fueling
I . Specifu method of onsite fueling for construction equipment (i.e. stationary tanks, truck
mounted tanks, wet hosing, etc.). If stationary tanks will be used, show their location on the
SWPPS site plan.
2. Describe type and size of tanks.
3. Describe containment methods for fuel spills and make reference to the SWPPS site
plan for location information.
4. If fueling occurs during evening hours, describe lighting and signage plan. Make reference to the
SWPPS site plan for location information,
Maintenance, Repairs, and Storage of Vehicles and Equipment
1. Identifu maintenance and repair areas and show their locations on the SWPPS site plan'
Use of drip pans or plastic beneath vehicles is required. A note to this effect must be shown
on the SWPPS site plan.
2. Describe method for collection, storage, and disposal of vehicle fluids.
3. lf an area is designated for vehicle maintenance, signs must be posted that state no vehicle
washing may occur in the area. A note to this effect must be shown on the SWPPS site plan'
Concrete Saw Cutting, Slurry, and Washwater Disposal
l. Assure that washout of concrete trucks is performed off-site or in designated concrete washout
areas only. Identify truck washout areas to assure such areas are not within a criticul aquifer
recharge area. lf they are, the washout area must be lined with an impervious membrane.
Show location information on the SWPPS site plan. Locate washout areaat least 50 feet from
sensitive areas such as storm drains, open ditches, or water bodies, including wetlands.
2. Specify size of sumps needed to collect and contain slurry and washwater. Show location
City of Kent Surlace Water Design Manual 2-29 Jdye2022
SECTION 2.3 PLANS REQUIRED FOR DRAINAGE REVIEW
information on the SWPPS site plan.
3. Identify areas for rinsing hand tools including but not limited to screeds, shovels, rakes,
floats and trowels. Show the locations of these areuN on the SWPPS site plan.
4. Describe methods for collecting, treating, and disposal of waste water from exposed
aggregateprocesses, concrete grinding and saw cutting, and new concrete washing and
curing water.
Handling of pH Elevated Water
New concrete vaults/structures may cause collected water to have an elevated pH. This water
cannot be discharged to storm or surface water until neutralized.
L Provide details on treating/neutralizing water when pH is not within neutral parameters.
Written approval from Ecology is required before using chemical treatment other than CO2 or
dry ice to adjust pH.
2. Provide details on disposal of water with elevated pH or of the treated water.
Application of Chemicals including Pesticides and Fertilizers
1. Provide a list of chemicals that may be used on the project site and the application rates.
2. Describe where and how chemicals will be applied. Show location information on the
SWPPS site plan.
3. Describe where and how chemicals wilt be stored. Show location information on the SWPPS
site plan.
B. SWPPS SITE PLAN
The site plan element of the SWPPS plan shall include all of the information required for the base
map (see Table 2.3.1.A , p.2-23), as well as existing and proposed roads, driveways, parking areas,
buildings, drainage facilities, utility corridors not associated with roadways, relevant critical
eress34 and associated buffers, and proposed final topography. A smaller scale may be used to
provide more comprehensive details on specific locations of each activity and specific prevention
measure. In addition to this information, the following items, at a minimum, shall be provided as
applicable:
1. Identiff locations where liquids will be stored and delineate secondary containment areas that
will be provided.
2. Identiff locations where construction materials and wastes will be generated and stockpiled.
3. Identifu location of fueling for vehicles and equipment if stationary tanks will be used.
4. Delineate containment areas for fuel spills.
5. Show location of lighting and signage for fueling during evening hours.
6. Delineate maintenance and repair areas and clearly note that drip pans or plastic shall be
used beneath vehicles. Also, clearly note that signs must be posted that state no vehicle
3a Relevantcriticalareas,forthepurposesofdrainagereview,includeaquaticareas,wetlands,floodhazardareas"erosionhazard
areas, landslide hazard areos, steep slope haZard areas, and critical aquifer recharge areas
City of Kent Surface Water Design Manual 2-30 Julye2022
SECTION 2.3 PLANS REQUIRED FOR DRAINAGE REVIEW
2.3.1.5
washing may occur in the area.
7. Delineate truck washout areas and identify the location of slurry/washwater sumps and
rinsing areas for tools.
8. Delineate where chemicals will be applied and identify where they will be stored.
9. Identifu where spill response materials will be stored.
C. POLLUTION PREVENTION REPORT
This report provides the specifics on pollution prevention and must include the following
information in addition to the activity-specific information specified in Subsection A above:
l. List the possible sources of pollution per Subsection A above and identify the BMPs to be
used for each source to prevent pollution. Include any supporting information (site
conditions, calculations, etc.) for the selection and sizing of pollution prevention BMPs.
2. Identify the personnel responsible for pollution prevention and clearly list the responsibilities
of each person identified. Contact information for these personnel must be clearly identified
in the report and on the SWPPS site plan'
3. Describe the procedures to be used for monitoring pollution prevention BMPs and for
responding to a BMP that needs attention, including keeping records/reports of all inspections
of pollution prevent BMPs (see Reference Section 8-E for examples of worksheets that may be
used).
D. SPIL PREVENTION AND CLEANUP REPORT
This report provides the specifics on spill prevention and cleanup and must include the
following information in addition to any activity-specific information in Subsection A
above related to spill prevention:
l. List the possible sources of a spill and identiff the BMPs to be used for each source to prevent a
spill.
2. Identifu personnel responsible for spill prevention and cleanup and clearly list the responsibilities
of each person identified. Contact information for these personnel must be clearly identified in
the report and on the SWPPS site plan. (On typical projects, the primary contact for SWPPS issues
will be the CSWPP supervisor, who may be managing other spill responders to assure compliance;
complex projects may warrant specialist personnel for specific site applications.)
3. Describe the procedures to be used for monitoring spill prevention BMPs and for
responding to a spill incident, including keeping records/reports ofall inspections and spills
(see Reference Section 8-E for examples of worksheets that may be used).
4. Identifu where spill response materials will be stored. Make reference to the SWPPS site
plan for location information.
5. Identify disposal methods for contaminated water and soil after a spill.
LANDSCAPE MANAGEMENT PLANS (IF APPLICABLE)
A landscape manogement plan is a Kent approved plan for defining the layout and long-term maintenance
of landscaping features to minimize the use of pesticides and fertilizers, and reduce the discharge of
suspendedsolids and other pollutants. General guidance for preparing landscape management plans is
provided in Reference Section 4-C.
City of Kent Surface Water Design Manual 2-31 Julye2022
SECTION 2.3 PLANS REQUIRED FOR DRAINAGE REVIEW
2.3.2
If a landscape management plan is proposed,
proposed project. The elements listed below
it must be submitted with the engineering plans for the
are required for evaluation of landscape management plans.
1. Provide a s/e vicinity map with topography.
2. Provide a site plan with topography. Indicate areas with saturated soils or high water tables'
3. Provide a plant list (provide both common and scientific names) that includes the following
information:
a) Indicate any drought-tolerant plants, disease resistant varieties, species for attracting beneficial
insects (ifany) and native plants.
b) For shrubs and groundcovers, indicate the proposed spacing.
c) For turf areas, indicate the grass mix or mixes planned. Indicate sun/shade tolerance, disease
susceptibility, drought tolerance, and tolerance of wet soil conditions.
4. Provide a landscape ptan. Indicate placement of landscape features, lawn areas, trees, and planting
groups (forbs, herbs, groundcovers, etc.) on the site.
5. Include information on soil preparation and fertility requirements'
6. Provide information on the design of the irrigation method (installed sprinkler system, drip inigation
system, manual, etc.)
7. Provide a landscape maintenance plan, including the following:
a) Physical care methods, such as thatch removal or aeration, and mowing height and frequency.
b) Type of fertilizer (including N-P-K strength) and fertilization schedule or criteria.
c) Type of chemicals to be used for common pests such as crane fly larvae, and the criteria or
schedule for application.
d) Any biocontrol methods.
8. Provide information about the storage of pesticides or other chemicals, and disposal measures that
will be used:
a) If applicable, indicate how the chemicals will be stored on the site between applications to prevent
contact with stormwater or spills into the storm drainage system.
b) Indicate how excess quantities of fertilizers or chemicals will be handled for individual
applications.
g. Provide an implementation plan (see Reference Section 4-C for guidance on preparing the
implementation plan).
PROJECTS IN TARGETED DRAINAGE REVIEW
This section outlines the specifications and contents of limited scope engineering plans allowed for
projects in Targeted Drainage Review. Table 2.3.2.A specifies the minimum required elements of the
iargeted techniial information report based on the type of permit or project, and on the three categories of
project characteristics subject to Targeted Drainage Review per Section 1.1 .2.2.
City of Kent Surface Water Design Manual 2-32 Iulye2022
bject Categry-f.(t)
Redevelopment projects (as
defined in 1.1) in Targeted
Drainage Review that propose
$100,000 or more of
improvements to an existing
high-use site
bjss!13!9sssl2(')
Projects in Targeted
Drainage Review that
propose to construct or
modify a 12" or larger
pipe/ditch, or receive
runofffrom a 12" or larger
pipe/ditch
Drainage Review
Type
Bsi@')
Projects that contain or are
adjacent to floodplains,
streams, lakes, wetlands or
closed depressions OR projects
within a Landslide Hazard
Area, Landslide Hazard
Drainage Area or Erosion
Hazwd Area. Projects in TDR
that contain or are adjacent
to aJlood, erosion, or
II. steep slope hazard area; or
are within a CDA or LHDA; or
propose >7,000 sf of land
disturbing activiry, (3 acres if
in Simplified DR).
Type of Permit or Project
N/Ao TIR Sections l,2,and6
(minimum)
r Simplified Site ESC Plan(3)
and SWPPS Plan
r Site Improvement Plan(sr
r TIR Sections 1,2,3,5,6,
7, and 8 (minimum)
. Simplified Site ESC
Plan(3)and SWPPS Plan
o ESC Plan(4) lor
conveyance work
o Site Improvement Plan(5)
Targeted Drainage
Review ONLY
r TIR Sections l,2, 3, 5, 6,
7, and 8 (minimum)
. Simplified Site ESC
Plan(3)
r [,$f P[a1(+) f61
conveyance work
o Site Improvement Plan(s)
N/Ar TIR Sections 1,2,and6
(minimum)
. Simptified Site ESC Plan(3)
and SWPPS Plan
r Site Improvement Plan(s)
Targeted Drainage
Review COMBINED
WITH Simplified
Site Drainage
Review
SNGLE-FAMILY
RESIDENTLALBUILDING
PERMITS
(SFRs)
&
SHORTPLATS
o TIR Sections 1,2,4,8, and l0
(minimum)
o [,g(] Plsn(a) and SWPPS Plan
for any site disturbance work
r Site Improvement Plan(5)
o TIR Sections 1,2,6,and8
(minimum)
o [,g[ P[an(a) nnd SWPPS Plan
for any site disturbance work
r Site Improvement Plan(s)
r TIR Sections 1,2,3,5,6,
7, and 8 (minimum)
r [g[ P[an(+) and SWPPS
Plan for any site
disturbance work
r Site Improvement Plan(5)
Targeted Drainage
Review ONLY
OTHERPROJECTSOR
PERMITS
Notes:
(r) The above plan elements are considered the recommended minimum for most development cases in Targeted Drainage Review. The
Development Review Engineer may add to these elements if deemed necessary for proper drainage review. Predesign meetings with the
Development Review Engineer are recommended to identify all required elements.
(2) For more detailed descriptions of project categories, see Section 1.1.2.2. If the proposed project has the characteristics of more than one
category, the plan elements under each applicable category shall apply.
(3) Simplified site ESC plans are an element of the simplified site drainage plan as explained in the Simplffied Site Drainage Requirements
booklet (detached Appendix C).
(4) E,SC plans shall meet the applicable specifications detailed in Section 2.3.1.3
(5) Site improvement plans shall meet the applicable specifications detailed in Section 2.3.1.2. The Development Review Engineer may allow
modified site improvement plans as desoibed in Section 2.3.1.2.
TABLE 2,3,2.A MINIMUM ENGINEERING PLAN ELEMENTS(')
FOR PROJECTS IN TARGETED DRAINAGE REVIEW
City of Kent Surface Water Design Manual 2-33 l:uly 2022
SECTION 2.4 PLANS REQUIRED AFTER DRAINAGE REVIEW
2.4 PLANS REQUIRED AFTER DRAINAGE REVIEW
This section includes the specifications and contents required of those plans submitted at the end of the
permit review process or after a permit has been issued.
PLAN CHANGES AFTER PERMIT ISSUANCE
If changes or revisions to the originally approved engineering plans require additional review, the revised
plans sf,all be submitted to the Permit Center as a Plan Revision for approval prior to construction. The
plan change submittals shall contain all of the following in PDF format:
o The appropriate Revision fom(s).
o The revised TIR or addendum.
o The engineering plans.
o Other information needed for review.
FINAL CORRECTED PLAN SUBMITTAL
During the course ofconstruction, changes to the approved engineering plans are often required to
address unforeseen field conditions or design improvements. Once construction is completed, it is the
applicant's responsibility to submit to the Economic and Community Development Department a final
corrected plan ("as-builts"), which is an engineering drawing that accurately represents the project as
constructed. These corrected drawings must be professionally drafted revisions applied to the original
approved plan, excluding the CSWPP plan, and must include all changes made during the course of
construction; the ESC plan, however, should not be included. The final corrected plan must be
stamped, signed, and dated by a licensed civil engineer registered in the State of Washington' A CAD
drawing file (.dwg) of the final corrected plan must be submitted along with paper copies. The CAD
file must contain all the pages of the plan set for road and drainage infrastructure, but need not contain
other sheets. A final corrected TIR, updated to include all changes made to the originally approved
TIR during the course of construction, must be submitted with the final corrected plan. In addition to
any design changes and supporting calculations and documentation, the final corrected TIR shall
include a final updated Stormwater Facility Summary Sheet (see Reference 8-D) and signed/recorded
copies ofall required easements and declarations ofcovenant. A copy ofany required Landscape
Management Plan (see Section 2.3.1.5) shall accompany the plans and TIR. The electronic copy of the
final corrected TIR may be in .pdf or other approved format.
FINAL PLAT, SHORT PLAT, AND BINDING SITE PLAN
SUBMITTALS
In addition to the requirements of the Economic and Community Development Department, submittals for
final recording of subdivisions and short plats must include the following information:
1. Indicate dimensions of all easements, tracts, building setbacks, tops of slopes, wetland boundaries,
and floodplains.
2. Include pertinent restrictions as they apply to easements, tracts, and building setback lines.
3. Include the dedication and indemnification clause as provided in Reference Section 8-G
accompanying Chapter 5 of the Kent Design and Construction Standards.
2.4.1
2.4.2
2.4.3
City of Kent Surface Water Design Manual 2-34 Iuly 2022
SECTION 2.4 PLANS REQUI RED AFTER DRAINAGE REVIEW
4. State the maximum amount of added impervious surface and proposed clearing per lot as
determined through engineering review. The maximum amount of impervious surface may be
expressed in terms ofpercentage oflot coverage or square feet.
5. Specifr roof downspout controls by lot based on the "sizing Credits for Roof Downspout Controls"
(see Section 1.2.3.2) as determined through engineering review and approval.
6. For a plat or short plat, record a note conditioning single-family residential permit approval on
compliance with approved roof downspout controls (see notes in Section 5.1).
7 . Include a recorded declaration of covenant and grant of easement for each lot on which flow
control BMPs are installed or stipulated per Core Requirement 9, Section 1.2.9.4.1, and each lot
for which flow control BMPs are installed in a separate dedicated tract per Section L2.9 .4.1 .
City of Kent Surface Water Design Manual 2-35 Iuly 2022
City of Kent Surface Water Design Manual 2-37 July 2022
CHAPTER3
HYDROLOGIC ANALYSIS AND DESIGN
KENT
W*gnrNGTor{
CITY OF KENT
Surface Water Design Manual
2022
CHAPTER3
City of Kent Surface Water Design Manual I:uJiy 2022
City of Kent Surface Water Design Manual July 2022
CHAPTER 3 HYDROLOGIC ANALYSIS AND DESIGN
3 HYDROLOGIC ANALYSIS AND DESIGN
The City of Kent has made no changes to Chapter 3 of the 2021King County Surface Water Design
Manual. Chapter 3 ofthe 2016 King County Surface Water Design Manual shall be used for all proposals
in the city of Kent.
City of Kent Surface Water Design Manual 3-l Iuly 2022
CHAPTER4 CONVEYANCE SYSTEM ANALYSIS AND DESIGN
City of Kent Surface Water Design Manual 3-2 Iuly 2022
City of Kent Surface Water Design Manual July 2022
City of Kent Surface Water Design Manual 3-4 Iuly 2022
CHAPTER 4
CONVEYANCE SYSTEM ANALYSIS AND DESIGN
KENT
WrgnrNGTor{
CITY OF KENT
Surface Water Design Manual
2022
CHAPTER 4
Conveyance System Analysis and Design.
4.1 Route Design Easment Requirements.................
4.1.2 Easment And Setback Requirements....................'.....
4.2 Pipes, Outfalls, and Pumps
4-1
.................... 4-1
4.2.2 OutfallSystems.......
.................... 4-2
4-1
4-2
4-8
4-84.3 Culverts And Bridges
City of Kent Surface Water Design Manual Iuly 2022
CHAPTER 4 CONVEYANCE SYSTEM ANALYSIS AND DESIGN
City of Kent Surface Water Design Manual 3-l htly 2022
City of Kent Surface Water Design Manual IuJy 2022
CHAPTER 4 CONVEYANCE SYSTEM ANALYSIS AND DESIGN
4 CONVEYANCE SYSTEM ANALYSIS AND
DESIGN
The City of Kent has made the following revisions to Chapter 4 of the 2021 KCSWDM. Apart from these
changes, the King County version of Chapter 4 applies for proposals in the City of Kent.
ROUTE DESIGN EASEMENT REQUIREMENTS
EASEMENT AND SETBACK REQUIREMENTS
This section shall read as follows:
Permanent onsite exclusive Easements for access, maintenance, and construction are required for all public
and private stormwater systems serving more than one property located outside of public right-of-way.
Private improvements such as buildings, fences, garages, carports, retaining walls, utilities, signs, light
standards, etc. are not allowed in these easements. Where an encroachment occurs, the developer shall
remove and relocate the conflicting private improvements. Easements shall be accessible by
construction equipment used to operate and maintain these facilities. When Easements are required,
then legal descriptions for same shall be submitted with a professional land surveyor stamp thereon' Also,
a current title report covering the properties to be encumbered by the Easements shall accompany said
description. Under no circumstances shall a Bill of Sale be placed on the City Council agenda for action
until all Easements have been approved and recorded.
When offsite Easements and/or onsite Easements for the extension of approved comprehensive stormwater
plans are required, these shall be approved and recorded prior to holding any preconstruction meeting.
The same conditions shall apply regarding legal descriptions and title reports.
A. Easements
The minimum easement widths are as follows:
l. Storm drain pipelines (under l0 feet deep) - 15 feet wide.
2. Storm drain pipelines (10 -15 feet deep) -20 feetwide. Note: large diameter or deeper sewers
may require wider easements as determined necessary by the Director.
3. Access andlor maintenance roads are required to all retention/detention facilities, control
manholes, and other drainage structures. The minimum roadway width shall be 12 feet. The road
shall be paved with an all-weather surface, and the pavement shall be designed to support a HS-20
loading maintenance vehicle.
4. The Easements for detention facilities or other structures shall extend a minimum of l2 feet
around the outside of the facility, and shall include access Easements as well.
B. Right-of-way
Where possible, utility extensions shall be located within City Right-of-way. When possible storm
drainage Retention/Detention facilities shall be located adjacent to City Right-of-way.
4.1
4,1.1
City Of Kent Surface Water Design Manual 4-l luJy 2022
Work inside County and State Right-of-way requires use permits from the appropriate agencies.
County and State Right-of-way Permits must be obtained by the developer/owner.
4.2 PIPES, OUTFALLS, AND PUMPS
4.2.I PIPE SYSTEMS
4.2.I.1 DESIGNCRITERIA
Acceptable Pipe Sizes - This section is replaced with the following:
"The minimum diameter of storm drain approved for mainline conveyance systems, or for driveway
culverts, shall be 12 inches. Minor laterals and connections to catch basins may be 8 inches in diameter if
approved by the City. The minimum diameter acceptable for private systems shall be 8 inches, excluding
downspouts,"
Allowable Pipe Materials - This section is replaced with the following:
The designer shall have the option of constructing storm sewers, drains, and culverts of the type of pipe
listed below within the limits specified. It is not necessary that all pipe on the project be of the same type;
however, all contiguous pipe shall be of the same type.
NOTES:
1. These are minimum depths. Actual depths must meet design loading requirements.
including maximum depths specified by the pipe manufacturer. Pipe cover in areas not
sub.iect to vehicular loads may be reduced to 3 inch minimum. . Regardless of stated
minimum cover, the designer shall mitigate for pipe buoyancy.
2. PVC pipe shall not be used as driveway culvert and on pipe runs where one or both ends are
not terminated at a structure.
PCP (Bell & spigot Gasket joint)
RCP
HDPE
PVC SDR 35
PVC SDR 21
Ductile Iron
Ribbed PVC
2
I
1.5
3
1.5
0.5
2
Yes
Yes
Yes
Yes, see Note 2 for exemption
Yes, See Note 2 for exemption
Yes
Yesn see Note 2 for exemption
Minimum Cover (ft)
(from top of bell)
(See Note I )
Public SystemsType of Pipe
City of Kent Surface Water Design Manual 4-2 July 2022
All materials used for construction of storm drainage systems and appurtenances shall be new and
undamaged. All materials used shall be subject to inspection by the City prior to use. The contractor shall
provide the City with shop drawings, manufacturer's specifications, and certificates of materials as
requested.
The materials referred to herein, shall conform to the applicable provisions of the Standard Specifications
and the manufacturer's recommended installation procedures.
l. Plain Concrete Storm Sewer Pipe (PCP) - All nonreinforced concrete pipe shall be ASTM Cl4-81
Class II or better with gasketed joints. Joints for concrete pipe shall use rubber gaskets per ASTM
C443.The maximum diameter for unreinforced pipe shall be 12 inches.
2. Reinforced Concrete Pipe (RCP) - All reinforced concrete pipe shall be ASTM C76'85a Class IV
or as specified. Elliptical and arch pipe shall be reinforced per ASTM C507 and ASTM C506
accordingly. Joints for reinforced concrete pipe shall use rubber gaskets per ASTM C443'
3. Protective Treatment - All steel pipe shall be coated with a protective asphalt treatment. The minimum
acceptable protective treatment shall be APWA Treatment 1.
4. pVC Pipe - PVC pipe conforming to ASTM 3034, SDR 35 and SDR 2l with rubber gasket joints
may be used where adequate cover can be obtained. Joints shall conform to ASTM D32 l2 using
elastomeric gaskets conforming to ASTM F477.
5. Ribbed PVC - PVC pipe conforming to ASTM FTSFVLTNI-B-9 with rubber gasket joints may be used
where adequate cover can be obtained. Joints shall conform to ASTM D3212 using elastomeric gaskets
conforming to ASTM F477.
6. HDPE Pipe - Non-corrugated solid walled High Density Polyethylene Pipe (HDPE) are manufactured
from resins meeting the requirements of ASTM D3350 and meeting or exceeding cell classification of
445574c,and a plastic pipe institute (PPI) designation of PE 4710. The pipe shall have a minimum
standard dimension ratio (SDR) of 32.5. HDPE pipe shall be joined into a continuous length by using
a butt-fusion joint method meeting the requirements of ASTM F2620 andthe manufacturer's
recommendations.
7 . Ductile Iron Pipe - DI pipe shall be Class 50 in accordance with USA Standard A-21.1(AWWA C-
5l ). All joints shall be push-on mechanical or flanged.
Pipe Alignment - Replace Requirement I with the following:
o,All storm drains smaller than 36 inches in diameter shall be designed with a straight alignment between
manholes or catch basins.
Storm Drainage Manholes
A. Location - Storm drainage manholes or catch manholes for access are required at the following
locations on the conveyance system:
l. All changes in slope.
2. Changes in pipe diameter.
3. Connections of storm pipes from manholes to the conveyance systems that are latger than24
inches in diameter.
The following exceptions may be connected to conveyance systems that are equal to a 48-inch-
diameter pipe using an approved saddle branch:
a. Connections of storm pipes from catch manholes that are 24 inches and smaller in diameter.
b. Connections of storm pipes from catch basins that are 18 inches and smaller in diameter'
City of Kent Surface Water Design Manual 4-3 July 2022
4. Changes in alignment.
5. Distances not greater than 400 feet on pipe runs'
6. Dissimilar pipe materials and/or change in pipe material.
7. In a trunk line smaller than 12 inches, at every other catch basin but no greater than 400 feet.
B. Flow Channels - Where necessary to maintain the hydraulic gradeline, channeled storm drain
manholes shall be used. The manhole shall be fully channeled to the crown of the pipe to accomplish
smooth flow and minimize turbulence at junctions. Catch or drop section manholes are permitted on
the trunk system.
C. Flow at Transition Manholes - To maintain the energy gradient and the velocity through grade
changes and changes in diameter at manholes, the invert of the downstream pipe shall be lowered. A
general method to achieve the required drop at manholes is to match crowns of the storm pipes. In
cases where slopes are at a minimum, an alternate approach is to use the 0.8 rule. Where possible' a
0.1 foot drop in all manholes is desirable.
D. Standard Manholes - The minimum diameter of manholes shall be 48 inches and shall be precast
reinforced concrete structures. All manholes shall be equipped with safety steps. Larger diameter
manholes are required for larger pipelines and special pipe configurations. The following table lists
the minimum diameter of manholes for various runs of straight pipelines. (See Standard Details 5-
8(a), 5-8(b) and 5-9 in Appendix B of the Kent Design and Construction Standards.)
Stormwater Inlets
Stormwater inlets, installed to intercept surface runoff, include catch basins, catch manholes, curb inlets,
and headwall structures. Catch type inlets are required to trap or minimize silts, sediment, and debris from
entering the main drainage systems. Curb type inlets without a drop section are permitted only where
approved by the City.
A. Catch Basin Locations for Street and Roadways - Catch basins or catch manholes shall be installed at
these locations:
1. At all street gutterline intersections in such a configuration as to minimize gutter flows through
pedestrian crossings.
2. At locations along gutters so as to provide a maximum gutter flow of 200 feet for street grades up
to 3o/o.
3. Where Type II catch basins are used for access to the trunk storm sewer, the maximum spacing
shall be 400 feet.
4. At all low points ofvertical curves and grade breaks.
5. At the inlet of minor ditches to the drainage system.
8 inches - 21 inches
24 inches - 36 inches
36 inches - 42 inches
42 inches - 60 inches
48 inches
54 inches
72 inches
96 inches
Manhole DiameterPipe Diameter
City of Kent Surface Water Design Manual 4-4 Iuly 2022
B. The low point of vertical curves or roadway grades require catch basins with through curb inlet to
minimize the potential for property damage. Cul-de-sacs located at the low end of roadway grades
also require catch basins with through curb inlet. (See Standard Detail 5-2(a) and (b) in Appendix B
of the Kent Design and Construction Standards.)
C. In all cases, the location, size, and number of catch basins to be installed shall be sufficient to ensure
that there is adequate capacity to efficiently collect the stormwater.
D. Catch Basin Locations for Private Developments - Catch basins or catch manholes are required at
these locations for private developments:
1 . At the junction of private storm drainage systems with City drainage systems.
2. At adequate locations throughout the development to provide efficient drainage of paved areas
and to ensure that surface water runoff to adjacent City streets and private property is minimized.
3. At locations as required to intercept natural drainage entering the site.
Castinss
A. Manhole frames and covers shall be cast gray iron or ductile iron. All manhole covers located outside
the street shall have locking lids. (See Standard Plans in Section 5.9 of the Kent Design and
Construction Standards.)
B. Public catch basin frame and grates shall be ductile iron and bolt down. (See Standard Plans in Section
5.9 of the Kent Design and Construction Standards.)
C. Public catch basins used in areas not designed to receive surface drainage or where required by the
Director shall have ductile iron bolt down frame with solid lid. (See Standard Plans in Section 5.9 of
the Kent Design and Construction Standards.)
D. Public catch basins used in low point of vertical curves or roadway grades and at the low end of
roadway grades in cul-de-sacs shall have through curb inlet frame and self-locking vaned grate.
(See Standard Plans in Section 5.9 of the Kent Design and Construction Standards.)
E. Public catch basins used in steep roadway grades of 60/o and greater shall have ductile iron self-
locking vaned grate with vaned grate frame. (See Standard Plans in Section 5.9 of the Kent Design
and Construction Standards.)
F. Public catch basins used in rolled curb and gutter shall have rolled curb frame and grate' (See
Standard Details (See Standard Plans in Section 5.9 of the Kent Design and Construction Standards.)
G. Block lettering is required on the top surfaces of storm drain castings, and shall read as follows:
..NO DUMPING!DRAINS TO STREAMS!"
Manhole and Catch Basin AdaPters
A. All aluminum surfaces in contact with the concrete or concrete pipe shall be treated to protect from
corrosion. The aluminum pipe to be treated shall be cleaned with solvent to remove contaminants'
After cleaning, the pipe shall be painted with 2 coats of paint conforming to Federal Specification TT-
P-645.
B. PVC pipe adapters shall be Kor-in-seal type flex joints or sand collars meeting ASTM D-303H-78
SDR35 specifications or other materials as approved by the Director to permit slight differential
movement,
City of Kent Surface Water Design Manual 4-5 July 2022
C. Ribbed PVC adapters shall be Ribbed PVC sand collars meeting ASTM D-303H-78 SDR35
specifications. Where the pipe enters square to the manhole or catch basin, double gaskets may be
used. The gaskets must fall within a l-inch tolerance ofthe inside and outside face of the wall of the
structure.
D. All pipe materials other than the above shall be mudded directly into the manholes and catch basins
using a smooth 45 degree bevel from the pipe to the structure. The allowable protrusion is 2 inches
inside the structure unless otherwise approved by the Director.
Pipe Bedding
Pipe bedding for storm drain and culvert pipe shall be 5/8-inch minus crushed rock. (Pea gravel is not
allowed). A continuous and uniform bedding shall be provided in the trench for all buried pipe. Bedding
material shall be tamped in layers around the pipe and to a sufficient height above the pipe to adequately
support and protect the pipe to 95Yo compaction ASTM D-1557, in accordance with the standard
specifications and details and in conformance with the following gradation:
L.A. wear 500 rev. 35o/o Max., Degradation 25%o min. Free from wood waste, bark, and other deleterious
material.
Bedding shall be placed 6 inches under or around the pipe and 6 inches over the pipe where, in the opinion
of the Director, existing material is found unsuitable. The pipe shall be protected from damage when
compacting. At least 2 feet of cover is required over the pipe prior to using heavy compaction equipment.
Where determined necessary by the Director, ballast material shall be used below bedding to stabilize the
trench. Ballast shall conform to the requirements of Pipe Bedding and Foundation Material in Section
3.20.A of the City of Kent Design and Construction Standards.
Backfill
Pipe trench backfill shall be in accordance with Section 4.7 .F of the City of Kent Design and Construction
Standards unless otherwise approved by the Director.
Cleanins
All storm drain pipe including the downstream system shall be thoroughly cleaned to remove any solids or
construction debris that may have entered the pipe system during construction.
The Contractor shall be responsible to ensure that materials flushed from the storm drain are trapped,
removed, and do not enter the downstream drainage system.
Storm Drain Marking
3/4 Inch
5/8 Inch
1/4 Inch
U.S. No.40
U.S. No.200
%o Fracture
Sand Equivalent
lO0Vo
95 - 1000
45 - 65V"
6-18
7.5 Max.
75 Min.
40 Min.
Sieve Size Passing
City of Kent Surface Water Design Manual 4-6 July 2022
The pavement adjacent to all new catch basins shall be marked with one of the standard pollution
prevention buttons shown below. The Contractor is responsible for installation of the buttons. Privately
maintained catch basins shall use marker b) and must be purchased by the contractor. Markers for publicly
owned catch basins, marker a), will be provided by the City. Public catch basin makers shall not be used
for privately maintained catch basins.
$o DU ft{P tN c
ltq'
a) Public catch basin marker b) Private catch basin marker
Repairs
All storm drain system installations shall be new and undamaged. Repairs by grouting or collars are not
acceptable for new piping systems. The contractor shall bear all costs for correction ofdeficiencies.
Television Inspection
All new City storm drain extensi ons,24 inches in diameter and smaller shall be TV camera inspected by
the City prior to acceptance. All construction must be completed and approved by the inspector prior to
the TV inspection. The manholes and catch basins must be set to grade, channeled, and grade rings set in
place prior to TV inspection. Castings and the top grade ring must be removed for paving and grouted in
place after paving.
It is the responsibility of the contractor to string each storm main when required prior to the inspection.
The string shall be a nylon cord of sufficient strength for the City equipment.
Pipe Design Between Structures - Add the following to the list of requirements:
4. Storm sewers with design velocities greater than l0 fps shall be designed with energy dissipation
structures as deemed necessary by the City. Pipes installed at a slope greater than20Yo require pipe
anchors.
Spill Control - Delete the following item:
e) ,,active spill control plan." This is not an acceptable option for spill control in the City of Kent-
4.2.2 OUTFALL SYSTEMS
4.2.2.I DESIGN CRITERIA
t
IO ST
City of Kent Surface Water Design Manual 4-7 luly 2022
General - Add the following to the list of requirements:
8. Points of discharge from culverts and storm sewers into ditches and swales 15"/o ot greater in grade
shall require the design and installation ofengineered energy dissipators.
4.3 CULVERTS AND BRIDGES
4.3.I CULVERTS
4.3.I.I DESIGN CRITERIA
General - Add the following to the list of requirements:
3. The minimum diameter of culvert for roadside ditches or other installations shall be 12 inches. Larget
diameter culverts shall be installed where required by appropriate hydraulic analysis.
Headwater - Requirement 3 is revised to read: "The maximum headwater elevation at design flow shall
be at least 12 inches below finished grade."
Inlets and Outlets -Requirement 4 is revised to apply to all pipes larger than l2 inches. Requirement 5
is revised to apply to all new culverts unless deemed unnecessary by the City.
City of Kent Surface Water Design Manual 4-8 Iuly 2022
CHAPTER 5
FLOW CONTROL DESIGN
KENT
WAgHTNGToT{
CITY OF KENT
Surface Water Design Manual
2022
CHAPTER 5
Flow Control Design -1
-1
-1
a
-J
-3
5
5
5
5
5
5
5.1.2 DetentionTanks..........
5.1.1 DetentionPonds........
5.1.5 ParkingLotDetention.....................
City of Kent Surface Water Design Manual Iuly 2022
CHAPTER 5 FLOW CONTROL DESIGN
City of Kent Surface Water Design Manual 4-4 Iuly 2022
City of Kent Surlace Water Design Manual July 2022
CHAPTER 5 FLOW CONTROL DESIGN
5 FLOW CONTROL DESIGN
The City of Kent has made the following revisions to Chapter 5 of the 2021' KCSWDM. Apart from these
changes, the King County version of Chapter 5 applies for proposals in the City of Kent.
5.1. DETENTION FACILITIES
5.1.1 DETENTION PONDS
5.1.1.1 DESIGN CRITERIA
General - The following items are added to this section:
6. All retention/detention ponds shall have a minimum of one foot free board above the
maximum design water surface elevation.
7. Outlets of all detention ponds shall be provided with suitable debris barriers designed to
protect the outlet from blockage or plugging'
8. The site and grading plans for open ponds shall clearly denote all elevations, dimensions,
cross-section views (a minimum of one through each direction), soil preparation requirements
and other information necessary to construct the system as designed. The design plans shall
also denote that the design engineer shall verify the pond volume and construction prior to
landscaping.
9. All ponds shall provide for the energy reduction of incoming flows.
Side Slopes - This section is replaced with the following
2.
Side slopes for earth-lined ponds shall not exceed three feet horizontal to one foot vertical,
unless specifically approved by the Director. See Section 6.4.4 for side slope requirements for
internal berms in combined ponds and wetponds.
Ponds constructed with rock walls or retaining walls shall be designed by a licensed structural
or civil engineer registered in the State of Washington. Walls shall not exceed 6 feet in
height.
Pond walls may be vertical retaining walls, provided: (a) they are constructed of reinforced concrete
per Section 5.1.3; (b) a fence is provided along the top of the wall; (c) at least 25o/o of the pond
perimeter will be a vegetated soil slope not steeper than 3H:1V in addition to one access for
maintenance equipment at a maximum 6: I slope; and (d) the design is stamped by a licensed
structural civil engineer.
Fencing is required for all ponds unless otherwise approved by the Engineer/Director.
Embankments - Requirements 1 and2 have been revised as follows and item 7 has been added:
1. Any embankment for a pond in excess of four feet must be designed by a licensed civil
engineer and approved by the City.
2. The top width of the berm shall be 15 feet, unless otherwise approved by the City. The outer
edge of the berm slope may be located within the outer 10 feet of critical area buffers or
)
4.
City of Kent Surface Water Design Manual 5-1 luly 2022
CHAPTER 5 FLOW CONTROL DESIGN
within pond landscape setbacks, as long as the landscaping on the berm and slope within
those buffers and setbacks meets the geotechnical engineer's requirements.
A licensed civil engineer experienced in soil mechanics shall inspect and certif, the
construction of the berm.
Fencing - This section is replaced with the following:
L Fencing is required for all ponds unless otherwise approved by the Engineer/Director.
2. Fences shall be 6 feet in height. For design examples, see WSDOT Standard PlanL-2, Type 1 or Type
3 chain link fence. Fence material shall be vertical metal balusters, 9 gauge stainless steel,
aluminized steel, or galvanized steel fabric. If galvanized, bonded vinyl coating is required. For steel
fabric fences, the following apply:
f) Vinyl coating shall be compatible with the surrounding environment (e.g., green in open, grassy
areas and black or brown in wooded areas). All posts, cross bars, and gates shall be coated the
same color as the vinyl clad fence fabric.
g) Fence posts and rails shall conform to WSDOT Standard Plan L-2 for Types 1, 3, or 4 chain link
fence, except that if galvanized, they must be coated with bonded vinyl.
3. Access road gates shall be 16 feet in width consisting oftwo swinging sections 8 feet in width.
Additional vehicular access gates may be required as needed to facilitate maintenance access.
4. Pedestrian access gates (if needed) shall be 4 feet in width'
5. For fences to be maintained by the City, fencing and gates shall be per details 5-22 and 5-23 of the
KDCS.
Signage - This section is replaced with the following:
Detention ponds, infiltration ponds, wetponds, and combined ponds to be maintained by the City of Kent
or privately maintained shall have a sign placed for maximum visibility from adjacent streets, sidewalks,
and paths. The sign shall meet the design installation requirements illustrated in standard plan 5-40 of the
Kent Design and Construction Standards Manual.
Right-of-Way - Requirement 2 has been revised as follows and item 3 has been added:
2. All storm water retention/detention systems and outlet control structures that service more than one
legally defined property are required to be located within a tract or recorded storm drainage easement.
In some cases, the City may approve locating detention piping within the street right-of-way. If the
detention facilities are not located adjacent to the roadway, a l2- foot-wide, all-weather surfaced
access road shall be constructed to the facility. This access road shall be located within atract or
easement and shall provide for unobstructed ingress and egress to the facility.
3. A written restriction shall be added to final plat drawings that "Prior approval must be obtained from
the Department of Public Works before any structures, fill or obstructions, including fences, are
located within any drainage easement or tract.
5.I.2 DETENTION TANKS
5.L.2.1 DESIGN CRITERIA
Materials - The following is added to this section:
7
City of Kent Surface Water Design Manual 5-2 Iuly 2022
CHAPTER 5 FLOW CONTROL DESIGN
5.1.5
5.1.6
Corrugated steel storage pipes shall be uniformly coated with APWA Treatment I asphalt or better
Corrugated aluminum or concrete pipe may be substituted without asphalt treatment. The use of
zinc chromate primer is prohibited unless approved by the City.
The Fire Authority shall be consulted during design of any underground facilities to ensure that
outrigger placement (pointload) requirements are met.
Stability - The following is added to this section:
The concrete structures shall be designed by a civil or structural engineer registered in the State of
Washington, and be designed for at least HS-20 traffic loading conditions. Concrete vaults shall be
designed in accordance with King County Surface Water Management Standard Details for
detention vaults. A City building permit is required for vault structures.
The Fire Authority shall be consulted during design of any underground facilities to ensure that
outrigger placement (pointload) requirements are met'
PARKING LOT DETENTION
Replace this section with the following:
1. Maximum depth for surface storage in parking lots shall be as follows
a. Commercial/multi-family parking and maneuvering areas - six inches.
b. Industrial truck loading/maneuvering areas - l8 inches.
2. Parking lots designed for detention shall not exceed slopes of seven percent in areas of
vehicular parking or maneuvering.
3. All parking lot ponds shall be designed and constructed in such a manner so as to provide 0.50
feet of freeboard between the maximum water surface elevation and adjacent driveways,
landscaping or adjacent properties.
4. The maximum water surface elevation of any parking lot pond shall be a minimum of six
inches below the finished floor elevation of adjacent buildings. All ponding systems shall be
designed to go to overflow conveyance prior to flooding structures.
5. Where parking lot ponding is to be utilized, the site grading and paving plans shall clearly
denote all critical elevations, ponding, dimensions, and any other necessary information to
construct the detention pond as designed. The design plan shall indicate that the respective
details are critical for the storm drainage detention system operation and that the site grading
must be verified.
ROOF DETENTION
Add the following items
6. All rooftop detention facilities shall be provided with overflow scupper drains at the maximum
water surface elevation.
7. Rooftop detention rings shall be installed in accordance with the manufacturer's specifications
and shall be designed to restrict rooftop runoff rates not to exceed two gallons per minute per
1000 square feet o[roof area.
City of Kent Surface Water Design Manual 5-3 July 2022
City of Kent Surface Water Design Manual luly 2022
CHAPTER 6
WATER QUALITY DESIGN
KENT
WA3HINGTON
CITY OF KENT
Surface Water Design Manual
2022
CHAPTER 6
Water Quality Design........
6.2 General Requirements For WQ Facilities......
6.7 Proprietary Facility Designs.......
6.2.3 Slopes, And Embankmetns........
6-
6-
6-
6-
City of Kent Surface Water Design Manual luly 2022
City olKent Surface Water Design Manual Iuly 2022
CHAPTER 6 WATER QUALITY DESIGN
6 WATER QUALITY DESIGN
The City of Kent has made the following revisions to Chapter 6 of the 2021 KCSWDM. Apart from these
changes, the King County version of Chapter 6 applies for proposals in the City of Kent.
6.2 GENERAL REQUIREMENTS FOR WQ FACILITIES
USE OF MATERIALS
This section is replaced with the following:
Galvanized materials in stormwater facilities and conveyance systems is discouraged. If galvanized materials
are used, bonded vinyl coating is required. Where other metals, such as aluminum or stainless steel, or plastics
are available, they shall be used.
6.2.3 SLOPES, AND EMBANKMENTS
SIDE SLOPES, FENCING, AND EMBANKMENTS - Add the following to the list of requirements:
"Any above-ground stormwater facility will be screened from public right of way and adjacent property
per city of Kent underlying zoning perimeter buffer requirements."
Requirement 4 is replaced with the following:
4. Non-residential commercial and industrial facilities that are privately owned and maintained, shall follow
the fencing requirements of this manual as well as any fencing requirements in other codes or regulations.
6.7 PROPRIETARY FACILITY DESIGNS
This section is replaced with the following:
Water quality facilities other than those identified in Chapter 6 are allowed on an experimental basis if it
can be demonstrated they are likely to meet the pollutant removal goal for the applicable receiving water,
Use of such facilities requires an experimental design adjustment to be approved by Kent according to
Section 1.4, "Adjustment Process.", Experimental and proprietary BMPs may be allowed to provide
pretreatment for filter BMPs or as the second WQ facility in the Two-Facility Treatment Train option.
Applications will be reviewed on a case-by-case basis. For an experimental or proprietary BMP to be
approved, it must be approved for use through the Washington Department of Ecology's TAPE Program.
The general use level designation (GULD) of the TAPE Program confers a general acceptance for the
specified applications (land uses). Manufactured treatment devices with a GULD may be used for new
development, re-development, or retrofit situations anywhere in Washington, subject to conditions that
Ecology places within the use designation document. Manufactured treatment devices with a GTJLD can
have an unlimited number of installations without the need for an adjustment.
City of Kent Surface Water Design Manual 6-l Jdy 2022
City of Kent Surface Water Design Manual luly 2022
APPENDIX
KENT
Vt'esrilHcroH
CITY OF KENT
Surface Water Design Manual
2022
APPENDIX A
Appendix A - Maintenance Requirements For Flow Control, Conveyance, And Facilities.
APPENDIX B
Appendix B - Master Drainage Plan Objectives, Criteria, Components, And Review Process
APPENDIX C
Appendix C - Simplified Drainage Requirements
APPENDIX D
Appendix D - Construction Stormwater
D.1 General CSWPP Requirements...
A-2
A-6
A-9
.. A-13
.. A-13
City of Kent Surface Water Design Manual July 2022
City of Kent Surface Water Design Manual Iily 20222
City of Kent Surface Water Design Manual Jily 2022
APPENDIXA
MAINTENANCE REQUIREMENTS FOR FLOW CONTROL,
CONVEYANCE, AND WATER QUALITY FACILITIES
KENT
APPENDIX A
MAINTENANCE REOUIREMENTS FOR FLOW CONTROL,CONVEYANCE, AND WATER QUALITY FACILITIES
WasxlHGToN
CITY OF KENT
Surface Water Design Manual
2022
City of Kent Surface Water Design Manual A-l July 2022
APPENDIX A
MAINTENANCE REQUIREMENTS FOR FLOW CONTROL,CONVEYANCE, AND WATER QUALITY FACILITIES
APPENDIX A - UAINTENANCE REQUIREMENTS FOR FLOW
CONTROLO CONVEYANCE, AND WATER
QUALITY FACILITIES
The City of Kent has made no changes to Appendix A of the 2021 KCSWDM. Users should refer to the
County document for guidance on the thresholds and standards for maintenance of all flow control facilities
and BMP's, conveyance systems, and water quality facilities required in this manual.
City of Kent Surface Water Design Manual A-2 Iuly 2022
City of Kent Surface Water Design Manual Iuly 20223
APPENDIX B
MASTER DRAINAGE PLAN OBJECTIVES,CRITERIA, COMPONENTS, AND REVIEW PROCESS
APPENDIX B
MASTER DRAINAGE PLAN OBJECTIVES, CRITERIA,
COMPONENTS AND REVIEW PROCESS
KENT
WAsHlNcror{
CITY OF KENT
Surface Water Design Manual
2022
The City of Kent does not adopt Appendix B of the 2021 KCSWDM.
City of Kent Surface Water Design Manual A-4 IuIy 2022
City of Kent Surface Water Design Manual luly 20225
APPENDIX B - TU.q,STER DRAINAGE PLAN OBJECTIVES,
CRITERIA, COMPONENTS, AND REVIEW
PROCESS
The City of Kent does not adopt Appendix B of the 2021 KCSWDM.
City of Kent Surface Water Design Manual A-6 luly 2022
City of Kent Surface Water Design Manual Iuly 20227
APPENDIX C
SIMPLIFIED DRAINAGE REQUIREMENTS
APPENDIX C
SIMPLIFIED DRAINAGE REQUIREMENTS
KENT
WAgHrNcroH
CITY OF KENT
Surface Water Design Manual
2022
City of Kent Surface Water Design Manual A-8 Iuly 2022
APPENDIX C _ SIMPLIFIED DRAINAGE
REQUIREMENTS
The City of Kent has made no changes to Appendix C of the 2021 KCSWDM. Users should refer to the
County document for guidance on simplified drainage requirements for smaller projects that qualify for
Simplified Drainage Review.
City of Kent Surface Water Design Manual A-9 July 2022
City of Kent Surface Water Design Manual l0 Iuly 2022
APPENDIX D
CONS TRUCTION STORMWATER POLLUTION PREVENTION
APPENDIX D
CONSTRUCTION STORMWATER POLLUTION
PREVENTION
KENT
WASHINGTOTI
CITY OF KENT
Surface Water Design Manual
2022
City of Kent Surface Water Design Manual A-11 Iuly 2022
City of Kent Surface Water Design Manual t2 Iuly 2022
APPENDIX D - CONSTRUCTION STORMWATER
POLLUTION PREVENTION
The City of Kent has made one minor change to Appendix D of the 2021 KCSWDM which describes the
required measures to be implemented during construction to prevent discharges of sediment-laden runoff from
the project site. It also describes effective management practices for spill control and chemical pollutants used
during construction that may be needed to supplement the required erosion and sedimentation control
-"ur*... Apart from this change, the King County version of Appendix D applies for proposals in the City of
Kent. The City's change to the County document is as follows:
D.l GENERAL CSWPP REQUIREMENTS
Erosion and Sediment Control lVleasures, Sediment Retention @age D-58 of the 2016 KCSVI/DM)'
The following item is added to this section:
City of Kent Surface Water Design Manual A-13 Iuly 2022
REFERENCES
i r.t
TEMPORARY CATCH BASIN PIPE DETAIL
DRAIhIACE GRATE
ffilt'{
GRATE FRA,ME
SETIIMENT
AF,IIJ SE*RI5
NRAINACE GRI.TE
{kEcrAr,lGiJL&EfiA]E 3H0.'stl
BEL*YJ ILET
CIIATE DEI,4CE
IEMPO,RARY CATUII
BA5IH SIPASS PIPE
0\,ERFLO',v Ff.t''ASg
FILTEREI ISATER
5E{:TION VIE$J
F]'iJTES:
1 TU F;HEVENT STAI'lDlf\i,ll WATER, lF,tlTALL
BYPA5S plpE A$ "5Hf,r'r'1.1,
t. lt'l5ERT BifrASS plPE ll.,lTcl llEliFLfiw CF CAT':H
aAslit r^J5tr.,t{i l" slAl\lEIEr{ f-i\,c cll +nEATElt
J. qEIIOYE BYPi.5S PIPE FRICE TS II{STALLATIBI'I
CF FINAL LIFT OF PA?EI,EI',IT.
4- EI.{sURE REI,{AII'IING HOLE 15 EEPAINM.
HETRIEVAL
s\'5TEfi, TY,f,
TEI.{PORARY CATCTI
BASIN BYPASS PIPE
OVEftFLOW
tiYpAS$, T\?
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TEMPORARY CATCH BASIN BYPASS PIPE DETAIL
f"JCT TD SCALE
City of Kent Surface Water Design Manual A-t4 Iuly 2022
City of Kent Surface Water Design Manual luly 2022
REFERENCES
KENT
WAgHINGTON
CITY OF KENT
Surface Water Design Manual
2021
City of Kent Surface Water Design Manual Iuly 2022
City of Kent Surface Water Design Manual IuJy 2022
RE,FERENCES
The 2021King County Surface Water Design Manual includes many reference materials. Some references are for
guidance and some are required worksheets to be utilized during the development application, review, and
approval process. The applicant is responsible to ensure that the most current materials are used in preparing a
permit application.
The City of Kent accepts certain reference materials as written in the 20 16 King County Surface Water Design
Manual. Other references are not applicable in Kent. Some reference materials have been revised to reflect
Kent-specific forms or development procedures. References from the King County Manual are identified
below along with how they are to be utilized in Kent (in bold).
Six separate maps are attached that must be utilized during the Development process (See Table of Contents
for list).
REF'ERENCES
l KCC 9.04 - Surface Water Runoff Policy
Refer to Kent City Code sections 7.05 and 7.07
2. Adopted Critical Drainase Areas
None currently listed
3. Other Adopted Area Specific Drainaee Requirements
Not applicable
4. Other Drainase Related Resulations and Guidelines
4.A. Grading Code Soil Amendment Standard
See Kent Design and Construction Standards and Kent City Code section 15.07
4.B. Clearine & Grading Seasonal Limitations
See Kent Design and Construction Standards and Appendix D (Construction Stormwater Pollution
Prevention)
4.C. Landscape Management Plan Guidelines
Utilize Reference 4.c from the King county Surface water Design Manual
4.D. Shared Facilitv Maintenance Responsibilitv Guidance
Refer to Kent Design and Construction Standards
5. Wetland Hydrology Protection Guidelines
Utilize Reference 5 from the King County Surface Water Design Manual in consultation with city
review staff
6. Hvdroloeic/Hvdraulic Design Methods
6.A. Infiltration Test Methods
City of Kent Surface Water Design Manual R-l July 2022
Utilize Reference 6.A from the King Counfy Surface Water Design Manual as applicable and in
consultation with cify review staff
6.8. Pond Geometry Equations
Utilize Reference 6.8 from the King County Surface Water Design Manual as applicable and in
consultation with city review staff
6.C. Introduction to Level Pool Routine
Utilize Reference 6.C from the King County Surface Water Design Manual as applicable and in
consultation with cify review staff
6.D. Supplemental Modeling Guidelines
Utilize Reference 6.D from the King County Surface Water Design Manual as applicable and in
consultation with city review staff
7. Eneineering Plan Support
7.A. Kine Countv Standard Map Symbols
Refer to Kent Design and Construction Standards
7.B. Standard Plan Notes and Examole Construction Sequence
Refer to Kent Design and Construction Standards
7.C. Stormfilter Facilitv Access and Cartridee Confieuration
Utilize Reference 7.C from the King County Surface Water Design Manual as applicable and in
consultation with city review staff
8. Forms and Worksheets
8.A. Technical Information Report (TIR) Worksheet
Utilize the TIR Worksheet provided in reference 8.A of the King County Surface Water Design
Manual or a TIR Worksheet provided by city review staff
8.B. Offsite Analysis Drainage System Table
Utilize the Offsite Analysis Drainage System Table from the King County Surface Water Design
Manual or an Offsite Analysis Drainage System Table provided by city review staff
8.C. Water Quality Facilitv Sizine Worksheets
Utilize the Water Quality Facility Sizing Worksheets provided in the King County Surface Water
Design Manual
8.D. Flow Control and Water Quality Facility Summary Sheet and Sketch
Utilize the Flow Control and Water Quality Facility Summary Sheet and Sketch provided in the
King County Surface Water Design Manual
8.E. CSWPP Worksheet Forms
Utilize the CSWPP Worksheet Forms provided in the King County Surface Water Design Manual
as applicable and in consultation with city review staff
8.F. Adjustment Application Form and Process Guidelines
Utilize Design and Construction Standards Deviation Request Application Form and Process
Guidelines provided by the Kent Economic and Community Development Department Permit
Center or Development Review divisions
8.G. Dedication and Indemnifrcation Clause - Fin
City oflKent Surface Water Design Manual R-2 Iuly 2022
Utilize Dedication and Indemnification Clause - Final Recording language as required by the Kent
Economic and Community Development Department Permit Center or Development Review
divisions
8.H. Bond Quantities Worksheet
Utilize Engineer's Cost Estimate as provided in Appendix D of the Kent Design and Construction
Standards
8.I. Maintenance and Defect Agreement
Utilize Maintenance and Defect Agreements provided by the Kent Economic and Community
Development Department Permit Center or Development Review divisions
8.J. Drainage Facilitv Covenant
Utilize Drainage Facility Covenant format as required by the Kent Economic and Community
Development Department Permit center or Development Review divisions
8.K. Drainase R elease Covenant
Utilize Drainage Release Covenant as applicable and as directed by the Kent Economic and
Community Development Department Permit Center or Development Review divisions
8.L. Drainage Easement
Utilize Drainage Easement standard procedure and format as applicable and as directed by the
Kent Economic and Community Development Department Permit Center or Development Review
divisions
8.M. Flow Control BMP Covenant and BMP Maintenance Instructions (Recordable format)
Utilize Reference 8.M as provided in the King County Surface Water Design Manual as applicable
and as directed by the Kent Economic and Communify Development Department Permit Center or
Development Review divisions
8.N. Impervious Surface Limit Covenant
Utilize Reference 8.N as provided in the King County Surface Water Design Manual as applicable
and as directed by the Kent Economic and Community Development Department Permit Center or
Development Review divisions
8.O. Clearing Limit Covenant
Utilize Reference 8.O as provided in the King County Surface Water Design Manual as applicable to
preserve native vegetative areas and as directed by the Kent Economic and Community
Development Department Permit Center or Development Review divisions
8.P. River Protection Easement
Utilize Reference 8.P as provided in the King County Surface Water Design Manual as applicable
and as directed by the Kent Economic and Community Development Department Permit Center or
Development Review divisions
8.Q. Leachable Metals Covenant
Utilize Reference 8.Q as provided in the King County Surface Water Design Manual as applicable
and as directed by the Kent Economic and Community Development Department Permit Center or
Development Review divisions
9. Interim Changes to Requirements
9.A. Blanket Adjustments
Blanket Adjustments will be added by reference to this manual and posted on the City's website. As
of January 2017,there are no applicable Blanket Adjustments to this manual.
City of Kent Surface Water Design Manual R-3 Iuly 2022
9.B. Administrative Changes
Administrative Changes will be added by reference to this manual and posted on the Cityos website.
As of January 2017,there are no applicable Administrative Changes to this manual.
10. Kent-Identified Water Oualitv Problems
None at this time. Refer to Washington Department of Ecology's Impaired Waterbody list for state
listed water quality problems.
I 1. Materials
1I.A. ryACANT)
Not applicable
II.B. (VACANT)
Not applicable
I LC. Bioretention Soil Media Standard Specifications
Utilize the Bioretention Soit Media Standard Specifications provided in the King County Surface
Water Design Manual as aPPlicable
l1.p (VACANT)
Not applicable
11.E. Roofing Erodible or Leachable Materials
Utilize the Roofing Erodible or Leachable Materials guidance provided in the King County Surface
Water Design Manual as applicable
12. (VACANT)
Not applicable
13. NACANT)
Not applicable
I 4. Suoolemental Aonroved Facilities
14.A. Approved Proorietary Facilities
Utilize the Approved Proprietary Facilities Specifications provided in the King County Surface
Water Design Manual as applicable
14.8. Approved Public Domain Facilities
None at this time
City of Kent Surface Water Design Manual R-4 Iuly 2022
MAPS
KENT
WASHIN6TON
CITY OF KENT
Surface Water Design Manual
2022
MAPS
Erosion Hazard Areas ..........
Flow Control Applications..................
Landslide Hazard/ Landslide Hazard Drainage Areas
Soils Wellhead Areas......
Water Quality Applications .........
M-1
M-2
M-3
M-4
M-5
M-6
M-7
Wetlands
Lower Mill Creek Regulatory Area
King County 100 Year 7 Day Isopoluvial Map...M-8
City of Kent Surface Water Design Manual lily 2022
City of Kent Surface Water Design Manual Jily 2022
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City of Kent Surlace Water Design Manual M-8 .Ilulv 2022
sTAl'ri oF WASI{INGTON, COUNTY OF KING }
AFFIDAVIT OF PUBLICATION
PUBLIC NOTICE
Rudi Alcott, being first duly sworn on oath that he is the
Vice President of Advertising for Sound Publishing,
which publishes the
Kent Reporter
a weekly newspaper, which newspaper is a legal ncwspaper of gcneral
circulation and is now and has been for more than six months prior to thc date
of publication hereinafter referred to, published in the llnglish language
continuously as a weekly newspaper in King County, Washington, 'I'he
Kent- Covington Reporter has been approved as a l,egal Newspaper by ordcr o[
the Superior Court of the State of Washington for King County.
The notice in the exact form annexed was published in regular issues o1'
the Kent- Covington l{eporter (and not in supplcment" form) which was rcgularly
disuibuted to its subscribers during the below statcd pcriod.'fhc anncxed
notice, a:
Public Notice #K8N9s7040
was published on June 24r 2022
The full amount of the fee charged for said foregoing publication is the
sumof $217 69
l{udi A}cott
Vice President, Advert ising
Subscribed and sworn to me this 8'h day of September,2022 .
'tiibbett, Notary Public for thc Statc of Washington, Residing in
ng, Washington ttlllt
DER
gsT,ARr
PUg.LIC
WA
;;.ri.'-'a&-.t*.:r*l :ii,:'.:r+o.'i+1ffii#g.:
CITY OF KENT
NOTICE OF ORDI.
NANCES
PASSED BY THE CITY
COUNCIL
The following are 6um-
maries of otdinances
passed by the Kent Cilv
Council on June 21.anna
ORDINANCE NO. 4430
- AN ORDINANCE of lhe
City Council of lhs Cityof Kent, Washington,
amending Chapter 2.34
of the Kent Ciiy Code.entitle.' "Municipal
Court". to adjust the
salarie$ for Judges. JUdg-es pro lem. and courl
conrmissioners. This or-
dinance shall take etfect
and be in force 30 days
from and after it5 pas-
sage, as provided by
law. Oncs this ordinanceis effective. llle salary
provisiqns shall apply
retroactively to July 1.
2022. All other provi-
sions. it any, shall applv
prospectively.
ORDINANCE NO. 4431
- AN ORDINANCE of the
Crty Council of lhe City
of Kenl. Washrnlrlon. va-
cating a portion ol Na-
don Slreet. also knowi)
as Naden A.venue. and
reserving an easenrent-
Thrs ordinanc€ shall take
effect and be in full force
five days after the date
of publicalion, as provid-
€d by law.
ORDINANCE NO.4432
- AN ORDINANCE of lhe
City Council of the Cily
of Kent. Washinglon. va-
cating a portion of right-
of-way consisting of an
alleyway situated be-
tw6en propertiss located
between Railroad Ave-
nue S. to the west and
Bridges Avenue S. to the
east, and E. Russell St.
lo the no(h and E. Mor-
Classified Proof
I ..'e "5r,.: .. . ' .
Proofbd by Jennifer Tribbett, 09108/7022 I l:12:28 arn Pagc: 2
Classified Proof
d*€iii *ffii;si'riY' ':l':i;;
ton Streel to the $outh.
and reseruing an ease-
ment and lhe right to
grant easements. This
ordinanco shall take ef-
fect and be in force 30
days konr and afler its
passage. as p,ovded by
law.
oRoINANCE NO. 4433
- AN OROINANCE of the
Ciiy Council of the City
of Kent. Washington,
amendrng the Parks and
Recrealion Elomant of
the city's Comprehen-
sve Plan related io th€
adoption oI lhe 2Q22-
2027 Pa'ir. and Open
Space Plan.
(CPA-2021-8). Thrs ordi-
narfce shall lake eftect
and be in force 3O days
from and after the date
of passage as provided
bv law
ORDINANCE NO. 4434
- AN ORDINANC€ of the
City Council ol lhe Cily
of Kent. Washington. re-
pealing Section 2 of Or
dinence No. 4234 thalhad adopted Kent's
2017 Surface Water De-
sign Manual. and adopt-ing Kenl's new 2O22
SurtacB Wat6r Design
Manual. This ordinance
shall take effect and be
in force 30 days from
and after its passage. as
provKled by law.
ORDINANCE NO. 4435
- AN OROINANCE of the
Cily Council o{ the City
ol Kent, Washinglon. re-
lalng to land us6 and
zonrng. speclfically the
rezorring ol a 2.23-acreparcel located al
24032 116th Ave SE.
Kent. Washinglon from
NCC (Neighborhood
Convenience Commer-
cial) to MRT-16 (Multi-
family Residential Town-
house Dislrict) Thrs
ordinanco shall take ef-
{€6t and be in force 30
days from and atter its
passage.
A copy ot th6 conrplete
lext of any ordrnance will
be mailed upon teqtrest
af the Crq/ Clerk
Kimbedey A. Komolo,
Crty Clerk
Kkonloto@KenlWA gov
?53-856-5725
#957040
06t24t22
Prooi'ed by Jennifer Tribbett, 09/08/2022 I l: l2:28 am Page: 3