HomeMy WebLinkAbout3602ORDINANCE NO. 60.9
AN ORDINANCE of the City Council of the
City of Kent, Washington, repealing Section 5, entitled
Standards for Stormwater Systems, of Ordinance No
3117, and adopting the Surface Water Design Manual
dated February, 2002, to implement the policies codified
in Chapters 7.05, 7 06 and 7.07 of the Kent City Code
WHEREAS, section five of the Kent Construction Standards entitled
"Standards for Stormwater Systems," originally adopted by Ordinance No 3117 on
May 18, 1993, has not been updated or amended since the date of adoption, and
WHEREAS, the public health, safety, and welfare of the residents of
the City of Kent require the proper design, construction, and maintenance of surface
water and stormwater control systems, and
WHEREAS, during the preparation of the Surface Water Design
Manual ("Manual"), staff met with the development community and a consulting firm
and the manual was subject to the SEPA process, and
WHEREAS, the Manual presents the City's strategy to reduce public
hazards due to floods, erosion and landslides and 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
Surface Water Design Manual
requirements, assists developers/engineers 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 Clear Water Acts, and
WHEREAS, on May 6, 2002, the Public Works Committee
recommended the Manual for adoption by the City Council, NOW THEREFORE,
THE CITY COUNCIL OF THE CITY OF KENT, WASHINGTON
DOES HEREBY ORDAIN AS FOLLOWS
SECTION 1. Real Section 5 of the Kent Construction Standards,
entitled "Standards for Stormwater Systems," which was originally adopted by
Ordinance No 3117, is hereby repealed in its entirety, PROVIDED, that all other
sections of the Kent Construction Standards adopted by Ordinance No 3117 shall
remain in effect unless previously or hereafter amended or repealed
SECTION Z Adoption The Surface Water Design Manual, attached
hereto as Exhibit A to this Ordinance, is hereby adopted
SECTION 3. Severability If any section, subsection, paragraph,
sentence, clause, or phrase of this ordinance is declared unconstitutional or invalid for
any reason, such decision shall not affect the validity of the remaining portions of this
ordinance
2 Surface Water Design Manual
SECTION 4. E fective Date This ordinance shall take effect and be in
force thirty (30) days from and after its passage as proviqqd
ATTEST -
APPROVED
CITY CLERK
FORM
-ARTHUT" FITZPATRICK,
ACTING CITY ATTORNEY
PASSED day of lw&'a_ 2002
APPROVED day of %1%494�_ '2002
PUBLISHED c2,15 day of 2002
I hereby certify that this is a true copy of Ordinance No 9� w
passed by the City Council of the City of Kent, Washington, and approved by the
Mayor of the City of Kent as hereon indicated
P \Gvd\Ordinance\SurfaceWaterDe ignMa al dac
(SEAL)
BI�E3 FSR, CITY CLERK
{Np�eYSimmons� AcTi.�c.
3 Surface Water Design Manual
' CITY OF KENT
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City of Kent
Surface Water Design Manual
(Chapter 5, Kent Construction Standards)
W A S 7 N70 7 O N
May 2002
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1 ACKNOWLEDGEMENTS
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This Manual is based on the 1998 King County Surface Water Design Manual Many individuals
associated with King County were involved in preparation of that Manual Acknowledgement of these
individuals is included at the front of that Manual
Individuals involved in preparation of the Kent Surface Water Design Manual are as follows
1 Mayor
Jim While
Kent City Council
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Tun Clark
Connie Epperly
Leona Orr*
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Julie Peterson*
Bruce White
Judy Woods
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Rico Yingling*
(* Public Works Committee Members)
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Chief Administrative Officer
Mike Martin
Public Works Director
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Don Wickstrom, P E
City Engineer
Gary Gill, P E
Environmental Engineering Manager
William Wolmski, P E
Project Manager
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Phil Noppe
Consultants
Ralph Nelson, P E - ENTRANCO
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Bruce Barker, P E - MGS Engineering Consultants
Gary Minton, PhD, P E - Resource Planning Consultants
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Technical Review
Mike Mactutis, P E
Dave Brock, P E
Mel Daley, P E DMP, Inc
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Dave Bockrath, POE Engineering
Dennis Alfredson, Schneider Homes
Tom Hemphill, ADS
GIS Mapping
Jim Cordova
Kurt Palowez
Document Production
Kenta Hadley
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PREFACE
HOW TO USE
' THIS DOCUMENT
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a � s m T 7
W A S H I N G O N
CITY OF KENT
Surface Water Design Manual
(Chapter 5 - Kent Construction Standards)
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PREFACE
HOW TO USE THIS DOCUMENT
This stormwater section is an addendum to the 1998 King County Surface Water Design Manual
(KCSWDM) and applies to development proposals within the City of Kent It includes all changes and
deletions to the KCSWDM adopted by the City of Kent and is to be used for guidance in drainage review
and design of stormwater facilities within the City
Addendum Organization
The information presented in this addendum is organized as follows
•
Preface How to Use This Document. This preface provides instructions for using the City of
Kent's addendum to the KCSWDM It also defines terns in the King County manual that are used
differently for the City of Kent, 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 1—Drainage Review Requirements: A completely revised Chapter I has been provided
in this addendum it is to be used instead of Chapter 1 in the 1998 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 eight Core and five Special Requirements
• Chapter 2—Dramage Plan Submittal: A completely revised Chapter 2 has been provided in this
addendum It is to be used instead of Chapter 2 in the 1998 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 1998 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 1998 KCSWDM This addendum to Chapter 4 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 5—Flow Control Design- The City of Kent has made minor changes to Chapter 5 of the
1998 KCSWDM This addendum to Chapter 5 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 1998 KCSWDM This addendum to Chapter 6 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 Drainage 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: King County Reference Sections 1, 2, 3, 4b, 6A, and 7 do not apply to the City of Kent
The King County version of Reference sections 4A, 5, and 6B apply to proposals in the City of Kent
The City has modified and replaced several references within Section 8 This document includes the
revised Sections 8-A through 8-J
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 Public Works Department:
• DDES (Department of Development and Environmental Services)
• DNR (Department of Natural Resources)
• SWM (Surface Water Management)
• 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 11, 14, and 15.
COUNTY DESIGNATIONS THAT DO NOT APPLY IN THE CITY
The following designations are used in the 1998 KCSWDM but are not currently 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
• 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 ci kent wa us/PublicWorks/Stormwatei The web site will not be used to make
i evisions in key policy areas— such as the thresholds and minimum requirements in Chapter 1 We
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encourage you to visit this web site for 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
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TABLE OF CONTENTS
W 7S 7 N G 7 O 74
CITY OF KENT
SURFACE WATER
' DESIGN MANUAL
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TABLE OF CONTENTS
Chapter 1 — Drainage Review And Requirements
tSection
Section 1 1
Drainage Review
5.1-3
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Section 1 1 1
Projects Requiring Drainage Review
51-8
Section 1 2 2 Core Reqmt #2
Section Core
Section 1 12
Drainage Review Types and Requirements
5 1-9
'
Section 1 1 3
Drainage Review Required By Other Agencies
51-16
Section 1 2 4 Core Reqmt #4
Section 1 1 4
Drainage Design Beyond Minimum Compliance
51-17
tSection
12 Core Requirements
5.1-18
'
Section 1 2 1 Core Reqmt #1
Discharge at the Natural Location
51-18
Section 1 27 Core Reqmt #7 Financial Guarantees and Liability
Section 1 28 Core Reqmt #8 Water Quality
Section 1 2 2 Core Reqmt #2
Section Core
Offsite Analysis
51-19
51-46
1 2 3 Reqmt #3
Flow Control
51-25
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Section 1 2 4 Core Reqmt #4
Conveyance System
51-36
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Section 1 25 Core Reqmt #5 Erosion and Sediment Control
5 1-40
'
Section 1 26 Core Reqmt #6 Maintenance and Operations
51-43
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Section 1 27 Core Reqmt #7 Financial Guarantees and Liability
Section 1 28 Core Reqmt #8 Water Quality
51-45
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Section 1 43 Adjustment Application Process
51-46
Section 13 Special Requirements
51-53
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Section 1 3 1 Special Reqmt #1 Other Adopted Area -Specific Requirements
5 1-53
Section 1 32 Special Reqmt #2 Development Within Flood plain/Floodway
51-54
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Delineation
Section 1 33 Special Reqmt
5.2 -
#3 Flood Protection Facilities
51-55
Section 1 34 Special Reqmt #4 Source Controls
51-55
Section 1 3 5 Special Reqmt #5 Oil Control 51-56
Section 1 3 6 Redevelopment Requirements 51-58
Section 14 Adjustment Process
5.1-61
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Section 1 41 Adjustment Authority
51-62
Section 1 42 Criteria for Granting Adjustments
5 1-62
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Section 1 43 Adjustment Application Process
51-62
Section 1 44 Adjustment Review Process
51-63
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Section 1 45 Appeal Procedure
5 1-64
Chapter 2 — Drainage Plan Submittal
Section 2.1 Plans for Permits and Drainage Review
5.2 -
Section 2.2 Plans Required with Initial Permit Review
52 -
2 -Section
Section 2 2 1 Subdivisions and Short Plats
52-5
Section 2 2 2 Commercial Site Development
52-7
Section 2 2 3 Single -Family Residential
52-7
Section 2 2 4
Other Permits
52-7
Section 2.3
Plans Required for Budding Permit Drainage Review
5.2 -
Section 2 3 1
Engineering Plan Specifications
52-8
Section 2 3 2
Projects in Targeted Drainage Review
52-28
Section 2 4
Plans Required After Drainage Review
5.2-31
Section 2 4 1
Plan Changes After Permit Issuance
52-31
Section 2 4 2
Final Corrected Plan Submittal
52-31
Section 2 4 3
Final Plat, Short Plat, and Binding Site Plan Submittals
52-31
Chapter 3 — Hydrological Analysis and Design
refer to the 1998 King County Surface Water Design Manual
Chapter 4 — Conveyance System Analysis and Design
Section 4 1 Design Criteria
54-1
Section 4 2 1 1 Design Criteria
54-2
Section 4 3 1 1 Design Criteria
54-7
Section 4 4 2 Datum Correlations
54-8
For all other sections, refer to the 1998 King County Surface Water Design Manual
Chapter 5 — Flow Control Design
Section 5 3
Detention Facilities
55-1
Section 5 3 1
Detention Ponds
55-1
Section 5 3 2
Detention Tanks
55-3
Section 5 3 3
Detention Vaults
55-3
Section 5 3 5
Other Detention Options
55-3
For all other sections, refer to the 1998 King County Surface
Water Design Manual
Chapter 6 — Water Quality Design
Section 6 1 3 Resource Protection Menu 56-1
Section 6 2 General Requirements for WQ Facilities 56-1
For all other sections, refer to the 1998 King County Surface Water Design Manual
Appendices
A — Kent Maintenance Requirements for Privately Maintained Drainage Facilities
King County Appendix B does not apply, See the 1998 King County Surface Water
Design Manual for Appendices C and D
References
4A — Landscape Management Plan Guidelines
5 — Bibliography of Supporting Studies and Research
6B — Pond Geometry Equations
8-A — Technical Information Report (TIR) Worksheet
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8-B — Offsite Analysis Drainage System Table
8-C — Flow Control and Water Quality Facility Summary Sheet and Sketch
8-D — Bond Quantities Worksheet — (contact the City of Kent Public Works Department)
8-E— Surface Water and Drainage Facilities Construction Bond
8-F — Declaration of Stormwater Facility Maintenance Covenant
8-G — Drainage Easements
8-H — Water Quality Memorandum
8-1 — Adjustment Application Form and Process Guidelines
8-J — Plat Dedication Clause — Final Recording
Maps (included as hard copy fold -outs and PDF files on CD)
Flow Control Applications Map
Water Quality Applications Map
Landslide Hazard / Landslide Hazard Drainage Areas Map
Erosion Hazard Areas Map
Soils / Wellhead Protection Area Map
Wetland Map
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CHAPTER 1
DRAINAGE REVIEW
AND REQUIREMENTS
WASHINGTON
CITY OF KENT
SURFACE WATER
DESIGN MANUAL
Section 1.1 Drainage Review 5.1-3
Section 1 1 1
Projects Requiring Drainage
51-8
51-19
Review
Section 1 1 2
Drainage Review Types and
51-9
Requirements
51-36
Section 1 1 3
Drainage Review Required By
51-16
51-40
Other Agencies
Section 1 1 4
Drainage Design Beyond
51-17
Minimum Compliance
51-45
Section 1.2
Core Requirements
5.1-18
Section 1 2 1 Core Requirement #1 Discharge 5 1-18
at the Natural Location
Section 12 2 Core Requirement #2 Offsite
51-19
Analysis
Section 1 2 3 Core Requirement #3 Flow
51-25
Control
Section 1 24 Core Requirement #4
51-36
Conveyance System
Section 1 25 Core Requirement #5 Erosion
51-40
and Sediment Control
Section 1 26 Core Requirement #6
51-43
Maintenance and Operations
Section 1 27 Core Requirement #7. Financial
51-45
Guarantees and Liability
Section 128 Core Requirement#8 Water
51-46
Quality
Section 1.3 Special Requirements
5.1-53
Section 1 3 1 Special Requirement #1 Other
51-53
Adopted Area -Specific
Requirements
Section 1 3 2 Special Requirement #2
51-54
Developments Within
FloodplamlFloodway
Section 1 3 3 Special Requirement #3 Flood
51-55
Protection Facilities
Section 1 3 4 Special Requirement #4 Source
51-55
Controls
Section 1 3.5 Special Requirement #5* Oil
51-56
Control
Section 1 3 6 Redevelopment Requirements
51-58
Section 1.4 Adjustment Process
5.1-61
Section 141 Adjustment Authority
51-62
Section 1 4 2 Cntena for Granting Adjustments
51-62
Section 1 4 3 Adjustment Application Process
51-62
Section 1 4 4 Adjustment Review Process
51-63
Section 1.4 5 Appeal Procedure
51-64
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CHAPTER I
DRAINAGE REVIEW AND REQUIREMENTS
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' 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,
7 06, and 7 07 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
t The information presented in Chapter 1 is organized into four main sections as follows
• Section 1 1, "Drainage Review"
'• Section 1 2- "Core Requirements"
• Section 1 3, "Special Requirements"
• Section 1 4, "Adjustment Process"
' Each section begins on an odd page so that tabs can be inserted by the user if desired for quicker
reference
Key Words and Phrases
Several key words and phrases have specific definitions as they are used in this manual, those of particular
importance in determining drainage requirements are listed below These and other terms are defined in
the "Definitions" section in the back of the King County Manual Many of these terms are also defined in
Section 1 1 below and when first used in this chapter
• Acceptable discharge point • Natural discharge area
• Closed depression • New impervious surface
• Construct or modify 0 Pollution -generating impervious surface
• Direct discharge 0 Pollution -generating pervious surface
• Drainage area 0 Project site
• Equivalent area 0 Redevelopment project
' Existing site conditions 0 Replaced impervious surface
• Flow durations 0 Single-family residential project
• Flow path • Site (see also onsite and offsue)
' High -use site 0 Surface flow
• Hydraulically connected 0 Threshold discharge area
City of Kent Surface Water Design Manual
(Chapter 5 — Kent Construction Standards) 5 1-1 May, 2002
1 1.1 DRAINAGE REVIEW
Drainage i eview is the evaluation by the Public Works permit review staff of a proposed project's
compliance with the drainage requirements of this manual During drainage review, Public Works permit
review staff may also consider the proposed 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.
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Guide to Using Section 1.1
The following steps are recommended for efficient use of Section 1 I
1 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 1. use the flow chart in Figure 1 1 2 A to determine
what type of drainage review will be conducted by Public Works The type of drainage review
defines the scope of drainage requirements that will apply to your project as summarized in Table
1 1 2 A
3 Check the more detailed threshold information in Section 1.1.2 to verify that you have determined the
correct type of drainage review
4 After verifying drainage review type, use the information in Section 1 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 13 of this chapter.
Note For Steps 2 through 4, it is recommended that you arrange a preapplication meeting with Public
Works permit review slaff io confirm the type of drainage i eview and scope ofdramage regua ements that
apply to your proposed project
❑ KEY DEFINITIONS
' May, 2002
Proper application of the drainage review thresholds in this section requires an understanding of the key
definitions listed below Other definitions can be found in the "Definitions" section of the King County
Manual
Acceptable discharge point An enclosed drainage system (i e, pipe system, culvert, or tightlme) 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
Base flood The flood having a one percent chance of being equaled or exceeded in any given year Also
referred to as the "100 -year flood" The base flood is determined for future flow conditions, except in
City of Kent Surface Water Design Manual
5 1-3 (Chapter 5 — Kent Construction Standards)
SECTION I I DRAINAGE REVIEW
areas where the FEMA Flood Insurance Study Includes detailed base flood calculations In that case, the
the FEMA data shall apply
Construct or modify: To Install a new drainage pipe/ditch or make Improvements to an existing drainage
pipe/ditch (for purposes other than routine maintenance, repair, or emergency modifications, and
excluding driveway culverts installed as part of single-family residential building permits) that either
serves to concentrate previously unconcentrated surface and stormwater runoff or serves to Increase,
decrease, and/or redirect the conveyance of surface and stormwater runoff
Contiguous Pollution -Generating Impervious Surface (CPGIS): a discrete patch of PGiS that is all
together as opposed to being separated in different locations on the project site The intent is to identify
those redevelopment projects that are replacing and/or adding enough impervious surface in one location
to allow for opportune installation of a water quality treatment facility The threshold of 5,000 square feet
or more of contiguous PGIS shall be applied to by Threshold Discharge Area
Erodible or leachable materials, wastes, or chemicals Substances that, when exposed to rainfall,
measurably alter the physical or chemical characteristics of the rainfall runoff (examples include erodible
soil, uncovered process wastes, manure, fertilizers, oily substances ashes, kiln dust, garbage dumpster
leakage, etc )
Erosion hazard area: Areas within the City of Kent underlain by soils which are subject to severe
erosion when disturbed Such soils include, but are not limited to those delineated in the Soil Survey, King
County Ai ea, Washington as having a moderate to severe, severe to very severe erosion hazard potential
Existing flooding 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 conditions: depend on what, if any, land conversion occurred on the site since December
1978, when Kent first required flow control for new development (Ordinance 2130) IF a drainage plan
has been approved by the City since December 1978 for any land conversion activity which includes the
addition of more than 5,000 square feet of new impervious surface, AND the plans indicate a stormwater
management system was designed according to standards in place at the time, AND the same system
shown on the approved plan was constructed and is still operating according to the design, THEN existing
site conditions are those created by the site improvement (In this case, including any drainage facilities
constructed per the approved engineering plans It is the responsibility of the project proponent to submit
such documentation to the Public Works Department ) OTHERWISE, existing site conditions are those
that were present on December 1978 as determined from aerial photographs, and if necessary, on
knowledge of individuals familiar with the area The intent is to mitigate unadressed impacts created by
site alterations or improvements, such as clearing, which have occurred since December 1978
Finished area Any enclosed area of a building that is designed to be served by the building's permanent
heating or cooling system
Floodway The channel of the river or stream and those portions of the adjoining floodplams which are
reasonably required to carry and discharge the base flood flow The portions of the adjoining floodplams
that are considered to be "reasonable required" are defined by the City flood hazard regulations as stated
in Chapter 14 of the Kent City Code
Flow duration 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)
Full 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
Geologic Hazard Areas Include Seismic Hazard Areas, Erosion Hazard Areas, and Landslide Hazard
Areas
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City of Kent Surface Water Design Manual ,
(Chapter 5 —Kent Construction Standards) 5 1-4 May, 2002
SECTION I 1 DRAINAGE REVIEW
Habitable budding Any residential, commercial, or industrial budding that is equipped with a permanent
heating or cooling system and an electrical system
High -use site: A commercial or industrial site that (1) has an expected average daily traffic (ADT) count
' equal to or greater than 100 vehicles per 1,000 square feet of gross building area, (2) is subject to petroleum
storage or transfer in excess of 1,500 gallons per year, not including delivered heating oil, or (3) is subject to
use, storage, or maintenance of a fleet of 25 or more diesel vehicles that are over 10 tons net weight (trucks,
buses, trains, heavy equipment, etc ) Also included is any road intersection 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,
excluding projects proposing primarily pedestrian or bicycle use improvements
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Impervious surface: A hard surface area which either prevents or retards the entry of water into the soil
mantle as under natural conditions prior to development, and/or a hard surface which causes water to run off
the surface in greater quantities or at an increased rate of flow from the flow present under natural conditions
prior to development Common impervious areas include, but are not limited to, rooftops, walkways,
patios, driveways, parking lots, or storage areas, concrete or asphalt paving, gravel roads, packed earthen
materials, and oiled macadam, or other surfaces which similarly impede the natural infiltration of surface
and stormwater runoff Open, uncovered flow control or water quality treatment facilities shall not be
considered impervious surfaces (see also "new impervious surface")
Landscape management plan An approved plan for defining the layout and long-term maintenance of
landscaping features to minimize the use of pesticides and fertilizers, and to reduce the discharge of
suspended solids and other pollutants Guidelines for preparing landscape management plans can be
found in Reference Section 4-A Submetal requirements are detailed in Section 2 3 1 4
Landslide hazard area: The following shall be classified as landslide hazard areas
1 Any existing active or dormant landslide
2 Areas delineated in the "Soil Survey, King County Area, Washington" as having a "severe"
limitation for building site development These soils consist of the following
• Alderwood gravelly sandy loam, 15 to 30 percent slopes (AgD)
• Alderwood and Kitsap sods, very steep (AkF)
• Beausue gravelly sandy loam, 15 to 30 percent slopes (BeD)
• Beausite gravelly sandy loam, 40 to 75 percent slopes (BeF)
• Everett gravelly sandy loam, 15 to 30 percent slopes (EvD)
• Indianola loamy fine sand, 15 to 30 percent slopes (InD)
• Kitsap silt loam, S to 15 percent slopes (KpQ
• Kitsap silt loam, 15 to 30 percent slopes (KpD)
• Ovall gravelly loam, 15 to 25 percent slopes (OvD)
• Ovall gravelly loam, 40 to 75 percent slopes (OvF)
• Ragnar fine sandy loam, 15 to 25 percent slopes (RaD)
• Ragnar-Indianola association, moderately steep (RdE)
3 Areas designated as quaternary slumps, earth flows, mudflows, or landslides on maps published
by the US Geological Survey, Washington Department of Natural Resources, or geologic
consultant reports completed for the City of Kent
4 Areas with all three of the following characteristics
slopes steeper than 15 percent,
slopes intersecting granular material over silts or clays, and
springs or ground water seepage or evidence of seasonal springs or ground water seepage
5 Slopes that are parallel or subparallel to planes of weakness (such as bedding planes, joint
systems, and fault planes) in subsurface materials
City of Kent Surface Water Design Manual
51-5 (Chapter 5— Kent Construction Standards)
SECTION 1 1 DRAINAGE REVIEW
6 Slopes subject to failure during seismic shaking
7 Areas potentially unstable as a result of rapid stream Incision or stream bank erosion
S Areas located in a canyon or on an active alluvial fan, presently or potentially subject to
Inundation by debris flows or catastrophic flooding
9 Any area with a slope of 40 percent or steeper and with a vertical relief of 10 or more feet A
slope is delineated by establishing its toe and top and measured by averaging the inclination over
10 feet of vertical relief
Landslide hazard drainage area. Specially mapped areas where the City has determined that overland
flows from new projects will pose a significant threat to health and safety because of their close proximity
to landslide hazard areas Such areas are delineated on the Landslide Hazard/ Landslide Hazard
Drainage Areas Map adopted with this manual (see map pocket inside cover)
Natural discharge location: The location where runoff leaves the project site under existing site conditions
Natural onsite drainage feature A natural swale, channel, stream, closed depression, wetland, or lake
New impervious surface. The addition of a hard or compacted surface such as roofs, pavement, gravel,
or dirt, or the addition of a more compacted surface such as the paving of pre-existing dirt or gravel
NPDES: National Pollutant Discharge Elimination System
Organic content: Measured on a dry weight basis using ASTM D2974
Peak discharges: Computed using KCRTS as detailed in Chapter 3
Pollution -generating impervious surface (PGIS): Those impervious surfaces considered to be a
significant source of pollutants in stonnwater runoff Such surfaces include those subject to vehicular use
or storage of erodible or leachable materials, wastes. or chemicals, and that receive direct rainfall or the
rum -on or blow-in of rainfall Metal roofs are also considered to be PGIS unless they are treated to prevent
leaching
Pollution -generating pervious surface (PGPS): Any non -impervious surface with vegetative ground
cover subject to use of pesticides and fertilizers Such surfaces include, but are not limited to, the lawn
and landscaped areas of residential or commercial sites, golf courses, parks, and sports fields
Project site: That portion of a property or properties subject to proposed project improvements including
those required by this manual
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SECTION 1 I DRAINAGE REVIEW
Redevelopment project: A project that proposes to add, replace, and/or alter impervious surface (for
purposes other than routine maintenance, resurfacing, regrading, or repair) on a site that is already
substantially developed (I e , has 35% or more of existing impervious surface coverage) The following
examples illustrate the application of this definition
Example of an Existing
Site Condition for a
Redevelopment Project
Existing Impervious
Area (35%)
L!4
0, t_Existing
Pervious
ing Area
(65%)
Example of a Proposed
Redevelopment Project
that Alters Existing
Impervious Surface
Impervious Area (35%)
h
g9 Pervious
Area
Parking (65%)
Example of a Proposed
Redevelopment Project
that Adds to Existing
Impervious Surface
Impervious Area (85%)
Parking
Replaced impervious surface- Any existing Impervious surface on the project site that Is proposed to be
removed down to bare soil or base course and replaced with pollution -generating impervious surface,
excluding Impervious surface removed for the sole purpose of Installing utilities
Roadway. The traveled portion of any public or private road or street
Single-family residential project: A project that constructs or modifies a single-family dwelling unit
and/or makes related onsite improvements, such as driveways, roads, outbuildings, play courts, etc , or a
project that creates single-family residential lots such as a plat or short plat
Site The legal boundaries of the parcel or parcels of land for which an applicant has or should have
applied for authority from Kent to carry out a development activity, Including any drainage improvements
required by this manual
Subject to vehicular use As used in the definition of pollution -generating surrface, means a surface,
whether paved or not, that is regularly used by motor vehicles The following are considered regularly -
used surfaces roads, unvegetated road shoulders, bicycle lanes within the traveled lane of a roadway,
driveways, parking lots, unfenced fire lanes, diesel equipment storage yards, and airport runways The
following are not considered regularly used surfaces road shoulders primarily used for emergency
parking, paved bicycle pathways, bicycle lanes adjacent to unpaved or paved road shoulders primarily
used for emergency parking, fenced fire lanes, and Infrequently used maintenance access roads
City of Kent Surface Water Design Manual
51-7 (Chapter 5— Kent Construction Standards)
SECTION 1 1 DRAINAGE REVIEW
Threshold discharge area: An onsite area draining to a single natural discharge location or multiple
natural discharge locations that combine within one -quarter -mile downstream (as determined by the
shortest flow path) The examples below illustrate this definition The purpose of this definition is to
clarify how the thresholds of this manual are applied to project sites with multiple discharge points '
Example of a Project Site
Example of a Project Site
Example of a Project Site
with a Single Natural
with Multiple Natural
with Multiple Natural
Discharge and a Single
Discharges and a Single
Discharges and Multiple
Threshold Discharge Area
Threshold Discharge Area
Threshold Discharge Areas
\ Natural
\.�M, atiaige--
, Natlral "Natural
Dischar'ge'j,__,tiseh
tUraI Natural
Dische;e_ Discharge—
iii Area
Areal • Area' 2,,
Area f Area 2
THRESHOLD
ISti;HQRGE' •
1HRE I-jqLp
D($G ARG6r�
THRESF�IOLD THRESHOLD
ISCHAf1GE DISCHAfiGt '
AR
\A "�
AREA t 2 W
- i( Shaded) 'z
EA:
ded)
�AllEA
hade)
`�
Natural'
Natural
Natural
• Discharge
Discharge
Discharge
' Location
Locations
Locations
1/4 Mile Downstream—'-----------------------------
=------
(shortest flow path) V
V V
Treatment train: 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 portion. Any portion outside the ordinary high water line of streams, lakes, and wetlands
Wetlands: 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 vegetation typically adapted for life in saturated soil conditions Wetlands generally include
swamps, marshes, and similar areas Wetlands do not include those articial 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 amentities 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 in the Wetlands Delineation Manaul, U S Army Corps of
Engineers Technical Report Y-87-1 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 (Cowardm et al 1979) (See Kent
City Code Chapter 11 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 l 05)
City of Kent Surface Water Design Manual
(Chapter 5 —Kent Construction Standards) 5 1-8 May, 2002
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SECTION I 1 DRAINAGE REVIEW
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 Adds 5,000 square feet' or more of new impervious surface, OR
2 Proposes to construct or modify a drainage pipe/ditch that is 12 inches or more in size/depth, or
receives surface and stormwater runoff from a drainage pipe/ditch that is 12 inches or more in
size/depth, OR
3 Contains or is adjacent to a floodplain, stream, lake, wetland', closed depression, erosion hazard
area' or landslide hazard area', OR
4 Is located within a Landslide Hazard Drainage Areae and adds 2,000 square feet or more of new
impervious surface, OR
5 Is a redevelopment project proposing $100,000' or more of improvements to an existing high -use
site, OR
6 Is a redevelopment project proposing $500,000 or more of site improvements and creates 5,000
square feet or more of contiguous° pollution -generating impervious surface through any
combination of new and/or replaced impervious surface
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 Plat)
Subdivision Developments (Plat)
I The threshold of 5,000 square feel or more of new impervious surface shall be applied by threshold discharge area and shall
include all impervious surface that will ultimately result from the proposed protect (e g , impervious surface that will result from
future homes within a plat or short plat)
2 Landslide Hazard Areas, Landslide Hazard Drainage Areas and Erosion Hazard Areas and Wetlands are defined in the
section titled "Key Definitions" above Maps have been included in the pocket of this manual showing locations of these
areas
3 This is the "protect valuation" as declared on the Public Works and Construction Permit applications submitted to the City
a Contiguous pollution -generating impervious surface (PGIS) means a discrete patch of PGIS that is all together as opposed to
being separated in different locations on the protect site The intent is to identify those redevelopment protects that are
replacing and/or adding enough impervious surface in one location to allow for opportune installation of a water quality
treatment facility The threshold of 5,000 square feet or more of contiguous PGIS shall be applied by threshold discharge
area
' City of Kent Surface Water Design Manual
May, 2002 5 1.9 (Chapter 5 — Kent Construction Standards)
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SECTION I I DRAINAGE REVIEW '
If drainage review is required for the proposed project, the type of drainage review must be determined '
based on project and site characteristics as described in Section 1 1 2 The type of drainage review defines
the scope of drainage requirements that must be evaluated for project compliance with this manual
1.1.2 DRAINAGE REVIEW TYPES AND REQUIREMENTS '
For most protects adding 5,000 square feet or more of impervious surface, the full range of core and '
special requirements contained in Sections 1 2 and 1 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 of drainage requirements to a project's size, location type of
development, and anticipated impacts to the local and regional surface water system,
• Small Site Drainage Review, Section 1 1 2 1 '
• Targeted Drainage Review, Section 1 1 2 2
• Full Drainage Review, Section 1 12 3 '
Each project requires only one of the above drainage review types, with the single exception that a project
that qualifies for Small Site Drainage Review may also require Targeted Drainage Review Figure
1 1 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 1 1 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
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(Chapter 5 — Kent Construction Standards) 5 1-10 May, 2002
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Is the project a single-family residential project (as
defined in Section 1 1) that adds 2,000 to 10,000 sf of Yes
new impervious surface AND clears < 2 acres OR <
35% of the site, whichever is greater?
M
Does the project add _> 2,000 sf of
new impervious surface within a No
Landslide Hazard Drainage Area
(LHDA) or >_ 5,000 sf outside of a
LHDA OR is it a redevelopment
project costing >_ $500,000 that
creates >_ 5,000 sf of contiguous
PGIS from new and/or replaced
impervious surface?
I Yes
FULL DRAINAGE REVIEW,
Section 1,1.2.3 '
SECTION I I DRAINAGE REVIEW
SMALL SITE DRAINAGE REVIEW
Section 1 1 2 1
Note The project may also be subject to Targeted
Drainage Review as determined below
Does the project have the characteristics of one or more of the following
categories (see the more detailed threshold language in Section 1 1 2 2)?
1 Projects that contain or are adjacent to floodplams, streams, lakes,
wetlands, closed depressions, landslide hazard areas or erosion hazard
areas, OR projects within a Landslide Hazard Drainage Area
2 Projects proposing to construct or modify a drainage pipelditch 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 an
existing high -use site
M
Yes
Reassess whether TARGETED DRAINAGE REVIEW
drainage review is
required per Section Section 1 1 2 2
1121
City of Kent Surface Water Design Manual
May, 2002 5 1-11 (Chapter 5 —Kent Construction Standards)
SECTION I 1 DRAINAGE REVIEW
City of Kent Surface Water Design Manual
(Chapter 5 —Kent Construction Standards) 5 1-12 May, 2002
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Small Site Targeted
Full
Drainage Drainage
Drainage
Review Review
Review
Single-family Small Site projects or other small projects that
All projects, including
residential are not subject to Full Drainage Review as
redevelopment
projects that add determined in Section 1 1 2 3, AND which
projects, that add >_
2,000 to 10,000 have the characteristics of one or more of the
5,000 sf (2,000 sf
sf of new following categories of projects
within a Landslide
impervious 1 Projects containing or adjacent to
Hazard Drainage
surface AND floodplains/ streams, lakes, wetlands, or
Area) of new
clear closed depressions, landslide hazard areas
impervious surface but
< 2 acres or < or erosion hazard areas, or projects within a
do not qualify for
35% of the site, Landslide Hazard Drainage Area
Small Site Drainage
whichever is 2 Projects proposing to construct or modify a
Review, OR
greater drainage pipe/ditch that is 12" or larger or
redevelopment
receives runoff from a 12" or larger
projects costing >_
drainage pipe/ditch
$500,000 that create >
3 Redevelopment projects proposing
5,000 sf of contiguous
>_ $100,000 in improvements to a
PGIS from new and/or
high -use site l')
replaced impervious
surface
Category Category Category
1 2 3
SMALL SITE REQUIREMENTS
CORE REQUIREMENT #1
*(2)
✓
✓
✓
Discharge at Natural Location
CORE REQUIREMENT #2
✓
V/
*(z)
�(3)
Offsite Analysis
CORE REQUIREMENT#3
*(2)
*(2)
*(2)
'1,/(3)
Flow Control
CORE REQUIREMENT #4
*(2)
V/
*(2)
v/
Conveyance System
CORE REQUIREMENT#5
✓
✓
✓
Erosion & Sediment Control
CORE REQUIREMENT #6
*(2)
✓
✓
✓
Maintenance & Operations
CORE REQUIREMENT #7
*(2)
✓(3)
V/(3)
V/(3)
Financial Guarantees & Liability
CORE REQUIREMENT#8
*(2)
*(2)
✓(3)
✓(3)
Water Quality
SPECIAL REQUIREMENT#1
✓(3)
*(2)
V/ (3)
Other Adapted Requirements
SPECIAL REQUIREMENT #2
V63)
V/(3)
Developments within
Floodplain/Floodway
SPECIAL REQUIREMENT #3
✓(3)
*(2)
V/(3)
Flood Protection Facilities
SPECIAL REQUIREMENT #4
V/(3)
V/(3)
V/(3)
V/(3)
Source Control
SPECIAL REQUIREMENT #5
V/(3)
✓(3)
Oil Control
1 Category 3 projects that install oil controls that construct or modify a 12 -Inch pipe/ditch are also Category 2 projects
2 May be applied by Public Works based on project or site-specific conditions
3 These requirements have exemptions or thresholds that may preclude or limit their application to a specific project
City of Kent Surface Water Design Manual
(Chapter 5 —Kent Construction Standards) 5 1-12 May, 2002
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SECTION 11 DRAINAGE REVIEW
1.1.2.1 SMALL SITE DRAINAGE REVIEW
Small Site Drainage Review is a simplified alternative to Full Drainage Review for small residential
projects adding less than 10,000 square feet of new impervious surface and restricting site clearing to less
than 2 acres or less than 35% of the site, whichever is greater The core and special requirements applied
under Full Drainage Review are replaced with simplified small 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 alternative 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 Small 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
r Small Site Drainage Review is allowed for any project that is subject to drainage review as determined in
"
a Section 1 1 1 and that meets all of the following criteria
H • The project is a single-family i esidennal projects, AND
° • The project adds 2,000 to 10,000 square feet6 of new impervious surface, AND
0
• The project clears less than 2 acres or less than 35% of the site, whichever is greater
Note Some projects qual Eying for Small Site Drainage Review may also require Targeted Drainage
Review if they meet any of the threshold criteria in Section 1 12 2
Any potential small site proposal may elect to go through Full Drainage Review described in Section
1123
Scope of Requirements
R IF Small Site Drainage Review is allowed, THEN the applicant may apply the simplified small site
e submittal and drainage design requirements detailed in Small Site Drainage Requirements adopted as
w Appendix C to the King County Manual (detached) and available as a separate booklet from King County
s Department of Natural Resources or Department of Development and Environmental Services These
requirements include simplified BMPs for flow control and erosion and sediment control Note An open
space tract or covenant may be required to preserve uncleared areas
Exemption from Core and Special Requirements
The simplified drainage requirements applied under Small Site Drainage Review are considered sufficient
to meet the overall intent of the core and special requirements in Sections 1 2 and 13, except under certain
conditions when a proposed project has characteristics that trigger Targeted Drainage Review (see the
threshold for Targeted Drainage Review in Section 1 12 2) and may require the involvement of a licensed
civilengineer Therefore, any proposed project that qualifies for Small Site Drainage Review as determined
above and complies with the small site drainage requirements detailed in Appendix C is considered exempt
s Single-family residential project is defined on page 5 14
6 The threshold of 10,000 square feet of new impervious surface shall be applied by threshold discharge area and shall
include all impervious surface that will ultimately result from the proposed project (e g , impervious surface that will result from
future homes within a plat or short plat)
City of Kent Surface Water Design Manual
May, 2002 5 1-13 (Chapter 5 —Kent Construction Standards)
SECTION 1 1 DRAINAGE REVIEW '
from all core and special requirements in Sections 12 and 1 3 except those which would apply to the project '
if it is subject to Targeted Drainage Review as specified m Section 1 12 2
1.1.2.2 TARGETED DRAINAGE REVIEW '
Targeted Drainage Review (TDR) is an abbreviated evaluation by Public Works 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 1
concerns that must be addressed by a licensed civil engineer or by Public Works 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
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 Public '
Works 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
Construct and modify is defined as to install a new drainage pipelditch or make improvements to an existing drainage
pipe/ditch (for purposes other than routine maintenance, repair, or emergency modifications, and excluding driveway culverts
installed as part of single-family residential building permits) that either serves to concentrate previously unconcentrated
surface and stormwater runoff or serves to increase, decrease, and/or redirect the conveyance of surface and stormwater
runoff
a A high -use site is defined as a commercial or industrial site that (1) has an expected average daily traffic (ADT) count equal to
or greater than 100 vehicles per 1,000 square feet of gross building area, (2) is subject to petroleum storage or transfer in
excess of 1,500 gallons per year, not including delivered heating oil, or (3) is subject to use, storage, or maintenance of a fleet
of 25 or more diesel vehicles that are over 10 tons net weight (trucks, buses, trains, heavy equipment, etc ) Also included is
any road intersection 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, excluding projects proposing primarily pedestrian or bicycle use improvements
City of Kent Surface Water Design Manual
(Chapter 5 — Kent Construction Standards) 5 1-14 May, 2002
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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 1 1 2 3, AND
that have the characteristics of one or more of the following project categories
T
• TDR Project Category #1. Projects that contain or are adjacent to a floodplain, stream, lake,
'
I'
wetland, closed depression, erosion hazard area or landslide hazard area, OR projects located within a
E
Landslide Hazard Drainage Area
H
°
• TDR Project Category #2: Projects that propose to conslr uc1 or modify' a drainage pipe/ditch that is
'
n
12 inches or more in size/depth or receives surface and stormwater runoff from a drainage pipe/ditch
that is 12 inches or more in size/depth
• TDR Project Category #3: Redevelopment projects that propose $100,000 or more of
'
improvements to an existing high -use site a
Scope of Requirements
'
IF Targeted Drainage Review is required, THEN the applicant must demonstrate that the proposed project
e
complies with the selected core and special requirements corresponding to the project category or
M
categories that best match the proposed project The project categories and applicable requirements for
,
S
each are described below and summarized in Table 1 12 A
Note If the pi oposed project has the characteristics of more than one project category, the requirements
of each 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 Public '
Works 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
Construct and modify is defined as to install a new drainage pipelditch or make improvements to an existing drainage
pipe/ditch (for purposes other than routine maintenance, repair, or emergency modifications, and excluding driveway culverts
installed as part of single-family residential building permits) that either serves to concentrate previously unconcentrated
surface and stormwater runoff or serves to increase, decrease, and/or redirect the conveyance of surface and stormwater
runoff
a A high -use site is defined as a commercial or industrial site that (1) has an expected average daily traffic (ADT) count equal to
or greater than 100 vehicles per 1,000 square feet of gross building area, (2) is subject to petroleum storage or transfer in
excess of 1,500 gallons per year, not including delivered heating oil, or (3) is subject to use, storage, or maintenance of a fleet
of 25 or more diesel vehicles that are over 10 tons net weight (trucks, buses, trains, heavy equipment, etc ) Also included is
any road intersection 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, excluding projects proposing primarily pedestrian or bicycle use improvements
City of Kent Surface Water Design Manual
(Chapter 5 — Kent Construction Standards) 5 1-14 May, 2002
1
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' SECTION I 1 DRAINAGE REVIEW
'
in addition, Public Works may require the applicant to demonstrate compliance with any one or more of
the seven core requirements in Section ] 2 based on project or site-specific conditions For example, if a
Category #1 TDR Project contains or is adjacent to a stream, lake, wetland, closed depression, or defined
Hazard Area, Public Works may require compliance with "Core Requirement #] Discharge at the Natural
'
Location" (Section] 2 1) This may in turn require compliance with "Core Requirement #2 Offsite
Analysis" (Section 1 2 2) if a tightline is required by Core Requirement #1 If a tighthne is found to be
'
unfeasible, Public Works may instead require a flow control facility per "Core Requirement #3 Flow
Control' (Section 1 2 3) If a tightlme is feasible, "Core Requirement 94 Conveyance System" (Section
12 4) would be required to ensure proper size and design Any required flow control facility or tightlme
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 floodplains, streams, lakes, wetlands, closed depressions, or geologic hazard areas as determined
by Public Works
'
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 floodplams, streams, lakes, wetlands, closed
depressions, Landslide Hazard Drainage Areas, Erosion Hazard Areas, or other area -specific drainage
requirements adopted by the City Such projects primarily include single-family residential projects in
Small Site Drainage Review
IF the proposed project meets the characteristics of TDR Project Category #I, THEN the applicant
must demonstrate that the project complies with the following five requirements
tn
Core Requirement 42 Offsite Analysis, Section 12 2
E
Q Core Requirement #5 Erosion and Sediment Control, Section 1 2 5
7
S • Special Requirement #1 Other Adopted Area -Specific Requirements, Section ] 3 1
• 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
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City of Kent Surface Water Design Manual
5 1-15 (Chapter 5 — Kent Construction Standards)
SECTION I 1 DRAINAGE REVIEW
TDR Project Category 42
This category is intended to apply selected core and special requirements to those projects that
propose to construct 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
• Core Requirement 41 Discharge at the Natural Location, Section 1 2 1
• Core Requirement 42 Offsite Analysis, Section 12 2
• Core Requirement #4 Conveyance System, Section 1 2 4
• Core Requirement 45 Erosion and Sediment Control, Section 1 2 5
• Core Requirement #6 Maintenance and Operations, Section 1 26
• Core Requirement 47 Financial Guarantees and Liability, Section 1 2 7
• Special Requirement #4 Source Control, Section 1 3 4
TDR Project Category 43
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 sites developed
prior to current water quality requirements These are referred to as high -use sites and are defined
below
High -Use Site Definition: A high -use site is any one of the following
• A commercial or industrial site with an expected average daily traffic (ADT) count equal to or
greater than 100 vehicles per 1,000 square feet of gross building area, OR
• A commercial or industrial site subject to petroleum storage or transfer in excess of 1,500 gallons
per year, not including 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 10 tons net weight (e g , trucks, buses, trains, heavy equipment, etc ),
OR
• A road intersection 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, excluding projects proposing primarily
pedestrian or bicycle use improvements
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 #1 Discharge at Natural Location, Section 12 1
• Core Requirement 45 Erosion and Sediment Control, Section 1 2 5
• Core Requirement #6 Maintenance and Operations, Section 1 2 6
• Care Requirement #7 Financial Guarantees and Liability, Section 1 2 7
• Core Requirement 48 Water Quality, Section 12 8
• Special Requirement 44 Source Control, Section 13 4
• Special Requirement #5 Oil Control, Section 13 5
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(Chapter 5 — Kent Construction Standards) 5 1-16 May, 2002
t 1.1.3 DRAINAGE REVIEW REQUIRED BY OTHER AGENCIES
Drainage review for a proposed project's impact on surface and stormwaters 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 conflicts in drainage requirements
9 Landslide Hazard Drainage Areas are delineated on a map adopted with this manual (see map pocket inside of back cover)
10 Pollubon-generating impervious surface (PCIS) is partially defined on page 5 1-4 and fully defined on page 5 1-49 Contiguous
pollution -generating impervious surface (PGIS) means a discrete patch of PGIS that is all together as opposed to being separated
' in different locations on the project site The intent is to identify those redevelopment projects that are replacing and/or adding
enough impervious surface in one location to allow for opportune installation of a water quality treatment facility
ii Replaced impervious surface is defined on page 5 1-4
' City of Kent Surface Water Design Manual
May, 2002 5 1.17 (Chapter 5 — Kent Constmetion Standards)
SECTION 1 I DRAINAGE REVIEW
Note In some cases, Public Works may 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 ar e proposed to meet oil control requirements, engineered plans and
calculations may not be necessary A plot plan showing catch basin locations may suffice
1.1.2.3
FULL DRAINAGE REVIEW
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Full Drainage Review is the evaluation by Public Works permit 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 sites
Threshold
Full Drainage Review is required for any proposed new and redevelopment projects that are subject to
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drainage review as determined in Section 1 1 1, AND which meet one or more of the following criteria
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❑ Projects which add 5,000 square feet or more of new impervious surface but which do not qualify for
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Small Site Drainage Review as specified in Section 1 12 1 (p 5 1-12), OR
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❑ Projects located within a Landslide Hazard Drainage Areas which add 2,000 square feet or more of
new impervious surface but which do not qualify for Small Site Drainage Review per Section 1.1.2.1,
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OR
Redevelopment projects proposing $500,000 or more of site improvements which create 5,000
square feet or more of contiguous pollution -generating impervious surface"° through any combination
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Scope of Requirements
IF Full Drainage Review is required, THEN the applicant must demonstrate that the proposed project
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complies with the following requirements
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All eight core requirements in Section 12
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All five special requirements in Section 13
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 can be found in Section 2 3
t 1.1.3 DRAINAGE REVIEW REQUIRED BY OTHER AGENCIES
Drainage review for a proposed project's impact on surface and stormwaters 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 conflicts in drainage requirements
9 Landslide Hazard Drainage Areas are delineated on a map adopted with this manual (see map pocket inside of back cover)
10 Pollubon-generating impervious surface (PCIS) is partially defined on page 5 1-4 and fully defined on page 5 1-49 Contiguous
pollution -generating impervious surface (PGIS) means a discrete patch of PGIS that is all together as opposed to being separated
' in different locations on the project site The intent is to identify those redevelopment projects that are replacing and/or adding
enough impervious surface in one location to allow for opportune installation of a water quality treatment facility
ii Replaced impervious surface is defined on page 5 1-4
' City of Kent Surface Water Design Manual
May, 2002 5 1.17 (Chapter 5 — Kent Constmetion Standards)
SECTION 1 1 DRAINAGE REVIEW
Seattle/King County Department of Public Health
Washington State
Department of Transportation
Department of Fish and Wildlife
Department of Ecology
Department of Natural Resources
United States Army Corps of Engineers
Onsite Sewage Disposal and Well permits
Developer/Local Agency Agreement
Hydraulic Protect Approval
Short Term Water Quality Modification Approval
Dam Safety permit
NPDES Stormwater permit
Forest Practices Class IV permit
Sections 10, 401, and 404 permits
1.1.4 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 property
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 mcreasmg the duration and magnitude of water level fluctuations Thus,
compliance with this manual should not be construed as mitigating all probable and significant stonnwater
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
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(Chapter 5 — Kent Construction Standards) 5 1-18 May, 2002
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1.2 CORE REQUIREMENTS
This section details the following eight core requirements
• Core Requirement # 1
Discharge at the Natural Location, Section 1 2 1
• Core Requirement #2
Offsite Analysts, Section 12 2
• Core Requirement 43
Flow Control, Section 1 2 3
• Core Requirement 44
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 47
Financial Guarantees and Liability, Section 12 7
• Core Requirement #8
Water Quality, Section 1 2 8
1.2.1 CORE REQUIREMENT #1:
DISCHARGE AT THE NATURAL LOCATION
RI All surface and stormwater runoff from a project must be discharged at the natural location so as not to be
e diverted onto or away from downstream properties The manner in which runoff is discharged from the
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T project site must not create a significant adverse impact to downhill properties or drainage systems (see
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"Discharge Requirements" below)
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 runoff at the natural location Diversions can also impact properties that
rely on runoff water to replenish wells and ornamental or fish ponds Projects that do not discharge at the
natural location will require an approved adjustment of this requirement (see Section] 4)
❑ DISCHARGE REQUIREMENTS
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Proposed projects must comply with the following discharge requirements (1, 2, and 3) as applicable
I Where no conveyance system exists at the abutting downstream property line and the natural (existing)
discharge is unconcentrated, any runoff concentrated by the proposed project must be discharged as
follows
a) IF the 100 -year peak disebarge12 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 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
12 Peak discharges for applying this requirement are determined using KCRTS as detailed in Chapter 3
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SECTION 1 2 CORE REQUIREMENTS
c) IF the 100 -year peak discharge is greater than 0 5 cfs for either existing or developed conditions,
or if a significant adverse impact to downhill properties or drainage systems is likely, THEN a
conveyance system must be provided to convey the concentrated runoff across the downstream
properties to an acceptable discharge point 13 Drainage easements for this conveyance system
must be secured from downstream property owners and recorded prior to engineering plan
approval
IF a proposed project or any natural 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 landslide
hazard area with slopes steeper than 15%, THEN a tightlme system must be provided through the
landslide hazard area to an acceptable discharge point unless one of the following exceptions applies
The tightlme system must comply with the design requirements in Core Requirement 44 and in
Section 4 2 2 unless otherwise approved by Public Works Drainage easements for this system must
be secured from downstream property owners and recorded prior to engineering plan approval
Exceptions: A tightlme is not required for any natural discharge location where 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) Public Works determines that a tightlme system is not physically feasible or will create a
significant adverse impact based on a sods report by a geotechnical engineer
For projects adjacent to or containing landslide hazard areas or erosion hazard areas, the applicant
must demonstrate that onsite drainage facilities and/or flow control BMPs will not create a significant
adverse impact to downhill properties or drainage systems
1.2.2 CORE REQUIREMENT #2: OFFSITE ANALYSIS
n Project proponents must submit an offsite analysis report that assesses potential offsite drainage impacts
e associated with development of the project site and that proposes appropriate mitigation measures for
nQi those impacts The initial permit submittal shall include, at minimum, a Level 1 downstream analysis as
r described in Section 1 2 2 1 below
Intent: To identify and evaluate offsite drainage problems that may be created or aggravated by the
proposed project, and to determine appropriate measures for preventing aggravation of those problems in
accordance with the requirements of this manual
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 identify existing or potential/predictable downstream problems so that
appropriate mitigation, as specified in Section 1 2 2 2, can be provided to prevent aggravation of these
problems A secondary component of the offsite analysis report is an evaluation of the upstream
drainage system to verify and document that 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
13 Acceptable 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, take, or wetland) where concentrated runoff can be discharged without
creating a significant adverse impact
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' ❑ EXEMPTION FROM CORE REQUIREMENT #2
A proposed project is exempt from Core Requirement #2 if any one of the following is true
' 1 Public Works determines there is sufficient information for them 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 5,000 square feet of new impervious surface, AND does not construct or
modify a drainage pipe/ditch that is 12 inches or more in size/depth or that receives runoff from a
drainage pipe/ditch that is 12 inches or more in size/depth, AND does not contain or lie adjacent to an
defined hazard area, OR
t3 The project does not change the rate, volume, duration, or location of discharges to and from the
project site (e g , where existing impervious surface is replaced with other impervious surface having
t similar runoff -generating characteristics, or where pipe/ditch modifications do not change existing
discharge characteristics)
1 1.2.2.1 DOWNSTREAM ANALYSIS
The downstream analysis must consider the existing conveyance system(s) for a minimum flow path
distance downstream of one-quarter mile and beyond as needed to reach a point where the project site area
constitutes less than 15% of the tributary area This minimum distance may be increased as follows
• Task 2 of a Level 1 downstream analysts (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 problems
1 in all cases, this information review shall extend one mile downstream of the project site The
existence of flooding, erosion, or nuisance problems may extend the one -quarter -mile minimum
distance for other tasks to allow evaluation of impacts from the proposed development to the
identified problems
• If a project's impacts to flooding, erosion, or conveyance system overflow problems are mitigated by
improvements to the downstream conveyance system, the downstream analysis will extend a
minium 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 site
'• At their discretion, Public Works may extend the downstream analysis beyond the minimum distance
specified above on the reasonable expectation of impacts
' The Level 1 downstream analysis is a qualitative survey of each downstream system and is the first step
in identifying flooding, erosion, or nuisance problems as defined below under "Downstream Problems
Requiring Special Attention " Each Level 1 analysis is composed of five tasks at a minimum
• Task 1 Define and map the study area
• Task 2 Review all available information on the study area
'• Task 3 Field inspect the study area
• Task 4 Describe the drainage system, and its existing and predicted problems
• Task 5 Propose mitigation measures
Upon review of the Level 1 analysis, Public Works 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 1 analysis
Levels 2 and 3 downstream analyses quantify downstream problems by providing information on the
severity and frequency of an existing problem or the likelihood of creating anew problem A Leve12
' City of Kent Surface Water Design Manual
May, 2002 5 1-21 (Chapter 5 —Kent Construction Standards)
SECTION 1 2 CORE REQUIREMENTS
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
A detailed description of offsite analysis scope and submittal requirements is provided in Section 2 3 1 1
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
❑ DOWNSTREAM PROBLEMS REQUIRING SPECIAL ATTENTION
While the flow control standards in Core Requirement 43 serve to minimize the creation and aggravation
of many types of downstream drainage problems, there are some types that are more sensitive to
aggravation than others depending on the nature or severity of the problem and which flow control
standard is being applied In particular, there are three 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 aggravation
1 Conveyance system overflow problems
2 Severe erosion problems
3 Severe flooding problems
Conveyance system overflow problems may not cause extensive property or environmental damage
Chronic flooding or erosion problems can result from the overflow of a constructed conveyance system that
is substandard or has become too small due to upstream development Such problems warrant additional
attention because of their chronic nature and because they result from the failure of a conveyance system to
provide a minimum acceptable level of protection (see definition below) Severe flooding and erosion
problems as defined below also warrant additional attention because they either pose a significant threat to
health and safety or can cause significant damage to public or private property
Conveyance System Overflow Problems (Type 1)
Conveyance system overflow problems in general are defined as any existing or predicted flooding or
erosion which does not constitute a severe flooding or erosion problem as defined below Conveyance
system overflow problems are defined as any flooding or erosion that results from the overflow of a
constructed conveyance system for runoff events less than or equal to a 25 -year event Examples include
inundation of a shoulder or lane of a roadway, overflows collecting in yards or pastures, shallow flows
across driveways, minor flooding of crawl spaces or unheated garages/outbuildings, and minor erosion
If a conveyance system overflow problem is identified or predicted downstream, the need for additional
mitigation must be evaluated as specified in Section 1 2 2 2 under "Problem -Specific 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 problems (exclusive of severe erosion or flooding problems) which may be identified
downstream, this manual does not require mitigation beyond the flow control standard applied in Core
Requirement #3 This is because to prevent aggravation of 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 unwarranted
for addressing nuisance problems However, if under some unusual circumstance, the aggravation of such
a problem is determined by Public Works to be a significant adverse impact, additional mitigation may be
required
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Severe Erosion Problems (Type 2)
Severe erosion problems are defined as downstream channels, ravines, or slopes with evidence of or
potential for erosion/incision sufficient to pose a sedimentation hazard to downstream conveyance systems
or pose a landslide hazard by undercutting adjacent slopes Severe erosion problems do not include
roadway shoulder filling or minor ditch erosion
If a severe erosion problem is identified or predicted downstream, additional mitigation must be
considered as specified in Section 1 2 2 2 under "Problem -Specific 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 (Type 3)
Severe flooding problems can be caused by conveyance system overflows or the elevated water surfaces
'
of ponds, lakes, wetlands, or closed depressions Severe flooding problems are defined as follows
• Flooding of the finished area14 of a habitable buildmg15, or the electrical/heating system of a habitable
building for runoff events 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 Such problems are referred to in this
manual as "severe budding flooding problems "
• Flooding over all lanes of a roadway or severely nnpacting16 a sole access driveway17 for runoff
events less than or equal to the 100 -year event Such problems are referred to in this manual as
"severe roadway flooding problems "
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if a severe flooding problem is identified or predicted downstream, the need for additional mitigation must
be evaluated as specified in Section 1 2 2 2 under "Problem -Specific Mitigation Requirements " This may
entail consideration of additional onsite flow control or other measures as needed to prevent creation of
'
significant aggravation of the problem
' 1.2.2.2 IMPACT MITIGATION
A proposed project must not significantly aggravate existing downstream problems or create new
problems as a result of developing the 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
Aggravation of an existing downstream problem means increasing the frequency of occurrence
and/or severity of the problem. Increasing peak flows at the site 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 of the 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 due to development only at return
14 Finished area, for the purposes of this definition, means any enclosed area of a building that is designed to be served by the
' building's permanent heating or cooling system
15 Habitable building means any residential, commercial, or industrial building that is equipped with a permanent heating or
cooling system and an electrical system
' 16 Severely impacting means the Flooding overlaps a culverted section of the driveway, posing a threat of washout or unsafe
access conditions due to indiscernible driveway edges, or the flooding is deeper than 6 inches on the driveway, posing a
severe impediment to emergency access
17 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
May, 2002 5 1-23 (Chapter 5 — Kent Construction Standards)
SECTION 1 2 CORE REQUIREMENTS
frequencies below the conveyance overflow return frequency, with the net result of 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 means 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 of flows for a range of return 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 at water 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 predeveloped and developed 2- and 10 -year peak flows The 100 -year peak
flow is only partially attenuated, and the flow increase may be enough to cause a "severe flooding problem"
as described on page 23 The potential for causing a new problem is often identified during the Level 1
downstream analysis, where the observation of a reduction in downstream pipe sizes, for example, may be
enough to predict creation of anew problem A Level 2 or 3 analysis will typically be required to verify
the capacity of the system and determine whether 100 -year flows can be safely conveyed
❑ SIGNIFICANCE OF IMPACTS TO EXISTING PROBLEMS
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, Public Works will make the determination as to whether the project's impact is
significant enough to require additional mitigation For the downstream problems defined in Section
1 2 2 1, this threshold of significant impact or aggravation is defined below
For conveyance system overflow problems, the problem is considered significantly aggravated if there is
any Increase in the project's contribution to the frequency of occurrence and/or severity of the problem for
runoff events less than or equal to the 25 -year event Note Inci eases in the project's contribution to this
type ofproblem are considered to be prevented ifsufficaent onsite flow control and/or offsite
improvements are provided as specified in Table 12 3 A
For severe erosion problems, the problem is considered significantly aggravated if there is any increase
In project's contribution to the flow duration'B of peak flows ranging from 50% of the 2 -year peak flow up
to the full 50 -year peak flow at the eroded area Note Increases in the project's contribution to this type
ofproblem are considered to be prevented mf Level 2 flow control or offsite improvements are provided as
specified in Table 12 3 A
For severe budding flooding problems, the problem is considered significantly aggravated if there is any
Increase in the project's contribution19 to the frequency, depth, and/or duration of the problem for runoff
events less than or equal to the 100 -year event
'a Flow duration means 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)
79 Increases in the proiecrs contribution are considered to be prevented if sufficient onsite flow control andlor offsite
improvements are provided as specified for "severe flooding problems' in Table 1 2 3 A (p 5 1-27)
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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 contribution to the frequency,
depth, and/or duration of the problem for runoff events less than or equal to the 100 -year event
'• The existing Jlooding20 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 10% (whichever is
greater) for the 100 -year runoff event
'• 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 runoff events less than or equal to the
100 -year event
• The "existing flooding" over all lanes of a sole access roadwajo21 is more than 3 inches deep or faster
than 5 feet per second for runoff events less than or equal to the 100 -year event, or is at any depth for
runoff events less than or equal to the 10 -year event
❑ PROBLEM -SPECIFIC MITIGATION REQUIREMENTS
' 1 IF a proposed project or threshold discharge area within a project drains to one or more of the three
types of downstream drainage problems defined in Section 12 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 demonstrating that the
proposed project will not create or significantly aggravate the identified downstream problem(s),
OR
b) Show that the natural discharge area or threshold discharge area draining to the identified
problem(s) qualifies for an exemption from Core Requirement 43 Flow Control, OR
c) Document that the area -specific flow control standard required in Core Requirement 43 is
adequate to prevent creation or significant aggravation of the identified downstream problem(s) as
indicated in Table 12 3 A with the phrase, "No additional flow control needed," OR
d) Provide additional onsite flow control necessary to prevent creation or significant aggravation of
the downstream problem(s) as specified in Table 1 2 3 A and further detailed in Section 3 3 5, OR
e) Provide offsite improvements necessary to prevent creation or significant aggravation of the
identified downstream problem(s) as detailed in Chapter 3 unless identified as not necessary in
Table 12 3 A, OR
f) Provide a combination of additional onsite flow control and offsite improvements sufficient to
' prevent creation or significant aggravation of the downstream problem(s) as demonstrated by a
Level 2 or Level 3 downstream analysts
2 IF it is identified that the manner of discharge from a proposed project may create a significant
adverse Impact as described in Core Requirement 41, THEN Public Works may require the applicant
to implement additional measures or demonstrate the impact will not occur
Intent: To ensure provisions are made (if necessary) to prevent creation or significant aggravation of the
three types of downstream problems requiring special attention by this manual, and to ensure compliance
with the discharge requirements of Core Requirement #1
In addressing downstream problems per Problem -Specific Mitigation Requirement 1 above, the easiest of
' the provisions to implement will often be that of additional onsite flow control This involves designing
the required onsite flow control facility to meet an additional set of performance criteria targeted to
' 20 Existing flooding, for the purposes of this definition, means 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
2i Sole access roadway means there is no other flood -free route for emergency access to one or more dwelling units
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SECTION 1 2 CORE REQUIREMENTS
prevent significant aggravation of specific downstream problems To save time and analysts, a set of
predetermined flow control performance criteria corresponding to each of the three types of downstream
problems is provided in Table 12 3 A and described in more detail in Chapter 3
Note that in some cases, the area -specific flow control standard applicable to the proposed project per
Section 1 2 3 1 is already sufficient to prevent significant aggravation of many of the defined downstream
problem types Such situations are noted in Table 12 3 A as not needing additional onsite flow control or
offsite improvements For example, if the Level 3 flow control standard is required by Section 12 3 1,
and a "conveyance system overflow problem" is identified through offsite analysts per Core Requirement
42, no additional onsite flow control is needed, and no offsite improvements are necessary
1.2.3 CORE REQUIREMENT 43: FLOW CONTROL
All proposed projects, including redevelopment projects, must provide onsite flow control facilities to
R mitigate the impacts of increased stormwater and surface water runoff generated by the addition of new
E impervious surface and any related land cover conversion These facilities shall, at a minimum, meet
M 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 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 of runoff generated by new development The level of
control varies depending on location and downstream conditions identified under Core Requirement #2
City of Kent Surface Water Design Manual
(Chapter 5 —Kent Construction Standards) 5 1-26 May, 2002
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May, 2002
SECTION 1 2 CORE REQUIREMENTS
Guide to Applying Core Requirement #3
Core Requirement #3 requires that onsite detention and/or infiltration facilities be constructed to control runoff
discharges from the project site These facilities must meet a minimum flow control performance standard asset forth in
Section 1 2 3 1, `Area -Specific Flow Control Standards ' and may need to be even larger to ensure that downstream
problems are not created or significantly aggravated as set forth in Section 1 2 2 2 `Problem -Specific Mitigation
Requirements " Table 1 2 3 A provides a quick guide for selecting the flow control performance criteria necessary to
meet both of these requirements
Area -specific Slow control standards target the level of clow control performance to the protection needs of specific
regions or areas of the city These areas are called flow control areas and are depicted on the Flow Control Applications
Map adopted with this Manual (see map pocket inside cover of Manual) Each flow control area has a basic flow control
standard that is specific to that area The performance criteria of that basic standard may need to be increased to address
a specific downstream drainage problem as explained in Step 4 below
Flow control implementation requirements are the minimum requirements for analyzing and designing flow control
facilities to achieve required performance and other protection goals
For efficient application of Core Requirement #3 the following steps are recommended
1 Determine the flow control standard that applies to the drainage basin where your project is located Generally, the
City of Kent has adopted the Level 2 flow control standard as the minimum standard to be applied citywide, but
there are areas within the City with different requirements They are shown on the Flow Control Applications Map
included with this manual and are described as follows
• Projects in the McSorley Creek Watershed shall apply Level 3 flow control
• Projects that provide for gravity stormwater discharges to the Green River shall follow the criteria specified by
the Green River Management Agreement (see item 5 under the Exemptions from Core Requirement #3 section )
• Projects that provide for stormwater discharges to the S 259th / 3rd Ave S Regional Detention Facility shall be
required to provide '/z of the detention volume required by the Green River Management Agreement (see item 7
under the Exemplions from Core Requirement #3 section )
• Projects in the Mill Creek Basin within the Kent Valley (generally below elevation 35' NGVD) and outside of
the Green River Natural Resources Area Subbasm (see flow control applications map) shall apply Level 1 flow
control with a pasture" assumption for predevelopment pervious areas
• Projects in areas where a Master Drainage Plan has been approved shall follow the provisions of the respective
plan (this includes "Kentview" and "The Lakes" areas)
2 Check the list of exemptions following Table 1 2 3 A to determine if and/or what portions of your project must
provide flow control facilities per Core Requirement #3
3 If flow control facilities are required, determine (for the flow control area identified above) the area -specific flow
control standard that applies to your project by consulting the detailed threshold information in Section 1 2 3 1 The
applicable flow control standard will determine the minimum flow control performance required for your proposed
project
4 if downstream problems were identified through offsite analysis per Core Requirement #2 and are proposed to be
addressed through onsite flow control use Table 12 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)
5 The design of flow control facilities that are located on the Valley floor shalt take into account the anticipated flood
levels downstream of the facility The project applicant shall demonstrate compliance with the minimum flow
control requirements to Public Works where variable flooding occurs by analyzing the facility performance over the
range of tailwater conditions anticipated, up to 100 -year flood levels See Section 12 3 2 for requirements
regarding projects in the Valley floor
6 Use Section 1 2 3 2 to determine the minimum requirements for implementing flow controls
City of Kent Surface Water Design Manual
5 1-27 (Chapter 5—Kent Construction Standards)
SECTION 1 2 CORE REQUIREMENTS
AREA -SPECIFIC STANDARD
Downstream Problems
Level 1 Flow Control
Level 2 Flow Control
Level 3 Flow Control
Identified through Offsite
Analysts per Core
Requirement #2
No problem identified
Match 2 -year and 10 -year
Match durations for 50% of 2-
Match durations for 50% of 2 -yr
Apply basic standard
peaks (Only applicable in the
yr through 50 -yr peaks
though 50 -yr peaks AND match
performance criteria
Kent Valley I Mill Creek Basin —
100 -year peaks
see flow control applications
map)
Type 1
Additional Flow Control
Additional Flow Control
No additional flow control or
Conveyance System
Hold 25 -yr peak to overflow Tr
Hold 25 -yr peak to overflow Tr
other mitigation is needed
Overflow Problem
peak'2i"'
peak(')(')
Type 2
Additional Flow Control
No additional flow control is
No additional flow control is
Severe Erosion Problem
Apply level 2 flow control(')(')
needed, but other mitigation
needed, but other mitigation
may be requiredVi
may be required(')
Type 3
Additional Flow Control
Additional Flow Control
Additional Flow Control
Severe Flooding Problem
Apply level 3 flow control If
Apply Level 3 flow control
If flooding is from a closed
flooding is from conveyance
If flooding is from a closed
depression, make design
system overflow, level 3 may
depression, make design
adjustments as needed to meet
be modified to match durations
adjustments as needed to meet
the special provision for closed
above the overflow Tr peak
the special provision for closed
depressions (1)i5i
rather than 50% of the 2 -year
depressionsi'iisi
peak If flooding is from a
closed depression, make
design adjustments as needed
to meet the 'special provision
for closed depressions"i3i(5)
Notes
More than one set of problem -specific performance criteria may apply if two or more downstream problems are identified through offsite
analysis per Core Requirement #2 If this happens, the performance goals of each applicable problem -specific criteria must be met
This can require extensive, 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 Level 3 flow control standard in place of the otherwise required area -specific standard Use of the Level 3 flow control
standard satisfies the specified performance criteria for all the area -specific and problem -specific requirements except if adjustments
are required per the special provision for closed depressions described below in Note 5
(3) Overflow T, IS the return period of 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, a T, of 2 years may be assumed
iii 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
A tightline system may be required regardless of the flow control standard being applied if needed to meet the discharge requirements
of Core Requirement #1 or the outfall requirements of Core Requirement #4, or is deemed necessary by Public Works where the risk of
severe damage is high
iso 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 of impervious surface area proposed by the project is greater than or equal to 10% of the 100 -year water
surface area of the closed depression, THEN use the point of compliance analysis technique described in Section 3 3 6 to verify that
water surface levels are not increasing for the return frequencies at which flooding occurs, up to and including the 100 -year frequency
If necessary, iteratively adjust onsite flow control performance to prevent increases Note The point of compliance analysis relies on
certain held measurements taken directly at the closed depression (e g, soils tests, topography, etc) If permission to enter private
property for such measurements is denied, Public Works may waive this provision and apply the Level flow control standard with a
mandatory 20% safety factor on the storage volume
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City of Kent Surface Water Design Manual '
(Chapter 5 —Kent Construction Standards) 5 1-28 May, 2002
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SECTION 1 2 CORE REQUIREMENTS
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❑ EXEMPTIONS FROM CORE REQUIREMENT #3
There are seven possible exemptions or departures from the requirement to provide a formal flow control
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facility per Core Requirement 43 The intent of these exemptions is to provide for situations where a
facility may not be practical or needed, where other alternatives to a facility can be lust as effective, other
arrangements govern, or where it makes sense to provide incentives for retaining native vegetation or for
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maximizing use of existing developed areas
1. Impervious Surface Exemption
A proposed project or any threshold discharge area within a project is exempt if less than 5 000
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square feet of new impervious surface will be added and the project or threshold discharge area is not
within a Landslide Hazard Drainage Area � If the project or threshold discharge area is located
within a Landslide Hazard Drainage Area, this exemption only applies to new impervious surface less
than 2,000 square feet
2. Impervious Surface Exemption Using Flow Control BMPs
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Any threshold discharge area within a proposed project is exempt if less than 10,000 square feet of
new impervious surface will be added, AND all of the following criteria are met
a) The area cleared to accommodate the proposed project must be less than 35% or less than 2 acres
of the threshold discharge area (whichever is greater), AND
b) if the project is a single-family residential project, flow control BMPs must be applied within
the threshold discharge area as specified in Small Site Drainage Requirements (detached
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Appendix C of the King County Manual), AND
c) For projects other than single-family residential projects, the new impervious surface within
the threshold discharge area must be comprised of either non -pollution -generating roofs that
comply with the roof downspout controls in Section 5 1, OR roads, trails, or driveways that
comply with the rural roadway dispersion requirements in Section 5 2 1, AND
d) The manner in which runoff is discharged from the project site must not create a significant
adverse impact per Core Requirement 41
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3. Peak Flow Exemption Using Flow Control BMPs
Any threshold discharge area within a proposed project is exempt if the project improvements within
the threshold discharge area generate less than a 0 1 cfs increase in the existing site conditions 100 -year
peak flow rate, AND all of the following criteria are met
a) If the project is a redevelopment project, flow control BMPs must be applied as specified in
Section 5 2, and the project improvements must not significantly impact a "severe erosion
problem" or "severe flooding problem" (see Section 1 2 2 1) and must not be located within a
Landslide Hazard Drainage Area, AND
b) If the project is a single-family residential project, the runoff from impervious surfaces must be
infiltrated or dispersed using flow control BMPs specified in Appendix C of the King County
Manual, and any areas of native vegetation assumed not to be cleared for the purposes of
computing the increase in 100 -year peak flow must be preserved within a tract or by covenant as
described in Appendix C of the King County Manual, AND
c) For projects other than redevelopment projects and single-family residential projects, the
new impervious surface within the threshold discharge area must be comprised of either non-
pollution -generating roofs that comply with the roof downspout controls in Section 5 1, OR roads,
22 Landslide Hazard Drainage Areas are delineated on a map adopted with this manual (see map pocket inside cover)
City of Kent Surface Water Design Manual
May, 2002 5 1-29 (Chapter 5 — Kent Construction Standards)
SECTION 1 2 CORE REQUIREMENTS
trails, or driveways that comply with the rural roadway dispersion requirements in Section 5 2 1,
AND
d) The manner in which runoff is discharged from the project site must not create a significant
adverse impact per Core Requirement 41
4 Peak Flow Exemption for Urban Redevelopment Projects
Any natural discharge area of a redevelopment project located within the City is exempt if the
project improvements within the natural discharge area generate less than a 0 1 cfs increase in the
existing site conditions 100 -year peak flow, AND all of the following criteria are met
a) The application of this exemption to natural discharge areas within a proposed project must not
result in more than a 0 4 cfs increase in the existing site conditions 100 -year peak flow rate for any
threshold discharge area of the project, AND
b) Flow control DMPs must be applied to the runoff from new impervious surfaces as specified in
Section 5 2 1, AND
c) The project improvements within the natural discharge area must not significantly impact a
"severe erosion problem" or "severe flooding problem" (see Section 1 2 2 1), AND
d) The manner in which runoff is discharged from the project site must not create a significant
adverse impact per Core Requirement 41
5. Direct Discharge Exemption
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 undetamed, provided 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 100 -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 capacity to carry the
resultant flows without overflowing, AND
3) Appropriate water quality treatment is provided as required
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
Release rates and/or detention requirements will be imposed by the City only when the Applicant's
downstream system analysis indicates that the existing stormwater drainage system does not have the
necessary capacity to carry the increased flows, AND appropriate off-site mitigation is not proposed
by the developer
The City of Kent does not recognize any other "major receiving waters" within the City that would be
candidate for the Direct Discharge Exemption
Standards are currently pending for the Washington Avenue Pump Station (refer to flow control map)
Until standards are approved and adopted, the provisions of the Green River Management Agreement
as described above shall apply
City of Kent Surface Water Design Manual
(Chapter 5 —Kent Construction Standards) 5 1-30 May, 2002
' SECTION 1 2 CORE REQUIREMENTS
6. Peak Flow Exemption 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 not automatically required to provide on-site detention storage, but
' 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 of Public Works
' 7. Area -specific Exemptions for Regional Facilities
Stormwater detention and treatment are not required for any development that discharges to the
Valley Regional Detention/Enhaneed Wetland Facility also known as the Green River Natural
Resource Area (see the 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
Standards are pending for the S 259th/3rd Avenue Regional Detention facility (refer to flow control
map) Until standards are approved and adopted provisions of the Green River Management
Agreement shall apply with the exception that only one-half of the Green River Management
Agreement flood storage volume shall be required
1.2.3.1 AREA -SPECIFIC FLOW CONTROL STANDARDS
a Protects subject to Core Requirement #3 must, at a minimum, comply with one of the three area-
_ specific flow control standards Level 1, Level 2, or Level 3, whichever applies per the threshold
Minformation detailed in this section These standards are described below
' ❑ LEVEL 1 FLOW CONTROL
' Level 1 flow control is a peak -matching performance standard in the 1998 King County Surface Water
Manual, primarily applied in areas where maintaining peak flows is sufficient to protect the natural and
constructed conveyance systems that are not sensitive to development -induced increases in runoff volumes
' and flow durations
The Level 1 Flow Control Standard is only applicable for the Kent Valley / Mill Creek Basin (see
flow control applications map)
' May, 2002
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City of Kent Surface Water Design Manual
5 1-31 (Chapter 5 —Kent Construction Standards)
SECTION 1 2 CORE REQUIREMENTS
Performance Criteria
Level 1 Flow Control: Match the developed peak discharge rates to the existing site conditions23 peak
discharge rates for 2- and 10 -year return periods
Intent
The use of level I flow control for the Kent Valley / Mill Creek Basin is intended to protect flow -carrying
capacity and limit increased erosion within the downstream conveyance system for runoff events less than
or equal to the 10 -year event Matching the 2- and 10 -year peak flows is intended to prevent increases in
return -frequency peak flows less than or equal to the l0 -year flow down to the 2 -year flow This level of
control is also intended to prevent creation of new "conveyance system nuisance problems" as defined in
Section 1 2 2 Volumes anticipated under the level 1 flow control standard assuming a "pasture"
predevelopment condition are comparable to those previously required under the "Valley Criteria'
Effectiveness in Addressing Downstream Problems
While the level 1 flow control standard provides reasonable protection from many development -induced
conveyance problems, (up to the 10 -year event), it does not prevent increases in runoff volumes or flow
durations that tend to aggravate the three types of downstream problems described in Section 1 2 2 1
Consequently, if one or more of these problems are identified through offsite analysis per Core
Requirement #2, additional onsite flow control and/or offsite improvements will likely be required (see
"Problem -Specific Mitigation Requirements" in Section 12 2 2 )
❑ LEVEL 2 FLOW CONTROL
Level 2 flow control is a duration -matching and peak -matching performance standard, which is
effective in preventing increases in existing erosion rates The City has determined that a duration -
matching standard is needed to prevent severe erosion and sedimentation damage caused by development -
induced increases in flow durations The Level 2 Flow Control also provides greater flood protection than
the Level 1 and is a regionally accepted standard for fisheries resource protection
Performance Criteria
it
Level 2 Flow Control: Match developed discharge durations to predeveloped durations for the range of
34 t_ predeveloped discharge rates from 50% of the 2 -year peak flow up to the full 50 -year peak flow, assuming
existing site conditions as the predeveloped condition In addition, match the developed peak discharge
r rates to the existing site conditions peak discharge rates for 2- and 10 -year return periods (See previous
section titled "Level 1 Flow Control" for a definition of "existing site conditions ")
Effectiveness in Addressing Downstream Problems
While the Level 2 flow control standard provides an excellent level of protection for preventing most
development -induced problems, it does not necessarily prevent increases in 100 -year peak flows that can
aggravate "severe flooding problems" as defined in Core Requirement #2 (see Section 12 2 1), nor does it
23Existing site conditions depend on what, if any, land conversion activity has occurred at the site since December 1978, when
Kent first required flow control for new development (Ordinance 2130) If a drainage plan has been approved by the City since
December 1978 (or since 1979 by the County) for any land conversion activity and the plans indicate a stormwater
management system was designed according to standards in place at the time, and the same system shown on the approved
plan was constructed and is still operating according to the design, Then existing site conditions are those created by the site
improvements (in this case, including any drainage facilities constructed per the approved engineering plans The project
proponent will be required to submit such documentation to the Public Works Department) Otherwise, existing site conditions
are those that were present in December 1978 (or since 1979 by the County) as determined from aerial photographs and, if
necessary, or knowledge of individuals familiar with the area The intent is to mitigate unaddressed impacts created by site
alterations or improvements, such as clearing, which has occurred since December 1978
City of Kent Surface Water Design Manual
(Chapter 5 —Kent Construction Standards) 5 1-32 May, 2002
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' SECTION 1 2 CORE REQUIREMENTS
necessarily prevent aggravation of all "severe erosion problems " Consequently, if one or more of these
problems are identified through offsite analysts per Core Requirement #2, additional onsite flow control
and/or offsite improvements will likely be required (see "Problem -Specific Mitigation Requirementsl' in
Section 1 2 2 2)
1
❑ LEVEL 3 FLOW CONTROL
' Level 3 flow control is a duration -matching and peak -matching performance standard for up to the
100 -year storm, which is effective in preventing significant increases in water surface levels of lakes,
wetlands, and closed depressions The standard is primarily applied in areas where the City has
' determined that a higher level of flow control is needed to prevent aggravation of existing documented
flooding or erosion problems, the City has designated such areas as Level 3 Flow Control Areas Level
3 flow control is required in the McSorley Creek Drainage Basin
' Performance Criteria
Level 3 Flow Control: Apply the Level 2 flow control standard AND match the developed 2-, 10- and
R 100 -year peak discharge rate to the corresponding 2-, 10- and 100 -year peak discharge rate for existing
Q site conditions (See previous section titled "Level 1 Flow Control" for a definition of "existing site
T conditions ")
Effectiveness in Addressing Downstream 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 problems defined in Core Requirement 42 The one
exception is when the wetland or lake is a closed depression with a "severe flooding problem," and the
proposed project is adding impervious surface area amounting to more than 10% 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 analysts" (see "Special Provision for Closed
Depressions" in Table 1 2 3 A, and see Section 3 3 6, "Point of Compliance Analysis")
1.2.3.2 FLOW CONTROL IMPLEMENTATION REQUIREMENTS
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Onsite vs. Offsite Implementation
All required flow control 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
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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
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Requirement 3" (item 6, page 5 1-31)
1 The regional or shared facility must be of adequate size and design to meet the current flow control
requirements for the proposed projects increased surface and stormwater runoff Note the current flow
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control requirements are those specified by Core Requirement 43 of this manual unless superseded by
other adopted area -spec fe flow control requirements per Special Requirement 41 (see Section 13 1)
2 The regional or shared facility must be fully operational at the time the proposed project is constructed
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in the case of a shared facility, the proposed project must comply with the terms and conditions of all
contracts, agreements, and permits associated with the shared facility
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3 The conveyance system between the project site and the regional facility must be approved by Public
Works and meet all the following criteria,
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City of Kent Surface Water Design Manual
5 1-33 (Chapter 5—Kent Construction Standards)
SECTION 1 2 CORE REQUIREMENTS
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 44, Conveyance
System, for the entire contributing drainage area, assuming build -out conditions to current zoning
for the "equivalent area" portion (defined in Figure 1 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 of a shared facility, the criteria are the same, except the conveyance system need only have
adequate capacity and erosion protection for buildout of the participating portion of the contributing
drainage area The particpahng portion includes those properties that have agreements for use of the
shared facility
FIGURE I.2.3.A EQUIVALENT AREA DEFINITION AND ILLUSTRATION
Equivalent area The area tributary to a direct discharge conveyance system that is contained within
an arc formed by the shortest, straight line distance from the conveyance system discharge point to the
furthermost point of the proposed project
Existing
Conveyance
System Discharge
Point
Major
Basin Boundary Receiving
Water
61
Equivalent =
Area
(gliaded' ,
Ar
c
Basin
Boundary
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, KCRTS, 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 robe 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 100 -
City of Kent Surface Water Design Manual
(Chapter 5 —Kent Construction Standards) 5 1-34 May, 2002
SECTION 1 2 CORE REQUIREMENTS
' 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 of conveying the 100 -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 50% of the flood
storage volume lost from the 100 -year floodplain when comparing pre- and post -development conditions
1 Compensatory flood 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)
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Land Cover Assumptions
Land cover assumptions for designing flow control facilities are detailed in Chapter 3 For residential
development (plats, short plats, and large single-family projects), flow control facilities must be sized for
the ultimate potential development of the site, this assumes that all forest and shrub cover (outside of
proposed impervious surface areas) will be converted to grass unless protected by an open space tract or
covenant
PFedeveloped pervious areas should be assumed to be forest or pasture, whichever was predominant in
1978. The grass assumption for pervious areas should only be made when the predeveloped land cover
(current or 1978 as applicable) is greater than 4 dwelling units per gross acre
Roof Downspout Controls in Subdivisions
All proposed single-family residential subdivision projects must, on a lot -specific basis, provide for or
implement one of three types of roof downspout controls in the order of preference specified in Section
5 1 These include downspout infiltration, dispersion, or a perforated stub -out connection
Sizing Credits for Roof Downspout Controls
When sizing flow control facilities serving single-family residential subdivisions, the following credits
may be applied
• Where roof runoff is infiltrated according to the requirements of Section 5 1 1, the roof area may be
discounted from the net impervious area used for sizing flow control facilities
• Where roof runoff is dispersed according to the requirements of Section 5 1 2 on lots 22,000 square
feet or larger, and the vegetated flow path of the roof runoff is 50 feet or longer, the roof area may be
modeled as grass surface rather than impervious surface when sizing flow control facilities
Note These credits do not apply when determining eligibility for exemptions from Core Requirement #3
Onsite Runoff Bypass
Proposed project runoff may bypass proposed onsite flow control facilities provided that all of the
following are true
1 Runoff from both the bypass area and the flow control facility converges within a quarter -mile
downstream of the project site discharge point, AND
City of Kent Surface Water Design Manual
5 1-35 (Chapter 5 —Kent Construction Standards)
SECTION 1 2 CORE REQUIREMENTS
2 The flow control facility is designed to compensate for the uncontrolled bypass area such that the net
effect at the point of convergence downstream is the same with or without bypass, AND
3 The 100 -year peak discharge from the bypass area will not exceed 0 4 cfs, AND
4 Runoff from the bypass area will not create a significant adverse impact to downstream drainage
systems or properties, AND
5 Water quality requirements applicable to the bypass area are met
Offsite Bypass Requirement
IF the existing 100 -year peak flow rate from any upstream offsite area is greater than 50% of the 100 -year
developed peak flow rate (undetained) for the project site, THEN the 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 #1 under "Discharge
Requirements "
Manifold Detention Facilities
A manifold detention facility 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 natural discharge points, and redistributes the runoff back 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 Storage
vaults/tanks also require excessive maintenance costs compared to ponds and are therefore not acceptable
for City-mamtamed retention/detention facilities
Underground vaults or tanks shall not be permitted to meet detention requirements for 1) new
developments on commercial or industrial parcels over 1 -acre in size and 2) all residential developments
In addition, underground vaults or tanks shall not be permitted for redevelopments where there is an
existing pond or where there is area available for an open pond, regardless of the size of the parcel area for
the proposed redevelopment
No credit will 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 supplies 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
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To work towards protection of the groundwater resource, the City of Kent, Covington Water District, and
Water District 111 have designated and mapped "Wellhead Protection Areas" These areas are shown on
the Sods/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 quality to the maximum extent
possible This shall predominantly be achieved through the construction of infiltration facilities as
described in Chapter 5 — Flow Control Design
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City of Kent Surface Water Design Manual
5 1-37 (Chapter 5 — Kent Construction Standards)
SECTION 1 2 CORE REQUIREMENTS
1.2.4 CORE REQUIREMENT 44: 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
R
e "Conveyance Requirements for New Systems," Section 1 2 4 1
M "Conveyance Requirements for Existing Systems," Section 1 2 4 2
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• "Conveyance System Implementation Requirements," Section 1 2 4 3
Intent: To ensure proper design and construction of engineered conveyance system elements
Conveyance systems are natural and engineered drainage facilities that collect, contain, and provide for the
flow of surface water and stormwater This core requirement applies to the engineered elements of
conveyance systems—primarily pipes, culverts, and ditches/channels
1.2.4.1 CONVEYANCE REQUIREMENTS FOR NEW SYSTEMS
All new conveyance system elements,24 both onsite and offsite, shall be analyzed, designed, and
constructed according to the following requirements All analyses shall examine the full range of
anticipated tatlwater conditions
Pipe Systems
I 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 the 25 -year design capacity,
provided the overflow from a 100 -year runoff event does not create or aggravate a "severe flooding
problem" or "severe erosion problem" as defined in Core Requirement #2, Section 12 2 Any
overflow occurring onsite for runoff events up to and including the 100 -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
3 The upstream end of a pipe system that receives runoff from an open drainage feature (pond, ditch,
etc ) shall be analyzed and sized as a culvert as described below
Culverts
1 New culverts shall be designed with sufficient capacity to meet the headwater requirements in Section
4 3 1 and convey (at minimum) the 25 -year peak flow, assuming developed conditions for onsite
tributary areas and existing conditions for any offsite tributary areas
2 New culverts must also convey as much of the 100 -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 1 2 2 Any overflow occurring onsite for runoff events up to and including
the 100 -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
3 New culverts proposed in streams with salmonids shall be designed to provide for fish passage based
on requirements published in the document "Fish Passage Design at Road Culverts — A design manual
24 New conveyance system elements are those that are proposed to be constructed where there are no existing constructed
conveyance elements
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(Chapter 5 — Kent Construction Standards) 5 1-38 May, 2002
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SECTION 1 2 CORE REQUIREMENTS
for fish passage at road crossings" by the Washington State Department of Fish and Wildlife Habitat
and Lands Program - Environmental Engineering Division
Ditches/Channels
1 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 100 -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 1 2 2 Any overflow occurring onsite for runoff events up to and including
the 100 -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 tighthne conveyance systems traversing slopes that are steeper than 15% and/or located in designated
Landslide Hazard Areas shall be designed with sufficient capacity to convey and contain (at minimum) the
100 -year peak flow, assuming full build -out conditions25 for all tributary areas, both onsite and offsite
Tightlme 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
1.2.4.2 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
I The existing system must be analyzed and shown to have sufficient capacity to convey and contain (at
minimum) the 10 -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 100 -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 12 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
25 Full buildout conditions means the tributary area is developed to its full zoning potential except where there are existing
environmentally sensitive areas, open space tracts, and/or native growth protection easements/covenants
City of Kent Surface Water Design Manual
May, 2002 5 I-39 (Chapter 5 — Kent Construction Standards)
SECTION 1 2 CORE REQUIREMENTS
section of pipe to match the capacity of other pipes in the system, Improving a pipe inlet, or enlarging
a short, constricted reach of ditch or channel
4 Modifications to an existing conveyance system or element that act to attenuate peak flows due to the
presence of upstream detention storage shall be made In a manner that does not significantly Increase
peak 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
1 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 of the drainage problem(s) being addressed.
1.2.4.3 CONVEYANCE SYSTEM IMPLEMENTATION REQUIREMENTS
Methods of Analysis and Design
Properly -sized conveyance elements provide sufficient hydraulic capacity to convey peak flows of the
return frequencies Indicated in Sections 12 4 1 and 12 4 2 Conveyance capacity shall be demonstrated
using the methods of analysis detailed m 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 runoff from
non -roof -top pollution -generating impervious surface must provide a spill control device as detailed in
Section 4 2 1 prior to discharge from the project site or into a natural onsite drainage feature 26 More
specifically, this requirement applies whenever a proposed project does either of the following
• Constructs a new onsite conveyance system that receives runoff from non -roof -top pollution -
generating impervious surface, OR
• 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 surface 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 If no facilities are
present, then the spill control device must be installed upstream from the final discharge point to the
downstream drainage system
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
26 Natural onsite drainage feature means a natural swale, channel, stream, closed depression, wetland, or lake
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(Chapter 5 —Kent Construction Standards) 5 1-40 May, 2002
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Outfalls to the Green River
New stormwater outfalls or modifications to existing stormwater outfalls discharging to the Green River
between River Mile 6 (South Boeing Access Road) and SR 18 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 construct
or modify outfalls Copies of the plan are available from King County DNR
27 Peak discharges shall be as computed using KCRTS as detailed in Chapter 3
City of Kent Surface Water Design Manual
5 1-41 (Chapter 5 — Kent Construction Standards)
SECTION 1 2 CORE REQUIREMENTS
2 No modifications to currently adopted standard roadway cross-sections in the King County Road
Standards are necessitated by the channel, AND
3 The channel will be accessible for maintenance (see Section 1 2 6), AND
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4 The channel will not be subject to erosion
Exceptions: The following are exceptions to the requirement for vegetative channels
• Conveyance systems proposed under roadways, driveways, or parking areas
• Conveyance systems proposed between houses in urban -zoned plats and short plats
• Conveyance systems conveying roof runoff only
Outfalls
An outfall is defined as a point where collected and concentrated surface and stormwater runoff is
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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 Public Works determines that erosion protection is being provided by
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other means or is not needed Details on outfall structures are included in Section 4 2 2
New Point Discharges Over Steep Slopes if Landslide Hazard Areas: Proposed outfalls that will 1)
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discharge runoff in a location where the natural (existing) discharge is unconcentrated over a slope steeper
than 15%, and/or 2) discharge runoff through a designated landslide hazard area must meet the following
criteria
P 1F the 100 -year peak discharge is less than or equal to 0 2 cfs21 under existing conditions and will
remain less than or equal to 0 2 cfs under developed conditions, THEN outfall runoff may be
discharged onto a rock pad shaped in a manner so as to disperse flow The outfall and rock pad must
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be located upstream from any Landslide Hazard Area buffer and no less than 50 feet from a defined
Landslide or Erosion Hazard Area unless otherwise approved by Public Works based on an
evaluation/report by a geotechnical engineer
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2 IF the 100 -year peak discharge is greater than 0 2 cfs but 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 runoff must
be conveyed to a dispersal trench or other dispersal system The dispersal trench or system must be
located upstream from any landslide hazard area buffer and no less than 50 feet from a defined
Landslide or Erosion Hazard Area unless otherwise approved by Public Works based on an
evaluation/report by a geotechnical engineer
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3 IF the 100 -year peak discharge is greater than 0 5 cfs for either existing or developed conditions,
THEN a tightlme conveyance system must be constructed to convey the runoff to the bottom of the
slope unless other measures are approved by Public Works based on an evaluation/report by a
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geotechnical engineer Tighthne systems must be designed such that existing base flow conditions are
not significantly changed and adequate energy dissipation is provided at the bottom of the slope
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Outfalls to the Green River
New stormwater outfalls or modifications to existing stormwater outfalls discharging to the Green River
between River Mile 6 (South Boeing Access Road) and SR 18 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 construct
or modify outfalls Copies of the plan are available from King County DNR
27 Peak discharges shall be as computed using KCRTS as detailed in Chapter 3
City of Kent Surface Water Design Manual
5 1-41 (Chapter 5 — Kent Construction Standards)
SECTION 1 2 CORE REQUIREMENTS
Interflow and Interception
Intel flow is near -surface groundwater that moves laterally through the soil horizon following the hydraulic
gradient of underlying relatively Impermeable soils When mterflow is expressed on the surface, it is
termed a spi mg of seepage 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 mterflow to downstream wetlands or streams, provision for passage of
unconcentrated flows must be made
Pump Systems
Pump systems may be used to convey water from one location or elevation to another within the project
site provided 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 from the project site that would not have discharged by gravity flow
under existing site conditions will require an approved adjustment to Core Requirement #1 (see Section
1 4, "Adjustment Process") These pump systems will be considered only when they are the sole
alternative to solving a flooding or erosion problem as defined in Section 12 2 Typical conditions of
approval for these systems are available in Reference Section 8-1 under "Adjustment Application Form
and Process Guidelines"
1.2.5 CORE REQUIREMENT #5:
EROSION AND SEDIMENT CONTROL
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 project
R site to downstream drainage facilities, water resources, and adjacent properties To prevent sediment
2 transport, Erosion and Sediment Control (ESC) measures are required and shall perform as described in
T Section 1 2 5 2 Both temporary and permanent erosion and sediment controls shall be implemented as
described in Section 12 5 3
Intent: To prevent the transport of sediment to streams, wetlands, lakes, drainage systems, and adjacent
properties Erosion on construction sites can result in excessive sediment transport to adjacent properties
and to surface waters Sediment transport can result in major adverse impacts, such as flooding due to
obstructed dramageways, smothering of salmonid spawning beds, and creation of algal blooms in lakes
1.2.5.1 ESC MEASURES
The following ESC measures shall be provided as specified below and as further detailed in the King
County Erosion and Sediment Control (ESC) Standards, adopted as Appendix D (detached) of the King
County Manual
• 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
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 1 to September 30) or for more than two days during the
wet season (October 1 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 10 feet of vertical relief shall be covered if they are to remain
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(Chapter 5 — Kent Construction Standards) 5 1-42 May, 2002
SECTION 1 2 CORE REQUIREMENTS
' 1.2.5.2 ESC PERFORMANCE
The above ESC measures shall be applied and maintained so as to prevent, to the maximum extent
possible, the transport of sediment from the project site 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 in the ESC Standards (detached Appendix D of the King County Manual) and
approved by the City However, the City may determine at any 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 sieve test of storm and surface water discharges indicates that sand -sized sediment (soil particles
coarser than the #200 sieve, 0 075 mm) is leaving the project site or entering onsite wetlands, streams,
or lakes, THEN corrective actions and/or additional measures beyond those specified in
Section 1 2 5 1 shall be implemented as deemed necessary by the City Note The City can require
' that the ESC supervisor have a 4200 sieve on site Also, "leaving the project site" will be interpreted
liberally For example, if this criterion is applied to individual lots within a subdivision, it may,
depending on the site, be appropriate to conduct the sieve test at the outlet of the drainage system
' i other than at the edge of the lot
2 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 1 2 5 1 can be required by the City
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City of Kent Surface Water Design Manual
5 1-43 (Chapter 5 —Kent Construction Standards)
unworked for more than 12 hours. The intent of these measures is to prevent erosion by having as
much area as possible covered during any period of precipitation
• 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
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Traffic Area Stabilization: Unsurfaced entrances, roads, and parking areas used by construction
traffic shall be stabilized to minimize erosion and tracking of sediment offsite
• 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 site 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
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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 of any disturbed
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areas However, areas at the perimeter of the site small enough to be treated solely with perimeter
protection do not require surface water controls Significant sources of upstream 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
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unmediately following rough grading
• Dust Control, Preventative measures to minimize wind transport of soil shall be implemented when
a traffic hazard may be created or when sediment transported by wind is likely to be deposited in
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water resources
' 1.2.5.2 ESC PERFORMANCE
The above ESC measures shall be applied and maintained so as to prevent, to the maximum extent
possible, the transport of sediment from the project site 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 in the ESC Standards (detached Appendix D of the King County Manual) and
approved by the City However, the City may determine at any 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 sieve test of storm and surface water discharges indicates that sand -sized sediment (soil particles
coarser than the #200 sieve, 0 075 mm) is leaving the project site or entering onsite wetlands, streams,
or lakes, THEN corrective actions and/or additional measures beyond those specified in
Section 1 2 5 1 shall be implemented as deemed necessary by the City Note The City can require
' that the ESC supervisor have a 4200 sieve on site Also, "leaving the project site" will be interpreted
liberally For example, if this criterion is applied to individual lots within a subdivision, it may,
depending on the site, be appropriate to conduct the sieve test at the outlet of the drainage system
' i other than at the edge of the lot
2 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 1 2 5 1 can be required by the City
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5 1-43 (Chapter 5 —Kent Construction Standards)
SECTION 1 2 CORE REQUIREMENTS
1.2.5.3 IMPLEMENTATION REQUIREMENTS
ESC Plan
As specified in Chapter 2, all proposed projects must submit a plan for providing ESC measures The
ESC plan shall include a detailed construction sequence as proposed by the design engineer and shall
identify required ESC measures All ESC measures shall conform to the details and specifications in the
ESC Standards unless an alternative is approved by Public Works (see "Alternative and Experimental
Measures" in the ESC Standards, detached Appendix D) The ESC plan shall be accompanied by any
calculations or information necessary to size ESC measures and demonstrate compliance with Core
Requirement 45 The City may require large, complex projects to phase construction and to submit
multiple ESC plans for the different stages of construction Development of new ESC plans is not
required for changes that are necessary during construction
Wet Season Construction
During the wet season (October 1 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
Construction within Streams, Lakes, or Wetlands and Buffers
Any construction that will result in disturbed areas 1) on or within a stream or associated buffer, or 2)
within a wetland or associated buffer, or 3) within 50 feet of a lake shall be subject to the "Sensitive Area
Restrictions" contained in the ESC Standards These provisions include phasing the project whenever
possible so that construction in these areas is limited to the dry season
Maintenance
All ESC measures shall be maintained and reviewed on a regular basis as prescribed in the ESC
Standards The applicant shall designate an ESC supervisor who shall be responsible for maintenance and
review of ESC measures and for compliance with all permit conditions relating to ESC as described in the
ESC Standards
Final Stabilization
Prior to obtaining final construction approval, the site shall be stabilized, structural ESC measures (such as
silt fences and sediment traps) shall be removed, and drainage facilities shall be cleaned as specified in the
ESC Standards
Flexible Compliance
Some projects may meet the intent of Core Requirement #5 while varying from specific ESC requirements
contained here and in the ESC 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 likely 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 More information on intent and general ESC principles is contained in the ESC Standards
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
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(Chapter 5 — Kent Construction Standards) 5 I-44 May, 2002
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' SECTION l 2 CORE REQUIREMENTS
' A NPDES General Permit for Construction (pursuant to the Washington State Department of
Ecology's Baseline General Permit for Stormwater) is required for projects that will disturb more than
five acres The five -acre threshold applies even where the five acres are to be disturbed in phases, as
' long as the construction is "part of a larger common plan of development or sale "
1.2.6 CORE REQUIREMENT #6: MAINTENANCE AND OPERATIONS
' a Maintenance and operation of all drainage facilities is the responsibility of the applicant or property
QE owner, except those facilities for which Kent is granted an easement, tract, or right-of-way and officially
T 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 operation28 of 1) conveyance systems within improved public road
' rights-of-way and 2) flow control and water quality facilities if these systems/facilities are associated with
a residential subdivision with five or more lots served by a public street, or a non-residential short plat or
long plat subdivision (t a commercial, industrial, etc ) Kent will not assume maintenance and operation
' of 1) stonnwater facilities associated with residential subdivisions with 9 or less lots served by a private
street, or 2) stormwater facilities associated with commercial or industrial developments located on single
parcels
' Kent will assume maintenance and operation of these facilities one year after final construction
approval by Public Works 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 to be maintained and operated by Kent must be located in a
tractor right-of-way dedicated to Kent Access roads serving these facilities must also be located in the
tractor right-of-way and must be connected to an improved public road right-of-way When allowed (see
section 1 2 3 2) they may be sited in private rights-of-way or roads if the easement includes provisions for
facility access and maintenance
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 normally assume maintenance responsibility
for conveyance systems which are outside of improved public road right-of-way
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downspout, footing, or yard drains, nor does Kent assume maintenance of those water quality facilities installed and integrated
into site landscaping
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Because of these factors, road and utility projects are allowed greater flexibility in meeting the intent of
Core Requirement #5 as described in the ESC 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
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 follow mg permits maybe
required and should be considered when implementing ESC measures
• A Class IV Special Forest Practices Permit is required by the Washington State Department of
Natural Resources for projects that will clear more than two acres of forest or 5,000 board feet of
umber All such clearing is also subject to the State Environmental Policy Act (RCW 43 21C) and
will require SEPA review
' A NPDES General Permit for Construction (pursuant to the Washington State Department of
Ecology's Baseline General Permit for Stormwater) is required for projects that will disturb more than
five acres The five -acre threshold applies even where the five acres are to be disturbed in phases, as
' long as the construction is "part of a larger common plan of development or sale "
1.2.6 CORE REQUIREMENT #6: MAINTENANCE AND OPERATIONS
' a Maintenance and operation of all drainage facilities is the responsibility of the applicant or property
QE owner, except those facilities for which Kent is granted an easement, tract, or right-of-way and officially
T 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 operation28 of 1) conveyance systems within improved public road
' rights-of-way and 2) flow control and water quality facilities if these systems/facilities are associated with
a residential subdivision with five or more lots served by a public street, or a non-residential short plat or
long plat subdivision (t a commercial, industrial, etc ) Kent will not assume maintenance and operation
' of 1) stonnwater facilities associated with residential subdivisions with 9 or less lots served by a private
street, or 2) stormwater facilities associated with commercial or industrial developments located on single
parcels
' Kent will assume maintenance and operation of these facilities one year after final construction
approval by Public Works 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 to be maintained and operated by Kent must be located in a
tractor right-of-way dedicated to Kent Access roads serving these facilities must also be located in the
tractor right-of-way and must be connected to an improved public road right-of-way When allowed (see
section 1 2 3 2) they may be sited in private rights-of-way or roads if the easement includes provisions for
facility access and maintenance
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 normally assume maintenance responsibility
for conveyance systems which are outside of improved public road right-of-way
May, 2002
1
2e Kent does not assume maintenance of lot drainage 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
5 1-45 (Chapter 5 — Kent Construction Standards)
SECTION 1 2 CORE REQUIREMENTS
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, "Maintenance
Requu ements for Privately Maintained Drainage Facilities," and as further prescribed in Chapter 6 for
water quality facilities 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 occurred and where waste was
disposed of shall also be kept by the owner and be available for inspection by the City
A "Declaration of Stormwater Facility Maintenance Covenant" must be completed for all private
developments This document is included with the manual as Reference 8-F The timing of completion of
this agreement varies depending on the type of development For stormwater facilities serving more than
one single family residential unit in a short plat serving less than four lots, the agreement shall be
completed prior to plat recording For all other developments or redevelopments (including commercial,
industrial, multifamily development or redevelopment or single family residential building), the agreement
shall be completed prior to engineering plan approval The responsibilities detailed in the document may
be assumed by a homeowners association or other legal organization as approved by the City of Kent
Kent may inspect all privately maintained drainage facilities for compliance with these requirements If
property owner(s) fail to maintain their facilities to acceptable standards, the City may issue a written
notice specifying the required actions If these actions are not performed in a timely manner, the City may
enter the property to perform the actions needed and bill the property owner(s) for the cost of the actions
In the event a hazard to public safety exists, written notice may not be required
If the proposed project is a residential subdivision development, all privately maintained conveyance
systems or other drainage facilities, which convey flows through private property, must be located in a
drainage easement dedicated to convey surface and stormwater Individual owners of the properties
containing such easements must maintain the drainage facilities through their property The legal instrument
creating drainage easements on private property must contain language that requires a private property
owner to obtain written approval from Kent prior to removing vegetation (except by routine mowing) from
any drainage easement containing open, vegetated drainage facilities (such as swales, channels, ditches,
ponds, etc ) A sample copy of the City of Kent's Drainage Easement is included with the Manual as
Reference 8-13
City of Kent Surface water Design Manual I
(Chapter 5 — Kent Construction Standards) 5 1-46 May, 2002
1
I
SECTION 1 2 CORE REQUIREMENTS
1.2.7 CORE REQUIREMENT 97:
FINANCIAL GUARANTEES AND LIABILITY
R All drainage facilities constructed or modified for projects (except downspout infiltration and dispersion
Q systems), and any work performed in the right-of-way, must comply with City of Kent's financial
14
T guarantee requirements
1
1
' May, 2002
Intent: To ensure financial guarantees are posted to sufficiently cover the cost of correcting, if necessary,
incomplete or substandard drainage facility construction work, and to warrant for one year the satisfactory
performance and maintenance of those newly -constructed drainage facilities to be assumed by Kent for
maintenance and operation Core Requirement 47 is also intended to ensure that a liability policy is
provided that protects the proponent and the City from any damages relating to the construction or
maintenance of required drainage facilities by private parties
Surface Water and Drainage Facilities Construction Bond
Prior to commencing construction, applicants that are required to construct drainage facilities pursuant to
this manual and/or any other requirements of the Kent City Code must post a Surface Water and Drainage
Facilities Construction Bond (see Reference 8-E) This guarantee must be an amount sufficient to cover
the cost of project -related work performed on- or off-site The exact amount of the bond shall be
documented in the approved engineers estimate Note Public IKorks may waive the requirement of this
guar antee on projects proposing only minor modifications or improvements to the drainage system (e g,
catch basin inserts, spill control devices, pipe replacements, etc) In addition, this guarantee may be
combined with other i equired guarantees
The bond shall be released in two stages as follows Generally, 90% of the bond is released upon
completion of the following
1 Payment of all required fees
2 Construction of the drainage facilities
3 Receipt of final construction approval from Public Works
4 Provide the City with As-budts meeting the requirements of the City of Kent Construction Standards
(including this manual)
The drainage facilities shall be maintained during a period of one year according to the City of Kent's
"Maintenance Requirements for Privately Maintained Drainage Facilities" At the end of the year, the
remaining 10% of the bond is released subject to the following requirements
I For plats, record the final plat
2 For tracts containing drainage facilities to be maintained by Kent and not located within the final plat,
deed the tract to Kent and set property corners in conformance with state surveying standards
3 For easements containing drainage facilities to be maintained by Kent and not located within the final
plat, provide easement documents and set temporary survey markers to delineate the easement
location
4 Receive a final City inspection to ensure the drainage facilities have been properly maintained and are
operating as designed
5 Correct any defects noted in the final inspection
6 Submit a letter to the City formally requesting a release of the bond
Following completion of these steps, the City will assume maintenance and operation of the drainage
facilities
City of Kent Surface Water Design Manual
5 1-47 (Chapter 5 —Kent Construction Standards)
SECTION 1 2 CORE REQUIREMENTS
1.2.8 CORE REQUIREMENT #8: WATER QUALITY
All proposed projects, including redevelopment projects, must provide water quality (WQ) facilities to
n treat the runoff from new and/or replaced pollution -generating impervious surfaces and pollution -
E,
r4i generating pervious surfaces These facilities shall be selected from one of the area -specific WQ menus
r described in Section 1 2 8 1 and implemented according to the applicable WQ implementation
requirements in Section 1 2 8 2
Intent: To require an efficient, cost-effective level of water quality treatment tailored to the sensitivities
and resource protection needs of the downstream receiving water to which the project site drains, or, in the
case of infiltration, protection of the receiving groundwater system
Guide to Applying Core Requirement #8
Core Requirement 48 requires that WQ treatment facilities be provided to remove pollutants from runoff
discharging from the project site in accordance with either the Basic WQ Menu or the Resource Stream
Protection menu found in Section 1 2 8 1
The WQ menus are a group of facility options designed to provide levels of treatment targeted to
resource protection needs
WQ implementation requirements are the minimum requirements for analyzing and designing WQ
facilities to achieve intended performance and other protection goals
Other Important Information about Core Requirement #8
Core Requirement #8 is the primary component of an overall water quality protection strategy required by
this manual Other requirements include the following
• Core Requirement #4 Conveyance System, Spill Control Provisions, Section 1 2 4—This provision
generally applies whenever a project constructs or replaces onsite conveyance system elements that
receive runoff from pollution -generating impervious surfaces The provision requires that runoff from
such impervious surfaces be routed through a spill control device prior to discharge from the project
site or into a natural onsite drainage feature
• Special Requirement 94 Source Control, Section 1 3 4—This requirement applies water quality
source controls from the King County Stormwater Pollution Control Manual to those projects
proposing to develop or redevelop a commercial, industrial, or multifamily site
• Special Requirement #5 Oil Control, Section 1 3 5—This requirement applies special oil controls to
those projects proposing to develop or redevelop a high -use site
State Standards
The stormwater and surface water discharging from an individual property or project shall be of such
quality as to meet Class "A" water quality standards of the State of Washington herein adopted as part of
this manual by reference No one shall introduce into the drainage waters any liquid or solid foreign
substances of biodegradable or other nature that shall cause the water quality to degrade from applicable
State Standards
City of Kent Surface Water Design Manual
(Chapter 5 — Kent Construction Standards) 5 1-48 May, 2002
SECTION 1 2 CORE REQUIREMENTS
' Key Definitions
• Pollution -generating impervious surface (PCIS) Definition Those Impervious surfaces considered
to be a significant source of pollutants in stormwater runoff Such surfaces include those subject to
vehicular use29 or storage of erodible or leachable materials, wastes, or chemicals,30 and that receive
direct rainfall or the run-on or blow-in of rainfall 31 Metal roofs are also considered to be PGIS unless
they are treated to prevent leaching
' Pollution -generating pervious surface (PGPS) Definition: Any non -impervious surface with
vegetative ground cover subject to use of pesticides and fertilizers Such surfaces include. but are not
limited to, the lawn and landscaped areas of residential or commercial sites, golf courses, parks, and
' sports fields
❑ EXEMPTIONS FROM CORE REQUIREMENT #8
' There are four possible exemptions from the requirement to provide a formal water quality facility per
Core Requirement #8
' 1. Surface Area Exemption for New Development
A proposed project or any threshold discharge area within a project is exempt if it meets all of these
criteria
' a) Less than 5,000 square feet of new PGIS32 will be added, AND
b) Less than 5,000 square feet of contiguous PGIS33 will be created through any combination of new
' and/or replaced impervious surface as part of a redevelopment project, AND
c) Less than 1 acre of contiguous PGPS' will be added and/or modified,35 OR there is a formal
agreement with the City to implement a landscape management plan36 for the PGPS areas on the
site, AND
d) The project does not propose greater than $100,000 in improvements to a high -use site
29 A surface, whether paved or not, shall be considered subject to vehicular use If It Is regularly used by motor vehicles The
following are considered regularly -used surfaces roads, unvegetated road shoulders, bicycle lanes within the traveled lane of
' a roadway, driveways, parking lots, unfenced fire lanes, diesel equipment storage yards, and airport runways The following
are not considered regularly used surfaces road shoulders primarily used for emergency parking, paved bicycle pathways,
bicycle lanes adjacent to unpaved or paved road shoulders primarily used for emergency parking, fenced fire lanes, and
infrequently used maintenance access roads
' 30 Erodible or leachable materials, wastes, or chemicals are those substances that, when exposed to rainfall, measurably alter
the physical or chemical characteristics of the rainfall runoff (examples Include erodible soil, uncovered process wastes,
manure, fertilizers, oily substances, ashes, kiln dust, garbage dumpster leakage, etc )
31 A covered parking area would be considered pollution -generating if runoff from uphill could regularly run through it, or if rainfall
could regularly blow in and wet the pavement surface The same parking area would not be included if it were enclosed by
walls or If a low wall and berm prevented stormwater from being blown in or from running onto the covered area
32 New PGIS means new Impervious surface (as defined on page 5 1-6) that is pollution -generating
t a3 Contiguous PGIS means a discrete patch of PGIS that Is all together as opposed to being separated In different locations on
the project site As used In this and other exemptions, the Intent Is to apply Core Requirement #8 to those redevelopment
projects that are replacing andlor adding enough impervious surface in one location to allow for opportune installation of a
' water quality facility
34 Contiguous PGPS means a patch of PGPS that is all together as opposed to being separated in different locations on the
project site
35 Modified PGPS means any existing PGPS that Is re -graded or re-contoured by the proposed project
36 Landscape management plan means a King County approved plan for defining the layout and long-term maintenance of
landscaping features to minimize the use of pesticides and fertilizers, and to reduce the discharge of suspended solids and
other pollutants Guidelines for preparing landscape management plans can be found in Reference Section 4-A Submittal
requirements are detailed in Section 2 3 1 4
City of Kent Surface Water Design Manual
May 2002 5 1-49 (Chapter 5 — Kent Construction Standards)
I
SECTION 1 2 CORE REQUIREMENTS
2. Cost Exemption for Redevelopment Projects
A redevelopment project or any threshold discharge area within a redevelopment project is exempt
if it meets all of these criteria
a) Less than $500,000 of total site improvements is proposed, AND
b) Less than 5,000 square feet of new PGiS will be added, AND
c) Less than I acre of contiguous PGPS will be added and/or modified, OR there is a formal
agreement with the City to implement a landscape management plan for the PGPS areas. AND
d) The project does not propose greater than or equal to $100,000 in improvements to a high -use
site
3. Standard Infiltration Exemption
A proposed project or any drainage area within a project is exempt if the runoff from pollution -
generating pervious and impervious surfaces is infiltrated in soils with a measured infiltration rate37
of less than or equal to 9 inches per hour
4. Soil Treatment Exemption
A proposed project or any drainage area within a project is exempt if the runoff from pollution -
generating impervious surfaces is infiltrated in soils that meet the "groundwater protection criteria"
outlined below
Groundwater Protection Criteria: The first 2 feet or more of the soil beneath an infiltration facility
must meet one of the following specifications for general protection of groundwater
a) The soil must have a cation exchange capacity38 greater than 5 and an organic content"' greater
than 0 5%, OR
b) The soil must be composed of less than 250/'o gravel by weight with at least 75% of the soil passing
the #4 sieve, and the portion passing the 44 sieve must meet one of the following gradations
• At least 50% must pass the #40 sieve and at least 2% must pass the 9100 sieve, OR
• At least 25% must pass the #40 sieve and at least 5% must pass the #200 sieve
1.2.8.1 WATER QUALITY MENUS
k Projects subject to Core Requirement #8 must provide a facility selected from either the Basic Water
' Quality menu or the Resource Stream Protection menu, whichever applies per the threshold
w information detailed in this section
t
Redevelopment projects subject to Core Requirement 48 need only apply the Basic WQ menu Note A
higher standard may be imposed by an adopted resource management plan through Special Requirement
41, Section 13 1, or the proposed project may apply a higher standard voluntarily
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 current fisheries, is that provided by the Resource Stream Protection
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)
37 Measured infiltration rate shall be as measured by the EPA method or the Double Ring Infiltrometer Method (ASTM D3385)
For some soils, an infiltration rate of less than 9 inches per hour may be assumed based on a soil texture determination rather
than a rate measurement For more details, see the requirements in Section 5 4 1
38 Cation exchange capacity shall be tested using EPA Laboratory Method 9081
39 Organic content shall be measured on a dry weight basis using ASTM D2974
City of Kent Surface Water Design Manual
(Chapter 5 —Kent Construction Standards) 5 1-50 May, 2002
I
ISECTION 1 2 CORE REQUIREMENTS
❑ BASIC WQ MENU
The Basic WQ 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 menu is for redevelopment projects that are subject to Core
Requirement #8
' 1.2.8.2 WATER QUALITY IMPLEMENTATION REQUIREMENTS
Methods of Analysis and Design
' Water quality treatment facilities shall be analyzed and designed as detailed in Chapter 6
IMay, 2002
40 For evaluation purposes, typical concentrations of TSS in Seattle area runoff are between 30 and 100 mg/L (Table 1, "Water
Quality Thresholds Decision Paper," King County Surface Water Management Division, April 1994)
City of Kent Surface Water Design Manual
51-51 (Chapter 5— Kent Construction Standards)
Treatment Goals and Options
'
The treatment goal for facility options in the Basic WQ menu is 80% removal of total suspended solids
(TSS) for a typical rainfall year assuming typical pollutant concentrations in urban runoff4" TSS is the
'
general performance indicator for basic water quality protection because it is the most obvious pollutant of
concern The basic WQ menu includes facilities such as wetponds, combined detention/wetponds,
biofiltration swales, 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
t
and 4 from Core Requirement #8)
❑ SENSITIVE LAKE PROTECTION MENU
tThe
City has determined that the Sensitive Lake Protection Menu does not adequately ensure removal of
metals, and may not meet water quality goals and standards Phosphorus is not a primary pollutant of
'
concern in the City, lake watersheds are nearly built -out, so future development is not expected to change
the trophic status or lead to declines in lake health due to excessive nutrient loadings
❑ RESOURCE STREAM PROTECTION MENU
1
The Resource Stream Protection menu is applied throughout Kent
Treatment Goals and Options
The treatment goal for facility options in the Resource Stream Protection menu is 50% reduction of total
zinc Zinc is an indicator of a wider range of metals typically found in urban runoff that are potentially
toxic to fish and other aquatic life The Resource Stream Protection menu includes options for using a
large sand filter or a combination of two facilities in series, one of which is either a sand filter or a leaf
compost filter See Chapter 6 for specific facility options and designs
'
Intent
Facility options in the Resource Stream Protection menu are intended to remove more metals than
expected from the Basic WQ menu Lower metal concentrations reduce the risk to fish of exposure to
both chronic and acutely toxic concentrations of metals such as copper and zinc
' 1.2.8.2 WATER QUALITY IMPLEMENTATION REQUIREMENTS
Methods of Analysis and Design
' Water quality treatment facilities shall be analyzed and designed as detailed in Chapter 6
IMay, 2002
40 For evaluation purposes, typical concentrations of TSS in Seattle area runoff are between 30 and 100 mg/L (Table 1, "Water
Quality Thresholds Decision Paper," King County Surface Water Management Division, April 1994)
City of Kent Surface Water Design Manual
51-51 (Chapter 5— Kent Construction Standards)
SCC HON 1 2 CORE REQUIREMENTS
Siting of Treatment Facilities
Required treatment facilities shall be located to treat the runoff from all contiguous PGIS exceeding the
threshold for application of Core Requirement 48 to redevelopment projects and all new PGIS on other
projects, except as allowed below under "Treatment Trades" and "Untreated Discharges " In addition, all
runoff from 1 acre or more of new and/or modified contiguous PGPS must also be directed to a treatment
facility
Any other onsite or offsite runoff draining to a proposed treatment facility must be treated whether it is
from a pollution -generating surface or not This is because treatment effectiveness is determined in part
by the total volume of runoff entering the facility
Treatment of Pervious Surfaces
Pollution -generating pervious surfaces subject to Core Requirement 48 need only be treated using the
Basic WQ menu It is also possible for the facility requirement to be waived if there is a good faith
agreement with Public Works to approve a landscape management plan that controls solids, pesticides,
and fertilizers leaving the site
Treatment Trades
Runoff from areas subject to water quality treatment requirements may be excused from the onsite
treatment requirement if a pre-existing area of impervious surface of equivalent size and pollutant
characteristics lying within the same watershed or stream reach tributary area is treated on the project site
Such substitution is subject to two restrictions
1 The pre-existing impervious surface is not currently being treated, is not required to be treated by any
phase of the proposed project, is not subject to NPDES or other permit requirements, and is not under
a compliance order or other regulatory action, AND
2 The proposal is reviewed and approved by Public Works
Untreated Discharges
If site topographic constraints are such that runoff from an area must be pumped to be treated by the water
quality facility, then Public Works may allow the area runoff to be released untreated, provided that these
conditions are met
I Treatment of the constrained area by filter strip, biofiltration, or a linear sand filter is not feasible, and
a "treatment trade" as described above is not possible, AND
2 The untreated area is less than 5,000 square feet of new PGIS and is less than 5,000 square feet of
contiguous PGIS being created through any combination of new and/or replaced impervious surface
as part of a redevelopment project, AND
3 Any PGPS within the area to be released untreated shall be addressed with a landscape management
plan unless otherwise exempt from Core Requirement #8
Use of Experimental Water Quality Facilities
Treatment 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 14,
"Adjustment Process " When sufficient data on performance has been collected and if performance is
acceptable, the new facility will be added to the appropriate water quality menu for common use
Owner Responsibility for Water Quality
Regardless of the means by which a property owner chooses to meet the water quality requirements of this
manual—whether a treatment facility, a train of facilities, a treatment trade, or an experimental treatment
City of Kent Surface Water Design Manual
(Chapter 5 —Kent Construction Standards) 5 1-52 May, 2002
SECTION 1 2 CORE REQUIREMENTS
facility—it is ultimately the responsibility of the property owner to ensure that runoff from their site does
not create water quality problems or degrade downstream beneficial uses It is also ultimately the
responsibility of the property owner to ensure that the discharge from their property is not in violation of
State and Federal laws
City of Kent Surface Water Design Manual
5 1-53 (Chapter 5 — Kent Construction Standards)
' 1.3 SPECIAL REQUIREMENTS
' This section details the following five special drainage requirements which may apply to the proposed
ptoject depending on its location or site-specific characteristics
'0 Special Requirement #1 Other Adopted Area -Specific Requirements, Section 1 3 1
• Special Requirement 42 Developments within Floodplain/Floodway, Section 1 3 2
• Special Requirement #3 Flood Protection Facilities, Section 13 3
' Special Requirement #4 Source Control, Section 13 4
• Special Requirement #5 Oil Control, Section 13 5
1
1.3.1 SPECIAL REQUIREMENT 41:
' OTHER ADOPTED AREA -SPECIFIC REQUIREMENTS
Other adopted area -specific regulations may be adopted and include additional requirements that have a
' more direct bearing on the drainage design of a proposed project An example is a basin plan or lake
management plan that is adopted by the City
• Basin Plans (BPs) The City may adopt basin plans to provide for the comprehensive assessment of
' resources and to accommodate growth while controlling adverse impacts to the environment A basin
plan may recommend specific land uses, regional capital projects, and special drainage requirements
for future development within the basin area it covers
'0 Lake Management Plans (LMPs) The City may adopt lake management plans to provide for
comprehensive assessment of resources and to accommodate growth while controlling adverse
' impacts from nutrient loading to selected lakes A lake management plan may recommend nutrient
control through special drainage and source control requirements for proposed projects within the area
it covers
Threshold Requirement
IF a proposed project is in a basin plan or THEN the proposed project shall comply
' lake management plan with the drainage requirements of the
basin plan or management plan,
respectively
' Application of this Requirement
The drainage requirements of adopted area -specific regulations such as basin plans shall be applied in
addition to the drainage requirements of this manual unless otherwise specified in the adopted regulation
Where conflicts occur between the two, the drainage requirements of the adopted area -specific regulation
t shall supersede those in this manual
Examples of drainage requirements found in other adopted area -specific regulations include the following
• More or less stringent flow control
• More extensive water quality controls
• Forest retention requirements
' City of Kent Surface Water Design Manual
May, 2002 5 1-55 (Chapter 5 —Kent Construction Standards)
SECTION 1 3 SPECIAL REQUIREMENTS
• infiltration restrictions
• Groundwater recharge provisions
• Discharge to a constructed regional flow control or conveyance facility
1.3.2 SPECIAL REQUIREMENT #2:
DEVELOPMENTS WITHIN FLOODPLAIN/FLOODWAY
Floodplams and floodways are subject to inundation during extreme events The 100 -year floodplams are
delineated to minimize flooding impacts to new development and to prevent aggravation of existing
flooding problems by new development Regulations and restrictions concerning development within a
100 -year floodplain are found in Chapter 14 09 of the Kent City Code
The State Flood Control Program has designated the Green River Valley as a Flood Control Zone Number
Two Washington State law requires a flood control zone permit for all new development in or on the
floodplain within the designated flood zone
A condition of the issuance of an approved flood control zone permit is the requirement to flood proof,
usually by land filling the proposed building site The proposed finished floor elevations of all new
buildings shall be located at least one foot above the existing 100 -year floodplain elevation The as -built
elevations must be verified and an Elevation Certificate must be completed and returned to the City of
Kent Public Works Department The floodplain elevations adopted by the City of Kent are delineated on
the Flood Insurance Rate Maps Copies of these maps are available for review at the Public Works
Department
The following requirements relate to mapping of the floodplain/floodway and compensatory storage
requirements
Threshold
Requirement
IF a proposed project contains or is adjacent
THEN the 100 -year floodplain boundaries
to a stream, lake, wetland, or closed
(and floodway, if available or if
depression, or if other Kent regulations
improvements are proposed within the 100 -
require study of flood hazards
year floodplam) based on an approved flood
hazard study (described below) shall be
delineated on the site improvement plans
and profiles, and on any final subdivision
maps prepared for the proposed project
IF a proposed project proposes to fill within a
THEN the development must provide
FEMA designated floodplain that is outside
offsetting storage for 50% of the pre -
of the floodway (also known as the floodway
development floodway fringe storage
fringe) AND if the proposed project is
capacity that was provided on the property
located in the Green River Valley
that is to be filled
IF a proposed project proposes to fill within a
THEN the development must provide
FEMA designated floodplain that is outside
offsetting storage for 100% of the pre -
of the floodway (also known as the floodway
development floodway fringe storage
fringe) AND if the proposed project is
capacity that was provided on the property
located outside of the Green River Valley
that is to be filled
City of Kent Surface Water Design Manual
(Chapter 5 — Kent Construction Standards) 5 1-56 May, 2002
1
1
1
1
1
1
i
t
1.3.3
1
1
1
1.3.4
May, 2002
SECTION 1 3 SPECIAL REQUIREMENTS
Application of this Requirement
If an approved flood hazard study exists, then It may be used as the basis for delineating the floodplain and
floodway boundaries provided the study was prepared in a manner consistent with this manual and other
Kent flood hazard regulations If an approved flood hazard study does not exist, then one shall be
prepared based on the requirements described in Section 4 4 2, "Floodplain/Floodway Analysts "
Projects proposing to fill or alter the floodplain and/or floodway must prepare a flood hazard study that
analyzes and documents the effect of the proposed development and mitigating measures
SPECIAL REQUIREMENT #3:
FLOOD PROTECTION FACILITIES
Developing sites protected by levees, revetments, or berms requires a high level of confidence in their
structural integrity and performance Proper analysis, design, and construction is necessary to protect
against the potentially catastrophic consequences if such facilities should fail
IF a proposed project either
• contains or is adjacent to a stream that
has an existing flood protection facility
(such as a levee, revetment, or berm),
OR
proposes to construct a new or to modify
an existing flood protection facility
Application of this Requirement
THEN the flood protection facilities shall be
analyzed and/or designed to conform with
the Federal Emergency Management
Administration (FEMA) regulations (44 CFR)
and the State Flood Control Program
The applicant is required to demonstrate conformance with FEMA regulations using the methods specified
in Section 4 4 2 In addition, certain easement requirements (outlined in Section 4 1) must be met to allow
City access for maintenance of the facility
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
City of Kent Surface Water Design Manual
5 1-57 (Chapter 5 — Kent Construction Standards)
SECTION 1 3 SPECIAL REQUIREMENTS '
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
Application of this Requirement
I
THEN the project must provide water quality '
source controls applicable to the proposed
project in accordance with the King County
Stormwater Pollution Control Manual
When applicable, structural source control measures, such as car wash pads or dumpster area roofing, shall
be shown on the site improvement plans submitted for engineering review and approval Other
nonstructural source control 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
1.3.5 SPECIAL REQUIREMENT #5: OIL CONTROL
Projects proposing to develop or redevelop a high -use site (defined below) must provide oil controls in
addition to any other water quality controls required by this manual Such sites typically generate high
concentrations of oil due to high traffic turnover or the frequent transfer of oil
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 of gross 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 10 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 turnover per square foot of
building 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 areas 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 of heating oil tanks
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City of Kent Surface Water Design Manual '
(Chapter 5 — Kent Construction Standards) 5 1-58 May, 2002
ISECTION 1 3 SPECIAL REQUIREMENTS
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IF a proposed project either
• develops a site that will have high -use
site characteristics (defined above), OR
• is a redevelopment project proposing
$100,000 or more of improvements to an
existing high -use site
High -Use Menu
THEN the project must treat runoff from the
high -use portion of the site using oil control
treatment options from the High -Use menu
(described below and detailed in Chapter 6)
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 BMP Oil control options include facilities that are
small, handle only a limited site area, and require frequent maintenance, as well as facilities that treat
larger areas and generally have less frequent maintenance needs Facility choices include catch basin
inserts, linear sand filters, and oil/water separators See Chapter 6 for specific facility choices and design
details
Application of this Requirement
For high -use sites located within a larger commercial center, only the impervious surface associated with
the high -use portion of the site is subject to treatment requirements If common parking for multiple
businesses is provided, treatment shall be applied to the number of parking stalls required for the high -use
business only However, if the treatment collection area also receives runoff from other areas, the
treatment facility must be sized to treat all water passing through it
High -use roadway intersections shall treat lanes where vehicles accumulate during the signal cycle,
including left- and right -turn lanes and through lanes, from the beginning of the left -turn pocket (see
Figure 1 3 5 A below) If no left -turn pocket exists, the treatable area shall begin at a distance equal to
three car lengths from the stop line If runoff from the intersection drains to more than two collection
areas that do not combine within the intersection, treatment may be limited to any two of the collection
areas
Note For oil control facilities to be located in public road right-of-way and maintained by Kent, only
coalescing plate or baffle oil/water separators shall be used unless otherwise approved by an adjustment
Methods of Analysis
The traffic threshold for the High -Use menu shall be estimated using information from Trip Generation,
published by the Institute of Transportation Engineers, or from a traffic study prepared by a professional
engineer or transportation specialist with experience in traffic estimation
City of Kent Surface Water Design Manual
5 1-59 (Chapter 5 —Kent Construction Standards)
SECTION 1 3 SPECIAL REQUIREMENTS '
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1.3.6 REDEVELOPMENT REQUIREMENTS
Slormwater management for redevelopment projects is addressed in many separate sections of this
manual In an effort to clarify the requirements for redevelopment, these sections have been compiled and
are presented below
Redevelopment projects are required to go through drainage review if they are subject to a Kent
development proposal, permit, or approval (such as those listed on page 5 1-6) AND
Propose $100,000 or more of improvements to an existing high -use site, OR
Propose $500,000 or more of site improvements and create 5,000 square feet or more of contiguous
pollution -generating impervious surface through an}, combination of new and/or replaced
impervious surface
Redevelopment Projects proposing $100,000 or more of improvements to an existing high -use site are
subject to "Category 3" Targeted Drainage Review, and require the following
Core Requirement #1- Discharge at the Natural Location
Core Requirement #5 Erosion and Sediment Control
Core Requirement #6 Maintenance and Operations
Core Requirement 47 Financial Guarantees and Liability
Core Requirement 48 Water Quality
Special Requirement #4 Source Control
City of Kent Surface Water Design Manual
(Chapter 5 —Kent Construction Standards) 5 1-60 May, 2002
' SECTION 1 3 SPECIAL REQUIREMENTS
Special Requirement 45 Oil Control
In addition, a "Category 3" project that installs oil controls that construct or modify a 12 -inch ditch ore
also a "Category 2" project that may also include the following
Special Requirement #1 other adopted requirements
43 flood protection facilities
Redevelopment Projects proposing $500,000 or more of site improvements and creating 5,000 square feet
_ or more of contiguous pollution -generating impervious surface through any combination of new
and/or replaced impervious surface are subject to full drainage review The requirements for these
' projects are listed in Table 1 1 2 A and include flow control (Core Requirement 43) and Water Quality
(Core Requirement #8)
1.3.6.1 FLOW CONTROL REQUIREMENTS
When analyzing a redevelopment project for flow control, flows must meet the area specific flow control
' standard by using KCRTS and comparing existing site conditions with proposed site conditions for the
applicable frequency and/or duration criteria
Redevelopment projects may be granted an exemption from Core Requirement 3 - Flow Control under
' one scenario - the Peak Flow Exemption Using Flow Control BMPs, as follows
IF the project improvements in the threshold discharge area generate less than a 0 1 cfs increase in
the existing site conditions 100 -year flow rate AND flow control BMPs are applied as specified in
Section 5 2, AND the project improvements do not significantly impact a "severe erosion
problem" or a "severe flooding problem", AND the project is not located in a Landslide Hazard
Drainage Area
1.3.6.2 WATER QUALITY CONTROL REQUIREMENTS
' Redevelopment projects may be granted an exemption from Core Requirement #8 - Water Quality Control
under two scenarios
1 Surface Area Exemption
' 1F a) less than 5,000 square feet of new PGIS will be added, AND b) less than 5,000 square feet
of contiguous PGIS will be created through any combination of new and/or replaced impervious
surface as part of the redevelopment project, AND c) less than 1 acre of PGPS will be added or
modified, OR there is a formal agreement with the City of Kent to implement a landscape
management plan for the PGPS areas, AND d) the project does not propose greater than $100,000
in improvements to a high use site
'
2 Cost Exemption
IF a) less than $500,000 of total site improvements is proposed, AND b) less than 5,000 square
feet of new PGIS will be added, AND c) less than 1 acre of contiguous PGPS will be added
and/or modified, or there is a formal agreement with the City of Kent to implement a landscape
management plan for the PGPS areas, AND d) the project does not propose greater than or equal
to $100,000 in improvements to a high use site
'
Redevelopment projects subject to Core Requirement #8 need only apply the Basic Water Quality Menu,
regardless of the location of the development Only one of the following facility options must be chosen
to meet the requirement (see Section 6 1 1)
Swale
• Biofiltration
' City of Kent Surface Water Design Manual
May, 2002 5 1-61 (Chapter 5—Kent Construction Standards)
SECTION 1 3 SPECIAL REQUIREMENTS
• Filter Strip
• Wetpond
• Wet vault
• Stormwater Wetland
• Combined Detention and Wetpond
• Sand Filter
In addition, these facilities need only be sized to treat the runoff from the new and/or replaced impervious
surface that is subject to Core Requirement #8 Any other onsite or offsite runoff draining to a proposed
treatment facility must be treated whether it is from a pollution -generating surface or not This is because
treatment effectiveness is determined in part by the total volume of runoff entering the facility
Redevelopment Projects involving greater than or equal to $100,000 in improvements to a high use site
are required to meet Special Requirement #5 —Oil Control Runoff from the high -use portion of the site
must be treated using oil control treatment options presented in Chapter 6
City of Kent Surface Water Design Manual
(Chapter 5 — Kent Construction Standards) 5 1-62 May, 2002
I
1.4 ADJUSTMENT PROCESS
For proposed protects subject to drainage review by Public Works, this process is provided for the
occasions when a project proponent desires to vary from one of the 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 2, Drainage Plan Submittal
+ Chapter 4, Conveyance System Analysis and Design
Chapter 5, Flow Control Design
* Appendix C of the King County Manual, Small Site Drainage Requirements (detached)
* Appendix D of the King County Manual, Ei osion and Sediment Control Standards (detached)
Requests for standard adjustments will be accepted only for permits pending approval or approved
permits that have not yet expired
• Complex Adjustments- Complex adjustments typically require more in-depth review because they
deal with more complicated requirements or requirements that affect basic City policies or other
agencies These adjustments deviate from the requirements contained in the following chapters and
sections of this manual
+ Chapter 1, Drainage Review and Requirements
* Chapter 3, Hydrologic Analysis and Design
* Chapter 6, Water Quality Design
Appendix D of the Kent Construction Standards, Maintenance Requirements for Privately
Maintained Facilities
Requests for complex adjustments will be accepted only for permits pending approval or approved
permits that have not yet expired
• Preapphcation Adjustments- This type of adjustment may be requested when the applicant needs an
adjustment decision to determine if a project is feasible or when the results are needed to determine if
a project is viable before funding a full application The approval of preapplication adjustments is tied
by condition to the project proposal presented at a preapplication meeting with Public Works
' 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 site specific, and that do not have
sufficient data 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
' City of Kent Surface Water Design Manual
May 2002 5 1-63 (Chapter 5 — Kent Construction Standards)
SECTION 1 4 ADJUSTMENT PROCESS
1.4.1 ADJUSTMENT AUTHORITY
The Kent Public Works Department shall have full authority to approve or deny all types of adjustments
for any proposed project subject to drainage review by Public Works
1.4.2 CRITERIA FOR GRANTING ADJUSTMENTS
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
Where it has been demonstrated that meeting the criteria for producing a compensating or comparable
result will deny reasonable use of a property, the applicant shall produce the best practicable alternative as
determined by the Director of Public Works The director or his/her designee shall assess the case to
affirm that denial of reasonable use would occur and to require the practicable alternative that best
achieves the spirit and intent of the requirement Public Works staff shall provide recommendations to the
director on the best practicable alternative to be required
Granting any adjustment that would be in conflict with the requirements of any other Kent department will
require review and concurrence with that department
Experimental Design Adjustments
Experimental design adjustments that request use of an experimental water quality facility or flow control
facility will be approved by Public Works on a limited basis if, upon evaluation, Public Works agrees the
following criteria are met
i The new design is likely to meet the identified target pollutant removal goal or flow control
performance based on limited data and theoretical considerations, AND
2 Construction of the facility can, in practice, be successfully carried out, AND
3 Maintenance considerations are included in the design, and costs are not excessive or are borne and
reliably performed by the applicant or property owner, AND
4 A share of the cost of monitoring to determine facility performance is contributed by the applicant or
property owner
Conditions for approval of these adjustments may include a requirement for setting aside an extra area and
posting a financial guarantee for construction of a conventional facility should the experimental facility
fail Once satisfactory operation of the experimental facility is verified, the set aside area could be
developed and the financial guarantee released
1.4.3 ADJUSTMENT APPLICATION PROCESS
Standard and Complex Adjustments
The application process for standard and complex adjustments is as follows
• Requests for standard and complex adjustments will be accepted only for permits pending approval or
approved permits that have not yet expired
Citv of Kent Surface Water Design Manual
(Chapter 5 — Kent Construction Standards) 5 1-64 May, 2002
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' SECTION 1 4 ADJUSTMENT PROCESS
The completed adjustment request application forms must be submitted to Public Works 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 and/or 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)
• A fee reduction may be requested if it is demonstrated that the adjustment request requires little or no
engineering review
Preapplicatlon Adjustments
The application process is the same as for standard and complex adjustments except that requests will be
accepted prior to permit application, but only if
• The applicant provides justification at a preapplication meeting with Public Works that an adjustment
' decision is needed to determine the viability of the proposed project, AND
• Sufficient engineering information to evaluate the request is provided
' Experimental Design Adjustments
The application process is the same as for standard and complex adjustments except that requests will be
accepted prior to permit application
Blanket Adjustments
There is no application process for blanket adjustments because they are initiated and issued solely by the
City
1.4.4 ADJUSTMENT REVIEW PROCESS
The general steps of the review process for specific types of adjustments are presented as follows
Standard and Complex Adjustments
• Public Works 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 Public Works
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 Public Works director or designee will review and either approve or deny the adjustment request
following Public Work's determination that all necessary information has been received from the
applicant
• Approvals of standard and complex adjustments will expire upon expiration of the permit to which
they apply
' Preapplication Adjustments
The review process is the same as for standard and complex adjustments except that approvals will expire
' one year after the approval date, unless a complete permit application is submitted and accepted
' May, 2002
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City of Kent Surface Water Design Manual
5 1-65 (Chapter 5 —Kent Construction Standards)
SECTION 1 4 ADJUSTMENT PROCESS
Experimental Design Adjustments
• Public Works staff will review the submitted material and inform the applicant as to whether
additional information is required to complete the review Public Works will also inform the
applicant as to how much time is estimated to complete the review
• The Public Works Director or designee will review and either approve or deny the adjustment request
in writing
Blanket Adjustments
Blanket adjustments will be established by Public Works based on
I A previously approved standard, complex, preapplication, or experimental design adjustment and
supporting documentation, AND
2 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
1.4.5 APPEAL PROCEDURE
The applicant may appeal the denial or approval conditions of an adjustment request by submitting a
formal letter to the Public Works Director within 15 working days of the decision TMs 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 Public Works Director
shall respond to the applicant in writing within 15 working days, this decision shall be final Aper -hour
review fee will be charged to the applicant for City review of an appeal
City of Kent Surface Water Design Manual 'I
(Chapter 5 — Kent Construction Standards) 5 1-66 May, 2002
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CHAPTER 2
DRAINAGE PLAN
SUBMITTAL
W A S H I N G T O N
CITY OF KENT
SURFACE WATER
DESIGN MANUAL
Section 2.1
Plans for Permits and
5.2-3
Drainage Review
Section 2.2
Plans Required with Initial
5.2-5
Permit Submittal
Section 2 2 1
Subdivisions and Short Plats
52-5
Section 2 2 2
Commercial Site
52-7
Development
Section 2 2 3
Single -Family Residential
52-7
Section 2 2.4
Other Permits
52-7
Section 2.3
Plans Required for
5.2-8
Drainage Review
Section 2 3 1
Engineering Plan
52-8
Specitications
Section 2 3 2
Projects in Targeted
52-28
Drainage Review
Section 2.4
Plans Required After
52-31
Drainage Review
Section 2 4 1
Plan Changes After Permit
52-31
Issuance
Section 2 4 2
Final Corrected Plan
52-31
Submittal
Section 2 4 3
Final Plat, Short Plat, and
52-31
Binding Site Plan Submittals
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CHAPTER 2
DRAINAGE PLAN SUBMITTAL
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' City of Kent Surface Water Design Manual
Chapter 5 — Kent Construction Standards 52-1
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This chapter details the drainage related submittal requirements for engineering design plans as part of a
permit application to the Kent Public Works 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, S, 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
• Section 2 1, "Plans for Permits and Drainage Review"
• Section 2 2, "Plans Required with Initial Permit Application"
• Section 2 3, "Plans Required for Drainage Review"
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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
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Chapter 5 — Kent Construction Standards 52-1
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' 2.1 PLANS FOR PERMITS AND DRAINAGE REVIEW
The Kent Public Works 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 of the types of engineered drainage plans required for engineering review at vanous
permit review stages Detailed requirements are presented in Sections 2 2 and 2 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 three
stages tentative, preliminary, and final Subdivisions and short plats will require a "Tentative
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
nght-of-way, site 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 Kent Public Works 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 (see Section 2 3 1 0)
' 3 An erosion and sediment control (ESC) plan (see Section 2 3 1 3)
Note A landscape managementplan is also included if applicable (see Section 2 3 1 4)
' 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 ofthese
' plans should be confirmed during the project preapphcation meeting with the Kent Public Works
Department For other permits, such as single-family residential permits, the scope of the targeted
engineering analysis is usually determined during Kent Public Works Department engineering review
Projects without major drainage improvements may be approved to submit a modified 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 12 for
further information
Plans Required for Small Site Drainage Review
' Small site drainage plans are a simplified form of site improvement and ESC plans (without a TIR),
which can be prepared by a non -engineer from a set of pre-engineered design details Small site drainage
plans are only allowed for projects in Small Site Drainage Review
' For single-family residential permits, the level and scope of drainage plan requirements are determined by
the Kent Public Works Department during drainage review Some projects qualifying for Small Site
Drainage Review may also require Targeted Drainage Review
City of Kent Surface Water Design Manual
May, 2002 5 2-3 Chapter 5 — Kent Construction Standards
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City of Kent Surface Water Design Manual '
Chapter 5 — Kent Construction Standards 52-4 May, 2002
DRAINAGETABLE 2.2.1.A
Type of Permit or Project
Plans Required with
Type of
Plans Required for,
Initial Land Use Permit
Drainage Review(5)
Building Permit
Application
Drainage Review
SUBDIVISIONS AND SHORT
Plat Map(1)
Full or Targeted Drainage
. Tentative Plat/Short Plat
PLATS'
Tentative Plat/Short Plat
Review(2)
Drainage Package(1)
Drainage Package
• Engineering Plans(3)
- Conceptual Drainage
Plan
- Level 1 Downstream
Analysis
- Surveyrropographic
Information
COMMERCIAL
Engineering Plans(2) (3)
Full or Targeted Drainage
Engineering Plans(3)
Review
Small Site Drainage Review
Small Site Drainage
OR
Plans(4)
SINGLE-FAMILY
Conceptual Drainage
Small Site Drainage Review
• Small Site Drainage
RESIDENTIAL BUILDING
Plan(1)
AND Targyeted Drainage
Plans(4)
PERMITS
Review(z) OR
. Engineering Plans(3)
Full or Targeted Drainage
Engineering Plans(3)
Review(2)
OTHER PROJECTS OR
Project -specific (contact
Full or Targeted Drainage
Engineering Plans(3)
PERMITS
the Kent Public Works
Review(2)
Department
Notes
(1) 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
(4)Specifications for submittal of small site drainage plans are found in Appendix C of the King County Manual, Small
Site Drainage Requirements (detached)
(5) Refer to Chapter 1, 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 small site drainage review process
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City of Kent Surface Water Design Manual '
Chapter 5 — Kent Construction Standards 52-4 May, 2002
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' 2.2 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 engineeringplans will vary depending on permit type For subdivisions and
short plats, this submittal usually follows the City's approval of the Tentative Plat/Short Plat Drainage
Package For commercial budding 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 requited to be submitted with the moral permit application, they must be
' accompanied by the app opriate suppoi ling documents (e g, required application forms, an
environmental checklist, etc) For more details, see Reference Section 8 accompanying Chapter 5 of the
Kent Construction Standards
' Design Plan Certification
All tentative 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 Registration for Professional
Engineers and Land Surveyors
' 2.2.1 SUBDIVISIONS AND SHORT PLATS
1 Applications for tentative subdivisions and short plats must include a tentative plat/short plat drainage
package consisting of the following
I 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 brief narrative describing existing and proposed site conditions, including inventoried or
delineated wetlands, creeks, ponds, steep or unstable slopes, erosion hazard areas and
landslide hazard areas Include a description of the existing use of the site and the proposed
use of the site after development
b) Two simple drainage plans are required for a Conceptual 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 l" = 30', for
example, not 1/8" = 1'), and must contain a north arrow NOTE North arrows shall either
point to the top of the page or to the right side of the 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 will 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
' May, 2002
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City of Kent Surface Water Design Manual
5 2-5 Chapter 5 — Kent Construction Standards
SECTION 2 3 PLANS REQUIRED rOR DRAINAGE REVIEW
g) All manmade or natural features (streams, creeks, drainage ditches, railroad tracks, hills,
depressions, structures of all kinds, steep slopes, lakes, etc ) and the existing direction of
surface water flows shown by arrows pointing in the direction of flow
h) Setback dimensions from all property lines and from sensitive areas such as wetlands, steep
or unstable slopes, and Native Growth Protection Tracts
I) Existing and proposed building and landscape locations
�) Differentiate between developed portions of a parcel, and undeveloped / natural areas of a
parcel, and areas set aside for future development
k) Outside storage areas and types of surfaces for storage areas
1) The post -development drainage plan shall include a proposed conveyance system layout, and
the location of discharge points onto and off of the property, the total amount of impervious
surface created (including rooftops), the approximate budding 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 )
m) A legal description for the property and the Assessor's Tax Lot Number for the property
n) The name, address, and telephone number of the person preparing the Site Plan
a) A vicinity map that clearly shows the project location
b) The location and type of existing and proposed flow control facilities
c) The location and type of existing and proposed water quality facilities
d) The location and type of existing and proposed conveyance systems
2 A Level 1 Downstream Analysts as required in Core Requirement #2 and outlined under "TIR
Section 3, Offsite Analysis " The Level 1 Downstream Analysis is required for all short plats
except those meeting the exemptions outlined in Section 1 2 2 or qualifying for Small Site
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 Kent Public Works 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 Kent
Public Works Department 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 identify as undesignated)
City of Kent Surface Water Design Manual
Chapter 5 —Kent Construction Standards 5 2-6 May, 2002
' SECTION 2 3 PLANS REQUIRED FOR DRAINAGE REVIEW
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2.2.2 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
engineering plans Projects which may qualify for limited scope engineering design should request
Targeted Drainage Review during the preapplication meeting with the Public Works Department
2.2.3 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 2 (see detached Appendix C, Section
C S 3, note that erosion control, flow control not required)
2.2.4 OTHER PERMITS
' May, 2002
Othei permit applications will require project -specific information Initial submittal requirements can be
obtained by contacting the Kent Public Works Department
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2.3 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 an erosion and sediment control (ESC) 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 1-1 23
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 qualify for Small Site Drainage Review may be required to submit small site drainage plans
These are'simphfied 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 Small Site Drainage Requirements
booklet available at the King County Department of Development and Environmental Services and
appended to the King County Manual (detached Appendix C, Section C 5 3) contains the specifications
for small site drainage plans and details on the Small Site Drainage Review process
Note Projects in Small Site Drainage Review maybe required to submit engineering plans if they are
also subject to Targeted Drainage Review as determined in Section 1 12 2 and Appendix C of the King
County Manual Also, short plats in Small Site Drainage Review will be required to submit engineering
plans if roadway construction is a condition ofprelimmary approval
Design Plan Certification
All tentative 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 Registration for Professional
Engineers and Land Surveyors.
2.3.1 ENGINEERING PLAN SPECIFICATIONS
For drainage review purposes, engineering plans must consist of
1 A technical information report (TIR) as detailed in Section 2 3 1 0, AND
2 Site improvement plans as detailed in Section 2 3 12, AND
3 An erosion and sediment control (ESC) plan as detailed in Section 2 3 1 3
Also, if applicable per Section 1 2 8, a landscape management plan, as detailed in Section 2 3 1 4, must
be included
Projects in Targeted Drainage Review require a limited scope TIR with site improvement plans and an
ESC plan, as detailed in Section 2 3 2 The Kent Public Works 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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1 2.3.1.1 TECHNICAL INFORMATION REPORT (TIR)
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The full TIR should be a comprehensive supplemental report containing all technical information and
analysts necessary to develop the site improvement plan This report should contain all calculations,
conceptual design analysts, 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
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The TIR shall contain the following ten sections, preceded by a table of contents
1 Project Overview
2 Conditions and Requirements Summary
3 Offsite Analysts
4 Flow Control and Water Quality Facility Analysts and Design
5 Conveyance System Analysis and Design
6 Special Reports and Studies
7 Other Permits
8 ESC Analysts and Design
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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 This standardized format allows a quicker, more efficient
review of information required to supplement the site improvement plan,
The table of contents should include a list of the 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 of the proposal, predeveloped and developed
conditions of the site, site area and size of the improvements, and the disposition of stormwater runoff
before and after development The overview shall identify and discuss difficult site parameters, the
natural drainage system, and drainage to and from adjacent property, including bypass flows
The following figures are required
Figure 1. TIR Worksheet
Include a copy of the TIR Worksheet (see Reference Section 8-A accompanying Chapter 5 of the Kent
Construction Standards)
Figure 2. Site Location
Provide a map that shows the general location of the site Identify all roads that border the site and all
significant geographic features including all streams, lakes, wetlands, closed depressions, and Hazard
Areas
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SECTION 2 3 PLANS REQUIRED FOR DRAINAGE REVIEW
Figure 3 Drainage Basins, Subbasins, and Site Characteristics
This figure shall display
1 Acreage of all subbasms
2 All site characteristics
3 Existing discharge points to and from the site
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, 1"= 300' as a base for the figure (available
at the Kent Permit Center)
6 The length of travel (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
1 The project site
2 The area draining to the site
3 The drainage system downstream of the site for the distance of the downstream analysis
(see Section 12 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 concern), 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 Soil 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 B)
All urban plats and short plats (creating lots less than 22,000 square feet) must evaluate onsite soils for
suitability for roof downspout infiltration as detailed in Section 5 1 1 This soils report, as well as
geotechnical investigations necessary for proposed infiltration facilities, should 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 satisfy 1, 2, and 3 above
❑ TIR SECTION 2
CONDITIONS AND REQUIREMENTS SUMMARY
The intent of this section is to ensure all preliminary approval conditions and applicable requirements
pertaining to site 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 requirementsz,
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
❑ TIR SECTION 3
OFFSITE ANALYSIS
All projects in engineering review shall complete, at a minimum, an Offsite Analysts, except for projects
meeting the exemptions outlined to Section 12 2 The Offsite Analysts is usually completed as part of the
2 Any specific regulations related to floodplams, streams, lakes, wetlands, closed depressions, or geologic hazard areas
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SECTION 2 3 PLANS REQUIRED FOR DRAINAGE REVIEW
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initial permit application and review process, and is to be included in the T1R Note If offsite conditions
have been altered since the initial submittal, a new offsite analysis may be required
The primary component of the offsite analysis is the downstream analysis described in detail below
Upstream areas are included in this component to the extent they are expected to be affected by backwater
effects from the proposed project Other components of the offsite analysis could include, but are not
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limited to, evaluation of impacts to fish habitat, groundwater levels, groundwater quality, or other
environmental features expected to be significantly affected by the proposed project due to its size or
iproximity
to such features
Levels of Analysis
The offsne analysis report requirements vary depending on the specific site and downstream conditions
Each project submittal shall include at least a Level 1 downstream analysis Upon review of the Level 1
pi oblem Kent Public Works Department staff will determine whether a Level 2 or 3 analysts is
analysis, the Kent Public Works 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
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 distance) from the proposed project discharge location and shall extend upstream as necessary to
encompass the offsite drainage area tributary to the proposed project site For the purposes of Tasks
3, 4, and 5, the study area shall extend downstream to a point on the drainage system where the
proposed project site constitutes a minimum of 15 percent of the total tributary drainage area, but not
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May, 2002 5 2-11 Chapter 5 - Kent Construction Standards
Level 1 Analysis
The Level 1 analysis is a qualitative survey of each downstream system leaving a site This analysis
is required for all proposed projects and shall be submitted with the initial permit application
Depending on the findings of the Level 1 analysts, a Level 2 or 3 analysis may need to be completed
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or additional information may be required If further analysis is required, the applicant may schedule
a meeting with Kent Public Works Department staff
Level 2 or 3 Analysts
if problems are identified in the Level 1 analysis, a Level 2 (rough quantitative) analysis or a Level 3
(more precise quantitative) analysts may be required to further evaluate proposed mitigation for the
pi oblem Kent Public Works Department staff will determine whether a Level 2 or 3 analysts is
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required based on the evidence of existing or potential problems identified in the Level 1 analysts and
on the proposed design of onsite drainage facilities The Level 3 analysis is required when results
need to be as accurate as possible for example, if the site is flat, if the system is affected by
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downstream controls, if minor changes in the drainage system could flood roads or buildings, or if the
proposed project will contribute more than 15 percent of the total peak flow to the drainage problem
location The Level 2 or 3 analysts may not be required if the Kent Public Works Department
determines from the Level 1 analysis that adequate mitigation can be developed
Additional Analysis
Additional, more detailed hydrologic analysis may be required if the Kent Public Works Department
determines that the downstream analysis has not been sufficient to accurately determine the impacts of
a proposed project on an existing or potential 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
(Task 1), 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 distance) from the proposed project discharge location and shall extend upstream as necessary to
encompass the offsite drainage area tributary to the proposed project site For the purposes of Tasks
3, 4, and 5, the study area shall extend downstream to a point on the drainage system where the
proposed project site constitutes a minimum of 15 percent of the total tributary drainage area, but not
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SECTION 2 3 PLANS REQUIRED FOR DRAINAGE REVIEW
less than one-quarter mile (minimum flow path distance) The study area shall also extend upstream
of the project site a distance sufficient to preclude any back water effects from the proposed project
The offsite analysts shall include (1) a site map showing property lines, and (2) the best available
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, site 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 analysts, Kent has gathered information regarding
existing and potential flooding and erosion problems For all levels of analysts, all of the resources
described below shall be reviewed for existing/potential problems in the study area (upstream and one
mile downstream of the project site)
• Adopted basin plans (available at the Kent Public Works Department, King County Department
of Development and Environmental Services, King County Department of Natural Resources and
the library)
• Finalized drainage studies (available at the Kent Public Works Department)
• Basin Reconnaissance Summary Reports and 1 "=400' scale problem summary maps (available at
King County Department of Development and Environmental Services, King County
Department of Natural Resources and the library)
• Floodplam/floodway (FEMA) maps (available at the Kent Public Works Department and the
library)
• Other offsite analysis reports in the same subbasin, if available (check with the Kent Public
Works Department records staff)
• Sensitive Areas Folio (available at King County Department of Development and Environmental
Services—must be used to document the distance downstream from proposed project to nearest
stream wetland or lake identified in the folio)
• U S Department of Agriculture, King County Soils Survey (available at King County Department
of Development and Environmental Services and the library)
• Wetlands Inventory maps (City-wide map included with this manual, detailed maps available at
the Kent Public Works 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 Kent Public Works Department)
Potential/existing problems identified in the above documents shall be documented in the Drainage
System Table (see Reference Section 8-B accompanying Chapter 5 of the Kent Construction
Standards) and described in the text of the Level 1 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), the
United States Army Corps of Engineers (Corps), and the public works departments of other
municipalities in the vicinity of the proposed project site
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 of the
following existing or potential problems and drainage features
Level 1 Inspection:
I Investigate any problems reported or observed during the resource review
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SECTION 2 3 PLANS REQUIRED FOR DRAINAGE REVIEW
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2 Locate all existing/potential constrictions or lack of capacity in the existing drainage system
3 identify all existing/potential flooding or nuisance problems as defined in Section 1 2 2 1
4 Identify existing/potential overtopping, scouring, bank sloughing, or sedimentation
5 Identify significant destruction of aquatic habitat or organisms (e g , severe siltation, bank
erosion, or incision in a stream)
6 Collect qualitative data on features such as land use, impervious surfaces, topography. and
sod types
7 Collect information on pipe sizes, channel characteristics, drainage structures, wetlands,
streams, lakes, closed depressions, and Hazard Areas
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8 Verify tributary basins delineated in Task 1
9 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
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or 3 Inspection.
1 Perform a Level 1 Inspection
2 Document existing site conditions (approved drainage systems or pre -1978 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 an experienced surveyor
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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 a map (Task 1). in the narrative (Task 4), and in the Offsae Analysis Drainage Svslene
Table (see Reference Section 8-13)
Drainage System Descriptions The following information about drainage system components such as
pipes, culverts, bridges, outfalls, ponds, tanks, and vaults shall be included in the report
1 Location (corresponding map label and distance downstream/upstream from site discharge)
2 Physical description (type, size, length, slope, vegetation, and land cover)
3 Problems
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4 Field observations
Problem Descriptions All existing or potential problems (e g, ponding water, high/low flows,
siltation, erosion, 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 of three defined
problem types that require special attention per Core Requirement #2 (see Section 1 2 2 1) Special
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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 12 2 2
The following information shall be provided for each existing or potential problem
I the high low flows,
Description of problem (ponding water, or 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)
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SECTION 2 3 PLANS REOUIRED FOR DRAINAGE REVIEW
3 General frequency and duration of problem (dates and times the problem occurred, if
available)
4 Return 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 analyses may be required to
accurately identify the return frequency of a par titular downstream problem, see Section
333
5 Water surface elevation when the problem occurs (e g, elevation of budding foundation,
crest of roadway, elevation of septic dramfields, or wetland/stream high water mark)
6 Names and concerns of involved parties (optional for all levels of analysis)
7 Current mitigation of the problem
8 Possible cause of the problem
9 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 See Section 1 2 3 1 fol
more details on the effectiveness of flow control standards in addressing downstream
problems
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 problem -specific mitigation
requirements set forth in Section 1 2 2 2 To meet these requirements, the proposed project may need
to provide additional onsite flow control as specified in Table 1 2 3 A (see also Section 3 3 5), or other
onsite or offsite mitigation measures as described in Section 3 3 5
❑ TIR SECTION 4
FLOW CONTROL AND WATER QUALITY FACILITY ANALYSIS AND DESIGN
Existing Site Hydrology (Part A)
This section of the TIR should include a discussion of assumptions and site parameters used in analyzing
the existing site hydrology The definition of "existing site conditions" presented in Section 1 1 shall be
applied for this section
The acreage, soil types, and land covers used to determine existing flow characteristics, along with basin
maps, graphics, and exhibits for each subbasm affected by the development, should be included
The following information must be provided on a topographical map
1 delineation and acreage of areas contributing runoff to the site
2 flow control facility location
3 outfall location and description
4 overflow route
The scale of the map and the contour intervals must be sufficient to determine the basin and subbasm
boundaries accurately The direction of flow, the acreage of areas contributing drainage, and the limits of
development should all be indicated on the map
Each subbasm contained within or flowing through the site should be individually labeled and KCRTS
parameters referenced to that subbasm
All natural streams and drainage features, including wetlands and depressions, must be shown Rivers,
closed depressions, streams, lakes, and wetlands must have the 100 -year floodplain (and floodway where
applicable) delineated as required in Special Requirement #2 (see Section 13 2) and by applicable
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CONVEYANCE SYSTEM ANALYSIS AND DESIGN
This section should 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
information should be presented in a clear, concise manner that can be easily followed, 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
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environmental regulations
Developed Site Hydrology (Part B)
This section should provide narrative, mathematical, and graphical presentations of parameters selected
and values used for the developed site conditions, including acreage, soil types, land covers, roadway
layouts, and all constructed drainage facilities
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Developed subbasm areas and flows should be clearly depicted on a map and cross-referenced to
computer printouts or calculation sheets Relevant portions of the calculations should be highlighted and
tabulated in a listing of all developed subbasin flows
All maps, exhibits, graphics, and references used to determine developed site hydrology must be included,
maintaining the same subbasin labeling as used for the existing site hydrology whenever possible If the
boundaries of the subbasin have been modified under the developed condition, the labeling should be
modified accordingly (e g, Subbasm "Am' is a modified version of existing Subbasm "A")
Standards C)
Performance (Part
The design engineer shall include brief discussions of the following
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. The area -specific flow control standard determined from the Flow Control Applications Map per
Section 1 2 3 and any modifications to the standard to address onsite or offsite drainage conditions,
and
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The applicable conveyance system capacity standards per Section 12 4
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Flow Control System (Part D)
This section requires an illustrative sketch of the flow control facility and its appurtenances This sketch
inust show basic measurements necessary to calculate the storage volumes available from zero to the
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maximum head, all orifice/restnctor sizes and head relationships, and control structure/restnctor
orientation to the facility
The applicant should include all computer printouts, calculations, equations, references, storage/volume
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tables, graphs, and any other aides necessary to clearly show, results and methodology used to determine
the storage facility volumes KCRTS facility documentation files, "Compare Flow Durations" files, peaks
files, return frequency or duration curves, etc , should be included to verify the facility meets the
performance standards indicated in Part C
Water Quality System (Part E)
This section provides an illustrative sketch of the proposed water quality facility (or facilities), source
controls, oil controls, and appurtenances This sketch (or sketches) should show overall measurements
and dimensions, orientation on the site, location of inflow, bypass, and discharge systems, etc
The applicant should include all 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
' ❑ TIR SECTION 5
CONVEYANCE SYSTEM ANALYSIS AND DESIGN
This section should 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
information should be presented in a clear, concise manner that can be easily followed, 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
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SECTION 2 3 PLANS REQUIRED FOR DRAINAGE REVIEW
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 should also be provided
❑ TIR SECTION 6
SPECIAL REPORTS AND STUDIES
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,
Kent may require the preparation of special reports and studies to address further the site characteristics,
the potential for impacts associated 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
analysis Topics of special reports may include any of the following
• Geotechnical/sods
• Wetlands
• Floodplams
• Slope protection/stability
• Groundwater
• Fluvial geomorphology
• Erosion and deposition
• Anadromous fisheries impacts
• Structural design
• Geology/Geologic Hazard Areas
• Hydrology
• Water quality
• Structural fill
❑ TIR SECTION 7
OTHER PERMITS
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
of the TIR 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 to Section 1 1 3
❑ TIR SECTION 8
ESC ANALYSIS AND DESIGN
This section must include all hydrologic and hydraulic information used to analyze and design the erosion
and sediment control (ESC) facilities, including final site stabilization measures The TIR shall explain
how proposed ESC measures comply with the Erosion and Sediment Control Standards (Appendix D of
the King County Manual) and show compliance with the implementation requirements of Core
Requirement 45, Section 12 5
The following information must be included
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
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SECTION 2 3 PLANS REQUIRED FOR DRAINAGE REVIEW
Section 1 2 5 and its applicability to the proposed project
2 Include all hydrologic and hydraulic information used to analyze and size the ESC facilities
shown in the engineering plans Describe the methodology, and attach any graphics or sketches
used to size the facilities
3 Identify areas with a particularly high susceptibility to erosion because of slopes or soils 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
4 Identify any ESC recommendations in any of the special reports prepared for the project If
these recommendations are not included in the ESC plan, provide justification
5 If proposing exceptions or modifications to the standards detailed in the Erosion and Sediment
Control Standards (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
❑ TIR SECTION 9
BOND QUANTITIES, FACILITY SUMMARIES, AND DECLARATION OF COVENANT
Bond Quantities Worksheet
Each plan submittal requires a construction quantity summary to establish appropriate bond amounts
Using the Bond Quantities Worksheet (Contact the City of Kent Public Works 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, which are normally not bonded, 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 81/2" x II " 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
Declaration of Covenant
(Privately Maintained Flow Control and WQ Facilities Only)
A declaration of covenant (see Reference 8-F) must be signed and recorded with the City of Kent before
any permit with privately maintained flow control or water quality facilities is approved
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❑ TIR SECTION 10
OPERATIONS AND MAINTENANCE MANUAL
For each flow control and water quality facility 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, what it does,
and how itworks In addition, the manual shall include a copy of the Stormwater Facility Maintenance
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Guide (see Appendix D of the Kent Construction Standards) and provide an outline of maintenance tasks
and the recommended frequency each task should be performed This is especially important for water
quality facilities where proper maintenance is critical to facility performance For this reason, most of the
water facility designs in Chapter 6 include "maintenance considerations" important to the performance of
each facility
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SECTION 2 3 PLANS REQUIRED FOR DRAINAGE REVIEW
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 Information necessary for persons trained in engineering to review the plans, as well as
those persons skilled in construction work to build the project according to the design engineer's Intent
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 NGVD, USGS and USC and GS 1947 (adjusted to the 1929 datum), 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 C of the King County Manual
Horizontal control for all plats, binding site plans, and short plats shall reference the North American
Datum of 1927 as the coordinate base and basis of bearings All horizontal control for these projects must
be referenced to a minimum of two City of Kent Survey Horizontal Control monuments If two horizontal
control monuments do not exist within one mile of the project, an assumed or alternate coordinate base
and basis of bearings may be used Horizontal control 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
• A base map (described in Table 2 3 1 A), and
• Site plan and profiles (see Section 2 3 1 1)
Note Site improvement plans must also include gradtngplans if onsite grading extends beyond the
roadway
Modified Site Improvement Plan
The Kent Public Works Department may allow a modified site improvement plan for some projects in
Targeted Drainage Review (see Section 2 3 2) or where mayor 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 11 " 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 the Kent Public Works Department
❑ GENERAL PLAN FORMAT
Site improvement plans should use APWA Standard Map Symbols as appropriate, and must include
Standar d Plan Notes (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 archive quality reproducibles, mylar or equal
2 Drafting details shall generally conform to APWA Standard Map Symbols with lettering size (before
reduction) no smaller than Leroy 80 (Leroy 100 is preferred) Existing features shall be shown with
dashed lines or as half -toned (screened) to clearly distinguish existing features from proposed
improvements
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SECTION 2 3 PLANS REQUIRED FOR DRAINAGE REVIEW
3 Each submittal shall contain a project information/cover sheet with the following
a) Title Project name and Kent Public Works Department file number
b) Table of contents (if more than three pages)
c) Vicinity map
d) Name and phone number of utility field contacts (e g , water, sanitary sewer, gas, power,
telephone, and TV) and the One -Call number (1-800-424-5555)
e) Kent's preconstructionhnspection notification requirements
f) 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
i) Name and phone number of the applicant
1) Legal description
k) Plan approval signature block for the Kent Public Works Department
1) 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 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-of-way 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 of the plan set shall be stamped, signed, and dated by a licensed civil engineer registered
in the State of Washington 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)
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SECTION 2 3 PLANS REQUIRED FOR DRAINAGE REVIEW
8. The location and label for each section or other detail shall be provided
Buffers and setbacks required for wetlands, streams, lakes, and hazard areas shall be designated as
required by Kent City Code
10 All match lines with matched sheet number shall be provided
11 Ail 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)
13 The standard plan notes that apply to the project shall be provided on the plans (see Appendix A of
the Kent Construction Standards)
14 Commercial building permit applications shall include the designated zoning for all properties
adjacent to the development site(s)
❑ 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 ESC plans are all
developed from the same background information This base map shall include the information listed in
Table 2 3 1 A
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REQUIREMENTSTABLE 2.3.1.A BASE MAP
Feature
Requirements
Ground Surface
Provide topography within the site and extending beyond the property lines
Topography
Contour lines must be shown as described in "Plan View Site Plan and
Roadway Elements" (see Section 2 3 1 2)
Surface Water
Provide ground surface elevations for a reasonable "fan" around points of
Discharge
discharge extending at least 50 feet downstream of all 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, roads (low spots), closed depressions,
springs, seeps, swales, ditches, pipes, groundwater, and seasonal standing
water
Other Natural
Show the location and relative sizes of other natural features such as rock
Features
outcroppings, existing vegetation, and trees 12 inches in diameter and greater
that could be disturbed by the project improvements and construction activities
(within tree canopy), noting species
Flows
Provide arrows that indicate the direction of surface flow on all public and
private property and for all existing conveyance systems
Floodplains/
Show the flood plain/floodways as depicted on City of Kent maps or otherwise
Floodways
required by the City of Kent
General
Show the location and limits of all existing
Background
Property boundaries
Information
• Structures
• Easements (including dimensions)
• Total property (including dimensions)
• Roads and nght-of-way
• Sanitary sewers and water utilities
• Common open space
Public dedications
• Other manmade features affecting existing topography/proposed
improvements
Development
Delineate limitations to the development that may occur as identified on the
Limitations
TIR worksheet, Part S (see Reference S-A)
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SECTION 2 3 PLANS REQUIRED FOR DRAINAGE REVIEW
❑ SITE PLAN AND PROFILES
The design engineer shall provide plans and profiles for all construction, including but not Itmited to the
following information
Plan View: Site Plan and Roadway Elements
1 Provide property lines, right-of-way lines, and widths for proposed roads and intersecting roads
2 Pi ovide all existing and proposed roadway features, such as centerlines, edges of pavement and
shoulders, ditchlines, curbs, and sidewalks In addition. show points of access to abutting properties
and roadway continuations
3 Show existing and proposed topography contours at 2 -foot intervals (5 -foot intervals for slopes
greater than 15 percent, 10 -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, floodplams, drainage
tracts easements, and conveyance systems Contours shall extend 50 feet beyond property lines to
resolve questions of setback, cut and fill 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
identify all existing utility poles
5 Identify 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
8 For subdivision projects, use drawing scales of 1 "=50' For commercial, multi -family, or other
projects, use scales of 1 "=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
I Sequentially number all catch basins and curb inlets starting with the structure farthest downstream
2 Represent existing storm drainage facilities 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
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 corner
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
9 Clearly show on the plans all drainage easements, tracts, access easements, Native Growth Retention
Areas, Sensitive Area Tracts, Sensitive Area Setback Areas, and building setback lines Show
dimensions, type of restriction, and use
10 Using arrows, indicate drainage direction of hydraulic conveyance systems
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15 Provide profiles for all 12 -inch and larger pipes and for channels (that are not roadside ditches)
16 Show the location of all existing and proposed (if available or critical for clearance) gas, water, and
' sanitary sewer crossings
17 Show energy dissipater locations
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SECTION 2 3 PLANS REQUIRED FOR DRAINAGE REVIEW
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Plan
View: Other
1
Show the location, identification, and dimensions of all buildings, property lines, streets, alleys, and
easements
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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 Siormwater Pollution Control Manual and any
subsequent revisions
Profiles: Roadway and Drainage
1
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 of points of curve, tangent, and
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
6
Show all roadway drainage, including drainage facilities, that are within the right-of-way or easement
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
indicate the inverts of all pipes and culverts and the elevations of catch basin grates or lids It is also
desirable, but not required, to show invert elevations and grate elevations on plan sheets
8
For pipes that are proposed to be within 2 0 feet of finished grade, indicate the minimum cover
dimensions
9
Indicate roadway stationing and offset for all catch basins
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Indicate vertical and horizontal scale
Clearly label all profiles with respective street names and plan sheet reference numbers, and indicate
all profile sheet reference numbers on plan sheets, if drawn on separate sheets
12
Locate match points with existing pavements, and show elevations
13
Show all property boundaries
' 14 Label all match line locations
15 Provide profiles for all 12 -inch and larger pipes and for channels (that are not roadside ditches)
16 Show the location of all existing and proposed (if available or critical for clearance) gas, water, and
' sanitary sewer crossings
17 Show energy dissipater locations
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SECTION 2 3 PLANS REQUIRED FOR DRAINAGE REVIEW
18 Identify datum used and all benchmarks (may be shown on plan view Instead) Datum and
benchmarks must refer to established control when available
19 Use a vertical scale of 1 "=5' As an exception, vertical scale shall be 1 "=10' if the optional I"=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 underneath the plan view, are preferred but not required
❑ DETAILS
The design engineer shall provide details for all construction, including but not limited to the following
Flow Control, Water Quality, and Infiltration Facility Details
1 Provide a scaled drawing of each detention pond or vault 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 One cross section
must include the restrictor
5 Specify soils and compaction requirements for pond construction
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
of rock and thickness
11 Show bollard locations on plans Typically, bollards are located at the entrance to drainage facility
access roads
12 On the 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 of restrictor and control structures The plan view must show
the location and orientation of all inlet pipes, outlet pipes, and flow restrictors
14 Draw details at one of the following scales 1 "=1', 1 "=2', 111=4', 1 "=5', 1 "=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
vaults) shall include plan sheets that Include complete working drawings showing dimensions, steel
placement, and specifications 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
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2.3.1.3 EROSION AND SEDIMENT CONTROL (ESC) PLAN
This section details the specifications and contents for ESC plans Note that 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 Standmds (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 , pipe/catch 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 inap
(see Table 2 3 1 A), as well as existing and proposed roads, driveways, parking areas, buildings, drainage
facilities, utility corridors not associated with roadways, all streams, lakes, wetlands, closed depressions,
Hazard Areas and associated 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 of construction
(e g , clearing, grading, 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
1 Identify 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 specifically written for the
proposed project An example construction sequence is provided in Appendix D of the King County
Manual
5 Include ESC Standard Plan Notes (see Appendix A-9 of the Kent Construction Standards)
Clearing Limits
1 Delineate clearing limits
2 Provide details sufficient to install and maintain the clearing limits
Cover Measures
1 Specify 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 trine of year, soil type, gradient, or
' some other factor, specify the conditions that control the use of the different measures
4 Specify 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 are specified, provide typical detail sufficient for installation and maintenance
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May, 2002 52-25 Chapter 5 - Kent Construction Standards
SECTION 2 3 PL4NS REQUIRED FOR DRAINAGE REVIEW
7 Specify the seed mixes, fertilizers, and soil amendments to be used, as well as the application rate for
each item
Perimeter Protection
1 Specify the location and type of perimeter protection to be used
2 Provide typical details sufficient to install and maintain the perimeter protection
3 if silt fence is to be used, specify the type of fabric to be used
Traffic Area Stabilization
1 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 Specify the measure(s) that will be used to create stabilized construction roads and parking areas
Provide sufficient detail to install and maintain
Sediment Retention
1 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
4 Provide typical section views through pond and outlet structures
5 Provide typical details of the control structure and dewatering mechanism
6 Detail stabilization techniques for outlet/inlet
7 Provide details sufficient to install cell dividers
8 Specify mulch or recommended cover of berms and slopes
9 Specify the 1 -foot marker indicating when sediment removal is required
10 Indicate catch basins that are to be protected
11 Provide details of the catch basin 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
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 of the runoff to a non-erosive discharge point
8 Provide location and details of rock check dams
9 Provide front and side sections of typical rock check dams
Wet Season Requirements
Provide a list of all applicable wet season requirements
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Sensitive Areas Restrictions
1 Specify the type, locations, and details of any measures necessary to comply with requirements to
protect surface waters `
2 Specify the type, locations, and details of any measures necessary to comply with any additional
protection required to protect Hazard Areas
2.3.1.4 LANDSCAPE MANAGEMENT PLANS (IF APPLICABLE)
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Approved landscape management plans are allowed to be used as an alternative to the requirement to
formally treat (with a facility) the runoff from pollution -generating pervious surfaces subject to Core
Requirement #8 (see Section 1 2 8) A landscape management 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 suspended solids and other pollutants General guidance for
preparing landscape management plans is provided in Reference Section 4-A
If a landscape management plan is proposed, it must be submitted with the engineering plans for the
proposed project The elements listed below are required for evaluation of landscape management plans
1 Provide a site 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 (if any) 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 plan 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 irrigation
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 cranefly larvae, and the criteria or
schedule for application
d) Any biocontrol methods
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
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SECTION 2 3 PLANS REQUIRED FOR DRAINAGE REVIEW
9, Provide an Implementation plan (see Reference Section 4-A for guidance on preparing the
implementation plan)
2.3.2 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
targeted technical 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
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ISECTION 2 3 PLANS REQUIRED FOR DRAINAGE REVIEW
TABLE 2.3.2.A MINIMUM ENGINEERING PLAN ELEMENTS")
1' PROJECTS IN TARGETED1'
Type of Permit or Drainage Review Prosect Category 1(2) Prosect Category 2(2) Prosect Category 3(2)
Prosect Type Projects that contain or are Projects in Targeted Redevelopment projects (as
adjacent to floodplains, Drainage Review that defined in 1 1) in Targeted
streams, lakes, wetlands or propose to construct or Drainage Review that propose
closed depressions OR modify a 12" or larger $100,000 or more of
projects within a Landslide pipe/ditch, or receive improvements to an existing
Hazard Area, Landslide runoff from a 12" or high -use site
Hazard Drainage Area or larger pipe/ditch
Erosion Hazard Area
SINGLE-FAMILY Targeted • TIR Sections 1, 2, and 6 • TIR Sections 1, 2, 3, 5, N/A
RESIDENTIAL BUILDING Drainage Review (minimum) 6, 7, and 8 (minimum)
PERMITS ONLY • Small Site ESC Plan(3) . Small Site ESC Plan(3)
' (SFRs) • Site Improvement Plan(5) . ESC Plan(4) for
conveyance work
• Site Improvement
Plan(5)
1 Targeted • TIR Sections 1. 2, and 6 • TIR Sections 1, 2, 3, 5, NIA
g Drainage Review (minimum) 6, 7, and 8 (minimum)
SHORT PLATS COMBINED WITH . Small Site ESC Plan(3) . Small Site ESC Plan(3)
Small Site . Site Improvement Plan(5) . ESC Plan(4) for
Drainage Review conveyance work
• Site Improvement
'Plan(5)
_ OTHER PROJECTS OR Targeted • TIR Sections 1, 2, 6, and • TIR Sections 1, 2, 3, 5, • TIR Sections 1, 2, 4, 8, and
PERMITS Drainage Review 8 (minimum) 6, 7, and 8 (minimum) 10 (minimum)
' ONLY . ESC Plan(4) for any site • ESC Plan(4) for any • ESC Plan(4) for any site
disturbance work site disturbance work disturbance work
• Site Improvement Plan(5) • Site Improvement . Site Improvement Plan(5)
Plan(5)
Notes
(1) The above plan elements are considered the recommended minimum for most development cases in Targeted Drainage
Review The Kent Public Works Department may add to these elements if deemed necessary for proper drainage review
Predesign meetings with the Kent Public Works Department are recommended to identify all required elements
(2) For more detailed descriptions of prosect 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) Small site ESC plans are an element of the small site drainage plan as explained in the Small Site Drainage Requirements
booklet (detached Appendix C)
(4) ESC 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 Kent Public Works Department
may allow modified site Improvement plans as described in Section 2 3 1 2
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' 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
2.4.1 PLAN CHANGES AFTER PERMIT ISSUANCE
If changes or revisions to the originally approved engineering plans require additional review, the revised
' plans shall be submitted to the Kent Public Works Department for approval prior to construction The
plan change submittals shall contain all of the following
1 The appropriate Plan Change Order form(s)
' 2 One copy of the revised T1R or addendum
3 Three sets of the engineering plans
4 Other information needed for review
2.4.2 FINAL CORRECTED PLAN SUBMITTAL
During the course of construction, 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
1 responsibility to submit to the Kent Public Works 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 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 Specific requirements for "as -built" submittals are described in Appendix E of the Kent
Construction Standards
Disposition of Approved Engineering Plans for Subdivisions
Upon engineering plan approval of any subdivision, the Kent Public Works Department will make a set
of repi educible mylars (cost to be paid by the applicant) and return the original set to the applicant's
engineer The Kent Public Works Department will retain this reproducible set, using it to make copies for
public inspection, distribution, and base reference as required At the tune the development is accepted
' for maintenance by Kent, the Kent Public Works Department set of reproducibles shall be replaced by the
corrected original set for permanent public records at the City of Kent
'2.4.3 FINAL PLAT, SHORT PLAT, AND BINDING SITE PLAN
SUBMITTALS
' Any subdivision to be finalized, thereby completing the subdivision process and legally forming new lots,
requires a final submittal for approval and recording Short plats also require a final submittal for
approval and recording Submittals shall be accompanied by appropriate fees as prescribed by ordinance
Final submittals will be allowed only after the approval of preliminary plans (for subdivisions only) and
' any required engineering plans, and after the construction of any required drainage facilities
All final map sheets and pages shall be prepared by a professional land surveyor registered in the State of
Washington and shall conform with all state and local statutes
The final submittal for recording only applies to subdivisions (plats), and short plats This plan is required
by state and local statutes
City of Kent Stormwater Manual
Chapter 5 — Kent Construction Standards 5 2-31 May, 2002
SECTION 2 4 PLANS REQUIRED AFTER DRAINAGE REVIEW
May, 2002
In addition to the requirements of the Kent Public Works 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 floodplams
2 Include pertment restrictions as they apply to easements, tracts, and building setback lines
3 Include the dedication clause as provided in Reference Section S -K accompanying Chapter 5 of
the Kent Construction Standards
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 of percentage of lot coverage or square feet
5 Specify 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)
City of Kent Stormwater Manual
52-32 Chapter 5 — Kent Construction Standards
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' CHAPTER 3
HYDROLOGIC ANALYSIS AND DESIGN
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W A S H I N G T O N
CITY OF KENT
SURFACE WATER
DESIGN MANUAL
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CHAPTER 3
HYDROLOGIC ANALYSIS AND DESIGN
The City of Kent has made no changes to Chapter 3 of the 1998 KCSWDM Users should refer to the
' County document for guidance on hydrologic analysis and design
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City of Kent Surface Water Design Manual
Chapter 5 — Kent Construction Standards 5 3-2 May, 2002
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CHAPTER 4
CONVEYANCE SYSTEM
ANALYSIS AND
' DESIGN
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WASHINGTON
CITY OF KENT
SURFACE WATER
DESIGN MANUAL
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CHAPTER 4
CONVEYANCE SYSTEM ANALYSIS
AND DESIGN
The City of Kent has made several minor changes to Chapter 4 of the 1998 KCSWDM Apart from these
changes, the King County version of Chapter 4 applies to proposals in the City of Kent The City's
changes to the County document are documented below
1 Section 4.1 DESIGN CRITERIA, Easement and Setback Requirements (page 4-3 of tire 1998
KCSKDM)—This section shall read as follows
Permanent onsite 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
When Easements are required, then legal descriptions for same shall be submitted with a professional
land surveyors stamp thereon Also at this time. a current title report covering the properties to be
1 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
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When offsite Easements and/or onsite Easements for the extension of approved comprehensive
stormwater plans are required, these shall he approved and recorded prior to holding any
preconstruchon meeting Also the same conditions shall apply regarding legal descriptions and title
reports
A Easements
The minimum easement widths are as follows
1 Storm drain pipelines (under 10 feet deep) — 15 feet wide
2 Storm drain pipelines (10 —15 feet deep) — 20 feet wide Note large diameter or deeper
sewers may require wider easements as determined necessary by the Director
3 Access and/or 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 12 feet
around the outside of the facility, and shall include access Easements as well
City of Kent Surface Water Design Manual
Chapter 5 — Kent Construction Standards 54-1 May, 2002
SECTION 4 CONVEYANCE SYSTEM ANALYSIS AND DESIGN
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,
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
• Section 4 2 1.1 DESIGN CRITERIA, Acceptable Pipe Sizes (page 4-5 of the 1998 KCSWDM)—
This section shall read, "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 "
Section 4.2 1.1 DESIGN CRITERIA, Allowable Pipe Materials (page 4-5 of the 1998
KCSWDM)—This section shall read as follows
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
CMP
1
Yes
PCP (Bell & spigot Gasket ,joint)
2
Yes
RCP
1
Yes
HDPE
1.5
Private only
PVC SDR 35
3
Yes, except for culvert
PVC SDR 21
15
Yes, except for culvert
Ductile Iron
05
Yes
Ribbed PVC
2
Yes, except for culvert
NOTE These are minimum depths Actual depths must meet design loading requirements This
includes maximum depths specified by the pipe manufacturer Pipe cover in areas not subject to
vehicular loads may be reduced to 1 inch minimum
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
Plain Concrete Storm Sewer Pipe (PCP) - All nonreinforced concrete pipe shall be ASTM C 14-
81 Class II or better with gasketed joints The maximum diameter for unremforced pipe shall be
12 inches
City of Kent Surface Water Design Manual
May, 2002 54-2 Chapter 5 - Kent Construction Standards
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SECTION 4 CONVEYANCE SYSTEM ANALYSIS AND DESIGN
2 Reinforced Concrete Pipe (RCP) — All reinforced concrete pipe shall be ASTM C76 -85a Class IV
or as specified
3 Corrugated Steel Pipe —AII corrugated steel storm drain pipe shall be sixteen (16) gauge or
thicker Gaskets are required on all fittings and couplings Couplings shall be Type F
(See Standard State WSDOT Plan B -1 3b for coupler)
4 Protective Treatment — All steel pipe shall be coated with a protective asphalt treatment The
minimum acceptable protective treatment shall be APWA Treatment 1 Aluminum corrugated
metal pipe may be substituted for Treatment 1 corrugated metal pipe If smooth bore pipe is
specified to improve hydraulic performance, the pipe shall be Treatment 5 for both annular and
helical pipe Aluminum pipe shall be painted with 2 coats of zinc chromate pruner where it
contacts concrete Aluminum storm drain pipe may be used in place of corrugated steel pipe with
equivalent structural strength
5 Corrugation —AII corrugated and steel storm drain pipe 15 inches in diameter and less shall have
helical corrugations Pipe larger than 15 inches in diameter can be either helical or annular
6 Aluminum Pipe — Aluminum storm drain pipe may be used in place of corrugated steel pipe with
equivalent structural strength
7 PVC Pipe — PVC pipe conforming to ASTM 3034, SDR 35 and SDR 21 with rubber gasketjoints
may be used where adequate cover can be obtained PVC pipe cannot be used for culvert pipes
8 Ribbed PVC — PVC pipe conforming to ASTM F79H/UNI-B-9 with rubber gasket joints may be
used where adequate cover can be obtained Ribbed PVC pipe cannot be used for culvert pipes
9 HDPE Pipe — HDPE pipe is approved only for private onsite drainage systems
10 Ductile Iron Pipe — Di pipe shall be Class 50 in accordance with USA Standard A-21 1 (AW WA
C-51) All joints shall be push -on mechanical or flanged
Section 4 2.1.1 DESIGN CRITERIA, Allowable Pipe Joints (page 4-6 of the 1998 KCSWDM)—
Replace this section with the following
Rubber gaskets shall be used for all pipe joints except as described below The rubber gaskets shall
be factory manufactured for the particular type and brand of pipe used Coupling bands and rubber
gaskets are required for all steel or aluminum sewer pipe Coupling bands for CMP shall be type "F"
unless otherwise approved The manufacturer recommendation shall be adhered to for HDPE pipe
• Section 4.2.1.1 DESIGN CRITERIA, Pipe Alignment (page 4-7 of the 1998 KCSWDM)—
Replace Requirement 1 with, "All storm drains smaller than 36 inches in diameter shall be designed
with a straight alignment between manholes The use of fabricated pipe bends for larger diameters
requires approval by the City and shop manufactured fittings designed for the specific application "
• Section 4.2.1 1 DESIGN CRITERIA, Structures (page 4-8 of the 1998 KCSWDM)—The
following additional criteria shall apply to projects in the City of Kent
Storm Drainage Manholes
A Location —Storm drainage manholes or catch manholes for access are required at the following
locations on the conveyance system
1 All changes in slope
2 Changes in pipe diameter
City of Kent Surface Water Design Manual
Chapter 5 —Kent Construction Standards
5 4-3
May, 2002
SECTION 4 CONVEYANCE SYSTEM ANALYSIS AND DESIGN
May, 2002
Connections of storm pipes from manholes to the conveyance systems that are larger than 24
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
4 Changes in alignment
5 Distances not greater than 400 feet on pipe runs
6 Dissimilar pipe materials
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 gradelme, 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 ateunctions 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 stone 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 Construction Standards )
8 inches — 21 inches
48 inches
24 inches — 36 inches
54 inches
36 inches — 42 inches
72 inches
42 inches — 60 inches
96 inches
Stormwater Inlets
Stonnwater 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 gutterlme intersections in such a configuration as to minimize gutter flows
through pedestrian crossings
City of Kent Surface Water Design Manual
54-4 Chapter 5 — Kent Construction Standards
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ISECTION 4 CONVEYANCE SYSTEM ANALYSIS AND DESIGN
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 stonnwater
D Catch Basin Locations for Private Developments — Catch basins or catch manholes are required at
these locations for private developments
I 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
Castings
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 Details 4-5, 4-6[a] and 4-6[b] in Appendix B of the
Kent Construction Standards )
B Public catch basin frame and grates shall be ductile iron and bolt down (See Standard Details 5-1[a]
and 5-1[b] in Appendix B of the Kent 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 Detail 5-1[b] and 5-1[c]
in Appendix B of the Kent Construction Standards )
D Public catch basins used in low point of vertical curves or roadway grades and at the low end of
i oadway grades in cul-de-sacs shall have through curb inlet frame and self-locking vaned grate
(See Standard Detail 5-1[d], 5-2[a] and 5-2[b] in Appendix B of the Kent Construction Standards )
E Public catch basins used in steep roadway grades of 6% and greater shall have ductile iron self-
locking vaned grate with vaned grate frame (See Standard Detail 5-1[d] and 5-1[e] in Appendix B of
the Kent Construction Standards )
F Public catch basins used in rolled curb and gutter shall have rolled curb frame and grate (See
' Standard Details 5-1 [f] and 5-1 [g] in Appendix B of the Kent Construction Standards )
G Block lettering is required on the top surfaces of storm drain castings, and shall read as follows
"NO DUMPING DRAINS TO STREAMSi"
Manhole and Catch Basin Adapters
A All aluminum surfaces in contact with the concrete or concrete pipe shall be painted with 2 coats of
zinc chromate primer The aluminum pipe to be painted 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 (Pruner, Paint, Zinc Chromate, Alkyd Vehicle)
City of Kent Surface Water Design Manual
Chapter 5 —Kent Construction Standards 5 4-5 May, 2002
2 At locations along gutters so as to provide a maximum gutter flow of 200 feet for street
grades up to 3%
3 Where Type Il catch basins are used for access to the trunk storm sewer, the maximum
spacing shall be 400 feet
4 At all low points of vertical curves and grade breaks
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5 At the inlet of minor ditches to the drainage system
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
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Appendix B of the Kent 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 stonnwater
D Catch Basin Locations for Private Developments — Catch basins or catch manholes are required at
these locations for private developments
I 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
Castings
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 Details 4-5, 4-6[a] and 4-6[b] in Appendix B of the
Kent Construction Standards )
B Public catch basin frame and grates shall be ductile iron and bolt down (See Standard Details 5-1[a]
and 5-1[b] in Appendix B of the Kent 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 Detail 5-1[b] and 5-1[c]
in Appendix B of the Kent Construction Standards )
D Public catch basins used in low point of vertical curves or roadway grades and at the low end of
i oadway grades in cul-de-sacs shall have through curb inlet frame and self-locking vaned grate
(See Standard Detail 5-1[d], 5-2[a] and 5-2[b] in Appendix B of the Kent Construction Standards )
E Public catch basins used in steep roadway grades of 6% and greater shall have ductile iron self-
locking vaned grate with vaned grate frame (See Standard Detail 5-1[d] and 5-1[e] in Appendix B of
the Kent Construction Standards )
F Public catch basins used in rolled curb and gutter shall have rolled curb frame and grate (See
' Standard Details 5-1 [f] and 5-1 [g] in Appendix B of the Kent Construction Standards )
G Block lettering is required on the top surfaces of storm drain castings, and shall read as follows
"NO DUMPING DRAINS TO STREAMSi"
Manhole and Catch Basin Adapters
A All aluminum surfaces in contact with the concrete or concrete pipe shall be painted with 2 coats of
zinc chromate primer The aluminum pipe to be painted 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 (Pruner, Paint, Zinc Chromate, Alkyd Vehicle)
City of Kent Surface Water Design Manual
Chapter 5 —Kent Construction Standards 5 4-5 May, 2002
SECTION 4 CONVEYANCE SYSTEM ANALYSIS AND DESIGN
B PVC pipe adapters shall be Kor-m-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
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 maybe
used The gaskets must fall within a 1 -inch tolerance of the 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 Beddm
Pipe bedding for storm drain and culvert pipe shall be 5/8-mch 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 95% compaction ASTM D-1557, in accordance with the standard
specifications and details and in conformance with the following gradation
3/4 Inch
100%
5/8 Inch
95-100%
1/4 Inch
45-65%
U.S. No. 40
6-18
U.S. No 200
7.5 Max.
% Fracture
75 Min.
Sand Equivalent 40 Min.
L A wear 500 rev 35% Max, Degradation 25% 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 Foundation Material in Section 3 8 2 of the City of
Kent Construction Standards
Backfill
Pipe trench backfill shall be in accordance with Section 4 7 6 of the City of Kent Construction Standards
unless otherwise approved by the Director
Cleanme
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
City of Kent Surface Water Design Manual
May, 2002 54-6 Chapter 5 — Kent Construction Standards
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SECTION 4 CONVEYANCE SYSTEM ANALYSIS AND DESIGN
Storm Drain Stenciling
The pavement adjacent to all new catch basins shall be stencil painted with a standard pollution prevention
stencil such as the thermoplastic example below The Contractor is responsible for purchase and
installation of the stencils
No DUMPING! 1*1
DRAINS To STREAM
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 of deficiencies
Television Inspection
All new City storm drain extensions, 24 inches in diameter and smaller shall be TV camera inspected by
the City of Kent Utility Department 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
• Section 4.2.1.1 DESIGN CRITERIA, Pipe Design Between Structures (page 4-9 of the 1998
KCSWDM)—Add the following to the list of requirements,
4 Storm sewers with design velocities greater than 10 fps shall be designed with energy
dissipation structures as deemed necessary by the City Pipes installed at slope greater than
20% require pipe anchors
Section 4 2 1.1 DESIGN CRITERIA, Spill Control (page 4-10 of the 1998 KCSWDM)—Delete
Item d "An active spill control plan." This is not an acceptable option for spill control in the City
of Kent
• Section 4.2.2 1 DESIGN CRITERIA, General (page 4-27 of the 1998 KCSWDM)— Add the
following to the list of requirements:
5 Points of discharge from culverts and storm sewers into ditches and swales 15% or greater in
grade shall require the design and installation of special energy dissipators
• Section 4 3.1.1 DESIGN CRITERIA, General (page 4-35 of the 1998 KCSWDM)—Add the
following to the list of requirements
City of Kent Surface Water Design Manual
Chapter 5 — Kent Construction Standards 5 4-7 May, 2002
SECTION 4 CONVEYANCE SYSTEM ANALYSIS AND DESIGN
3 The inimmum diameter of culvert for roadside ditches or other installations shall be twelve (12)
inches in diameter Larger diameter culverts shall be installed where required by appropriate
hydraulic analysis
• Section 4.3.1.1 DESIGN CRITERIA, Headwater (page 4-35 of the 1998KCSWDM)—
Requirement 3 has been revised to read "The maximum headwater elevation at design flow shall be
at least 12 inches below finished grade "
• Section 4.3.1 1 DESIGN CRITERIA, Inlets and Outlets (page 4-36 of the 1998 KCSWDM)—
Requirement 4 is revised to apply to all pipe larger than 12 inches Requirement 5 shall apply to
all new culverts unless deemed unnecessary by the City
• Table 4.4.2 Datum Correlations to KCAS (page 4-75 of the 1998 KCSWDM)--The Kent datum is
NGVD, USGS and USC and GS 1947 (adjusted to the 1929 datum)
City of Kent Surface Water Design Manual i
May, 2002 54-8 Chapter 5 — Kent Construction Standards
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CHAPTER 5
FLOW CONTROL
DESIGN
a �� a v a
W A S H I N G T O N
'CITY OF KENT
SURFACE WATER
DESIGN MANUAL
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CHAPTER 5
FLOW CONTROL DESIGN
The City of Kent has made one minor change to Chapter 5 of the 1998 KCSWDM Apart from this
change, the King County version of Chapter 5 applies for proposals in the City of Kent The City's change
to the County document is as follows
• Section 5.3 DETENTION FACILITIES, Detention Ponds, Design Criteria, General (page 5-20
of the 1998KCSWDM)—The following items are added to this section
5 All retention/detention ponds shall have a minimum of one (1) foot tiee board above the
maximum design water surface elevation
6 Outlets of all detention ponds shall be piovided with suitable debris barrels designed to
pi otect the outlet fern blockage or plugging
7 The site and giadmg plans for open ponds shall clearly denote all elevations, dimensions,
cioss-section views (a minimum of one thiough each direction), sod piepaiation
iequnements, and other information necessaiy to construct the system as designed The
design plans shall also denote that the design engmeei shall vei ify the pond volume and
construction prior to landscaping
8 All ponds shall piovide for the energy reduction of incoming flows
• Section 5.3 DETENTION FACILITIES, Detention Ponds, Design Criteria, Side Slopes (page 5-
20 of the 1998 KCSWDM�—This section is replaced with the following
1_ Side slopes for earth -lined ponds shall not exceed three (3) feet holizontal to one (1) foot
vertical unless specifically approved by the Duector
2 Ponds constructed with rock walls or retaining walls shall be designed by a licensed
structural of civil engmeei registered in the State of Washington Walls shall not exceed
6' in height
3 All ponds shall include at least two (2) sloped surface with one (1) accessible for
maintenance equipment at a maximum 6 1 slope
• Section 5.3 DETENTION FACILITIES, Detention Ponds, Design Criteria, Embankments (page
5-20 of the 1998 KCS WDM)—Requirements 1 and 2 have been revised according to the following,
and item 7 has been added
1 Any embankment for a pond in excess of four (4) feet must be designed by a licensed civil
engmeei and approved by the City
2 The top width of the bei in shall be fifteen (15) feet, unless other wise approved
City of Kent Surface Water Design Manual
Chapter 5 — Kent Construction Standards
5 5-1
May, 2002
May, 2002
A licensed civil encuieei e-,perienced in soil mechanics shall inspect and certify the
constiuc.tion of the beim
• Section 5.3.1 DETENTION PONDS, Fencing (page 5-22 of the 1998 KCSWDM)—Requirement 4
has been revised according to the following
An access gate for access roads is required and shall be structurally and aesthetically acceptable
for the use and location proposed An acceptable alternative to control traffic shall be removable
posts spaced at a maximum four (4) foot center sin lieu of an access gate where fencing is not
required, said posts shall be locked into position Locks shall be keyed to City of Kent
Specifications (See Standard Detail 5-12(b) for access gates, 6-15(a), (b), (c), (d) and 6-16 for
Bollards in Appendix "B")
• Section 5.3.1 DETENTION PONDS, Right -of -Way (i)age 5-23 of the 1998 KCSWDM)—
Requirements 2 has been revised according to the following, and item 3 has been added
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 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 twelve (12) foot wide, all-
weather surfaced access road shall be constructed to the facility This access road shall be located
within an easement and shall provide for unobstructed ingress and egress to the facility
A written restriction shall be added to the final plat drawing 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 delineated flood plain area "
• Section 5.3.2 DETENTION TANKS, Materials (page 5-33 of the 1998 KCSWDM)— The
following is added to this section
Corrugated steel storage pipes shall be uniformly coated with APWA Treatment 1 asphalt
or better Corrugated aluminum or concrete pipe may be substituted without asphalt
treatment Aluminum shall be painted with two (2) coats of zinc chromate primer where it
contacts concrete
The City of Kent Fire Department shall be consulted during design of any underground
facilities to ensure that outrigger placement (pomtload) requirements are met
• Section 5.3.3 DETENTION VAULTS, Structural Stability (page 5-37 of the 1998 KCSWDAN)--
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 City of Kent Fire Department shall be consulted during design of any underground
facilities to ensure that outrigger placement (pomfload) requirements are met
• Section 5.3.5 OTHER DETENTION OPTIONS, Use of Parking Lots for Additional Detention
(page 5-51 of the 1998 KCS WDM)—Replace this section with the following
Maximum depth for surface storage in parking lots shall be as follows
City of Kent Surface Water Design Manual
5 5-2 Chapter 5 —Kent Construction Standards
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a CommeiciaFinal lti-fainly paikmg and maneuveiing areas —six (6) inches
b hndusti al truck loading/maneuvering areas — eighteen (18) inches
2 Paiking lots designed for detention shall not exceed slopes of 7% in areas of vehicular
parking or maneuvering
3 All pai king lot ponds shall be designed and consti ucted in such a manner so as to provide
0 50 feet of free board between the maximum water surface elevation and adjacent
driveways, landscaping or adjacent pi operties
4 The maximum water surface elevation of any paiking lot pond shall be a nunarnun of six
(6) inches below the finished floor elevation of adjacent buildings All purring systems
shall be designed to go to overflow conveyance pi mi to flooding snuctmes
5 Wheie pai king lot ponding is to be utilized, the site giading and paving plans shall cleat ly
denote all ci itical elevations ponding, dimensions and any othel necessary information to
constitict the detention pond as designed The design plan shall indicate that the
respective details are critical foi the storm drainage detention system operation and that
the site giading must be verified
'a Section 5.3.5 OTHER DETENTION OPTIONS, Use of Roofs for Detention (page 5-51 ofthe
1998 XCSWDAI)—Add the following items
6 The maximum allowable depth of any rooftop detention facility shall be three (3) inches
7 All rooftop detention facilities shall be provided with overflow scupper drains at the
maximum water surface elevation
8 Rooftop detention inngs shall be installed in accordance with the manufacturei's
specifications and shall be designed to iesti ict iooftop runoff rates not to exceed two (2)
gallons per nnnute per 1000 sq feet of i oaf areas
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Chapter 5 —Kent Construction Standards 5 5-3
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May, 2002
I CHAPTER 6
WATER QUALITY
DESIGN
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W A S H I N G T O 7
CITY OF KENT
SURFACE WATER
DESIGN MANUAL
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CHAPTER 6
WATER QUALITY DESIGN
The City of Kent has made two changes to Chapter 6 of the 1998 KCS WDM Apart from this change, the
King County version of Chapter 6 applies for proposals in the City of Kent The City's change to the
County document is as follows
• Section 6.1.3 RESOURCE STREAM PROTECTION MENU (page 6-10 ofthe 1998
KCSWDM)—Add the following
Experimental and propietary 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 propietary BMP to be approved, evidence must be
provided that the BMP is capable of removing dissolved metals from stormwater, and the designer
should show in the computations that the treatment train is capable of meeting the water quality
standards for Class "A" waters Because performance data is typically limited on these devices, if
they are approved for an installation the City may require the implementation of a monitoring plan to
assess the BMP performance.
• Section 6.2 GENERAL REQUIREMENTS FOR WQ FACILITIES, SETBACKS, SLOPES,
AND EMBANKMENTS, SIDE SLOPES, FENCING, AND EMBANKMENTS (page 6-20 ofthe
1998 KCSWDM)--Add the following to the list of requirements "Any above -ground stonnwater
facility will be screened from public right of way and adjacent property per the underlying zoning
perimeter buffer requirements in the Kent City Code "
City of Kent Surface Water Design Manual
Chapter 5 - Construction Standards
56-1
May 2002
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APPENDIX
W A S H I N G 7 O N
CITY OF KENT
SURFACE WATER
DESIGN MANUAL
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MAINTENANCE REQUIREMENTS FOR
PRIVATELY MAINTAINED DRAINAGE FACILITIES
INDEX
NO 1
PONDS CONSTRUCTED WETLANDS
D-1 &2
NO 2
INFILTRATIO
D-3
NO3
CLOSED DETENTION AND TREATMENT SYSTEMS
(PIPES I TANKS)
D-4
N04
CONTROL STRUCTURE / FLOW RESTRICTOR
D-5
NO 5
CATCN ASK S
D-6 & 7
NO 6
DEBRIS BARRIERS (e g, TRASH RACKS)
D-8
N07
ENERGY DISSIPATORS
D-9
NO 8
FENCING
D-10
NO.9
GATES
D-11
NO 10
CONVEYANCE SYSTEMS (PIPES AND DITCHES)
D-12
NO 11
GROUNDS (LANDSCAPING)
D-13
NO 12
ACCESS ROADS / EASMENTS
D14
NO 13
GRASS BIOFILTERS AND FILTER STRIPS
D15
NO.14
OIL / WATER SEPARATORS
D-16
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APPENDIX "D" ,
MAINTENANCE REQUIREMENTS FOR
'
PRIVATELY MAINTAINED DRAINAGE FACILITIES
NO. 1 - PONDS CONSTRUCTED WETLANDS
'
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Maintenance Condition When Maintenance
Maintenance Activity or
Components Defect Is Needed
Results After Maintenance
General Trash & Debris Any trash and debris which exceeds 1
Trash and debris cleared from site
cubic foot per 1000 square feet (this is
'
about equal to the amount of trash it
would take to fill up one standard size
office garbage can) In general, there
,
should be no visual evidence of
dumping
Poisonous Any poisonous vegetation which may
No danger of poisonous vegetation
'
Vegetation constitute a hazard to City personnel or
where City personnel or the public
the public Examples of poisonous
might normally be (Coordination
vegetation include tansy ragwort,
with Seattle/King County Health
'
poison oak, stinging nettles, devils club
Department)
Pollution Oil, gasoline, or other contaminants in
No contaminants present other than
any amount found that could 1) cause
a surface film
t
damage to plant, animal, or marine life,
2) constitute a fire hazard, or 3) be
flushed downstream during rain storms
Unmowed It facility is located in private residential
When mowmg is needed,
Grass/Ground area, mowing is needed when grass
grass/ground cover should be
'
Cover exceeds 18 inches in height In other
mowed to 2 inches in height
areas, the general policy is to make the
pond site match adjacent ground cover
and terrain as long as there is no
interference with the function of the
facility
'
Rodent Holes Any evidence of rodent holes if facility
Rodents destroyed and dam or berm
is acting as a dam or berm, or any
repaired (Coordination with
evidence of water piping through dam or
Seattle/King County Health
berm via rodent holes
Department)
Insects When insects such as wasps and hornets Insects destroyed or removed from
interfere with maintenance activities site
Tree Growth Tree growth does not allow maintenance Trees do not hinder maintenance
access or interferes with mamtenance activities Selectively cultivate trees
activity (1 e , slope mowing, silt removal, such as alders for firewood
vactoring or equipment movements) If
trees are not interfering with access,
leave trees alone
MAINTENANCE REQUIREMENTS FOR
PRIVATELY MAINTAINED DRAINAGE FACILITIES
NO. 1 - PONDS CONSTRUCTED WETLANDS CON'T
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Maintenance
Components
Defect
Condition When Maintenance
Is Needed
Maintenance Activity or
Results After Maintenance
Side Slopes
Erosion
Eroded damage over 2 inches deep
Slopes should be stabilized by using
--
of Pond
where cause of damage is still present
appropriate erosion control
or where there is potential for
measure(s) e g, rock reinforcement.
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continued erosion
planting of grass, compaction
Storage Area
Sediment
Accumulated sedunent that exceeds
Sediment cleaned out to designed
10% of the designed pond depth
pond elevation is reseeded if
necessary to control erosion
Storage or
Bare area
Bare areas greater than one square foot
Complete grass covered bottom to
treatment
are to be resodded or seeded using
avoid erosion and to enhance
area
grass specifications in construction
pollutant removal
Vegetation gaps, or
In constructed wetlands, significant
Replace with new stock
'
channelized flow
open areas in the area of emergent
—
vegetation
Pond Dikes
Settlements
Any part of dike which has settled 4
Dike should be built back to the
inches lower than the design elevation
design elevation
Emergency
Rock Missing
Only one layer of rock exists above
Replace rocks to design standards
Overflow/
native soil in area five square feet or
Spillway
larger, or any exposure of native soil
Rock Filter
Plugging
When during storm, height of water on
Replace rock media
upstream face is 25% or more higher
than the height of water on the
a
downstream face
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MAINTENANCE REQUIREMENTS FOR
PRIVATELY MAINTAINED DRAINAGE FACILITIES
NO.2 - INFILTRATION
Maintenance
Components Defect
General Trash &
Poisonous
Vegetation
Pollution
Unmowed
Grass/Ground
Cover
Rodent Holes
Insects
Storage Area Sediment
Condition When Maintenance
Is Needed
See "Ponds" Standard No 1
See "Ponds' Standard No 1
See "Ponds" Standard No 1
See "Ponds" Standard No 1
Maintenance Activity or
Results After Maintenance
See "Ponds" Standard No 1
See "Ponds" Standard No 1
See "Ponds" Standard No 1
See "Ponds" Standard No 1
See "Ponds" Standard No 1 See "Ponds" Standard No 1
See "Ponds" Standard No I See "Ponds" Standard No I
A percolation test pit or test of facility Sediment is removed and/or facility
indicates facility is only working at is cleaned so that infiltration system
90% its designed capabilities works accordmg to design
Sheet Cover
Sheet cover is visible and has more
Sheet cover repaired or replaced
(If Applicable)
than V. - inch hole in it
Sump filled with
Any sediment and debris filling vault to
Clean out sump to design depth
Sediment and
10% of depth from sump bottom to
Debris
bottom of outlet pipe or obstructing
(If Applicable)
flow into connector pipe
Filter Bags Filled with
Sediment and debris fill bag more than
Replace filter bag or redesign system
Sediment and
/, full
Debris
Rock Trench Standing Water
When water is observed in the
Remove rock, and determine cause of
monitoring well more than 72 hours
slow infiltration rate remove and
after a storm
replace soil underneath the trench
D-3
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MAINTENANCE REQUIREMENTS FOR
PRIVATELY MAINTAINED DRAINAGE FACILITIES
NO.3 - CLOSED DETENTION AND TREATMENT SYSTEMS (PIPES/TANKS)
Maintenance Condition When Maintenance Maintenance Activity or
Components Defect Is Needed Results After Maintenance
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D-4
Storage Area Plugged Air Vents
One-half of the cross section of a vent
Vents free of debris and sediment
-
is blocked at any point with debris and
sediment
Debris and
Accumulated sediment depth exceeds
All sediment and debris removed
Sediment
10% of the diameter of the storage area
from storage area
for .%: length of storage vault or any
point depth exceeds 15% of diameter
Example — 72-mch storage tank would
require cleaning when sediment reaches
depth of 7 inches for more than '/
length of tank
Joints Between
Any crack allowing material to be
Tank/pipe repaired or replaced to
Tank/Pipe Section
transported into facility
design
Manhole Cover Not in Place
Cover is missing or only partially in
Manhole is closed
place Any open manhole requires
maintenance
Locking
Mechanism cannot be opened by one
Mechanism opens with proper tools
Mechanism Not
maintenance person with proper tools
Working
Bolts into frame have less than '/� mch
of thread (may not apply to self-locking
lids)
Cover Difficult to
One maintenance person cannot
Cover can be removed and
Remove
remove lid after applying 80 ponds of
reinstalled by one maintenance
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lift Intent is to keep cover from
person
sealing off access to maintenance
Ladder Rungs
Ladder is unsafe due to Trussing rungs,
Ladder meets design standards and
_
Unsafe
misalignment, rust or cracks
allows maintenance persons safe
access
'
Catch Basins
See "Catch Basins" Standard No 5
See "Catch Basins" Standard No 5
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MAINTENANCE REQUIREMENTS FOR
PRIVATELY MAINTAINED DRAINAGE FACILITIES
NO.4 - CONTROL STRUCTURE/FLOW RESTRICTOR
Maintenance Condition When Maintenance
Components Defect Is Needed
General Trash and Debris Distance between debris build up and
(Includes bottom of onfice plate is less than 1 '/2
Sediment) feet
Maintenance Activity or
Results After Maintenance
debris removed
Structural Damage Structure is not securely attached to Structure securely attached to wall
manhole wall, outlet pipe structure and outlet pipe
should support at least 1,000 pounds of
up or down pressure
Cleanout Damaged or
Gate Missing
Orifice Plate Damaged or
Missing
Orifice Plate Performance
Overflow
Pipe
Manhole
Catch Basin
Obstructions
Structure is not in upright position Structure in correct positron
(allow up to 10% from plumb)
Connections to outlet pipe are not Connections to outlet pipe are
watertight and show signs of rust watertight, structure repaired or
replaced and works as designed
Any holes — other than designed holes Structure has not holes other than
— in the structure designed holes
Cleanout gate is not watertight or is
Gate is watertight and works as
missing
designed
Gate cannot be moved up and down by
Gate moves up and down easily and
one maintenance person
is watertight
Cham leading to gate is trussing or
Cham is in place and works as
damaged
designed
Gate is rusted over 50% of its surface
Gate is repaired or replaced to meet
area
design standards
Control device is not working properly
Plate is free of all obstructions and
due to missing, out of place, or bent
works as designed
onfice plate,
When actual release rate at any one
Correct problem with appropriate
onfice is not within 75% to 125% of
action such as replacement of the
the design release rate
onfice plate
Any trash or debris blocking (or
Pipe is free of all obstructions and
having the potential of blocking) the
works as designed
overflow pipe
See "Closed Detention Systems"
See "Close Detention Systems"
Standard No 3
Standard No 3
See "Catch Basms" Standard No 5
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MAINTENANCE REQUIREMENTS FOR
PRIVATELY MAINTAINED DRAINAGE FACILITIES
NO. 5 - CATCH BASINS
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Maintenance
Components Defect
Condition When Maintenance
Is Needed
Maintenance Activity or
Results After Maintenance
General Trash & Debris
Trash or debns of more than % cubic
No trash or debris located
(Includes
foot which is located immediately in
immediately in front of catch basin
Sediment)
front of the catch basin opening or is
opening
blocking capacity of basin by more
'
than 10%
Trash or debris (in the basin) that
No trash or debris in the catch basin
exceeds 1/3 the depth from the bottom
Frame is sitting flush on top slab
of basin to invert of the lowest pipe into
'
or out of the basin
Cracks in Basin
Trash or debris in any inlet or outlet
Inlet and outlet pipes free of trash or
Walls/Bottom
pipe blocking more than 1/3 of its
debris
height
cracks or maintenance person fudges
'
Dead animals or vegetation that could
No dead animals or vegetation
generate odors that would cause
present within the catch basin
complaints or dangerous gases (e g ,
methane)
Deposits of garbage exceeding 1 cubic
foot in volume
No condition present which would
attract or support the breeding of
—
insects or rodents
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Structural Damage
Comer of frame extends more than 3/.
Frame is even with curb
to Frame and/or
inch past curb face into the street (if
Top Slab
applicable)
Top slab has holes larger than 2 square
Top slab is free of holes and cracks
inches or cracks wider than 'I/ inch
(intent is to make sure all material is
running into the basin)
Frame not sitting flush on top slab, 1 e ,
Frame is sitting flush on top slab
separation of more than % inch of the
'
frame from the top slab
Cracks in Basin
Cracks wider than '/z inch and longer
Basin replaced or repaired to design
Walls/Bottom
than 3 feet, any evidence of soil
standards
particles entering catch basin through
cracks or maintenance person fudges
'
that structure is unsound Basin
replaced to design
or repaired standard
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MAINTENANCE REQUIREMENTS FOR
PRIVATELY MAINTAINED DRAINAGE FACILITIES
NO. 5 - CATCH BASINS CON'T
Maintenance Condition When Maintenance Maintenance Activity or
Components Defect Is Needed Results After Maintenance
Cracks wider than Y, inch and longer No cracks more than % inch wide at
than 1 foot at the joint of any the joint of inletloutlet pipe
mlet/outlet pipe or any evidence of soil
panicles entering catch basin through
cracks
Settlement/ Basin has settled more than 1 inch or Basin replaced or repaired to design
Misalignment has rotated more than 2 inches out of standards
alignment
Fire Hazard Presence of chemicals such as natural No flammable chemicals present
gas, oil and gasoline
Vegetation Vegetation growing across and No vegetation blocking opening to
blocking more than 10% of the basin basm
opening
Pollution Nonflammable chemicals of more than No pollution present other than
%, cubic foot per three feet of basin surface film
length
Catch Basin Cover Not in Place Cover is nussing or only partially in Catch basin cover is closed
Cover place Any open catch basin required
maintenance
Locking Mechanism cannot be opened by one Mechanism opens with proper tools
Mechanism Not maintenance person with proper tools
Working Bolts into frame have less than 1/21nch
of thread
Cover Difficult to One maintenance person cannot Cover can be removed by one
Remove remove lid after applying 80 lbs or lift, maintenance person
intent is to keep cover from sealing off
access to mamtenance
Ladder Ladder Rungs
Unsafe
Metal Grates
Trash and Debris
Damaged or
Missing
Ladder is unsafe due to tmssing rungs, Ladder meets design standards and
misalignment, rust, cracks, or sharp allows maintenance person safe
edges access
Grate with opening wider than 7/8 inch Grate openings meet design
standards
Trash and debris that is blocking more Grate free of trash and debris
than 20% of grate surface
Grate missing or broken member(s) of Grate is in place and meets design
the grate standards,
D-7
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Bars are missing or entire barrier is Bars in place according to design
missing
Bars are loose and rust is causing 50% Repair or replace barrier to design
' deterioration to any part of barrier standards
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MAINTENANCE REQUIREMENTS FOR
PRIVATELY MAINTAINED DRAINAGE FACILITIES
NO. 6 — DEBRIS BARRIERS (e.g., Trash Racks)
Maintenance
Condition When Maintenance
Maintenance Activity or
Components
Defect Is Needed
Results After Maintenance
General
Trash and Debris Trash or debris that is plugging more
Barrier clear to receive capacity flow
than 20% of the openings in the barrier
Metal
Damaged/Missing Bars are bent out of shape more than 3
Bars in place with no bends more
Bars mches
than'/. inch
Bars are missing or entire barrier is Bars in place according to design
missing
Bars are loose and rust is causing 50% Repair or replace barrier to design
' deterioration to any part of barrier standards
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MAINTENANCE REQUIREMENTS FOR
PRIVATELY MAINTAINED DRAINAGE FACILITIES
NO.7 -ENERGY DISSIPATORS
Maintenance Condition When Maintenance Maintenance Activity or
Components Defect Is Needed Results After Maintenance
Eternal
Rock Pad Missing or Moved Only one layer of rock exists above Replace rocks to design standard
Rock native soil in area five square feet or
larger, or any exposure of native soil
Dispersion Pipe Plugged with Accumulated sediment that exceeds Pipe cleaned/flushed so that it
Trench Sediment 20% of the design depth matches design
Not Discharging Visual evidence of water discharging at Trench must be redesigned or rebuilt
Water Properly concentrated points along trench to standards
(normal condition is a "sheet flow" of
water along trench) Intent is to
prevent erosion damage
Perforations Over %: of perforations in pipe are Clean or replace perforated pipe
Plugged plugged with debns and sediment.
Water Flows Out Maintenance person observes water Facility must be rebuilt or redesigned
Top of Distributor flowing out during any storm less than to standards
Catch Basin the design storm or it is causing or
appears likely to cause damage
Receiving Area Water in receiving area is causing or No danger of landslides
Over -Saturated has potential of causing landslide
problems
Internal
Manhole/ Worn or Damaged Structure dissipating flow deteriorates Replace structure to design
Chamber Posts, Baffles, to % of original size or any standards
Sides of Changer concentrated work spot exceeding one
square foot which would make
structure unsound
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MAINTENANCE REQUIREMENTS FOR
PRIVATELY MAINTAINED DRAINAGE FACILITIES
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Deteriorated Paint Part or parts that have a rusting or Structurally adequate posts or parts
or Protective scaling condition that has affected with a uniform protective coating
Coating structural adequacy
Opening in Fabric Openings in fabric are such that an 8- No openings in fabnc
inch diameter ball could fit through
NO. 8 — FENCING
Maintenance
Condition When Maintenance
Maintenance Activity or
Components Defect
Is Needed
Results After Maintenance
General Missing or Broken
Any defect in the fence that permits
Parts in place to provide adequate
Parts
easy entry to a facility
security
'
Parts broken or missing
Broken or missing parts replaced
Erosion
Erosion more than 4 inches high and
No opening under the fence that
12-18 inches wide permitting an
opening under a fence
exceeds 4 inches in height
Wire Fences Damaged Parts
Posts out of plumb more than 6 inches
Posts plumb to within 1 '/2 inches
Top rails bent more than 6 inches
Top rail free of bends greater than 1
inch
Any part of fence (including posts, top
Fence is aligned and meets design
—
rails, and fabric) more than 1 foot out
standards
of design alignment
'
Missing or loose tension wire
Tension wire in place and holding
wire
Missing or loose barbed wire that is
Barbed wire in place with less the
sagging more than 2 '/2 inches between
inch sag between posts
posts
Extension arm missing, broken, or bent
Extension arm in place with no bends
out of shape more than 1 '/, inches
larger than'/. mch
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Deteriorated Paint Part or parts that have a rusting or Structurally adequate posts or parts
or Protective scaling condition that has affected with a uniform protective coating
Coating structural adequacy
Opening in Fabric Openings in fabric are such that an 8- No openings in fabnc
inch diameter ball could fit through
MAINTENANCE REQUIREMENTS FOR
PRIVATELY MAINTAINED DRAINAGE FACILITIES
NO.9 - GATES
Maintenance
Condition When Maintenance
Maintenance Activity or
Components Defect
Is Needed
Results After Maintenance
General Damaged or
Missing gate or locking devices
Gates and locking devices in place
Missing Members
Broken or missing hinges such that gate Hinges intact and lubed Gate is
cannot be easily opened and closed by working freely
a maintenance person
Gate is out of plumb more than 6 Gate is aligned and vertical
inches and more than 1 foot out of
design alignment
Missing stretcher bar, stretcher band, Stretcher bar, bands, and ties in
and ties place
Opening in Fabric See "Fencing" Standard No 8 See "Fencing" Standard No 8
D-11
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MAINTENANCE REQUIREMENTS FOR
PRIVATELY MAINTAINED DRAINAGE FACILITIES
NO. 10 — CONVEYANCE SYSTEMS (Pipes and Ditches)
Maintenance
Condition When Maintenance
Maintenance Activity or
Components
Defect
Is Needed
Results After Maintenance
Pipes
Sediment & Debris
Accumulated sediment that exceeds
Pipe cleaned of all sediment and
20% of the diameter of the pipe
debris
Vegetation
Vegetation that reduces free movement
All vegetation removed so water
of water through pipes
flows freely through pipes
Damaged
Protective coating is damaged, rust is
Pipe repaired or replaced
causing more than 50% deterioration to
any pan of pipe
Open
Trash & Debris
Trash and debris exceeds 1 cubic foot
Trash and debris cleared from debris
Ditches
per 1,000 square feet of ditch and
slopes
Sediment
Accumulated sediment that exceeds
Ditch cleaned/flushed of all sediment
20% of the design depth
and debris so that is matches design
Vegetation
Vegetation that reduces free movement
Water flows freely through ditches
of water through ditches
Erosion Damage to
See "Ponds" Standard No 1
See "Ponds" Standard No 1
Slopes
Rock Lining Out of
Maintenance person can see native soil
Replace rocks to design standard
Place or Missing
beneath the rock ]ming
Catch Basins
See "Catch Basins" Standard No 5
See "Catch Basins" Standard No 5
Debris
See "Debris Barriers" Standard No 6
See "Debns Bamers" Standard No
Barriers
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D-12
MAINTENANCE REQUIREMENTS FOR
PRIVATELY MAINTAINED DRAINAGE FACILITIES
NO. 11 — GROUNDS (Landscaping)
Maintenance Condition When Maintenance Maintenance Activity or
Components Defect Is Needed Results After Maintenance
General Weeds Weeds growing in more than 20% of Weeds present in less than 5% of the
(non-poisonous) the landscaped area (trees and shrubs landscaped area
only)
Safety Hazard Any presence of poison ivy or other No poisonous vegetation present in a
poisonous vegetation landscaped area
Trash or Litter Paper, cans, bottles, totaling more than Area clear of litter
1 cubic foot within a landscaped area
(trees and shrubs only) of 1,000 square
feet
Trees & Damage Limbs or parts of trees or shrubs that Trees and shrubs with less than 5%
Shrubs are split or broken which affect more of the foliage with spht or broken
than 25% of the total foliage of the tree limbs,
or shrub
Trees or shrubs that have been blown Tree or shrub in place free of injury
down or knocked over
Trees or shrubs which are not Tree or shrub in place and adequately
adequately supported or are leaning supported, remove any dead or
over, causing exposure of the roots diseased trees
D-13
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MAINTENANCE REQUIREMENTS FOR
PRIVATELY MAINTAINED DRAINAGE FACILITIES
NO. 12 - ACCESS ROADS/EASEMENTS
Maintenance
Condition When Maintenance
Maintenance Activity or
Components
Defect Is Needed
Results After Maintenance
General
Trash and Debris Trash and debris exceeds 1 cubic foot
Trash and debris cleared from site
per 1,000 square feet, ie, trash and
debris would fill up one standard size
'
garbage can
Blocked Roadway Debris which could damage vehicle
Roadway free of debris which could
'
tires (glass or metal)
damage tires
Any obstructions restricting the access
Obstruction removed to allow at least
to a 10 to 12 foot width for a distance
a 12 foot access
of more than 12 feet or any point
restricting access to less than a 10 foot
width
'
Road
Settlement When any surface defect exceeds 6
Roads surface uniformly smooth with
Surface
Potholes, Mush, inches in depth and 6 square feet in
no evidence of settlement, potholes,
Spots, Ruts area In general, any surface defect
much spots, or ruts
which hinders or prevents maintenance
access
Vegetation in Road Weeds growing in the road surface that Road surface free of weeds taller
Surface are more than 6 inches tall and less than than 2 inches
6 inches apart within a 400 square foot
area
Shoulders Erosion Damage
and Ditches
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Weeds and Brush
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Erosion within 1 foot of the roadway Shoulder free of erosion and
more than 8 inches wide and 6 inches matching the surrounding road
deep
Weeds and brush exceed 18 inches in
height or hinder maintenance access
D-14
Weeds and brush cut to 2 inches in
height or cleared in such a way as to
allow mamtenance access
MAINTENiANCE REQUIREMENTS FOR
PRIVATELY MAINTAINED DRAINAGE FACILITIES
NO. 13 — GRASS BIOFILTERS AND FILTER STRIPS
Maintenance Condition When Maintenance Maintenance Activity or
Components Defect Is Needed Results After Maintenance
General Trash & Debns See "Ponds and Constructed Wetlands See "Ponds and Constructed
Standard No 1" Wetlands Standard No 1"
Poisonous
See "Ponds and Constructed Wetlands
See "Ponds and Constructed
Vegetation
Standard No 1"
Wetlands Standard No 1",
Pollution
See `Ponds and Constructed Wetlands
See "Ponds and Constructed
Standard No 1"
'
Wetlands Standard No 1"
Curb Cuts
Flow entry points for filter steps are to
Debris blocks the entry of the
be kept clear of debris
stormwater,
Grass Heights
Mowing is needed if the grass exceeds
To maintain a thick growth of grass
eight inches, and is not to be cut to less
at the proper treatment height
than four mches
'
Side Slopes Erosion
Eroded damage over %z inch deep
Level areas and resod or reseed using
where cause of damage is still present
specifications in this manual '
or where there is potential for
continued damage
Bare Area
Any visibly bare area regardless of size
Avoid erosion and reduction of '
is to be reseeded, and resodded if
treatment efficiency
greater than 1 square foot in area
Standing Water
Areas where water is found standing
Standing water can kill the grass and '
for a day or more Cause must be
possibly breed mosquitos
determmed and corrected
'
Check Dams
Dislodged dam or portion of the dam,
Damaged check dams can result m
significant leakage of stormwater
damage to the treatment area
around, beneath or through a dam
D-15
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MAINTENANCE REQUIREMENTS FOR
PRIVATELY MAINTAINED DRAINAGE FACILITIES
NO. 14 - OIL/WATER SEPARATORS
Maintenance
Components
Defect
Condition When Maintenance
Is Needed
Maintenance Activity or
Results After Maintenance
Storage Area
See "Closed Detention Systems
See "Closed Detention Systems
Standard No 3"
Standard No 3"
Manhole
See "Closed Detention and Treatment
See "Closed Detention and
Systems Standard No 3"
Treatment Systems Standard No 3"
Catch Basins
See "Catch Basins Standard No 5"
See "Catch Basins Standard no 5"
Cleaning
Excessive Buildup
No Specific Condition
Clean at least once every three
Frequency
of Petroleum and
months
Floatables
Coalescing
Plugging
When debris has plugged 10% or more
Remove debris
Plates
of the vertical cross-section of the
Plates
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1 SURFACE WATER
DESIGN MANUAL
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1
The City of Kent accepts References 4A, 5, and 6B as provided in the 1998 King County Surface
Water Design Manual and they are included herein. References 8-A through 8-J are also provided
herein. Some of the references in reference section 8 have been revised by the City. Other
1 reference sections in the 1998 King County Manual (1, 2, 3, 4b, 6A, and 7) are not relevant to the
City of Kent Surface Water Design Manual.
Six separate maps relevant to the Manual are included as hard copies and PDF files on disk (See
Table of Contents for list).
1 REFERENCES
1
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Chapter 5 — Kent Construction Standards 5 R -I
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May, 2002
4A — Landscape Management Plan Guidelines
1
5 — Bibliography of Supporting Studies and Research
6B — Pond Geometry Equations
8-A —Technical Information Report (TIR) Worksheet
8-B — Offsite Analysis Drainage System Table
'
8-C —Flow Control and Water Quality Facility Summary Sheet and Sketch
8-D — Bond Quantities Worksheet — (contact the City of Kent Public Works Department)
1
8-E — Surface Water and Drainage Facilities Construction Bond
8-F — Declaration of Stormwater Facility Maintenance Covenant
'
8-G — Drainage Easements
8-H Water Memorandum
— Quality
8-1 — Adjustment Application Form and Process Guidelines
'
8-3 — Plat Dedication Clause — Final Recording
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REFERENCES
4A - LANDSCAPE MANAGEMENT PLAN
GUIDELINES
May, 2002
City of Kent Surface Water Design Manual
5 R-2 Chapter 5 — Kent Construction Standards
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1 REFERENCE 4-A
' Guidelines for preparing a landscape management
plan
Landscape management plans have the potential to significantly reduce the pollutant load washing off
managed green spaces For this reason, landscape management plans that incorporate key pollution prevention
elements and which are consistently implemented can be used in lieu of water quality treatment facilities (see
' Section 1 2 8) Submittal requirements for obtaining an approved landscape management plan are given in
Chapter 2
GENERAL CONSIDERATIONS
Studies of pollutant transport have consistently shown that forested lands consistently produce lower pollutant
loads—of solids, phosphorus and metals—than do lands used for residential, industrial or agricultural
purposes "Loading" refers to the total weight of a pollutant leaving a particular area or site It is measured by
determining both the concentration of a pollutant and the amount of flow leaving a site Since the Puget Sound
area was largely forested before settlement, lakes and streams in the area have developed biotic regimes in
response to this low pollutant loading --clear, cool waters supporting salmon and other aquatic life When the
input of pollutants increases, lakes and streams often shift to a more biologically productive mode, often with a
concomitant loss of clear water and a shift or even a decline in fish species
When forests are converted to cities, this increase in pollutant load needs to be managed in order to maintain
the beneficial uses of lakes and streams One way to manage pollutants is to treat stormwater before it enters a
water body Biofiltration swales, wetponds and sand filters, as well as other facilities, can be used to provide
' this treatment Another approach to manage pollutant loads is to prevent the pollutants from entering
stormwater in the first place
Our best models on how to keep nutrients and pollutants from entering storm water are from the original,
unaltered landscape—the forests Forests have a soft, absorptive duff layer, as well as dense vegetative
cover, especially near the ground surface Nutrients are provided in the form of slow-release organic
materials, or leaves, needles and woody material Rainfall runoff is greatly reduced from the levels seen in
' developed landscapes These factors help to keep the total load of nutrients and sediments transported to
receiving waters low
' ELEMENTS OF A SUCCESSFUL LANDSCAPE
MANAGEMENT PLAN
Good planning, tailored to the specific conditions of the site, as well as good follow-through, are both essential
in controlling the pollutants generated when forests are replaced with lawns, gardens or other landscape
features This section will focus on planning Follow-through, or implementation, will be discussed in the
' next section
1 I. PLAN CONTENTS
A landscape management plan for any particular site works best if developed with the specific site
characteristics in mind Soil type, slope, exposure, depth to groundwater as well as the particular suite of
plants chosen for the site all should help direct the specific make-up of the plan. However, there are some
basic principles that all sites should consider in order to be successful in controlling the export of soil or
1998 Surface Water Design Manual 9/1/98
KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
organic matter, fertilizers and pesticides in stormwater runoff Landscape management plans should address
each of the general principles given in Table 1, tailoring them to fit the specific site situation
Each of the five basic principles is expanded upon in the following section The recommendations discussed
under each principle are intended as a framework for a variety of site situations, from individual homes to large
parks and golf courses Thus, not every landscape management plan may be able to apply each of the listed
recommendations In addition, landscapes are managed for different purposes, some more formal than others
It may be that some recommendations will not be appropriate for very formal sites and thus not adopted, in
favor of other management practices that better fit the uses for which the site is intended In the end, the extent
to which a landscape management plan is successful depends on the ability of the practices chosen to retain
soil, fertilizers and pesticides on the site and away from water resources throughout the entire year
Table 1 Basic principles to reduce pollutant transport from landscaped areas
1 Minimize bare soil areas
2 Reduce water demand
3 Reduce extent of turf area—manage remaining turf for low -impact
4 Choose plants with sustainability in mind
5 Manage fertilizer and pesticide use wisely
Principle 1 Minimize bare soil areas
Bare soil areas are one source of solids that can be mobilized and carred downstream by rainfall Minimizing
bare soil areas makes it less likely that solid particles will be dislodged by rainfall Some pointers on how to
manage landscapes to minimize bare soil are given below
a) Estabhsh dense plantings of pest -resistant groundcover to shade out weeds. Some easy -care
recommendations are rock rose (Cistus sp ), snowberry (Symphoricarpus alba), salal (Gaultheria
shallon) and kinmckimck (Arctostaphylos uva-ursi)
b) If bare soil areas are required, as in plant beds or ball diamonds, surround the bare area with an area of
grass or groundcover to filter out solids that may be picked up by stormwater runoff
• The denser the grass or groundcover, the better it works to capture solids in runoff
• Try to make the filtering area as level as possible Avoid low spots, where runoff can concentrate
and create channels
• In general, filter areas should be about one-fourth as long (along the flow path) as the area
contributing low, assuming that slopes are gentle (less than about 10 percent) For flat, level areas
without dips, this length can be reduced.
c) Repair promptly bare patches in lawns or groundcovers that could contribute solids to stormwater
runoff
d) Don't place bark or loose mulch on slopes where it can be carred to stormdrains
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Principle 2 Reduce water demand
Reducing the need for irrigation reduces the potential movement of pollutants, conserves water and saves
money
a) Use drought tolerant or native vegetation
b) Install underground imgahon systems timed to water at night or dnp irrigation systems
c) Increase the organic content of soils to improve water -retention capability
d) Allow for longer water retention by terracing sloped areas
Principle 3 Reduce turf area and manage remaining turf for low -Impact
Turf requires care to look good In addition to mowing, turf areas typically require water, fertilizer and weed
and disease control However, some practices can reduce or minimize the amount of chemical controls
needed
a) Amend soil with organic matter to a depth of 8 -12 inches before the lawn is established Till the
organic matter into the native soil
b) Decide if all lawn area needs the same level of upkeep let some areas have a less formal look if
possible, and reduce fertilizer and pesticide use in those areas
c) Rely on tmgatton and lawn aeration as the primary tools to maintain healthy turf
d) Remove thatch each year to increase water penetration to grass roots and reduce runoff
e) Plant groundcovers rather than grass in shady areas Turf grasses usually need at least partial sun to
remain vigorous
Principle 4 Choose plants with sustainability In mind
Plants differ in their ability to cope with different soils, rainfall conditions, pest and diseases and
microclimates Choosing resilient plant species, plants with adaptations for particular environments or creating
optimal microenvironments are all techniques that can be used to create landscapes that require less
intervention Less watering and less need for pesticide and fertilizer application means less potential for
pollutants to leave the site
a) Choose disease resistant plants
b) Choose drought -resistant groundcovers, shrubs and trees in areas with poor soil or little shading
c) Group plants in clusters with tree, shrub and groundcover layers to create a better micro -environment
and to supply organic matter back to the soil
d) Include plants in the landscape that are important for beneficial insects such as parasitic wasps If
beneficial insects have nothing to sustain them, they won't stick around to control pests when you
need them
e) Use dense plantings or close spacing to shade out weeds rather than herbicides
f) Use plants with fibrous roots on steeper slopes or erosion -prone areas i Some good choices include
• *New Zealand flax (Phormium penax)
• Ornamental grasses, lawn grasses
• *Rock rose (Cetus sp )
• Rosa rugosa
Note that the County's Sensitive Areas Code (21a) defines and protects steep slopes and landslide hazard areas
from encroachment Generally, cleanng of vegetation is prohibited in areas with slopes of 40% or more
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KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
• Salmonberry (Rubus spectabiks) -- native
• Snowberry (S}mphoricarpus alba)-- native
* not hardy in all areas of the County
a) Use wetland plants 1n areas with seeps or a high water table
b) Attend to installation details Write enforceable planting specifications that include details such as
soil preparation, plant spacing, plant condition and size, planting depth, transplant handling and
irrigation Inspect the job during planting to prevent shortcuts such as blowing the soil mixture
around root balls rather than digging the roots into amended native soils
Principle 5 Manage fertilizer and pesticide use wisely
Many landscape plants and turf simply won't do well without fertilization and some amount of pest
management It's therefore important for landscape management plans to address when and how these actions
will be taken
a) Keep plants healthy by building healthy soil using composted organic material Healthy plants can better
resist diseases and insect pests
b) Tailor fertilizer make-up to lawn needs Adjust application rate and timing of fertilizer applications to
avoid carry -off in storm runoff
c) Reduce the phosphorus (P) concentration in fertilizers when possible by using a low phosphorous
formulation or formulations containing only nitrogen or potassium Added phosphorus is often not needed
for health foliage growth, only for encouraging profuse blooms
d) Use an integrated pest management approach to control pests Keep current about non -chemical controls
as a first -defense against pests
e) Encourage a diverse insect community in your landscape Beneficial insects can help control pests,
especially pests of trees and shrubs
f) Target pesticide application to the specific pest of concern Avoid pesticide "mixes" targeting genetic
problems (such as weed and feed) unless you actually need each of the formulations for a current problem
g) Only apply pesticides during the life -stage when the pest is vulnerable
h) Use fungicides very sparingly—they disrupt the base of aquatic food webs If you need to use fungicides
spray formulations with faster break -down times Consult a golf course management text for information
on the attributes of various fungicides (and other pesticides). Balough and Walker, 1992, Golf course
management and construction by Lewis Publishers is one source of information
1) Tolerate some weeds.
References
"Weed management for lawns and gardens " Washington Toxics Coalition Fact Sheet, 1989
"Least toxic lawn management." The B1oIntegral Resource Center (BIRC), P O Box 7414, Berkeley, CA
94707
Washington State Cooperative Extension publications on lawn care, Bulletin Office, Cooperative Extension,
Cooper Publication Building, Washington State University, Pullman, WA 991645912
Selected tittles include "Turf grass diseases" and supplement (EB0713 and EB0713S), "European
crane fly"(EB0856), "Fertilizer guide western Washington" (FG0041), "Disease control in home
lawns" (EB0938), "Home lawns" (EB0482)
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II PLAN IMPLEMENTATION
A landscape management plan, no matter how good, will not reduce pollutants in runoff if it is not
implemented And implementation often means that the plan needs to be modified over time since as plants
grow and as the cycle of pests change, the original plan may not fit the site The following must be addressed
before a landscape management plan can be approved
1 Identify who will be responsible for assuring the management plan is earned out
2 Identify how the applicant will assure that grounds crews or homeowners have the training and/or
resources required to implement the plan and keep up to date on advances in landscape care practices
and products
3 Agree to keep records of fertilizer and pesticide application, including rate of application, area treated
and disposal or storage of residue
4. Agree to certify each year that the landscape management plan for the protect in question has been
earned out, and that needed amendments or updates have been made
5 Provide the plan to County maintenance or inspection personnel on request
6 Agree to pay an annual fee (based on time expended) to allow the County to administer the
certification process, including review of plans, tracking of information, periodic field inspections and
sampling
1995 Surface Water Design Manual
9/l/9S
REFERENCES
' 5 - BIBLIOGRAPHY OF SUPPORTING STUDIES AND
RESEARCH
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BIBLIOGRAPHY OF
SUPPORTING STUDIES & RESEARCH
Several studies, issue papers and reports were prepared by King County Surface Water Management (SWM) Division
staff and consultants in support of the 1996 revisions to this manual This section presents a bibliography of these
documents which are available from the SWM Division
FLOW CONTROL ISSUE PAPERS
' Director's Briefing -Summary of Key Issues to the King County Surface Water Design Manual Update Rick Schaefer, R W
Beck and Associates, Linda Holden, Jeff Stem, King County Surface Water Management September, 1993 Bnefing paper
' presenting an overview of proposed changes in flow control, water quality and the review process Each section establishes
the need for a change, evaluates options for implementing the change, recommends an opuon, and discusses effects of the
recommendation
' Comparison of Current and Proposed Detention Standards Linda Holden, Jeff Stern, King County Surface Water
Management September, 1993 Comparison of current and proposed peak and duration standards, in terms of peak flow
increases, flooding frequency, and impacts to resources
' Detention Issue Paper Malcolm Leytham, Northwest Hydraulic Consultants, Linda Holden, Kelly Whiting, King County
Surface Water Management April, 1994 Provides an overview of proposed detention -related changes, including
• comparison of alternative design techniques - Discusses advantages and disadvantages of various hydrological models
including the "Y&W" method, SCS/SBUH 24-hour event method, SCS 7 -day event method, HSPF Version 10, and
HSPF Runoff Files
' components of design - Discusses use of various models for design of conveyance systems, RID facilities and other
miscellaneous hydraulic structures, with emphasis on applicability of KCRTS
• impact analysis - Presents impacts on detention sizing for several development case studies, comparing SBUH 24-hour
method and KCRTS for a variety of detention performance standards
The "Runoff Files" Implementation of HSPF Malcolm Leytham, Northwest Hydraulic Consultants, Linda Holden, Kelly
Whiting, King County Surface Water Management Apnl, 1994 Provides details on the Runoff Files method, including
principles and background, application of runoff files for facility design, and responses to some common questions and
concerns
Retention/Detention Standards Benefits and Limits in King Count) Basins Rhett Jackson, Derek Booth, King County
' Surface Water Management July, 1993 A discussion on the range of RID standards available, the role of management
objective and design methodology on their effectiveness, and fundamental limitations of onsite R/D on a basin -wide scale
Includes a companson of KCRTS and SBUH standards and effectiveness
Rationale For a "Threshold of Concern" in Stormwater Release Rates Derek Booth, King County Surface Water
Management March, 1993 Discusses selection of "50% of the 2 -year storm" as the lower threshold for duration control for
stream protection detention standards
' A Comparison of 7 -Day and 24 -Hour Detention Pond Design Standards - The Consequences of Inadequate Detention
Rhett Jackson, King County Surface Water Management August, 1992 A comparison of the effectiveness of 1990 Design
Manual 24-hour SBUH ponds and SBUH 7 -day ponds ("Barker method") as specified in the East Lake Sammatmsh Basin
' Plan, including effects on stream channels, water quality, stream and wetland habitats, and proposed capital improvement
projects
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WATER QUALITY ISSUE PAPERS
Incentives analysis offive case studies Gaynor Landscape Archaechs/Designers, Inc September, 1992 Five case
studies exploring better ways to integrate stormwater facilities into sites—emphasis is on aesthetic enhancements
The selection and sizing of treatment BMPs in new developments to achieve water quality objectives. 1993 Prepared
by Gary Minton, Resource Planning Associates with the assistance of Herrera Environmental Consultants and R W
Beck A summary of the literature and some ongmal analysis related to the size and performance of water quality
facilities
Water quality thresholds decision paper Louise Kulzer, King County Surface Water Management, April 15, 1994
Explores the need to revise the 5,000 square pollution -generating impervious surface foot threshold used to trigger
water quality facilities. Roof runoff quality explored Summaries stormwater from a number of local studies in Table
High use! Oil control decision paper Jennifer Gaus, King County Surface Water Management October, 1994
Examines the intensity of vehicle use and other "high use" land uses which would generate a concentration of oil in
stormwater treatable via oillwater separators Based on assumptions of uniform oil loss per vehicle. Redevelopment
water quality controls also discussed Identifies land use types affected and benefits of better oil control
Water quality credits decision paper Sheryl Corngan, John Heal, Louise Kulzer King County Surface Water
Management November, 1994 Identifies actions that reduce pollutant loading and presents example cases to show
effect of source reduction versus stormwater treatment on annual phosphorus loading
WATER QUALITY BENCH TESTS & MODELING STUDIES
Oil leachate tests for various adsorbant filter media Randy Brake, King County Surface Water Management May
1994 Presents results of bench tests exploring the release of oil into water from six oil absorbant media once oil
saturated Developed a standardized testing protocol
Infiltration and pollutant removal characteristics of a proposed sand filter configuration John Koon, King County
Surface Water Management May 1994 DRAFT, revision write-up expected November 1995. Presents results of
infiltration plugging potential and pollutant removal (TSS, turbidity & TP) from sand column tests using mortar sand.
Silty alluvial Duwamosh valley sediments used to determine plugging potential
Sand Filter sizing and costing Linda Holden, King County Surface Water Management May, 1995 Extensive
exploration of the effect of various sand filter design parameters and criteria on facility size using the KCRTS model
Summary of options & recommendation, supported by spreadsheets detailing results of various options
Infiltration, hydraulic conductivity and pollutant removal characteristics of sand filter materials John Koon, King
County Surface Water Management, March, 1995. Field Notes. Report expected November, 1995. Sand column tests
expanded and modified to detertnme hydraulic conductivity in addition to infiltration rate Pollutant removal (TSS,
turbidity, TP) of various fast and slow draining sands
Sand filter sand specifications John Koon, Kong County Surface Water Management June 16, 1995 Memo to Louise
Kulzer Documents sand rmxes examined and logic for the sand specification recommended, including vendor
availability
OTHER ISSUE PAPERS AND STUDIES
i King County Surface Water Design Manual Update - Cost Analysis Bruce Johnson, King County Surface Water '
l Management December, 1995 Assessment of costs associated with proposed flow control and water quality changes
Infiltration Issue Paper Steve Foley, King County Surface Water Management April, 1994 Summarizes and discusses '
recent changes to portions of the manual dealing with infiltration, including revisions intended to increase the use of
infiltration, provide increased water quality protection, and improve the functioning of infiltration facilities.
Temporary Erosion and Sedimentation Control Decision Paper Thor Tyson, King County Surface Water Management. '
November, 1993 Discussion and recommendations on recently implemented TESC issues, including maintenance standards,
February 1996 Draft Surface Water Design Manual ,
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IKING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
TESC contact persons, wet season requirements and extensions, performance standards, stream and wetland protection and
BMP revisions
Summary of Proposed Changes to the Variance Process Amy Carlson, Don Althauser, King County Surface Water
Management February, 1994 Discussion of changes proposed to the variance process with the goals of increasing flexibility
and predictability and reducing review ume and costs
' Shared Surface Water Facilities Matrix Management Group May, 1994 Outlines a proposal to provide developers with
the option of constructing shared surface water detention and treatment facilities as an alternative to currently required on-site
facilities, including a discussion on financing options
Application of Surface Water Control to Roadways Rick Schaefer, R W Beck and Associates December, 1992 Identifies
provisions of the current Design Manual that fail to address design challenges inherent in linear (roadway) projects, and
recommends alternative methods of surface water quantity and quality control
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REFERENCES
1 6B - POND GEOMETRY EQUATIONS
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Reference Section 6-B
Pond Geometry Calculations
<Known>
Volume
(V)
Pond Depth
(D)
Side Slope
(Ss)
Length -to -Width Ratio
(R)
<Find>
Bottom Area of Rectangular Pond
<Solution>
Y = depth of section measured from bottom, from zero to D
Wo= width at pond bottom
The pond width (W) at any depth, Y
Wy =W0+2SSY Eq 1
The pond length (L) at any depth, Y
Ly=RWo+2SSY Eq 2
The pond area at any depth, Y
Ay =LYWy=(RWo+2SSY)(Wo+2SSY Eq 3
or,
Ay =RWo +(R+1)2WOSSY+4SsYZ Eq 4
The equation for the pond -full volume (V) is obtained by integrating between Y=0
and Y=D
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D
V= J(RWo +(R+1)2W,SSY+4S2Y2 �Y Eq 5
0
or,
V=[RWD
02Y+(R+1)WOSSY2+4SsY'] Eq 6
3 a
or,
V=RDW 02+SSD2(R+l)W0+3S,2D3 Eq 7
Where
V = Volume of rectangular pond R = Length -to -width ratio
D = Depth Ss = Side Slope
Wo= Bottomwrdth
Rearrange equation to solve for Wo using quadratic equation, 0 = ax 2 +bx+c
O=RDWo +SSD2 (R+1)W0 +4SsD' —V
3
Q.
Use Quadratic Equation to solve for positive solution of Wo, x = —b± b' —4ac
2a
—SSD2 (R+1)±P,.D2 (R+1)12 —4RD(3 S2 D' —V1
W0 = V 2RD l J Eq. 9
Use Equation 2 for Length of pond at Y=O,
L0 = RW0
Use Equation 3 for Area of pond at Y=O,
AD = L0W0 = RW02
2
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1 8-A - TECHNICAL INFORMATION REPORT (TIR)
WORKSHEET
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FIGURE 1, PAGE 1 OF 2
CITY OF KENT PUBLIC WORKS DEPARTMENT
TECHNICAL INFORMATION REPORT (TIR) WORKSHEET
Project Owner I I Project Name
Address
Location
Township
Range
Section
Phone
Project Engineer
Company
Address/Phone
❑ Subdivision
❑ Short Subdivision
❑ Grading
❑ Commercial
❑ Other
Community
Drainage Basin
❑ River
❑ Stream
❑ Cutical Stream Reach
❑ Depressions/Swales
❑ Lake
❑ Steep Slopes
❑ DFW HPA
❑ COE 404
❑ DOE Dam Safety
❑ FEMA Floodplam
❑ COE Wetlands
❑ Floodplam
❑ Wetlands
❑ Seeps/Springs
❑ High Groundwater Table
❑ Groundwater Recharge
❑ Other
❑ Shoreline Management
❑ Rockery
❑ Structural Vaults
❑ Other
PART 7 SOILS
Sod Type Slopes Erosion Potential Erosive Velocities
❑ Additional Sheets Attached
City of Kent Surface Water Design Manual
May, 2002 5 R-6 Chapter 5 — Kent Construction Standards
REFERENCES '
Figure 1, page 2 of 2 '
City of Kent Public Works Department
TECHNICAL INFORMATION REPORT (TIR) WORKSHEET
PART 8 DEVELOPMENT rr
REFERENCE LIMITATION/SITE CONSTRAINT '
ClEl
Additional Sheets Attached
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Brief Description of System Operation
1
Facility Related Site Limitations '
Reference Facility Limitation
❑ Cast in Place Vault ❑ Other ❑ Drainage Easement
❑ Retaining Wall ❑ Access Easement
❑ Rockery > 4' high ❑ Native Growth Protection Easement
❑ Structural on Steep Slope ❑ Tract '
❑ Other
I or a civil engineer under my supervision have visited the site Actual site conditions as observed were incorporated into this worksheet
and the attachments To the best of my knowledge the information provided here is accurate
City of Kent Surface Water Design Manual
Chapter 5 — Kent Construction Standards 5 R-7 May, 2002
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MINIMUM ESC REQUIREMENTS
MINIMUM ESC REQUIREMENTS
DURING CONSTRUCTION
FOLLOWING CONSTRUCTION
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❑
Sedimentation Facilities
❑ Stabilize Exposed Surface
❑
Stabilized Construction Entrance
❑ Remove and Restore Temporary ESC Facilities
❑
Perimeter Runoff Control
❑ Clean and Remove All Silt and Debris
❑
Clearing and Grading Restrictions
❑ Ensure Operation of Permanent Facilities
❑
Cover Practices
❑ Flag Limits of SAO and Open Space Preservation Areas
❑
Construction Sequence
❑ Other
❑
Other
'
PART
❑
I SURFACE WATER SVSTEM
Grass Lined Channel ❑
Tank
❑ Infiltration Method of Analysis
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❑
Pipe System ❑
Vault
❑ Depression
❑
Open Channel ❑
Energy Dissipater
❑ Flow Dispersal Compensation/Mitigation
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❑
Dry Pond ❑
Wetland
❑ Waiver of Eliminated Site Storage
❑
Wet Pond ❑
Stream
❑ Regional Detention
Brief Description of System Operation
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Facility Related Site Limitations '
Reference Facility Limitation
❑ Cast in Place Vault ❑ Other ❑ Drainage Easement
❑ Retaining Wall ❑ Access Easement
❑ Rockery > 4' high ❑ Native Growth Protection Easement
❑ Structural on Steep Slope ❑ Tract '
❑ Other
I or a civil engineer under my supervision have visited the site Actual site conditions as observed were incorporated into this worksheet
and the attachments To the best of my knowledge the information provided here is accurate
City of Kent Surface Water Design Manual
Chapter 5 — Kent Construction Standards 5 R-7 May, 2002
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REFERENCES
8-13 - OFFSITE ANALYSIS DRAINAGE SYSTEM
TABLE
City of Kent Surface Water Design Manual
May, 2002 5 R-8 Chapter 5 — Kent Construction Standards
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8-C - FLOW CONTROL AND WATER QUALITY
FACILITY SUMMARY SHEET AND SKETCH
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REFERENCES
City of Kent Surface Water Design Manual
Chapter 5 — Kent Construction Standards 5 R-9 May, 2002
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KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
STORMWATER FACILITY SUMMARY SHEET
ENGINEER
JDFVELOPER
Name
Name
Firm
Firm
Address
Address
Phone
Phone
Developed Site- Acres Number of lots
Number of detention facilities on site Number of infiltration facilities on site
ponds ponds
vaults vaults
tanks tanks
Flow control provided in regional facility (give location)
No flow control required Exemption number
A
Basin
Number & type of water quality facilities on site
biofiltration swale (regular/wet/ or
continuous inflow?)
combined detention/WQ pond
(WQ portion basic or large?)
combined detention/wetvault
compost filter
filter strip
flow dispersion
farm management plan
landscape management plan
oil/water separator (baffle or coalescing plate?)
catch basin inserts Manufacturer
pre -settling pond
pre -settling structure Iv
flow -splitter catchbasm
asin
sand filter (basic or large?)
sand filter, linear (basic or large?)
sand filter vault (basic or large?)
stormwater wetland
wetpond (basic or large?)
wetvault
DESIGN INFORMATION INDIVIDUAL BASIN
A B C D
Water Quality design flow
Water Quality treated volume or we and Vr
1 1998 Surface Water Design Manual 9/1/98
1
KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
DESIGN
INFORMATION, cont'd
TOTAL INDIVIDUAL BASIN
Drainage basm(s)
Onsite area
A B C D
Offsite area
Type of Storage Facility
Live Storage Volume
Predevelo ed Runoff Rate
2 -year
I0- ear
100- ear
Developed runoff rate
2-vear
10-vear
1 00 - ear
Type of restnctor
Size of onfice/restnction
No 1
No 2
No 3
No 4
FLOW CONTROL & WATER QUALITY FACILITY SUMMARY SHEET SKETCH
All detention, infiltration and water quality facilities must Include a sketch per the following criteria
I Heading for the drawings should be located at the top of the sketch (top nght-hand corner)
The heading should contain
• North arrow (point up or to left) • D9#
• Plat name or short plat number • Address (nearest)
• Date drawn (or updated) • Thomas Brothers page, grid number
2 Label CBs and MHs with the plan and profile designation Label the control structure in writing or abbreviate
with C.S Indicate which structures provide spill control
3 Pipes-- indicate
Pipe size
Pipe length
Flow direction
Use s single heavyweight line
4 Tanks-- use a double, heavyweight line and indicate size (diameter)
5. Access roads
• Outline the limits of the road
• Fill the outline with dots if the road is gravel Label in writing if another surface
6 Other Standard Symbols
• Bollards 0 • • • ■ ■
• Rip rap 000000
000000
Fences --x---x---x--x---x---x--
• Ditches—D---D--- D--- D
7 Label trash racks In witting
8 Label all streets with the actual street sign designation If you don't know the actual street name, consult the plat
map
9 Include easements and lot lines or tract limits when possible
10 Arrange all the labeling or writing to read from left to right or from bottom to top with reference to a properly
oriented heading
I 1 Indicate driveways or features that may impact access, maintenance or replacement
911198 1998 Surface Water Design Manual '
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REFERENCES
8-D - BOND QUANTITIES WORKSHEET
(CONTACT KENT PUBLIC WORKS)
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City of Kent Surface Water Design Manual
May, 2002 5 R-10 Chapter 5 — Kent Construction Standards
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' 8-E - SURFACE WATER AND DRAINAGE
FACILITIES CONSTRUCTION BOND
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REFERENCES
City of Kent Surface Water Design Manual
Chapter 5 — Kent Construction Standards 5 R-11 May, 2002
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MAIL TO
'K E N T PROPERTY MANAGEMENT
WpS„,„G,o„ City of Kent
2204 TH Avenue South
i Kent, Washington 98032
Attn
i SURFACE WATER AND DRAINAGE
FACILITIES CONSTRUCTION BOND
STATE OF WASHINGTON) PROJECT NAME
)SS PROJECT NUMBER
COUNTY OF KING ) KIVA #
KNOW ALL MEN BY THESE PRESENTS, that we as Principal,
i and the , a Corporation organized and
existing under and by virtue of the laws of the State of and authorized
i to transact the business of surety in the State of Washington, as surety, are held and firmly bound
unto the City of Kent, in the penal sum of for
payment of which, well and truly to be made, we bind ourselves and each of our heirs, executors,
iadministrators and assigns, Jointly and severally, firmly by these presents
The condition of the foregoing obligation is such that the above described principal is about
' to construct , hereinafter referred to as "Improvements," that
may cause damage and disruption to certain lands and/or public rights-of-way within the City
limits of Kent and in the County of King in accordance with approved plans on file with the City
iof Kent Engineering Department
The principals of this bond agree to the following terms and conditions
I Said Improvements and their appurtenances thereto shall be constructed in accordance
with the approved plans,
2 Construction shall be completed within _ days after construction begins unless the
i City Engineer, for good cause shown, has granted an extension of time,
3 All construction and restoration shall be in accordance with the City of Kent Standards,
APWA Standards and Surface Water and Drainage Ordinance as set forth by City of Kent
Ordinance Nos 1142, 1672, and 3208 and as may hereafter be amended
4 Traffic control shall be implemented in accordance with the US Department of
i Transportation Manual on Uniform Traffic Control Devices as may hereafter be amended as set
forth in Part VI within said manual
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The Construction Inspector of the City of Kent shall be given twenty-four (24) hours notice
prior to the commencement of any work
IU \userdata%fomu\369E-00sk doc
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The condition of this obligation is such that if the principal shall construct said improvements
in accordance with the terms and conditions set forth herein this obligation shall terminate
Otherwise, it shall remain in full force and effect This obligation shall be released after final
acceptance of the improvements by the City and upon receipt by the City of the required
maintenance bond for the one-year maintenance period, otherwise to remain in full force and
effect
Signed, sealed and dated this day of
APPROVED AS TO FORM:
IOGVR A. L VICH,—CITY ATTORNE
CITY OF KENT
ATTEST
COUNTERSIGNED
PRINCIPAL
ADDRESS
PHONE
PRESIDENT
SECRETARY
BONDING COMPANY
*By
`AUTHORIZATION FOR ATTORNEY-IN-FACT ATTACHED HERETO
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IREFERENCES
8-F - DECLARATION OF STORMWATER FACILITY
MAINTENANCE COVENANT
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City of Kent Surface Water Design Manual
May, 2002 5 R-12 Chapter 5 — Kent Construction Standards
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IAFTER RECORDING MAIL TO:
IProperty Management
City of Kent
I 220 4th Avenue South
Kent, WA 98032
Attn:
Reference Number of Related Document
Grantor(s):
Grantee(s): City of Kent
Abbreviated Legal Description
Additional Legal Description is on Page(s) of Document
I Assessor's Tax Parcel No
Project-
' DECLARATION OF STORMWATER FACILITY
MAINTENANCE COVENANT
(INDIVIDUAL)
IN CONSIDERATION OF THE City of Kent (" City" ) approval for.
Iproperty legally described as follows. relating to real
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the undersigned Grantor(s) declares that the above-described
property is subject to a privately maintained stormwater drainage,
' detention, and/or stormwater treatment system (the, " Stormwater
Facilities" ), and also covenants and agrees as follows
DUTIES OF GRANTOR(S)-
IStormwater covenant
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1 Grantor(s) shall regularly inspect and maintain/repair the
private Stormwater Facilities on the said -described property in
accordance witn the standards specified in th
Construction Standards, specifically including ppendix D
(" Maintenance Requirements for Privately Maintain
Facilities" ), as now collectively enacted or hereafter amended,
which are incorporated by this reference as if fully set forth
herein (the, " City Construction Standards" ).
_' Grantor(s) shall inspect the Stormwater Facilities as often as
conditions require, but in any event at least once each year
Grantor(s) shall, within four weeks after each inspection,
maintain/repair the Stormwater Facilities as required by the
City Construction Standards.
3 Grantor(s) shall inspect each element of the Stormwater
Facilities whenever the City's Public Works Director
(" Director" ), in his/her sole discretion, determines that
unacceptable conditions exist within or adjoining to the
Stormwater Facilities. Similarly, the Director, in his/her sole
discretion, may require the Grantor(s) to complete the
maintenance/repair of the Stormwater Facilities within a shorter
time period than allowed in Section 2, above.
4 Grantor(s), in effecting this maintenance/repair, shall restore
the Stormwater Facilities to like new condition, or if that is
not practical, to an acceptable condition to the extent listed
and/or described in the City Construction Standards.
5 Grantor(s) is hereby required to obtain written approval from the
Director prior to grading, filling, piping, cutting or removing
vegetation (except for routine and minor landscape maintenance)
in open vegetated drainage facilities (such as biofiltration
swales, channels, ditches, ponds, etc.) or performing any
alterations or modifications to the Stormwater Facilities.
Grantor(s) shall obtain all necessary permits and provide all
required land surveys as required by the City Construction
Standards.
6 Grantor(s) shall assume all responsibility for the implementation
and cost of any maintenance and/or repairs to the Stormwater
Facilities.
RIGHTS OF THE CITY
Stormwater covenant
2 of 4 '
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1 The City shall have ingress and egress rights to the said -
described property for inspection and monitoring of the
Stormwater Facilities in order to determine performance,
operational flows or defects in the Stormwater Facilities, all
' in accord with the City Construction Standards.
2 If the City determines that, pursuant to the City Construction
Standards, the Stormwater Facilities require maintenance and/or
repair work, the Director shall deliver written notice to the
Grantor specifically describing the required maintenance and/o--
repair
nd/orrepair The notice shall also set a reasonable time in which
Grantor must complete the described work. The notice shall also
state that the City or its authorized agent may perform the
authorized maintenance and/or repair if the Grantor(s) falls to
complete the maintenance and/or repair within the time allowed
3 If the Grantor(s) does not _complete the required maintenance
and/or repair within the time allowed as set forth in the
Director's notice, the City of its authorized agent will not
commence the maintenance and/or repair work described in the
1 Director's notice until at least seven (7) calendar days after
the expiration of the time allotted to Grantor to make the
maintenance and/or repair. However, if the Director determines,
at his or her sole discretion, that an imminent danger exists,
the City's obligation to provide written notice shall be deemed
waived, and the City or its authorized agent may immediately
begin the required maintenance and/or repair work.
4 If the City or its authorized agent performs the required
maintenance and/or repairs to the Stormwater Facilities,
Grantor(s) shall reimburse the City all its costs incurred in
completing the maintenance and/or repairs within thirty (30)
' calendar days of Grantor's receipt of the City's invoice for
that work. overdue payments shall accrue interest at the rate
of twelve percent (12%) per annum.
5 If the Director determines, in his/her sole discretion, that the
Stormwater Facilities, if originally constructed in accordance
' with the City's approve design, need further modifications,
Grantor(s) authorizes the City to enter the Stormwater
Facilities property in order to make these modifications.
Stormwater covenant 3of4
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Any notice or consent required to be given or otherwise provided '
for by the provisions of this agreement shall be effective either
upon personal delivery or three (3) calendar days after mailing by
Certified Mail, return receipt requested
This Covenant is intended to protect the value and desirability of
the property described above, including the larger parcel(s), if '
any, benefited by the Stormwater Facilities. Further, this
Covenant shall inure to the benefit of all the citizens of the City
and shall bind Grantor(s), and its heirs, successors and assigns '
GRANTOR GRANTOR '
(Address): (Phone):
purposes therein mentioned
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Print Name
Notary Public in and for the State ,
Washington, residing at
My Commission Expires
Stcrmwater covenant
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STATE OF WASHINGTON )
)SS
COIINTY OF RING )
I, the undersigned, a Notary
Public in
and for the State of Washington,
hereby certify that on this
day of
20_,
'
personally appeared before me
and
to me known to
be the individual(s) described in
'
and who executed the foregoing instrument and
acknowledged that they signed and
sealed the same as their free and
voluntary act and deed for the uses and
'
purposes therein mentioned
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Print Name
Notary Public in and for the State ,
Washington, residing at
My Commission Expires
Stcrmwater covenant
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REFERENCES
' 8-G - DRAINAGE EASEMENT
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' City of Kent Surface Water Design Manual
Chapter 5 —Kent Construction Standards 5 R-13 May, 2002
1
AFTER RECORDING MAIL TO
Property Management
City of Kent
220 4th Avenue South
Kent, WA 98032
Attention
Reference Number of Related Document
Grantgr(s)
Grantee(s) City of Kent
Abbreviatea Legal Description
Additional legal Description is on Page(s)_of Document
Assessor's Tax Parcel No
Project Name
Easement
Individual
THIS INSTRU14EIlT made this day of 2e by and
between hereinafter called
"Grantor" and CITY OF KENT, a municipal corporation of King County,
State of Washington, hereinafter called "Grantee"
WITNESSETH That said Grantors for and in consideration of
and/or other valuable consideration
receipt of which is hereby acknowledged by said Grantors, do by
these presents grant, bargain, sell, convey, and confirm forever
unto the said Grantee, its successors and/or assigns, an easement
for with necessary appurtenances,
including use of incidental areas immediately adjacent for the
installation, operation, maintenance, ectsnding, construction,
altering, reconstructing and repair over, through, across under and
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The said Grantee shall have the right without prior
upon the following described
property
situated in King County,
necessary, to enter upon said property and immediate adjacent areas
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Washington, more particularly
described
as follows
1
repair and reconstructing of said or making any
connections therewith without incurring any legal obligation or
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The said Grantee shall have the right without prior
institution of suit or proceeding at law, at times as may be
necessary, to enter upon said property and immediate adjacent areas
_
with the necessary equipment for the purposes of altering
installation operation, maintenance, extending, constructing,
repair and reconstructing of said or making any
connections therewith without incurring any legal obligation or
liability therefore, providing that said altering, installation,
--
operation maintenance, extending, constructing, repair and
reconstructing of said shall be accomplished
in such a manner that the private improvements existing within this
easement area including said incidental areas shall not be
disturbed or destroyed, or in the event they are disturbed or
destroyed, they will be replaced in as good a condition as they
�93G [NO 19:G page ] rf ±
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were immediately before the property was entered upon by the
Grantee.
The Grantor shall retain the right to use the surface of this
easement including said incidental areas so long as said use does
not interfere with the uses heretofore defined. Under no
circumstances shall any cement concrete or any structures be placed
or erected on this easement. This easement shall be a covenant
running with the land forever and shall be binding on the Grantor's
successors, heirs, and assigns.
Dated this day of 20
STATE OF WASHINGTON )
)SS
COUNTY OF KING )
I, the undersigned, a Notary Public in and for the State of
Washington, hereby certify that on this day of
20_, personally appeared before me
to me known to be the individual(s)
described in and who executed the foregoing instrument and
acknowledged that they signed and sealed the same as their free and
voluntary act and deed for the uses and purposes therein mentioned.
Print Name
Notary Public in and for the State of Washington,
residing at
My Commission Expires
492C.IN0A92G Page 2 of 2
REFERENCES
i 8-H - WATER QUALITY MEMORANDUM
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' May, 2002
City of Kent Surface Water Design Manual
5 R-14 Chapter 5 — Kent Construction Standards
i DESIGNATION OF WATER BODIES
IN THE CITY OF KENT WITH
RESPECT TO STORMWATER
' TREATMENT
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Prepared for Entranco Engineers
Under contract with the City of Kent
to update the City's Stormwater Manual
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Prepared by
Gary R.Minton, PhD, PE
Resource Planning Associates
Seattle, Washington
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' September 28, 2000
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The City of Kent intends to adopt the King County surface water design manual' With respect to
stormwater treatment, the County manual identifies four levels of treatment "basic" ,
"biologically significant (sensitive) streams", "sensitive lakes", and "sphagnum bogs" For each
treatment level there is a menu of treatment options `Basic" is the lowest treatment level,
roughly corresponding to the treatment efficiency achieved by the BMPs identified in the
Department of Ecology's 1992 stormwater manual2 `Banc" is applied to any water body unless
it has been specifically designated into one of the other three groupings The County's current
designation is presented in a fold -out plate contained within its manual
The question for the City of Kent is how shall it designate its surface water bodies, which in tum
determines the treatment level Which lakes, if any, should be classified as "sensitive"? Which
streams, if any, should be classified as "biological significant"? The City has no sphagnum bogs
and therefore that grouping is not considered here
Designation of particular receiving waters for higher levels of treatment should be based on the
consideration of several factors Amongst these are
• How King County designated its water bodies
• City ordinances, policies and broad community objectives with respect to water quality and
aquatic health
• Existing management plans for specific water bodies
• Water quality data
• 303d listing of the Department of Ecology
• Ecology's updated manual, Final Draft
• Fisheries and aquatic habitat data
• The effectiveness of "basic" treatment systems to protect the City's creeks and lakes
KING COUNTY DESIGNATION PROCESS
Streams: The County developed a procedure to identify regionally and locally significant
resource areas Three criteria were used for regionally significant streams
"Watershed functions are not appreciably altered from predevelopment conditions
The diversity and abundance of aquatic and associated terrestrial habitat are of consistently
high quality and are dispersed throughout the system, and,
Freshwater life, particularly salmonids, approach or exceed the abundance and diversity of
equivalent undisturbed systems and make a significant contribution to the regional fishery
resource o f Puget Sound"
Three criteria were used for locally significant streams,
"Watershed functions have been altered (but conditions) are adequate for spawning and
rearing of salmonids and these functions include migration corridors connected to
regional significant streams ,
The diversity and abundance of aquatic and associated riparian habitats are good but not
exceptional , and,
Freshwater life, particularly salmonids, are supported at one more species and life stages
at population levels that are low but sustainable"
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Concerning creeks within watersheds relevant to the City, the County designated Covington and
Jenkens Creeks as regionally significant It did not identify any streams as locally significant
suggesting that the criteria of Booth et al 3 was not used at least for this category The above
criteria are considered later in this memorandum when reviewing the available data on fish
' habitat in the City It is of interest to note that existing water quality did not factor into the
County's designations
' Lakes: The procedure" followed by King County for the designation of lakes has three criteria
1 Trophic Status Index (TSI),
2 Resource value, which relates to public access and wetland resource information;
3 Land use, expressed as percent of the watershed that is forested now and in the future
The evaluation paper" considered 40 lakes, including Lake Meridian but not Lake Fenwick The
40 lakes were scored 3, 2, 1, and 0 3 representing the highest score with respect to potential
sensitivity In its manual King County identifies three "sensitive" lakes Cottage, Beaver, and
Desire These designations apparently did not flow from the procedures paper Although Beaver
and Desire received scores of 3, Cottage received a score of only 2 yet was designated
"sensitive" Further, several other lakes with scores of 3 were not designated "sensitive" Lake
Merdian was given a score of 2
' CITY ORDINANCES, POLICIES
City Ordinance 7 05 140 requires that stormwater containing pollutants discharges have a permit
The ordinance states that determination of whether pollutants will be present in the stormwater is
to be determined by the public works director using a variety of sources of information including
the water quality standards for the State (Chapter 173-201 WAC) However, the ordinance does
not specify whether the pollutants are to be decreased, or controlled or mitigated in any manner
The City's comprehensive plan (1995) notes that in 1985 the City in conjunction with the
establishment of the stormwater utility adopted the following water quality goal "Reduce the
environmentally detrimental effects of present and future runoff in order to maintain or improve
stream habitat wetlands, particularly water quality, and protected water -related uses"
The comprehensive plan (1995) established Goal LU -23, entitled "Protect and enhance water
resources for multiple benefits, including recreation, fish and wildlife resources and habitat, flood
' protection, water supply, and open space" Sub -policies LU -23 1 and LU -23,2 speak to protecting
wetlands Sub -policy LU -23 4 states that the City shall "Mamtam rivers and major and minor
streams in their natural state" To implement this sub -policy requires that stormwater discharges
be treated to a high level
EXISTING WATERSHED OR BASIN PLANS
There are two significant lakes in the City Fenwick and Meridian Lake Fenwick has a lake
management plan The plan does not call for inordinately high (more than "basic") levels of
treatment in new developments. Further, the watershed is essentially developed Hence, requiring
' higher levels of treatment for any new developments would not be of any use In light of the
management program underway for Lake Fenwick, it should not receive special designation at
this time It is possible in the future that the City might conclude that the management plan needs
to be revisited If that occurs, special designation could be considered at that time There is no
management plan for Lake Meridian because water quality has not been a problem
WATER QUALITY DATA
Lakes: Given the above conclusion concerning Lake Fenwick, only Lake Meridian is left for
consideration Like Lake Fenwick, the watershed of Lake Meridian is essentially developed
Only about 15% of the watershed is available for developments Recent data6 indicate that the
trophic state of Lake Meridian continues to be in the oligotrophic-mestrophic stage. In recent
years water quality has improved Average summer total phosphorus concentrations have
decreased from about 18 ug/L in 1994 to about 9 ug/L in 1998 The dominant alga has been the
blue-green Anabaena Nonetheless, it is unlikely that development of the remaining 15% of the
watershed will have a noticeable impact on the water quality of the lake Hence, Lake Meridian
should not be designated "sensitive",
Streams: The relevant data are of samples taken from creeks during storms The City recently
sampled two storms at 15 stations' While there are many pollutants in stormwater that are
potentially toxic, the constituents of particularconcern to fish are the metals Stations where the
metals standards were exceeded in either storm are summarized below
Storm 1. Chronic standard for copper exceeded in upper Meridian Valley Creek
Chronic standard for mercury exceeded in one of three stations in Big Soos Creek, two
of three stations in McSorley Creek, and at the mouths of Mullen Slough and Mill
Creek -Auburn
Acute and chronic criteria for zinc exceeded one of three stations in McSorley Creek
Storm 2 Neither zinc or copper standards exceeded at any station Mercury was not analyzed
Metals standards depend on water hardness the higher the hardness, the higher the numeric
standard The City's data indicate that the hardness is highly variable, ranging from 19 to 94
between the stations over the two storms An examination of the data for dissolved zinc indicate
a weak inverse relationship to hardness that is, the higher the hardness the lower the
concentration of dissolved zinc This likely reflects the differences in the proportions of
stormwater and base stream flow at each station during each storm The hardness of stormwater
tends to be significantly lower than base stream flows Hence, the greater the storm, the lower the
hardness in the stream during the storm because the majority of the flow in the stream is of storm
runoff This analysis suggests that during large storms, particularly during the early period of
each storm when concentrations tend to be highest, that the metals standards are most likely to be
exceeded
It is also important to note that the City's data are event mean concentrations (EMC), composites
of several individual samples taken over several hours However, the acute standard is the
average concentration during any one hour period Consequently, it is possible that the acute
standard was exceeded at other stations than those listed above Taking this into consideration,
assume that peak metals concentrations were double the observed EMC If this were the case, the
following exceedances for copper and zinc may have occurred
Storm 1: Acute standard for copper exceeded in upper Meridian Valley Creek.
Acute standard for zinc exceeded at one of three stations in Meridian Valley Creek
Storm 2 Neither zinc or copper standards exceeded at any station
The above analysis indicates that doubling the observed concentrations has a minor effect Stated
differently, with the exceptions noted the EMCs were considerably below the standard The
chronic standard is not considered as it is the average concentration over a four day period
It is of interest to note that the concentrations in Spnngbrook Creek, below its confluence with
Mill Creek, were not notably high relative to the standards Mill Creek is on the Department of
Ecology 303d (see below) list for metals Samples were not taken in Mill Creek
The above analysis leads to the conclusion that there is too little data to base a decision on
designation It is important to understand that with regard to metals standards, an exceedance is
allowed only once over a three year period Approximately 150 storm runoff events occur over a
three year period Failing to meet the standard of only one metal during only one of 150 events
constitutes a violation
303D LISTING OF THE DEPARTMENT OF ECOLOGY
A water body lake that receives 303d designation means that water quality standards are not
being met for the particular water quality parameters listed for the particular water body Creeks
within the City that have received this designation are segments of Soos Creek and its
tributaries, and Mill Creek Parameters specified for the specified segments of the Soos Creek
system include dissolved oxygen, fecal coliform, and temperature Meridian Valley Creek, a
tribitutary of the Soos, is not a designated segment The parameters specified for Mill Creek are
dissolved oxygen, fecal coliform, temperature, dissolved chromium, dissolved copper, total
mercury, and dissolved zinc
Stormwater treatment systems are generally ineffective in reliably reducing fecal coliform in
stormwater and have little relevance to temperature They do remove organic matter which
affects dissolved oxygen in streams, and metals However, temperature and dissolved oxygen
problems in Mill and Soos Creek likely occur during dry -weather and in the summer months, and
have no relationship to storm events
1 Based on its 303d listing Mill Creek should be designated "sensitive" with particular attention to
the control of dissolved metals Irrespective of the City's designation for Mill Creek, only
treatment technologies capable of removing dissolved metals should be allowed. For example, a
wet vault should not be acceptable method of treatment As metals are not specified for Soos
Creek, is tributaries within the City (Soosette Creek and Big Soos) should not be designated
"sensitive" based on the 303d list
Despite the fact that the water quality of Lake Meridian appears to be acceptable (identified as
' oligotrophic-mestrophic), the lake is designated on Ecology's 303d list It is listed for fecal
coliform and total phosphorus However, as previously noted little of the watershed remains
undeveloped Hence, requiring higher levels of phosphorus removal by new developments will
' be of little benefit If it is the view of the City that listing by the Department of Ecology is valid
then the City should proceed to develop a management plan If however the City believes the
listing is not valid, it is recommended that the City seek de -listing
' DEPARTMENT OF ECOLOGY'S NEW STORMWATER MANUAL
Ecologyjust recently published the Final Draft of its new manual 12. The final manual is to be
adopted by the end of the year. Ecology has defined two levels of treatment basic and enhanced
Basic treatment represents AKART and includes those treatment systems and sizes described in
Ecology's 1992 manual'- With respect to enhanced treatment, Ecology has generally followed
the concept originally developed by King County However, Ecology has included two unique
concepts with respect to streams First, Ecology identifies enhanced treatment has the removal of
dissolved metals (King County focuses on total metals, represented by zinc) Secondly, Ecology
has specified that enhanced treatment shall be used for all discharges that enter "fish bearing
streams" or "streams that flow to fish bearing streams" Whether this concept will remain in the
final manual is not know However, to follow Ecology's lead would mean that enhanced
treatment would be required of all discharges entering all of the City's streams
FISHERIES AND AQUATIC HABITAT DATA
Data are synthesized from several sources The City's creeks have recently been scored' with
respect to two habitat indices the B -IBI, index of biological integrity, and the RBI, rapid
bioassessment of habitat The former is based on invertebrates, the latter is based on several
habitat metrics Rating ranges for each creek are summarized in Table 1 For the B -IBI, above 40
is considered "excellent, 30 to 40 is considered "good", 20 to 30 is considered "fair", and less
than 20 is considered "poor"
Following the criteria used by King County, the data in Table 1 suggest Big Soos could be
classified as "locally significant"
TABLE 1 HABITAT DATA SUMMARY
CREEK
B -IBI
RBP
Mill
14-20
58 to 139
Springbrook
18
116
Garrison
14-20
120 to 150
Soosette
16 to 22
59 to 132
Meridian Valley
18 to 22
97 to 131
Big Soos
26 to 32
148 to 170
McSorley
20 to 22
114 to 151
Three studies have examined fish habitat conditions910 ii Harza (1999) rated various stream
segments and identified potential limiting factors The ratings are summarized in Table 2
TABLE 2 SUMMARY OF CREEK RATING CONCLUSIONS
CREEK
RATING
Big Soos
Good
Midway
Poor to good depending on
reach
Mill mouth -Auburn
Good
Mullen Slough
Good
Soosette
Poor to excellent depending
on reach
Meridian Valley
Poor to excellent depending
on reach
Clark lake outlet
Poor
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The 1999 study did not include the Sprmgbrook-Mill-Garrison system as it had been examined in
the 1996 study However, the 1996 study did not include ratings The 1996 study did observe
that coho were found most frequently in the upper reaches and not on the valley floor
' The above information appears to be too limited to draw firm conclusions concerning
designation Since portions of the Soosette and Meridian Valley Creeks are rated "excellent',
perhaps special protection should be applied to the entirety of each creek It makes little sense to
' apply the designation to only a portion of each creek
THE EFFECTIVENESS OF "BASIC" TREATMENT SYSTEMS
Since King County initially identified its menus of advanced treatment for streams and lakes
(about 1995) there have been additional data generated on the performance of certain treatment
systems It is reasonable to ask whether in fact some if not all of the "basic" treatment systems
provide adequate protection even for streams like Mill Creek
The question is whether "bask" treatment is insufficient to protect streams in those watersheds
within City where all land within each watershed will be essentially 100% developed In this
case, the assumption is that if the effluent from a "basic' treatment system does not meet
receiving water standards, that there likely will be receiving water violations This is because
almost all of the stream flow during a storm in heavily developed watersheds will be runoff from
the developed lands
To address this question, performance data were compiled of individual treatment systems that
have been studied in the Pacific Northwest PNW data are available from only eight facilities
three grass swales 13,14,15, two wet ponds 16, and three sand filters"," However, the sizes of the
two ponds do not fit either current King County or Ecology criteria One pond is very small and
one is very large, about 7% and 150% of the King County "bask' facility, respectively The
large pond is in essence an "enlarged" pond according to King County criteria There are no
PNW data for constructed wetlands Wet vaults are not considered as they do not remove
dissolved metals and therefore should not be allowed as stand-alone treatment systems
Copper and zinc are used to assess the effectiveness of "basic' treatment systems with respect to
' streams Receiving water standards are based on the dissolved fraction Unfortunately, for the
eight research facilities identified above, influent and effluent samples were evaluated for
dissolved metals for only the swales and one of the sand filters
Presented in Figure 1 is a plot of the zinc data for individual storms for the three swales and two
of the three sand filters Data are not included for one of the filters (the Lakemont filter) because
the report does not present data for individual storms For the three swales, Figure 1 presents data
for dissolved zinc However, only total zinc is available for the two remaining sand filters (only
the Lakemont study analyzed dissolved) Therefore, these data were multiplied by 0 5, assuming
that 50% of the total zmc was in the dissolved form This is a reasonable assumption on average
the dissolved fraction is about 50% of the total although it can range from 20% to 80% for
individual storms Sand filters are able to remove dissolved zinc 17,19
The standard for dissolved metals depends upon the hardness rising with increasing hardness
However, only Koon15 measured hardness it was found to vary from 18 to 50 with an average of
about 25 The acute standard for zinc at a hardness of 25 is 0 032 mg/L Within the hardness
range of 15 to 50, the acute standard for zinc ranges from 0 021 to 0 058 mg/L (the chronic
standard is about 90% of the acute)
1
A similar graph was prepared for copper Figure 2 The acute standard for copper at a hardness of
25 is 0 0038 mg/L Within the hardness range of 15 to 50, the acute standard ranges from 0 0032
to 0 008 mg/L (the chronic standard is about 70% of the acute)
A comparison of the standards for each metal to the performances represented in Figures 1 and 2
leads to the following observations
For most of the storms sampled in the swale studies the influent concentrations of zinc and
copper were less than their respective standard It is possible that this is also the case for the
sand filters if it were assumed that the dissolved/total ratio was about 0 25 rather than 0 50
But the low ratio is unlikely But it does suggest that metals concentrations from residential
properties may be generally less than the standard all three swales were located in
residential developments
Where the influent concentrations of zinc or copper exceeded their respective standards, grass
swales are not able to meet either standard Of 20 storms sampled, the removal efficiency
was zero or negative in eight events for both metals Sand filters appear to be more effective
and reliable than swales However, even sand filters may not be able to reduce zinc below the
standard when the hardness is less than about 20 and the influent concentration is above about
0 2 mg/L Swales probably cannot reduce the copper concentration to below the standard
irrespective of the hardness or influent concentration
King County has identified a performance goal of removing 50% by "enhanced" treatment
systems Grass swales are not able to meet the 50% performance goal for either copper or
zinc However, the "basic" sand filter appears able to remove 50%, meaning a sand filter
provides "enhanced" treatment
The findings of the study 17of the Lakemont sand filter are consistent with the above observations
A range of concentrations was given in the report of the filter which serves a residential area
influent values for dissolved zinc ranged from 0 0016 to 0 016 mg/L, effluent concentations
ranged from 0 0006 to 0 0014 mg/L However, it appears likely that the influent concentration
never rose above the standard Dissolved copper was also reduced influent values ranged from
0 0013 to 0 0068 mg/L, effluent concentations ranged from 0 0008 to 0 0013 mg/L It is likely
that the influent concentration for dissolved copper did at times exceed the standard
SUMMARY
The following streams could be considered for special designation
• Mill Creek ,justification, placement on the 303d list
• Soosette Creek justification, value of current fishery
• Meridian Valley Creek .justification, value of current fishery
• Big Soos value of habitat (B -IBI and RBP)
It is possible that upper Gamson Creek should be designated "sensitive" based on the RBP score
Alternatively, it could be decided that "enhanced" treatment will be used throughout the City
The logic of this decision is that "basic" treatment is likely insufficient to meet water quality
standards in those watersheds that are currently or in the future will be heavily developed Also,
the City is attempting to maintain the current relatively healthy fishery, a g. Soosette Creek
Requiring enhanced treatment throughout the City is consistent with the new manual of the
Department of Ecology that calls for such treatment for all "fish-beanng streams"
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Irrespective of the designation, only systems capable of reliably removing dissolved metals
should be allowed This precludes the following stand-alone systems listed in the King County
manual biofiltration swales, filter strips, and wet vaults The restriction also prevents the use of
several proprietary, prefabricated devices as stand-alone systems Stormceptor, Vortechs, V2B1,
' Downstream Defender, CDS, and BaySaver These systems can be used as pretreatment units
The City should consider the inclusion of two proprietary, prefabricated devices StormFilter and
' StormTreat Both are capable of removing dissolved metals However, performance data are
very limited on both systems Neither have sufficient data to meet the Puget Sound protocol20
Therefore, their inclusion could be considered provisional until the protocol is met Or they need
not be included in the manual, but allowed in a few sites on the condition that data are collected
REFERENCES
1 King County, 1998, Surface Water Design Manual
2 Ecology (Department of), 1992, Stormwater Manual for the Puget Sound Watershed
3 Booth, D et al, undated, Regionally and Locally Significant Resource Areas, King County,
provided by Louise Kulzer of King County
4 Walton, S and T Bennett, December 1994, Development of a Lake Protection Standard
Listing (Draft), King County, provided by Louise Kulzer of King County
5 Richard Chase, personal communication
6 King County, 1999, Lake Monitoring Report
7 Data provided by Richard Chase
8 Taylor Associates, 2000, City of Kent Water Quality Monitoring Program, 1999-2000
Ambient Monitoring (Draft), prepared for the City of Kent
9 Harza Engineers, 1999, 1998 Fish Habitat and Population Survey, prepared for the City of
Kent
10 Harza Engineers, 1996, Meridian Annex Area, Fish Habitat and Fish Population Survey,
prepared for the City of Kent
11 Harza Engineers, 1995, Final Report, Comprehensive Fisheries Assessment of the Mill
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Creek, Garrison Creek, and Sprmgbrook Creek System, prepared for the City of Kent
12 Ecology (Department of), August, 2000, Stormwater Management in Washington State (Final
Draft)
13 Goldberg, J et al , 1993, Dayton Avenue Swale Biofiltration Study, City of Seattle
'
14 Khan, Z , 1992, Biofiltration Swale Performance, Recommendations, and Design
Considerations, Metro Seattle
15 Koon, J et al , 1995, Evaluation of Water Quality Ponds and Swales in the Issaquah/East
1
Lake Sammamish Basins, King County
16 Comings, K, 1998, Stormwater Pollutant Removal by two Wet -Ponds in the Lake
Sammamish Watershed, Masters Thesis, U of Washington,
17 Shapiro and Associates, 1999, Lakemont Storm Water Tratment Facility Monitoring Program
— Final Report, City of Bellevue
18 Horner, R et al., 1995, Design, Construction, and Evaluation of a Sand Filter Stormwater
Treatment System, Alaska Marine Lines
19 Harper, H H , et al , 1993, Treatment Efficiencies of Detention with Filtration Systems", St
Johns River Water Management District, Florida
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20 Protocol for the Acceptance of Unapproved Stormwater Treatment Technologies for use in
the Puget Sound Watershed, Washington Chapter APWA, November, 1999
008
007
J
E 0 06
0 005
E2
c 0 04
v
u
to 003
V
y
002
c
`w 0 01
0 016
0 014
m
E 0 012
c
2 001
m
0 00E
U
0 0 DOE
U
C 0 004
N
J
.0 D M
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Figure 1 Zinc Removal
PNW data - 3 swales, 2 sand filters
0 005 01 015 02 025 03 035 04 045 05
Influent concentration (mg/L)
Total Zn times 0 5 (fitters) _ Dissolved zinc (swales)
Figure 2 Copper Removal
PNW data - 3 swales 2 fillers
W"
0 005 001 0 015 002 0 025 003 0 035 004
Influent concentration (mg/L)
Total Cu (Imes 0 5 (fliers) - Dissolved copper (swales)
w
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' 8-1- ADJUSTMENT APPLICATION FORM AND
PROCESS GUIDELINES
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REFERENCES
City of Kent Surface Water Design Manual
Chapter 5 — Kent Construction Standards 5 R-15 May, 2002
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SURFACE WATER DESIGN MANUAL
REQUIREMENTS 1 STANDARDS
ADJUSTMENT REQUEST
Project Name
File No Enginer/Planner Name
Project Address
Design Engineer
Applicant
Signature Date
Signature Date
Engineering Finn Name
Address City, State, Zip
Address City, State, Zip
INSTRUCTIONS TO APFUCANTIDESIGN ENGINEER:
1 Please be sure to Include all plans, sketches, photos, and maps which may assist in complete review and consideration of this adjustment
request Failure to provide all pertinent information may result in delayed processing or denial of your request Please submit this request and
all applicable fee
1 REFERTO SECTION 1.4 IN CHAPHRI OFTHESLWACE WATER DESIGN MANUAL FORADJUSTMENTS
1 DESCRIPTION OF ADJUSTMENT REQUEST O Standard O Complex O 6yerimental O Blanket O Pre -application
1 APPLICABLE SECTION(S) OF STANDARDS
1 JUSTIFICATION (see attachments, pages _ to
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4U I HUKILA I IUN NUNA I UKtb
DirectorlDesngnee Detemanation:
❑ Approval ❑ Conditional Approval (see below)
❑ Denial
tpproval Signed Date (F)wmenial & Blanket adiustments only)
CCf DiTKM OF APPRO,IAL
❑ See attached memo dated
Signed
Date
KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
REFERENCE 84
ADJUSTMENT PROCESS GUIDELINES
1.0 PREAPPLICATION ADJUSTMENT PROCESS
This process is used when the applicant needs an adjustment decision to determine if a
project is feasible or the results are needed to determine if a project is viable before funding
a full application. Preapplication adjustment requests will be accepted when 1) an issue is
raised or a potential constraint is identified at a preapplication conference with DDES, and
2) sufficient engineering information to evaluate the request is provided. A higher
preapplication adjustment fee will apply to these requests, and any unused adjustment fee
will be credited towards the permit application fee.
Steps in the processing of a preapplication adjustment shall include:
• A DDES preapplication conference is scheduled at which the applicant provides
justification that a decision on the adjustment will effect viability of the project. An
example could include a need to divert flows due to a downstream problem.
• King County may request additional information and site visits due to the limited data
and lack of prior project review.
• A preapplication deposit is required and fee for review will be an hourly rate billing -
applied against the deposit. Any unused fees could be returned to the applicant. Any
fees in excess of the deposit must be paid prior to the issuance of a decision.
• For approved preapplication adjustment, the applicant can apply that approval to the
applied for permit proposal provided conditions of the approval are met, the proposal has
not substantially changed and the applicable regulations have not changed. This will be
determined by DDES.
The criteria for granting a preapplication adjustment are the same as for a Standard or
Complex adjustment. However, preapplication adjustments will be tied by condition to the
project proposal resented at the preapplication meeting. The appeal process is also the same
as for a Standard adjustment or a Complex adjustment. This approval will expire 1 year
after the approval date, unless a complete permit application is submitted and accepted.
1998 Surface Water Desio Manual 9/1/98
IKING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL
1 3.0 FEE REDUCTION
1 This process is used for adjustments that are determined to meet either of the conditions A or
B identified below The DDES Director or designee shall be responsible for making the
1 determination for a fee reduction
A Minor adjustment requests that are defined as issues requiring no engineering review to
1 determine appropriateness These include
• New or revised standard specifications for engineering and construction which are
1 cited in the Manual (e.g . APWA standard specifications for public works
construction, WSDOT standard specifications),
1 • Minor design alternatives that meet the stated intent in the Manual,
• Identified errors in the Manual
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1 B Blanket Adjustments (See Reference Section 10-A Blanket Adjustments, for approved
Blanket Adjustments).
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REFERENCES -
8-J - DEDICATION CLAUSE - FINAL RECORDING
May, 2002
City of Kent Surface Water Design Manual
5 R-16 Chapter 5 — Kent Construction Standards
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DEDICATION
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Know all people by these presents that we, the undersigned owners of interest in the
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land hereby subdivided, hereby declare this plat to be the graphic representation of
the subdivision made hereby, and do hereby dedicate to the use of the public forever
all streets and avenues not shown as private hereon and dedicate the use thereof for
all public purposes not inconsistent with the use thereof for public highway
purposes. Also the right to make all necessary slopes for cuts and rills upon the lots
shown thereon in the original reasonable grading of said streets and avenues, and
further, the undersigned owners of the land hereby subdivided , waive for
themselves , their heirs and assigns and any person or entity deriving title from the
undersigned , any and all claims for damages against the City of Kent, its
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successors and assigns which may be occasioned by the establishment , construction
, or maintenance of roads and/ or drainage systems within this subdivision . Also
tract(s) " (and) " is hereby dedicated to the use of the public forever for any and
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all public purposes. This subdivision , dedication , waiver of claims and agreement
to hold harmless is made with the free consent and in accordance with the desires of
the said owners. Also the specific conditions and/or agreements that are conditions
of this plat are made a part hereto and the owners and their assigns do hereby agree
to and/or comply with all these conditions .
IN WITNESS WHEREOF we set our hands and seals.
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W A S H I N G T O N
1 CITY OF KENT
SURFACE WATER
DESIGN MANUAL
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