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RESOLUTION NO. i�L?
A RESOLUTION of the City of Kent,
Washington, adopting as City Policy the plans,
procedures and objectives of Comprehensive Sewer-
age Plan of 1980.
WHEREAS, URS, consulting engineers , have prepared a
Comprehensive Sewerage Plan for the Sewer Division of the
Department of Public Works of the City of Kent.
WHEREAS, said Comprehensive Sewerage Plan provides for
an orderly expansion and improvement of the plats and facilities
of the Sewer Division of the Department of Public Works of the
City of Kent, NOW, THEREFORE
THE CITY COUNCIL OF THE CITY OF KENT, WASHINGTON, DO
HEREBY RESOLVE AS FOLLOWS:
Section 1. The Comprehensive Sewerage Plan for the Sewer
Division of the Department of Public Works of the City of Kent
prepared and promulgated by URS, consulting engineers, 1981 be
and the same hereby is adopted as official policy of the City
of Kent.
Section 2. The City Council reserves the right to es-
tablish its own order of priority which may or may not be the same
as those suggested in the Comprehensive Sewerage Plan.
PASSED by the City Council of the City of Kent, Washing-
ton at a regular meeting this 2nd day of February, 1981.
ISABEL HOGAN, MAYO
ATTEST:
MARIE JEN E , CITY CLERK
I .
P OVED AS TO FORM: n
/i r--A 1
I hereby certify that this is a true copy of Resolution
No, c % , passed by the City Council of the City of Kent,
Washington, the 2nd day of February, 1981.
(SEAL)
Mari __ nsen, dity Clerk
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a
Comprehensive Sewerage Plan
J prepared by URS Company
REVIEW ®RAFT
I_T
URS is a multi-disciplinary profes-
sional services company consisting
of engineers, architects, planners,
economists, biologists, and other
professionals with related skills.
Its objective is to provide excel-
lence in service to meet the needs
of growing communities and the
businesses and industries in these
communities.
URS
Fourth and Vine Building
Seattle, Washington 98121
(206)623-6000
-_1
COMPREHENSIVE SEWERAGE PLAN
FOR
CITY OF KENT
WASHINGTON
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Prepared by
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URS Company
Seattle, Washington
_1 April 1980
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TABLE OF CONTENTS
Page
CONTENTS 1
TABLES
FIGURES
SUMMARY AND RECOMMENDATIONS iv
CHAPTER 1 - INTRODUCTION 1-1
Authorization 1-1
Purpose and Scope 1-1
History of Sewerage Service 1-2
Planning Area Description 1-3
Land Use 1-10
CHAPTER 2 - POPULATION AND FLOW PROJECTIONS 2-1
Population Projections 2-1
Flow Projections 2-4
CHAPTER 3 - DESIGN CRITERIA 3-1
CHAPTER 4 - EXISTING SYSTEM 4-1
Existing Facilities 4-1
Pump Stations 4-4
CHAPTER 5 - COMPREHENSIVE PLAN DEVELOPMENT 5-1
System Extension 5-1
Improvements to Existing Facilities 5-1
Financing Alternatives 5-13
Comprehensive Plan 5-15
Staffing Requirements 5-19
CHAPTER 6 - OTHER JURISDICTIONS AND PLANNING EFFORTS 6-1
REFERENCES
APPENDIX A - INFILTRATION/INFLOW ANALYSIS
APPENDIX B - COST ESTIMATES B-1
APPENDIX C - POPULATION PROJECTION CALCULATIONS C-1
APPENDIX D - SEWER CONSTRUCTION STANDARDS D-2
i
LIST OF TABLES
TABLE PAGE
1 Comprehensive Sewerage Plan vii
1-1 Major Industrial Wastewater Sources 1-13
2-2 Sewage Quantity Factors 2-5
2-3 Peak Flow Factors 2-5
3-1 Minimum Slopes 3-2
5-1 Lift Station Upgrading 5-11
5-2 Horseshoe Acres Alternative Costs 5-12
5-3 Comprehensive Sewerage Plan 5-17
5-4 Personnel Requirements 5-19
6-1 Major Contracts and Agreements 6-4
B-1 New Pipe Unit Cost B-2
B-2 Rehabilitation Program B-3
B-3 Other system improvements B-4
C-1 Population Projections C-1
ii
LIST OF FIGURES
FIGURE PAGE
1-1 Vicinity Map 1-4
1-2 Study Area 1-5
1-3 Topography and Drainage Basins 1-6
1-4 King County Agricultural Lands 1-12
2-1 Assumed Population Projections 2-3
4-1 Metro-owned Facilities 4-2
5-1 Unsewered Areas 5-2
6-1 Neighboring Sewer Service Jurisdictions 6-2
C-1 AAM District Boundaries C-2
iii
SUMMARY AND RECOMMENDATIONS
The purpose of the comprehensive sewerage plan is to allow
an orderly and cost-effective development of facilities to serve
existing and future users in the Kent sewer service area. The
plan describes the study area, projects populations and flows,
evaluates the existing system and proposes improvements to the
system. Plans are also presented to serve presently unsewered
areas.
POPULATION AND FLOW PROJECTIONS
The Kent area is currently growing very rapidly and projec-
tions indicate that the rapid growth will continue through the
decade. Projections developed by the Puget Sound Council of
Government estimate an average annual growth rate in the entire
study area of 4. 3 percent. The growth rate of the population ac-
tually served by the sewer system will likely be much as pre-
sently existing but unsewered developments connect to the system.
Sewage flow metering is generally not provided so average
. � per capita and per acre factors are presented to determine average
and peak flowrates from areas under study.
EXISTING SYSTEM
-� The Municipality of Metropolitan Seattle (Metro) has respon-
sibility for major interceptors and sewage treatment/disposal fa-
cilities for the City of Kent and therefore these are not discussed
in this report.
iv
The sewage collection and interceptor system serving
the City of Kent is in general adequately sized and in good
condition with the exception of the oldest parts of the system.
Infiltration and inflow is very high to much of the down- -
town system and the City is proceeding with a rehabilitation/
replacement program to correct this problem. A number of iso-
lated lines located throughout the system have been identified
as undersized to handle present or projected future flows.
There are presently three sewage lift stations operating
in the Kent system. The Aero-Kent station, serving the east-drain-
ing Soos Creek area, is presently undercapacity. The Horseshoe
Acres pump station presently has sufficient capacity but projected
rapid growth in the area could cause flows to exceed the capacity
within 5 years. The Linda Heights station is adequately sized to
handle the maximum projected flows from its drainage area.
Comprehensive Plan
The comprehensive plan is divided into two elements: system
extensions and improvements to existing facilities.
General schemes for sewer service to presently unsewered
areas are discussed and laid out as a part of the plan. Since
system extensions are financed solely by developers with no par-
ticipation by the City of Kent, no cost or financial information
is presented.
A number of improvements to existing facilities are proposed
which will result in an excellent system which is adequately sized
v
to accommodate considerable growth in outlying areas as well
as in the central business district. These improvements are
summarized on Table 1 .
The short-term program includes those projects which are
needed as soon as possible to provide adequate capacity for
existing flows or to reduce excessive infiltration and inflow
to the system. Financing for the short-term program should in-
clude use of cash-on-hand and sales of revenue bonds.
The intermediate-term program includes those projects
which will become necessary during the study period. These
projects should be financed by sale of either existing assets
or additional revenue bonds.
vi
TABLE 1
COMPREHENSIVE SEWERAGE PLAN
Cost
Project Date 1980 Dollars
Short-Term Program
Project No. 1 Summer-Fall 1980
Rehabilitation Projects 1-6, 26 $ 667, 000
Total $ 667, 000
Project No. 2 Fall-Winter 1980
Improvements Projects G, H, J, K $ 279, 000
Aero-Kent Interim Upgrade 12, 000
Total 291 , 000
Project No. 3 Fall-Winter 1980
Rehabilitation Projects 16-21 $ 494, 000
Improvement Projects A-F 317, 000
Total $ 811 , 000
Project No. 4 Spring 1981
Rehabilitation Projects 22-25,27 $ 837, 000
Improvement Project I 140, 000
Total $ 977, 000
Project No. 5
Rehabilitation Projects 7-15 Summer 1981 $ 480, 000
Total $ 480, 000
Total Short-Term Program Cost $3 , 226, 000
Intermediate-Term Program
Horseshoe Acres P.S. Upgrade Summer 1983 $ 84, 000
and Force Main
Telemetry System Upgrade Summer 1983 34, 000
Aero-Kent P.S. Upgrade Summer 1983 46, 000
Update Comprehensive Plan 1984-1985 30, 000
Total Intermediate-Term Program Cost $ 194, 000
* Refer to Appendix B for specific project descriptions
vii
CHAPTER 1
INTRODUCTION
This report presents the results and conclusions of studies
undertaken to update the existing comprehensive sewerage for plan
for the City of Kent, Washington. Considerable changes have oc-
curred in the Kent area since the original plan was completed in
1963 ; most notably the abandonment of local treatment of wastewater
in favor of regional treatment and disposal by the Municipality of
Metropolitan Seattle (Metro) .
These changes necessitate updating the plan to meet the
changing conditions.
AUTHORIZATION
This study was prepared under terms of an agreement between
the City of Kent and URS Company with notice-to-proceed dated
August 28, 1979.
PURPOSE AND SCOPE
The purpose of this study is to develop a comprehensive
sewerage plan that allows an orderly and cost-effective develop-
ment of facilities in the area served by the City of Kent. The
scope of work is summarized as follows:
( 1 . Review existing planning data and summarize material
relevant to sewerage planning in the study area.
1-1
2. Evaluate existing population, density, and wastewater,
flow characteristics and project these conditions into
the future.
3 . Develop design criteria on which to develop the plan. -
4. Evaluate the existing system with respect to future
needs and identify problem areas.
5. Develop and evaluate alternative plans to address
identified problems and select best overall plan.
6. Develop scheduling and financial information necessary
to implement the selected plan.
7. Meet all other requirements of the State of Washington
(WAC 173-240 ) and King County (King County Code Chapter
13.24 ) .
8. Coordinate the entire planning process closely with
City of Kent personnel.
HISTORY OF SEWERAGE SERVICE
The sewerage system of the City of Kent has been designed and
constructed in accordance with the growing needs of the City as
development occurred. As early as 1963 URS, (at that time known
as Hill & Ingman) completed an engineering report on sewage
collection, treatment, and disposal for the City of Kent. The
report resulted in recommendations which enabled the city to
continue to meet it ' s growing sewerage needs.
1-2
In July , 1967 the City turned over the treatment responsi-
bility to the Municipality of Metropolitan Seattle, and thereby
became a part of the Metro system. In 1966 the first utility
local improvement district (ULID) was established. Throughout
the late sixties and early seventies the City, as well as
Metro, constructed a number of large interceptors including the
Mill Creek Interceptor and the Garrison Creek Interceptor.
In 1977, a wastewater facilities plan (URS, 1977) was
completed which provided planning data and background material,
and which delineated the ultimate service area for City of Kent
facilities.
PLANNING AREA DESCRIPTION
The study area for this plan is. the Kent sewer service
area as franchised by King County plus additional adjacent areas
which potentially could be served by the Kent system. The area
encompasses most of the incorporated City of Kent and some
unincorporated areas of southern King County. A portion of the
City of Kent located west of Interstate 5 and provided sewer
service by the Des Moines Sewer District is not included in the
study area. The area generally coincides with the maximum feasible
service area as delineated in the Facilities Plan (URS, 1977) .
Figure 1-1 shows the location and Figure 1-2 the boundaries
of the study area. The area is bounded by Interstate 5 on the
1-3
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Fig. 1 1. Vicinity Map
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Study Area Miles
Present Kent City Limits r : Franchised Service Area North
Figure 1-2 Study Area
west (Des Moines and Lakehaven Sewer Districts ) , Cascade Sewer
District to the east, Auburn to the south and Renton and
Tukwila to the north.
Topography
The principal topographic features in the study area are the
upland plateaus rising from both sides of the Green River Valley.
The valley is about 2-1/2 miles wide with an elevation ranging
from 30 to 40 feet above sea level before it ascends to two
bordering hills. The West Hill rises quickly to about 300-400
feet with a steep slope while the East Hill rises to about 400
feet with a much more gentle slope. Because of the East Hill ' s
gentle slope, more development has occurred there.
A topographic map which shows the major drainage basins
and patterns, is presented on Figure 1-3.
Soils and Geology
The geology of Puget Sound area is primarily the result
of processes which occurred during the Vashon period of the
Fraser glaciation about 15, 000 years ago. The major portion of
the region was covered by ice several thousand feet thick. As
the ice moved, it compressed the underlying earth and carved it
into the valleys and ridges present today. In addition, the
glacier scraped away the pre-existing soil of the area. As the
ice retreated, it left behind the geologic and soil types charac-
teristic of glaciated areas. The geology of the planning area is
predominantly very dense and consolidated material called glacial
1-6
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Study Are a�
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Fig. 1 - 3. Topography and Drainage Basins
till over varying thicknesss of clay, sand, or bedrock. The till
is overlain by erosional material .
The surficial geology of the Green River Valley is composed
entirely of alluvial-sedimentary material deposited by streams
and lakes. This alluvium deposit is of recent origin, accumulat-
ing over the past 4, 500 years, and it contains numerous and
various layers of clay, silt, sand, gravel, and peat. The
Green-Duwamish Valley alluvium deposit offers some of the most
fertile soils in the Pacific Northwest.
Several fault lines exist on the slopes of the East and West
Hills, northeast, northwest, and Southwest of the Kent city
limits. These fault lines lend evidence to the instability of
the region 's slopes.
Soil composition and characteristics are very important
when planning optimum land uses for a region. Developing improper
uses for certain soil types will require expensive man-made
alterations of the natural environment in order to sustain the
desired incompatible land use. The nature of the soil determines
drainage bearing capacity, and wastewater "accommodability" .
The Kent lowland soils are generally homogeneous consisting
of primarily three nearly identically textured, alluvial soils:
the Puyallup, Puget, and Sultan soils. There are several different
types of soil within each individual soil series. The East and
West Hills, like the lowland, are basically homogeneous, but they
consist primarily of one soil series, the Alderwood series.
1-8
Generally, the soil types of the Kent lowland are unsuit-
able for urban and industrial uses. Because of the drainage
problems and low bearing capacity of valley soils, home sites,
septic tank filter fields, sewage lagoons, sanitary landfills
and heavy building sites are unsuitable land uses unless expen-
sive, man-made compensations are constructed. The Alderwood
soil of the East and West Hills are generally unsatisfactory
for septic tank filter fields, but do offer a high bearing ca-
pacity suitable for home and heavy building sites.
Most of the planning area, in the valley and on the hill-
sides, has soil that is moderately or severely limited for
septic tank/drainfield use. Development in such areas requires
sewers.
Climate
The planning area has a west coast marine climate which is
characterized by mild, wet winters and cool, relatively dry sum-
mers. Nearly all precipitation occurs as rainfall, although
snow does occur each year. Approximately 75 percent of the rain-
fall occurs between October 1 and April 1 , with a mean annual
precipitation of 34 . 1 inches in Kent.
Temperatures are moderated by the proximity of Puget Sound.
The mean annual temperature in Kent is 51 . 7 degrees F. while the
maximum and minimum recorded readings are 100 degrees and -5 de-
grees respectively.
1-9
The prevailing winds in the region are from the south in
the fall and winter, gradually shifting to the north in late
spring and summer.
LAND USE
Existing land use in the planning area includes a mix
of residential, commercial and light industrial uses. Most
recent residential growth has occurred in the East Hill and
West Hill areas. The South Valley area is primarily commercial
with some residential development. Industrial development is
concentrated in the North Valley.
Historically, the Green River Valley has been used for
agricultural purposes because of its excellent soil. However,
because of its location near Seattle-Tacoma, major freeways,
the Sea-Tac Airport, and two railway lines, recent decisions to
locate new freeways to improve accessibility, new utilities,
and flood control devices constructed in the Valley, it has
been transformed into an industrial area with residential areas
lining the hillsides. Increasing property taxes force many
farmers to sell their lands to speculators when farming was no
longer profitable. Much of the undeveloped land is owned by
speculators who lease land to farmers until it is ready for
development to a more intensive land use. Most of the valley
floor is zoned for industrial use now. The trend for the
valley to become more urbanized will likely continue.
1-10
A recent King County election (November, 1979) authorized
the sale of bonds for the purpose of buying development rights to
designated agricultural lands within the county to maintain them
as open land. Sales of these rights by property owners is
voluntary. Designated land within the study area is shown on
Figure 1-4. The effect of this decision on the three parcels
within the study area is presently unpredictable.
1
County policies do, however, address these designated
agricultural areas as follows:
o "Sewer and water local improvement district assessments
on agricultural land are frequently detrimental to the
operation of farms in King County. "
o In order to preserve agricultural lands, sprawled
development should be avoided and the urban center
development concept should be adopted.
o "King County shall approve those connections to sewer
interceptors reviewed by the County only when such
action shall not adversely affect the agricultural
potential of the District" .
o "King County shall not approve rezone applications for
more intensive use classifications for any of the
Agricultural Lands of County Significance" as designated.
o Boundaries surrounding designated "Agricultural Land of
County Significance" can be revised when the County
1-11
Council finds that the continued economic viability of
farm operations conducted in these subareas changes.
o Agriculture lands designated shall not be divided into
parcels of less than 10 acres.
The major industries which contribute wastewater to the
sewage system are listed in Table 1-1 along with average discharge
flows.
TABLE 1-1
MAJOR INDUSTRIAL WASTEWATER SOURCES
Average Flow
(mgd)
National Can . 25 - . 50
Flow Research . 25 - . 50
Heath Plating . 18 - .25
Boeing Aerospace Company . 18
Heath Techna . 07 - . 12
Borden Chemical . 05
Davis Walker . 05
Northwest Metal Products . 05
J
Reference: City of Kent records.
J
J
1 -13
CHAPTER 2
POPULATION AND FLOW PROJECTIONS
This chapter will present estimates of future sewage flows
within and from the Kent area upon which the existing facilities
are evaluated and new facilities are designed. The estimates of
future flows are based on projected land use and population as
discussed below.
POPULATION PROJECTIONS
Short-Term Growth
Estimates of future study area population presented in this
report are based on projections developed by the Puget Sound
Council of Governments (PSCOG, 1979) with input and review
provided by the Kent Planning Department. The Activity Allocation
Model (AAM) used by PSCOG presents projections based on "districts"
which approximately coincide with census tracts. Since the
boundaries of the AAM districts do not exactly coincide with the
study area boundary, the amount of projected growth for each AAM
district which will occur within the study area must also be
estimated.
By assuming uniform growth throughout each AAM district,
projected populations were allocated to the study area and to the
subaras within the study area. Projected populations for years
1980 through 2000 are shown on Figure 2-1 . Appendix C includes
additional details of the forecast on a district by district basis.
2-1
The future population actually served by the sewerage system
depends not only on area population growth, but also on the rate
with which service is provided to existing unsewered areas.
These projections are therefore very uncertain. An analysis of
the existing service area indicates that about 50 percent of
the total service area population is currently served. Assuming
that this proportion will increase to 80 percent by 1990 and 90
percent by 2000, the population actually served by the sewer
system can be estimated as shown on Figure 2-1 .
Saturation Population
The design of all but the largest sewerage facilities is
usually based on the saturation, or ultimate population of the
area which drains to the facility under design. The saturation
population is that which would occur if the area were fully
developed and no further growth could be accommodated. Although
this condition may never actually exist, the concept does provide
a conservative design basis.
Estimation of saturation population is based on existing
land use zoning regulations which specify the maximum development
density in a given area. If not otherwise specified, the
maximum single-family housing density was assumed to be 4
units/acre.
Based on data developed by the City of Kent (City of Kent,
1977) , single family residential' areas are assumed to average
3. 2 residents per unit. Multi-family areas are assumed to
average 2. 0 residents per unit.
2-2
80,000
Total
Study Area Actual
Year Population Population Served
1980 39,300 19,700
1990 60,100 48,300
70,000 2000 73,500 66,100
60,000
oc
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a off®
o Q,
50,000 J/
C9
� o�'j o f
® QJ
QO
40,000
G�P
30,000
f 20,000
_1
10,000
1975 1980 1985 1990 1995 2000
IYear
Fig. 2 - 1. Assumed Population Projections
J
The above discussion provides the information necessary to
determine the design population of any subarea within the study
area based on land use zoning regulations.
The saturation population of the entire study area is esti-
mated at about 130, 000 people.
FLOW PROJECTIONS
Sewage flow metering generally is not provided in the Kent
system and therefore accurate values for total or percapita flow
are not available. Estimates of total sewage flow based on
metered potable water use, are made, however, for the purpose of
payment to Metro for providing treatment. For the one year
period from July 1978 through June 1979, the estimated average
sewage flow was 2. 6 mgd.
More detailed estimates could be made based on a detailed
accounting of water use. Since most of the design problems are
based on conditions well into the future, the value of such a
complex accounting procedure would be limited because of the
large variability of water use patterns and the possibility that
future conditions will be considerably different than the present.
2-4
It is therefore concluded that average production values can
be used rather than values specific to Kent. During the actual
design of collection facilities, a more detailed analysis of
possible sewage flows within the tributary area should be made.
Factors used to determine design flows from given areas
are presented in Tables 2-2 and 2-3. Table 2-2 shows the factors
used for average volume of flow from residential, commercial and
industrial areas. Table 2-3 shows peaking factors (ratio of
peak instantaneous flow to average flow) and infiltration/inflow
rates to be used for design.
TABLE 2-2
SEWAGE QUANTITY FACTORS
Residential 60 gpcd*
Light Industrial 2, 000 gpad*
Heavy Industrial 4, 000 gpad
Light Commercial 3,000 gpad
Heavy Commercial 7, 000 gpad
* gpcd - gallons per capita per day
gpad - gallons per acre per day
_ 1
TABLE 2-3
PEAK FLOW FACTORS
Peaking Factors
100 Ac 1000 Ac 5000 Ac 10000 Ac
Residential 4. 0 3. 0 2. 2 1 . 9
Light Industrial 3. 2 2. 7 2. 0 ___
Heavy Industrial 2. 5 2. 2 1 . 7
Commercial 1 . 75 1 . 75 --- ---
Peak Infiltration 600 gpad
Peak Inflow 500 gpad
2-5
CHAPTER 3
DESIGN CRITERIA
The purpose of this chapter is to present the design criteria
upon which the performance of the existing system is evaluated
and upon which the planning of new facilities is based. Criteria
specified by the State of Washington (Department of Ecology,
1978) are adopted where applicable. Specific construction
standards are presented in Appendix D.
DESIGN PERIOD
Proposed plans for collection sewers are based on facilities
and capacities needed for the saturation development of the
tributary area. Larger interceptor and trunk lines and pumping
stations are designed based on the economic life of the facility
with consideration given to phased construction.
DESIGN OF SEWERAGE FACILITIES
The recommendations contained herein are for preliminary
design of interceptors, trunk sewers, force mains, inverted
siphons and pumping stations. Lateral sanitary sewer systems
are not, in general, considered unless they affect the major
facilities.
Trunk Sewers and Interceptors
Trunk and interceptor sewers must be designed with sufficient
capacity to carry the peak flow from the ultimate development of
3-1
the tributary area. This flow represents the sum of the several
loadings calculated separately, for each section of sewer or
tributary area. The loadings consist of peak flow of sanitary
sewage, peak flow of industrial waste, groundwater infiltration,
and any special quantities that must be considered. Excess storm -
flows would be bypassed through existing outfalls where possible.
The ability of a sewer to transport suspended solids con-
tained in sewage is related to the velocity of flow in the sewer.
A velocity of 2 feet per second is generally considered to be the
minimum which will keep pipe surfaces clean and free of deposited
material. Table 3-1 gives the minimum allowable slope for
various sizes of sewers to obtain a cleansing velocity under
average flow conditions.
TABLE 3-1
MINIMUM SLOPES FOR VARIOUS SIZED SEWER PIPE
NECESSARY TO MAINTAIN A CLEANSING VELOCITY
Pipe Size
In Inches Slope
8" 0.40%
10" 0.28%
12" 0.22%
15" 0. 15%
18" 0. 12%
21 " 0. 10%
24" 0. 08%
27" 0. 07%
30" 0. 06%
3-2
Pumping Stations
Wastewater pumping stations are generally constructed
underground, either as factory assembled package units or special
design models. Capacities of permanent pumping stations are
based on the peak flow of all sewers tributary to the individual
station. Pump stations are frequently designed to allow for
staged increases in pumping capacity by the installation of
additional units.
Pumps are usually driven by electric motors, are of a
non-clog design, and the number of units is sufficient to pump
the design peak flow with any one unit out of service. The pump
stations may be designed as a constant speed or a variable speed
drive. A constant speed pump station does not run continuously,
pumping only as necessary to convey the inflow to the station
while a variable speed will run almost continuously at a varied
pumping rate consistent with the rate of inflow.
In the past, stations were provided with overflow mechanisms
to discharge sewage in the event of a power or mechanical failure.
As the discharging of raw sewage is no longer allowed in an
effort to further enhance the environment, the problems caused by
electrical and mechanical failures have been eliminated. Mechanical
failures are eliminated by providing a duplication of pumping
capabilities in each pump station. Electrical power failures are
controlled by providing electrical power plants with failure
alarm systems. The implementation of these safeguards allows for
the deletion of overflow capabilities.
3-3
areas. Certain lateral and interceptor sewers will also have
to be located on easements following natural drainage courses.
Pumping stations will be placed so as to allow each access
for periodic maintenance. Suitable areas are existing street
rights-of-way or purchased sites abutting rights-of-way with
guaranteed access. Consideration must also be given to the
availability of an adequate power supply.
The location of the sewer lines in relation to other utili-
ties must also be considered. There may be some conflict in
final sewer locations due to interference with water mains,
drains, and electrical conduits. In most cases, however, sewer
lines would pass beneath the other utilities, especially in the
case of water mains, where it is desirable to have the sanitary
sewer a minimum of three feet below the water main.
3-4
CHAPTER 4
EXISTING SYSTEM
The purpose of this chapter is to describe the existing
sewerage system serving Kent and to identify problems associated
with the system. The enclosed comprehensive plan maps show
the existing system in detail along with proposed improvements
which are discussed in Chapter 5.
TREATMENT AND DISPOSAL
The treatment and disposal of wastewater will not be discussed
in detail in this report. Wastewater flows from the Kent area
are routed to a series of interceptors owned by the Municipality
of Metropolitan Seattle (Metro) which transport the waste north
to Metro's Renton treatment facilities. Final disposal, after
secondary treatment, is to the Duwamish River. Metro facilities
within Kent are shown on Figure 4-1 .
COLLECTION SYSTEM
The sewage collection and interceptor system serving
the City of Kent is in general adequately sized and in good
condition with the exception of the oldest parts of the system.
The general flow pattern of the system is from the high
elevation hill areas toward the Metro interceptors in the
J
4-1
center of the valley and from the south end of the system down
the valley to the north.
The West Hill area is mostly served by gravity and is
all tributary to a 24" line crossing the Green River on Kent-Des
Moines Road. One small area near Interstate 5 between S . 240th
and S . 252nd streets is tributary to the Linda Heights pump
station which pumps the waste into the gravity system.
The Soos Creek area naturally drains to the southeast into
the Soos Creek drainage basin. The proposed Metro Soos Creek
Interceptor was intended to carry these flows to the Auburn
Interceptor although construction of this line is uncertain. The
area is presently drained to the Del-Webb pump station which
pumps the wastewater through a 12" force main along Kent-Kangley
Road into the main gravity system.
The East Hill area is all served by gravity. Most of the
area, as well as flows from the Soos Creek area, flow westward
into large mains in the downtown area. The northern part of the
East Hill system drains into the North Valley system through a
main along S. 218th Street.
The South Valley system generally drains by gravity into
the Auburn Interceptor. One low elevation area (Horseshoe Acres )
i
is served by a pump station.
IThe North Valley system, which includes mostly industrial
�.J land use, is served by a network of large, lateral lines draining
to the Metro interceptor system in the center of the valley.
4-3
The sewage collection and interceptor system is gener-
ally in good condition. Most of the system has been planned
and constructed within the last 15 years and was designed based
on projected ultimate development conditions and according to
design specifications which require construction of a quality
system.
The major problem area is in the downtown area which
contains some of the oldest sections of the system. Studies
(see Appendix A) have indicated that infiltration and inflow to
these sections is high. In order to address this problem, the
City is proceeding with a rehabilitation/replacement program
which has thus far replaced several thousand feet of older
sewer line. Selection of lines to be replaced was based on a
television survey of the entire downtown area. About 60
percent of the project has been completed on the program to
date with additional expenditures of $2 . 5 million expected to
complete the lines.
A number of other existing lines have been identified by
the City Engineer as undersized and increased capacity at these
locations is needed in the short term. These problem areas are
shown on the enclosed plan maps and are listed in Appendix B.
PUMP STATIONS
The use of sewage pump stations in the Kent sewerage
system has been generally discouraged by the City due to the
relatively high cost of construction, operating and maintaining
4-4
the facilities. In order to connect to the sewerage system
using a pump station, the city requires that the station serve
a minimum of 250 single-family residences or their revenue
equivalent. No privately owned pump stations are allowed.
There are presently three sewage pump stations operating in
the Kent system.
1 . Horseshoe Acres P.S. - This station serves the Horse-
shoe Acres area in south Kent along the Green River.
The station includes two-650 gpm pumps which were de-
signed to be increased to 1000 gpm capacity. The wet
well and force main were also designed for a total flow
2000 gpm. Loss of power to the station will result
in the overflow of sewage to the gravity system at
the corner of 83rd Avenue South and South 259th Street.
Both residential and commercial growth is occurring
very rapidly. The area that is within the city limits
is expected to be fully developed within the next five
years (J. Harris, Kent Planning Dept. , 1980) with an
estimated peak flow of 940 gpm. Since this exceeds
the present single-pump capacity .of the station, an
upgrading of the station may be required in the near
future. Chapter 5 presents a gravity system alternative
to expanding the present pump system.
2. Linda Heights P.S. - This serves about 300 residences
in the West Hill area. The station consists of
two-400 gpm pumps which can be operated with on-site
diesel power if required. This station is presently
adequately sized for full development of its tributary
area.
3. Aero-Kent P.S. - The Aero-Kent station is located in
the far southeast corner of the service area and serves
that portion of the area which is within the Soos Creek
watershed draining to the east. Two 450 gpm pumps pre-
sently are included which can be modified to 1000 gpm
capacity. The wet well and force main are both sized
to handle a maximum flow of the 2000 gpm. Current
peak inflows to the station equal or exceed the
single-pump capacity and therefore, any additional
growth within the tributary area (presently about 700
connections ) will make upgrading the station necessary.
The upgraded station will be adequate to handle about
1500 connections. When this level of development is
reached, a new pump station will be required if
service to the Kent system is to continue.
4-5
All of the pump stations are equipped with a radio-telemetry
alarm system monitored 24-hours per day. The city owns a mobile
generator capable of operating the two stations which do not have
standby power. First priority is given to the Aero-Kent station
which has no gravity overflow.
4-6
CHAPTER 5
COMPREHENSIVE PLAN DEVELOPMENT
The previous chapters of this report have been concerned
with the presentation and evaluation of information to be used
for planning the orderly development and maintenance of sewerage
facilities for the City of Kent service area.
The purpose of this chapter is to present an analysis of
future needs and required facilities along with a plan of construc-
tion including scheduling, cost estimates, and a plan for financing
these projects.
Required facilities can be classified as either exten-
sions of existing facilities to serve presently unsewered areas
or replacement and improvement of existing facilities to handle
future requirements. These facilities will be discussed separately .
SYSTEM EXTENSION
Figure 5-1 shows the presently unsewered areas with the
study area boundaries. The unsewered area has been divided into
subareas for purposes of the following discussion. In general,
the unsewered areas are subject to moderate to severe limitations
on septic tank suitability (USDA, 1973 ) and therefore sewers will
generally be required prior to extensive development.
Jpatterns are basically Drainage p Y established and the basic
Jinterceptor network has been constructed to receive flows from
5-1
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Study Area Boundary
Presently Sewered Area Miles
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/4 0
Fig. 5 - 1. Unsewered Areas
these areas when development occurs. Most of the areas are
located along the outer fringes of the natural drainage basins
tributary to the Green River Valley, and can be served by
extending the existing gravity system.
Following is a discussion of each of the subareas shown
on Figure 5-1 and the facilities required to serve these
areas. Referenced facilities are shown on the enclosed
Comprehensive Plan maps. In some areas, only interceptor
sewers and trunk sewers are actually shown on the maps.
These facilities provide the basic framework or foundation
for the provision of sewer service to these areas within the
City' s approved service area.
Area I
Area I includes approximately 400 acres of potential
industrial and 120 acres of residential-agricultural land in
the northeast corner of the study area. The area is generally
undeveloped and, until development is proposed, definite
location for interior lines cannot be established.
The Cascade Sewer District has recently constructed the
Springbrook Trunk line which runs through this area. Negotiation
of an agreement is currently underway which would allow the
City of Kent to connect to this line. The portion of Area I
north of S. 192nd Street and south of S . 188th Street, and the
area east of the Valley Freeway, could be served directly by
ithis line. Alternatively, the existing 15" line crossing 84th
�J
5-3
Avenue S. at 192nd Street could be extended into this area
to bypass the Cascade S.D. line. Since no agreement has
yet been made with respect to compensation for use of the
Cascade S .D. line, no comparative cost analysis can be done
at this time to evaluate these alternatives.
The part of Area I north of S . 188th Street can be served
to to the north to the existing 36" Metro line on S . 180th
Street.
The southern part of Area I is tributary to several
existing lines along East Valley Road particularly an 18"
crossing on S. 206th Street.
Area II
Area II includes approximately 920 acres of residential
land, much of which has been developed. This area is generally
tributary to the Garrison Creek Interceptor which runs through
the area.
A proposed collection system has been laid out for this
area which will provide service for the existing development.
Under the proposed system, Area II can be served by 8" - 10"
lines.
Area III
Area III consists of approximately 300 acres of relatively
undeveloped residential land. This area is tributary to the
5-4
Garrison Creek Interceptor and to the 100th Avenue South Inter-
ceptor. Sufficient routes are available to serve this area
using 8" lines.
Area IV
Area IV is located in the southern part of the East Hills
and drains naturally south. Since much of the area is not yet
developed, specific collection system locations are not estab-
lished at this time. The part of the area west of 104th Avenue
S.E. would generally drain to the west to the existing system.
The remainder of the area would drain to the south to the Mill
Creek Interceptor.
Area V
Area V includes about 1 ,200 acres of potential residential
land located in the southeastern corner of the study area. The
total area is naturally tributary to the Soos Creek drainage
basin to the east and Metro has proposed a Soos Creek Interceptor
to collect these east draining flows. However, at this time it
appears that this interceptor will not be constructed soon. In
the meantime, development in the area can be served by the
present system of pumping from the Aero-Kent pump station through
Jan existing force main to the Kent system.
Upgrading of this pump station to a larger capacity will be
required before additional significant development occurs in the
area as discussed in the previous chapter.
�.l
5-5
Area VI
Area VI consists of about 720 acres of residential land
which is tributary to the north. The collection system for
this area will consist primarily of 8" lines with some 10" -
15" in the area approaching the beginning of the Mill Creek
interceptor. Two inverted siphons are proposed to cross the
upper part of the Mill Creek Canyon ( 104th and 108th Avenue
S.E. ) .
Area VII
Area VII consists of approximately 2000 acres in the
Valley floor from the Green River south to S. 277th Street. Of
this area, approximately 700 acres are within the city limits
of Kent. This total area has very poor drainage for the most
part. Most of the area outside the City is zoned for agricul-
tural purposes, and may remain in this classification, although
some development has occurred. If development continues, a
system of serving the area has been developed which will not
require pumping. The proposed system has been predicated on
the assumption that if development occurs, substantial fill
will be necessary in the very low areas to obtain surface
drainage and thus would provide sufficient cover over the
piping system. Because of the minimum slopes which must be
maintained, slow development with low flows could result in the
necessity for frequent cleaning until adequate flows are
accumulated.
5-6
The part of the area north of the Green River in the
Horseshoe Acres area is tributary to the existing pump station
and can be served from that point.
Area VIII
Area VIII covers approximately 225 acres including the
surface area of Lake Fenwick. Much of the area surrounding the
lake is too steep to permit building. Approximately 100 acres
could possibly be developed if sewer service were provided.
The natural drainage pattern is southeasterly from the south
end of the Lake. If the valley floor to the east and south
develops, the valley system would provide a route to the Auburn
Interceptor. If this does not occur, the most economical
method of discharge would be through a tight line to the
Auburn Interceptor. A six inch line would be adequate to serve
the area. A careful analysis will be needed to determine if
potential customers would provide adequate flow to maintain
minimum velocities.
Area IX
Area IX represents the Star Lake area in the southwestern
corner of the study area. Most of this area is within the
Lakehaven Sewer District and the District 's Comprehensive Sewer
Most of this area is within the Lakehaven Sewer District and
the District 's Commprehensive Sewer Plan ( 1977) does include
future service for the area. At the request of D.O.E. , the
Jarea was considered as a possible addition to the Kent service
area in the "201 " Facilities Plan (URS, 1977) . Service to the
J
5-7
J
Lakehaven system would require pumping while gravity ser-
vice eastward to the Kent system can be provided.
The Facilities Plan concluded, on the basis of total
dollars, that the least cost alternative for this area is
construction of a tight line leading eastward to the Auburn
Interceptor along S. 277th Street. A gravity line to the
Auburn Interceptor was also considered which cold be cost
effective if significant development occurs on the Valley
floor.
Area X
Area X is located on the west hill north of Kent-Des
Moines Road and extends east to the Green River. The major
feature of the area is the City of Seatle sanitary landfill
which has removed much of the area from potential development
in the near future. In addition, much of the remaining land is
very steep and probably unsuitable for development.
Approximately 70 acres of residential land located uphill
from the landfill along Military Road are presently unsewered.
This area could be served by an 8" lin connecting to an existing
line on Military Road. One section of this line would require
about an 800 foot section of inverted siphon to cross a dip in
the roadway. This may be avoided if easements can be acquired
to contour around the low elevation area.
The low elevation area between Kent-Des Moines Road and
the river in the south part of the area could be served by
5-8
gravity with a line along Frazer Road which crossed the river
and leads north along 53rd Ave. South to join an existing 24"
line which runs north from S. 228th Street.
Area XI
Area XI is located in the northwestern corner of the
study area and covers approximately 1 ,200 acres. Much of the
land in this area is unsuitable for development because of
steep slopes. The lower portion along the river is primarily
agricultural land and may be preserved as such.
The northern part of the area could be served by a collec-
tor line along Frager Road, on the west side of the river.
Further studies are planned to evaluate a number of
alternative plans to handle flows in this collector. These
alternatives include:
o Inverted siphon river crossing at S. 200th Street and
gravity line to 68th Avenue South.
_j
o Gravity service to the City of Tukwilla system.
J
o Gravity or pumped transmission to a proposed Metro
Jriver crossing at S. 212th Street.
JThe upland portion of Area XI could be served by lines
along Orillia Road and S. 212th Street. The proposed Metro
river crossing at S . 212th Street would be required.
5-9
I
IMPROVEMENTS TO EXISTING FACILITIES
This section discussed projects to upgrade and improve
existing facilities to more effectively serve existing areas
and to provide additional capacity to serve new service areas. _
The major project in this category is the remainder of
the downtown sewer rehabilitation project which is currently
underway. In general , lines subject to excessive infiltration
are being replaced with new equalized lines. The remaining
cost of the downtown projects (listed in Appendix B, Table B-2)
is $2. 5 million ( 1980 dollars ) .
As discussed in Chapter 4, several undersized lines
have been identified which will require replacement or parallel-
ing soon. These lines are shown on the enclosed comprehensive
plan maps and are listed in Appendix B. It is assumed that
undersized lines would be replaced with larger lines rather
than paralleled to avoid the high costs of additional manholes.
Detailed engineering studies should be performed at the design
phase to determine the optimum method of increasing capacity
and to confirm the choice of line size based on tributary area
conditions. The estimated cost of increasing the capacity in
these lines is $736, 000 ( 1980 dollars ) .
One of the major maintenance problem areas in the system
is the 18 inch sewer crossing the Milwaukee Railroad line
on James Street. Vibrations have caused frequent shearing of
connections and pipe joint problems. Rather than replace this
5-10
line, as has been done previously, an alternate route is
proposed which connects to Metro 's 27 inch line on N. 4th
Avenue.
In an additional area, on Kent-Kangley Road near Kent-
Meri-dian High School, modification to the existing system drainage
pattern should be considered to minimize improvement costs. An
existing 1700-foot section of 8" line is inadequate to handle
present and projected future flows. Rather than replacing the
entire line, most of the flow could be diverted through a 500
foot-12" line to the Mill Creek Interceptor on 97th Place
South. The remainder of the line could then serve the local
area.
The previous chapter indicated that the capacity of two of
the City' s sewage lift stations will be inadequate in the near
future. Table 5-1 Shows estimated cost for upgrading the stations:
TABLE 5-1
LIFT STATION UPGRADING
STATION CAPACITY-GPM COST
Existing New ( $1980 )
Horseshoe Acres 650 1300 13, 000
Aero-Kent 450 1000 46,000
-� Aero-Kent (Interim) 450 700 12, 000
JThe cost given for expansion of the Aero-Kent pump station
to 1000 gpm includes addition of an emergency generator unit
since the larger motor ( 40 hp) could not operate on the mobile
generator used by the City during emergencies. An upgrade to 700
5-1 1
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gpm capacity could be accomplished using a motor small enough ( 20
hp) to still operate on the mobile generator. This upgrade would
offer a low cost, interim way to provide additional capacity for —
several years. Due to the capability of the Horseshoe Acres
station to overflow some influent to the sewer system by gravity,
no generator was included in the cost.
Increasing the pump station capacity at Horseshoe Acres will
also require that the capacity of over 4000 feet of gravity pipe
downstream of the force main be increased to handle the increased
Pump flows as well as peak flows from the surrounding area. The
cost of these new pipes is estimated at $249, 000.
An alternative system was also laid out which includes
extending the existing 10-inch force main westward along South
258th Street emptying directly into the Auburn Interceptor.
Diverting these flows away from the gravity system would eliminate
the need to increase the capacity of the system as discussed above.
Cost estimates for these alternatives shown below indicate
that the cost of the force main extension is significantly less
than the gravity system.
TABLE 5-2
HORSESHOE ACRES ALTERNATIVE COSTS ( 1980 DOLLARS)
Annualized
Alternative Capital Cost Capital Cost*
Pump to Gravity System** 265, 000 $31 , 000/yr.
Pump to Auburn Interceptor*** 84, 000 10, 000/yr.
*Computed at 10% interest for 20 year period.
**Includes new lines required downstream.
***Includes extension of force main.
5-12
FINANCING ALTERNATIVES
One of the major goals of the comprehensive planning
process is the development of a financial plan to implement the
recommended sewerage facilities plan. A number of methods are
possible available to finance sewer improvements including sale
of existing assets, bonds, grants, taxes, and other charges.
These methods will be briefly discussed below.
Developer Extension
In general, developers are required to finance mains
connecting their developments with the existing sewer system.
After construction and approval, ownership of the mains is
tranferred to the City. If the developer is required to
oversize a line to account for future development, late-comers
charges may be allowed by the City over a limited time period
to help reimburse the extra cost. The City of Kent itself will
not provide reimbursement or financing to developers.
City council and King County approval must be received
prior to sewer extension to areas outside of the Kent city
limits.
Sale of Assets
The conversion of capital assets no longer useful to the
City into cash for construction of required facilities is
probably the best method of financing available for some
projects because no burden is placed on the taxpayer or property
5-13
owner. The sale of the old sewage treatment plant site located
south of S. 212th Street and West of 68th Avenue S. is one such
source of funds. Although a detailed assessment of this
property has not been performed recently, the 68-acre plot is
probably worth in excess of $3 million. Sale of the property,
either to another city department or to an external interest,
would require approval of the City Council.
Bond Sales
The sale or revenue, general obligation, and special
assessment bonds is a common method of financing sewage improve-
ments. Revenue bonds, which are repaid from operating revenues
and utility local improvement district (ULD) property assessments,
are particularly useful for financing improvements benefiting
specific areas within the jurisdiction. An approval vote is
not required for revenue bonds.
General obligation bonds are often used to finance improve-
ments which benefit a jurisdiction as a whole since taxes from
the entire jurisdiction are usually used to repay the bonds. A
vote of the people is generally required to issue general
obligation bonds.
State and Federal Grants
A variety of grant programs have been available in the
past to fund construction of sewer facilities although the
current availability of grant funds is limited. Public Law
92-500 funds, administered by EPA, provide up to 75 percent
5-14
of the construction costs of certain sewerage projects although
most of the projects recommended in this plan would not be
grant eligible. In addition, these funds have been essentially
"tied up" for several years by the state prioritization of
projects. Associated with these EPA grants is the Washington
State Referendum 26 program which provides up to 15% matching
money for EPA funded projects. The possibility does exist
that, in the future, some of these funds , will be released to
assist with non-federally funded projects.
Other federal agencies have grant programs to provide
assistance for sewer construction projects meeting various
requirements. These agencies, including the Economic Development
Administration (EDA) , Housing and Urban Development (HUD) and
the Farmers Home Administration (FHA) , should be monitored for
new programs and funding opportunities.
COMPREHENSIVE PLAN
This section will discuss the recommended plan for completing
the improvements previously recommended and shown on the enclosed
comprehensive plan maps.
Projects included in the proposed plan can be classified as
either short-term or intermediate-term improvements. This classi-
fication is shown on Table 5-3.
i
I
JShort-Term Program
The short term program includes those projects which are
P needed as soon as possible to provide adequate capacity for
J
J 5-15
TABLE 5-3
COMPREHENSIVE SEWERAGE PLAN
Cost
Project Date 1980 Dollars
Short-Term Program
Project No. 1 Summer-Fall 1980
Rehabilitation Projects 1-6, 26,28 $ 667,000
Total _ 667, 000
Project No. 2 Fall-Winter 1980
Improvements Projects G, H, J,K $ 279, 000
Aero-Kent Interim Upgrade 12, 000
Total 291 , 000
Project No. 3 Fall-Winter 1980
Rehabilitation Projects 16-21 $ 494, 000
Improvement Projects A-F 317,000
Total 811 , 000
Project No. 4 Spring 1981
Rehabilitation Projects 22-25,27 $ 837, 000
Improvement Project I 140,000
Total $ 977, 000
Project No. 5
Rehabilitation Projects 7-15 $ 480, 000
Total T 4 00, 000
Total Short-Term Program Cost $3, 226, 000
Intermediate-Term Program
Horseshoe Acres P.S. Upgrade Summer 1983 $ 84, 000
and Force Main
Telemetry System Upgrade Summer 1983 34, 000
Aero-Kent P.S. Upgrade Summer 1983 46, 000
Update Comprehensive Plan 1984-1985 30, 000
Total Intermediate-Term Program Cost $ 194, 000
*Refer to Appendix B for specific project descriptions.
5-16
existing flows or to reduce excessive infiltration and inflow to
the system. These projects include the remainder of the rehabili-
tation program and the capacity upgrade projects discussed to the
system. Completion of this program will result in an excellent
system which is adequately sized to accommodate considerable
growth in outlying areas as well as in the central business
district.
The schedule shown on Table 5-3 is based on completing the
short-term program as soon as possible. The schedule is
accelerated due to the rapid rate of economic inflation currently
occurring and which is likely to continue to some degree. The
implementation of these projects as scheduled is very important
and a significant cost penalty will be paid if delays occur.
Due to the time frame for the short-term program, financing
options which can be implemented quickly should be adopted.
Sufficient cash-on-hand is available to finance Project No. 1 .
Subsequent projects should be financed with revenue bonds. The
bonds should be sold as soon as possible and the money reinvested
until needed. These bonds can be repaid with revenues from
operations and with proceeds from the future sale of the old
sewage treatment plant site.
Intermediate-Term Program
The intermediate-term program includes those projects which
Jwill become necessary during the study period. The schedule shown
on Table 5-3 is only a rough indication of when the improvements
5-17
will be needed and the actual need and availability of funds
should be evaluated at that time. However, the updating of the _
comprehensive sewerage plan should not be delayed beyond 1984.
System Extension -
The previous discussion in this chapter indicated that
the entire study area can be served by the Kent sewerage system.
The Star Lake area (Area IX) is presently not within the Kent
service area although gravity service of this area by Kent should
be given careful consideration prior to further development of
sewer facilities. Service by the City of Tukwila for parts of
Area XI should also be considered before development.
System extensions to serve presently unsewered areas
should continue to be financed by the developers without City
participation. These projects will be constructed when required
and therefore, no construction schedule is provided.
All developer construction must be performed according to
the standards presented in Appendix D and other city and county
requirements. Approval of any extension by the City Council
and by King County must be received before construction.
As noted previously, the Comprehensive Plan maps in some
areas show only interceptor and trunk sewers. These facilities
provide the basic framework or foundation for the provision of
sewer service to those areas which are within the City 's present
or future approved service area.
5-18
STAFFING REQUIREMENTS
The purpose of this section is to estimate future per-
sonnel requirements for the operation and maintenance of the
Kent sewer system. These estimates should be used in the fu-
ture, along with actual operating data, as an aid in determining
when additional staff is required.
The estimates presented below were drawn from an EPA study
(EPA, 1973) which surveyed personnel needs in 54 cities of less
than 150, 000 population throughout the United States. The ac-
tivities considered in these estimates include supervision and
administration, maintenance of sewer lines and appurtenances,
maintenance of lift stations, and inspection of newly constructed
sewer main lines and appurtenances. The estimates are shown
on Table 5-4 based on population projections presented previously.
TABLE 5-4
PERSONNEL REQUIREMENTS
Projected Estimated
Population Personnel
Year Service Requirement
1980 19, 700 12
1985 34 , 000 16
1990 48, 300 20
1995 58, 000 23
2000 66, 100 26
�1
5-19
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CHAPTER 6
OTHER JURISDICTIONS AND PLANNING EFFORTS
The purpose of this chapter is to discuss the effect of
the City of Kent Comprehensive Sewerage Plan on neighboring
and overlapping jurisdictions and its effect on other planning
efforts.
Figure 6-1 shows the physical relationship of the study
area to neighboring sewer service jurisdictions. Major agree-
ments entered into by the City which concern sewer service
are listed on Table 6-1 .
City of Kent Comprehensive Plan
Throughout the development of this plan, Kent ' s Comprehen-
sive Plan and zoning regulations have been closely coordinated
with and no conflicts exist.
County General Sewerage Plan
King County has primary land use and utility planning
responsibility for unincorporated portions of the study area.
The King County Sewerage General Plan ( 1979) delineates "local
service areas" which represent the maximum extent to which
sewer service may be required. These boundaries are used by the
County in their review and approval process for sewer project
plans. Extension of sewers to serve areas beyond these boun-
daries requires amendment to the plan before County approval
can be given.
6-1
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SEWER
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Some of the future sewer lines shown in this plan do extend
into and provide service to areas not presently within the local
service area. The major areas in this category are presently
unincorporated agricultural areas west and south of the Green
River. The future use of this land is presently undecided and
the purpose of this plan is to provide an adequate method to
serve the areas if they are developed in the near future.
Appropriate approvals would be required before any such facilities
were constructed.
Municipality of Metropolitan Seattle (Metro)
Metro is the agency responsible for treatment of all
wastewater from the study area as well as other jurisdictions
in the Puget Sound area. Major transmission trunk lines
leading to treatment facilities are also financed and maintained
by Metro.
Metro is presently developing a facilities plan to address
the needs through year 2000. As a part of this study, a
number of alternative means of treatment and disposal of waste
are being considered. Planning data from the City of Kent and
other jurisdictions is being used as a basis for Metro' s
pinning, presumably including the 1979 Kent Facilities Plan
-� from which the study area for this plan was drawn. Therefore,
there should be no conflict between the two plans.
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Presented below is a list of other agencies associated with
wastewater planning, financing and operation in the area.
o Environmental Protection Agency (EPA) - the lead
federal agency responsible for financing the planning
and construction of wastewater collection and treatment
systems; reviews plans and evaluates environmental
impacts of each project.
o U.S. Army Corps of Engineers - responsible for navigable
waters; issues permits for construction in tidelands.
o Washington State Department of Ecology (DOE) - the lead
state agency responsible for environmental matters;
determines water quality criteria and effluent limitations;
administers the National Pollutant Discharge Elimination
System (NPDES) ; assists in funding of publicly owned
waste treatment systems.
-� o Washington State Department of Fisheries - responsible
for the food-fish resources of the State; in conjunction
with the Department of Game issues a hydraulics permit for
construction either in or affecting fish habitats in fresh
or saltwater areas.
o Washington State Department of Game - responsible for
wildlife throughout the State; jointly responsible with
the Department of Fisheries for issuance of hydraulic
permits.
6-5
Washington State Department of Social and Health
Services (DSHS ) - reviews and regulates engineering
designs, reports and plans for construction of new
waste treatment plants or expansions to existing
plants.
o City of Renton shares the northern boundary of the study
area and is a participant in the Metro system.
o City of Tukwila shares the northern boundary of the
study area and is a participant in the Metro system.
o City of Auburn is also part of the Metro system. The -
City forms the southern boundary of the study area and
shares with the City of Kent Metro's valley interceptor
and treatment facilities.
o Cascade Sewer District to the east of the study area is a
Metro participant and shares interceptor facilities with
Kent.
o Des Moines Sewer District borders the study area to the
west where drainage basins cross over the study area
boundary. Des Moines provides sewer service to a
portion of the City of Kent which lies west of Interstate 5.
o Lakehaven Sewer District borders the study area to the
southwest.
6-6
o King County is the local government, other than
the City of Kent, in whose jurisdiction a substan-
tial part of the study area is located.
No conflict in .service area delineation with neighboring
cities and districts should result from this plan since most of
the boundaries have previously been agreed upon. At the request
of the State of Washington, the Star Lake area was considered as
a potential service area for the City of Kent in a recent Facili-
ties Plan (URS, 1977) . This area is currently within the Lake-
haven Sewer District, although the natural drainage is toward
the Kent system and has therefore been included in the study area.
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REFERENCES
King County General Sewerage Plan, Adopted January 15,
1979.
Kent Planning Department, Kent Comprehensive Plan, September
1976.
Kent Planning Department, Kent Comprehensive Plan - Resource
Data, Draft #1 , March, 1976.
King County Ordinance 76-1073, King County Agricultural
Districts, 1976.
U.S. Department of Agriculture, Soil Survey of King County,
Washington, November, 1973.
Lakehaven Sewer District, Star Lake Facilities Plan, April,
1976.
URS Company, City of Auburn, Comprehensive Sewerage Plan
update draft, 1979.
Hill & Ingman, An Engineering Report on Sewage Collection,
Treatment and Disposal for the City of Kent, 1903.
URS Company, 1977. City of Kent 201 Wastewater Facilities
Plan.
U.S. Environmental Protection Agency, Manpower Requirements
for Waste Water Collection Systems in Cities and Towns up
1 to 150, 000 in Population, Grant Award No. T-900211 , June,
1973.
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APPENDIX A
INFILTRATION/INFLOW ANALYSIS
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APPENDIX A
INFILTRATION/INFLOW ANALYSIS
PREVIOUS STUDIES
In conjunction with sewer system projects constructed by the
City of Kent, and other agencies, the City has conducted four
infiltration/inflow analyses for substantial portions of the City's
sewage collection and interceptor system. These studies were
undertaken at various times as a part of the process of establishing
eligibility for Environmental Protection Agency and Department of
Ecology construction grants.
Three of the infiltration/inflow analyses were completed in
1973 , and were prepared for the following projects:
. 1
1 . Garrison Creek Interceptor
1 2. Linda Heights Pump Station Modifications
3. 100th Avenue Interceptor
The fourth study was completed in 1974 and was prepared in
conjunction with the Metro-Auburn Interceptor project. This study
Icovered the older portion of the City' s collection system, and a
:J substantial amount of relatively new system. For the most part, the
systems studies are located in the Valley floor and are subject to
high groundwater tables.
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A-1
The results of these studies have been summarized and are being
presented in' a condensed form for the purpose of unifying the findings
of the studies previously completed.
SUMMARY OF COMPLETED I/I ANALYSIS -
As mentioned, four independent I/I analyses have been completed
on portions of the City of Kent sewerage system. The areas studied
comprise approximately 40 percent of the total system. A summary of
each study is as follows:
Garrison Creek Interceptor
This study cover the Plats of Stockton and Kenton Firs located
southeasterly of 124th Avenue on S.E. 100th Street, and was done in
conjunction with the Garrison Creek Interceptor project constructed
by the City. The study covered approximately 6, 360 feet of 8" pipe
and 700 feet of 12" and 18" interceptor, all installed in the period
from 1969 to 1972. This system is in the East Hill area of the City
at an elevation of approximately 4001 .
An analysis of the flows showed that this system is entirely
free of infiltration and inflow.
100th Avenue Interceptor
This study consisted of an analysis of the system tributary to
the Hines pumping station then in existence. The 100th Avenue
Interceptor constructed by the City has subsequently intercepted the
flow to the pump station and now flows by gravity to the Garrison
A-2
The results of these studies have been summarized and are being
presented in a condensed form for the purpose of unifying the findings
of the studies previously completed.
SUMMARY OF COMPLETED I/I ANALYSIS
As mentioned, four independent I/I analyses have been completed
on portions of the City of Kent sewerage system. The areas studied,
which collectively comprise approximately 40% of the total system
are shown on Figure 7. A summary of each study is as follows:
Garrison Creek Interceptor
This study cover the Plats of Stockton and Kenton Firs located
southeasterly of 124th Avenue on S.E. 100th Street, and was done in
conjunction with the Garrison Creek Interceptor project constructed
by the City. The study covered approximately 6, 360 feet of 8" pipe
and 700 feet of 12" and 18" interceptor, all installed in the period
from 1969 to 1972. This system is in the East Hill area of the City
at an elevation of approximately 400 ' .
An analysis of the flows showed that this system is entirely
yy free of infiltration and inflow.
100th Avenue Interceptor
This study consisted of an analysis of the system tributary to
the Hines pumping station then in existence. The 100th Avenue
{ Interceptor constructed by the City has subsequently intercepted the
.J flow to the pump station and now flows by gravity to the Garrison
A-3
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Metro-Auburn Interceptor
The I/I Analysis performed for this project was by far the most
extensive study of the four completed to-date, both from the stand-
point of amount of pipe, and from flows generated. -
The area studied consisted of three independent sub-systems ;
the James Street system, the Smith Street system, and the South
system. The sub-systems serve all of the old area of the City and a
substantial amount of more recent developments. The age of the pipe
ranges from more than sixty years old, to virtually new construction.
The older pipe is vitrified clay and some concrete, while the new
pipe is concrete with some cast iron and PVC.
The size and approximately combined length of the pipe in the
three systems is as follows:
Size Approximate Length
6" 8, 540 '
8" 76, 650'
1011 7, 250'
12" 14, 550 '
14" 3, 500 '
18" 190'
24" 3,000 '
The footage of pipe in each sub-system is as follows:
James Street System 12, 820 ' = 2. 43 mi .
Smith Street System 66, 900 ' = 12. 67 mi .
South System 35, 660' = 6. 75 mi .
Total 21 . 85 miles
The James Street System is located primarily in the valley
floor, and while some of the pipe is relatively new, much of the
A-4
system is of older construction. Variations from theoretical
anticipated flows range from 700% during dry periods to 1150% during
wet periods.
Approximately 50% of the Smith Street System is located on
Scenic Hill at elevations of from 50 ' to 350 ' above mean sea level.
The Scenic Hill system is geographically part of the East Hill System.
While a substantial amount of the pipe in this area is old, infiltra-
tion and inflow were found to be insignificant. The remaining
portion of the system is old pipe located in the valley floor.
Variations from theoretical flows range from 550% to 700%; the
decrease in variation from the James Street System being attributed
to the influence of the lack of I/I from the Scenic Hill area.
Virtually all of the South system is located in the valley
floor, but has been constructed within the past 15 years. A substan-
tial portion of the South System, tributary to the Horseshoe Acres
pump station, was constructed in 1971 and 1972. The flow analysis
of the South System showed an insignificant amount of infiltration
and inflow.
While the study of the three sub-systems showed a large amount
of infiltration and inflow, it also disclosed that the problem is
A basically confined to the older portions of the system. Pipe
installed within the past 15 to 20 years is generally in good
condition with little or no infiltration/inflow problems. This is
particularly true in the higher elevations.
A-5
Despite the presence of a large amount of extraneous flows in
the older parts of the system, a cost-effective analysis disclosed
that these flows were not excessive by definition. A significant
factor in this analysis was the low cost of treatment by Metro.
RELATIONSHIP TO TOTAL SYSTEM -
The City of Kent System consists of approximately 445, 000 feet
of pipe, or some 115 miles. Of this amount, 171 ,500 feet, or 32. 5
miles have been studied under the previously summarized I/I Analyses.
This constitutes slightly under 40% of the total system. Also, of
this 32. 5 miles, 12. 5 miles have been constructed since 1970, or 22%.
The City of Kent is located in two characteristically different
areas with relationship to topography and groundwater conditions;
the Green River Valley floor, and the uplands on each side which
fall fairly abruptly to the valley. The East Hill System and the
West Hill System already referred to are in the higher elevation
areas above the valley. These areas are well-drained and not
subject to any significant high groundwater conditions. Of the
total 84 miles of system, approximately 53 miles are located on the
East and West Hills. Of this 53 miles, approximately 16. 5 miles
have been investigated under previous studies, or approximately
one-third. The remaining 31 miles of the total system are located
on the valley floor. Of this 31 miles, approximately 16 miles have
been investigated under the previous studies, or approximately 50%.
CONCLUSIONS
From the information developed in the studies summarized, it
was evident that the portions of the systems investigated on the
A-6
East and West Hill are free from excessive infiltration/inflow.
As noted, approximately one-third of East and West Hill systems
were investigated. These areas were studied in conjunction
with construction projects and as such, could be considered
randomly selected since location, pipe condition or groundwater
conditions were not factors involved in their selection. Since
virtually all of the systems located on the East and West Hills
are above 150 feet elevation, were installed within the past 20
years for the most part, and are subject to similar groundwater
conditions, it is concluded that the portions studied are
representative of the entire higher level systems and not
subject to excessive infiltration or inflow.
This conclusion is further substantiated by inteviews with
Sewer Department personnel. A large amount of the East and
West Hill Systems have been televised with no indication of
infiltration conditions. Also, the Department conducts a
thorough and continuous inspection program which has revealed
that the only areas where infiltration/inflow conditions exist
are in the old portion of the downtown system already investigated
under previous studies. Also, for the past ten years, the City
has televised all new construction prior to acceptance. As a
part of their construction practices, all pipe installed is
fill tested for leakage prior to installation, and the completed
sewer is air tested prior to television and acceptance.
All side sewers are also pressure tested. Since a substantial
amount of the Hill systems were installed in accordance with
these procedures, the likelihood of leakage is further minimized.
A-7
In the valley system, approximately 50% of the same 31 miles of
sewer pipe comprising the system have been investigated. The pipe
subject to infiltration and inflow has been noted under previous
studies. The portion of the system not covered under the previous
studies, approximately 15 miles, is located primarily in the valley
northerly of S. 240th Street. Virtually all of this system has been
installed subsequent to 1967, and was installed utilizing construc-
tion methods previously noted. For the most part, this portion of
the system consists of larger diameter pipe ( 12" through 27" ) .
Routine maintenance and inspection of the system by the Sewer
Department has not disclosed any evidence of infiltration or inflow.
Although the area is subject to high groundwater conditions, the
City Sewer Department Superintendent has stated that this system is
in very good condition and he has found nothing that would indicate
any significant amount of infiltration or inflow.
In summary, an assessment of the studies completed to date
indicates that the only areas subject to infiltration and inflow in
the entire Kent Sewage Collection System have been identified. Al-
though the quantities of infiltration and inflow in the deteriorated
portions of the old system were found to be substantial, it was
determined that it was not cost effective to correct the problem due
to the low cost of treatment by Metro. These findings were concurred
with by the agencies reviewing the studies.
Although rehabilitation and/or replacement was not found to be
cost effective, the" City is proceeding with a program of replacement
in the problem area, and has replaced several thousand feet of old
A-8
pipe within the past few years. This is a continuing program, and
it is anticipated by the City that within the next few years, all of
the older pipe will be replaced under the improvement program.
A-9
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APPENDIX B
COST ESTIMATES
APPENDIX B
COST ESTIMATES
This appendix presents cost estimation data and itemizes
estimated costs for the projects discussed in this plan. The
costs shown are presented in 1980 dollars based on an Engineering
News Record construction costs index (ENR) of 3200.
Unit cost estimates for replacement of existing pipe were
based on average low-bid costs from recent projects in the
downtown Kent area. A considerable degree of variation exists
between projects. Actual bid construction costs will vary
depending on specific problems and conditions encountered and
on economic conditions at the time of construction. A 25
percent allowance was added to the bid costs to account for
engineering and other allied costs. Replacement costs for
pipes ranging in size from 6" -12" were found to average about
the same since the actual costs of pipe represents only a small
part of the total cost. An average unit cost of $88/linear
foot is used in the estimates. For lines larger than 14" , and
average cost of $120/linear foot was used.
JUnit costs for new pipes are shown on Table B-1 .
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TABLE B-1
NEW PIPE UNIT COST
SIZE $/LF
8 64
12 70
15 77
18 86
24 104
27 113
30 129
36 147
42 165
These costs are based on sewer extension project bids for
the south King County area. These costs were updated using on an ENR
index of 3200 and include a 30 percent allied cost allowance.
B-2
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APPENDIX C
POPULATION PROJECTION CALCULATIONS
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Study Area z 3150
Boundary
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Fig. C - 1. AAM District Boundaries
APPENDIX D
CITY OF KENT
SEWER CONSTRUCTION STANDARDS
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