Storm Drain Report 001ni?aizoos Is:a2 Fax znszsaazon saav a oaooeue
CITY OF YELM
THURSTON COUNTY
WASHINGTON
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STEVENS STREET NW ROADWAY IMPROVEMENTS
STORM DRAINAGE
TECHNICAL INFORMATION REPORT (TIR)
G&O #06436
NOVEMBER 2006
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CONSULTING ENGINEERS
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CITY OF YELM
THURSTON COUNTY WASHINGTON
STEVENS STREET NW ROADWAY IMPROVEMENTS
STORM DRAINAGE
TECHNICAL INFORMATION REPORT (TIR)
GBcO N06436
NOVEMBER 2006
Gray ~ Osbo~e, Imo.
GONSULPNG ENGINEERS
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TABLE OF CONTENTS
PROJECT OVEAYIEW ........................................................................................................ ...1
Drainage BasinS .................................................................................................... ...2
Site Hydrology ...................................................................................................... ...2
Infiltration Design ..................................................................................... ...4
Water Quality Treatment ...................................~...,,.......,......................... ...5
CONDITIONS AND REQUIREMENTS SDMMARY ................................................................ ...6
Mitdmum Requuement #1 - Prepazation of Stormwater Site Plans .................... ...6
Minimum Requrement #2 -Construction Stormwater Pollution Prevention
Plan (SWPP) .........................................~...,..................................................... ...7
Minimum Requirement #3 -Source control of Pollution .................................... ...7
Minimum Requirement #4 - Preservation of Natural Drainage Sys[ems
and Outfal]s ...........................................~..,,..................................................... ...B
Minimum Requirement #6 - On-Site Stormwater Management .......................... ... 8
Minimum Requirement #6 -Runoff Treatment BMPs ........................................ ...8
Water Quality System ..............................~.,.....,........................................ ...8
Minimum Requirement #7-Flow Control ........................................................... ...8
Design Criteria-Performance Standards ................................................ ...8
Flow Control System ................................................................................ ...8
Conveyance System Analysis and Design ................................................ ...9
Minimum Requirement #8 -Wetlands Protection ............................................... ...9
Minimum Requirement #9 - Basin/W atershed Planning ..................................... ...9
Mimimum Requirement # ] 0 -Operation and Maintenance ................................. ...9
Optional Guidance #] -Financial Liability ......................................................... ...9
Bond Quantities, Facility Summaries, and Declaration of Covenant....... ...9
Optional Guidance #2 -Off-Site Analysis and Mitigation .................................. ...9
SPECUL REPORTS AND STUDIES ..................................................................................... .lO
OTHER PERMRS .............................................................................................................. . 10
LIST OF TABLES
No. Table Paee
t Parameters Used for bfodeling and Resulrs ............................................................4
2 Detention Model Results for Roadway Improvements ...........................................6
3 Bioswale Design .................................................~....,...........,...................................6
LIST OF FIGURES
No, Fieure Follows Paee
1 Location Map ...........................................................................................................1
2 Drainage Basins ..................................................... ..........................1
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LIST OF APPENDICES
Appendix A - Sto[mwa[er Modeling (SBUH)
Appendix B - Stonnwater Modeling (W WHM)
Appendix C -Design Drawings
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PROJECT OVERVIEW
The Stevens Sveet NW Roadway Improvements comprise approximately 9501ineal feet
of road improvements from NW Edwards Svee[ to North 1" Sveet (see Figure 1). The
project is sepazated into a northwest Bagmen[ and a southeast segment for stormwe[er
modeling. This Technical Information Reporl (TIR) is for both segments. The
notthwestem segment is approximately 7501ineal feet from N W Edwazds Sveet to the
former railroad crossing. The southeast segmrnt is approximately 2001ineal feet from
the former railroad crossing to North 1"Street. This project provides for the roadway
reconstruction and includes storm drainage improvements, curbs, gutters, sidewalks,
landscaping, illumination, and water system improvements.
The segment of Stevens Street NW evaluated ih this study lies in the Yelm Creek
watershed. Surface flows generally drain northeasterly m Yelm Creek. The road is in
flat terrain, and crosses one high point at the former railroad tracks (Ste. 8+10). This rise
creates two separate subbasins within the project. The two subbasins associated with the
project aze shown in Figure 2. Based on the geotechnical investigation, the land in the
area consists of cobbley sends end gravels. Because of the relative flat slopes N and
adjacent to the project, and the porous soils, there is little offsite flow onto the project
comdor. Currently off-site surface water adjacent to the project generally infiltrates.
Runoff from an existing paved pazking lot on the northeast side of the project flows away
from Stevens Street and runoff from a parking lot near the Stevens Sveet /Railroad
Sveet intersection flows into an existing off-site storm system, with some flows also
i~(vating. Runoff from the gravel/paved mini-storge lot (southeast subbasin) cutrently
flows onto North la Street, then easterly towazds Yelm Creek via existing storrtt pipes
and/or existing roadside ditches. There is no existing on-site storm collection,
conveyance, vestment, or detention facilities. Runoff from the existing roadway
(northwest subbasin) infilvates into the ground adjacent to the roadway.
The project consists of reconstructing the roadway to create a 32-foot wide, paved road
with curbs end 5-foot sidewalks on each side. Planter strips will also be located along a
majority of the project. In accordance with the DOT Highway Runoff Manual, ]992
Depamnent of Ecology Stormwater Management Manual, end the 2001 DOE Stormwater
Management Manual the improvements will require both vestment and detention. The
existing road is typically I S feet wide, with some sections of narrow gravel shoulder on
either side. The increase in impervious area that creates the detention requirement comes
from both the widened pavement and the sidewalk. The project adds 20,251 squaze feet
of impervious area. This means the project will have to comply with Minimum
Requirements #1 through #9, which include detention and water quality vestment.
The City of Yelm has directed that the requirements of the 2001 Washington State
Department of Ecology Stormwater Management Manual for Western Washington will
be applied to this project The project proposes adding more then 5,000 square feet of
new impervious area; therefore, Minimum Requrements #1 through #10 apply. Initial
modeling was completed using accepted methods for the Highway Runoff Manual and
City aJYelm 1
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CITY OF YELM
STEVENS STREET IMPROVEMEN75
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the 1992 DOE manual (SBUH model). The fatal facility design was modeled ruing the
model adopted in the 2001 DOE Manual (W WfIIvQ. This report uses the outlino of the
2001 DOE manual for the following minimum requirements. The City of Yelm has
permit authority for the projec[ for general permits.
DRALNAGE BASINS
This report identifies two subbasins, northwest and southeast. The Stevens Street NW
Roadway Improvements are included completely in these two segments and are discussed
in this report, These two drainage basins are identified through topographic and existing
conveyance informatien for this portion of the project. These basins are described es
follows:
The northwest basin is shown in Figure 2. The northwest bazin is
approximately 1.05 acres total, with approximately 0.01 acres westerly of
Stevens Sueet NW contributing off-site flow to the project. Stotmwater
runoff flows generally to the east toward Yelm Creek. Surface flows at
the northwest end of the project are collected by the existing system of
catch basins in NW Edwards Street prior to flowing into the projec[ area.
This existing storm system flows easterly towards Ye1m Creek.
The southeast basin is shown in Figure 2. The southeast basin is
approximately 0.22 acres, with approximately 0 acres contributing off-site
flow to the project. Stormwater runoff flows generally to the southeast
towards North First Street. Surface flows are collected in catch basins and
roadside ditches and wnveyed to Yelm Creek.
SITE IdYDROLOGY
Design storms areaway to characterize and quantify the likelihood of a rainfall event.
Different models use different methods to determine these events. Two methods used for
modeling in Washington State are the single event method and a continuous record
method. The single event method employs a 24-hour storm event and is derived from the
azea NOAH atlas. A certain amount of precipitation over a 24-hour period is related to
the probability of that event happening based on historical precipitation data. For Yelm,
the statistical likelihood of having a storm event with _ inches of rainfall in a
24-hour period is once every 2 years. 'T'his then is known az a 2-year, 24-hour event. The
computer model commonly used far the single-event method is called the Santa Barbara
Urban Hydrograph (SBUH) and is based on the Soil Conservation Service mode] TR-55.
The continuous record method does not limit the thne to 24 hours or any other set "event"
timeframe. Instead, the continuous method examines historical precipitation records and
determines the statistcal likelihood of wet weather conditions that may last for several
days, or for less then 24 hours. This requires more detailed datA input for precipitation
records and much more computing power. The continuous record model used by the
2 Ciy ofYelm
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2001 DOE Manual is the Western Washington Hydrology Model (W WHM) and is a
version of the Hydrologic Simulation Program Fortran (HSPF). Current regulations may
use one or the other, or both.
Initial planning and design for this project was completed using the SBUH model.
Modeling far final design was completed using W WHtvf.
A Santa Bazbaza Urban Hydrograph (SBUH) s[ormwater modeling program war used to
initially siu the infiltration facilities. The program takes impervious surface, soil type,
slope, length of flaw path, rainfall amount and rainfall pattem, end determines the peak
runoff rates. The slope of the subbasins ranges from 0.5 to 3 percent, and averaged
approximately 2 percent,
The rainfall pattem in the northwest cozresponds to a Type 1 A storm hyetograph. This
has a gradual inuease at first, followed by a spike when it reins heavily for ao hour or
two, and then tapers off to a low level of rain for the remainder of the 24 hours. The Soil
Conservation Service developed the distribution of rainfall over the period of the 24-hour
storm from actual rainfall data. Based on the precipimtion charu in the 2001 Westem
W ashing[on DOE Manual, a 2-year, 24-hour storm in the Ciry of Yelm is _ inches of
rain; a 10-yeaz, 24-hour storm drops _ inches of rain; and a 100-year, 24-hour storm
drops _ inches of rain. See the table below for the input parameters.
To comply with the 2001 DOE mauuel, the basins were modeled using the Westem
Washington Hydrology Mode] (W WHM) developed by DOE. For front design, modeling
data from both methods are presented below.
Ciry oJYelm 3
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TABLE 1
Parameters Used for Modeling and Results
In171tration Desfgo
The infilvation requirement limits flows for various storm design events. The developed
project 2-year flow is limited to one-half the predeveloped project 2-year flow. Off-site
flows ere included in the routing but are not pan of sizing detention facilities. The
4 Clry ofYelm
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As well as calculating peak runoff rates, stormweter modeflng programs route the design
storms through a specified i~ltraticn facility. The SBUH model was used for initial
sizing and the final design was routed through W WHM to assure compliance with the
2001 DOE Manual.
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developed 10- end 100-year flows ere limited to the predeveloped respective design
events.
Detention in the northwest subbesin is designed to use storage capacity in the infiltration
pipes and voids in the gravel backfill. The infiltration pipes will typically be located
below the treatment swales in the planter strips.
Calculations for the detention suing ere included in Appendices,
TABhE 2
Detection Model Results for Roadway Improvements
Since the road is typically crowned at the center, end surface water flows to both sides,
each side of the road must have a bioswale for atormwater treatment For modeling
purposes, this Swale typically will be a standard 10-inch-deep, 2-foot, 6-inch-wide ditch.
with the fore slope of 3:1, end the back slope being 2:1 (or flatter) as shown on the plans.
The biosweles are required to treat the peak flow from the 6-month storm. The peak
6-month storm flows for each subbasin were determined using the SBUH model. Table 3
shows the calculations for the quantity of water each bioswale is required to treat.
The required bioswale dimensions were calculated using the design procedure outlined in
the 2001 Western Washington DOE Manual which is equivalent or more stringent than
[he WSDOT Highway Runoff Manual. I[ was assumed that the grass would be S inches
tall. The runoff can be up to 2 inches below the height of the grass, making the
maximum depth of water in the Swale 3 inches. Manning's n, a constant that accounts for
flow retardance was selected to be 0.25 because a Swale is a straight, vegetated channel.
The slope of the swales were taken to be 2 perccnt. Water must remain in the swales for
_ minutes to receive adequate treatment. Using the velocity of the water, the
retention time was used to determine the required length of the swales.
The necessary bioswele length and width was calculated for the north cad south slopes of
the basin. Bioswales will be required on both sides of the road, and each is required to
have the dimensions listed below.
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TABLES
Bioswale Design
The Wazhington State Department of Transportation Highway Runoff Manual requires
that the peak flow from [he 6-month, 24-how storm, of 64 percent of the volume of the
2-year, 24-hour storm, be treated prior to discharge into surface waters. This applies to
pollution generating impervious surface runoff Impervious surface is defined az "A hazd
surface area which either prevenu or retards [he entry of water into the soil. Common
impervious surfaces include roof tops, walkways, patios, driveways, pazking lots or
storage areas, concrete or asphalt paving, gravel roads, packed earthen materials, and
oiled suffaces," Because the road and shoulder are used by motor vehicles, they will be
classified az pollution generating. The WSDOT Highway Runoff Manual Minimum
Requirement 4: Water Quality Control requves that "a project adding new impervious
surface shall achieve a net gain in the level of water quality treatment" and [hat the
designer shall "investigate the practicability of providing water quality veatment for
runoff from any existing impervious areas." Based on the findings of that investigation,
"BMPs for existing impervious runoff will be implemented whenever the investigation
demonstrates that it would be more practicable az demonstrated in the practicability
analysis." In the absence of a practicability analysis, it was assumed that the entire
existing impervious area will require treatment.
The City of Yelm has directed that the requirements of the 2001 Washington State
Department of Ecology Stormwater Management Manual for Wcstem Washington will
be applied to this project. The project proposes adding more Shan 5,000 square fee[ of
new impervious area; therefore, Minvnum Requirements #1 tfuough #10 apply. Initial
modeling was completed using accepted methods for the Highway Runoff Manual and
the 1992 DOE manual (SBUH model). The final facility design was modeled using [he
model adopted in the 2001 DOE Manual (W WHM). This report uses the outline of the
2001 DOE manual for the following minimum requirements. The City of Yelm has
pemit authority for the project for general permits.
MINIMUM REQUIREMENT #1 - PREPARATION OF STORMWATER SITE
PLANS
The project has a total of new and existing impervious surface that exceeds 5,000 square
fee[ and therefore must prepare a Stormwatez Site Plan for local government review. The
Stormwater Site Plan must prepared in accordance with Chapter 3 of the 2001 DOE
6 Clry oJYelm
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CONDITIONS AND REQUIItEMENTS SUMMARY
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Manunl. This TIR along with the plans and speci&cetions for the project include the
information required in a Stormwater Site Plan which includes the following:
• Project Overview
• Existing Conditions Summary
• Off•Site Analysis Report
• Permanent Stotmwater Controls
• Operation and Maincenance Requirements
• Bond Quantity Worksheet
MINIhIUM REQUIREMENT #2 -CONSTRUCTION STORMWATER
POLLUTION PREVENTION PLAN (SWPP)
The project is a redevelopment project and therefore must comply with Conswction
SWPP Elements #I through #]2.
The elements include the following:
• Element #1: Mazk Cleating Limits
• Element #2: Establish Constction Access
• Element #3: Convol Flow Rates
• Element #4: Install Sediment Convols
• Element #5: Stabilize Soils
• Element #6: Protect Slopes
• Element #7: Protect Drain lnlers
• Element #g: Stabilize Channels end Outlets
• Element #9: Convol Pollutants
• Element #10: Control Dewatering
• Element #1l: MainteinBMPs
• Element #12: Manage the Project
Convect documents will require the convector to submit plans meeting the minimum
elements and erosion and sediment convol requirements prior [o begiming Conswction.
The City will approve these plans. Clearing and grubbing ofthe-right-of-way is
anticipated for this project. The City is in the process of obtaining all the required
right-of-way for the project. Work will be performed within the fight-of-way. Best
management practices will be used to convol runoff from the project area Please see the
plans far specific best management practices employed for the project.
MIMMUM REQUIREMENT #3 -SOURCE CONTROL OF POLLUTION
Source control of pollution will be included in the best management practices during
Conswction. No new potential polluting sources will result from this project. Water
Ctry ofYelm 7
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quality vestment is being provided for stormwater runoff from the roadways and is
discussed in greater detail under Minimum Requirement #4 below.
MINIMUM REQUIREMENT #4 -PRESERVATION OF NATURAL DRAINAGE
SYSTEMS AND OUTFACES
All proposed collection end conveyance systems will continue to use the natural drainage
system for the basin. No alteration of natural drainage patterns will occur.
MINIMUM REQUIREMENT #5 - ON-SITE STORMWATER MANAGEMENT
This requirement includes the use of i~ltration, dispersion, or retention of stotmwater on
site to the maximum extent feasible without causing flooding or erosion impacts. On-site
stormwater management will be used for this project given the underlying soil types and
nature of the project.
MINIMUM REQUIREMENT #6-RUNOFF TREATMENT BMPS
Water Qualify System
The project includes more than 5,000 squaze feet of pollution generating surface in each
of the two threshold dischazge areas (subbasins) which requires treatment facilities and
onsite stortnwater BMPs. Treatment of the stormwater runoff occurs in roadside
bioswalw and before infiltration and/or discharge. On-site BMPs include construction
erosion and sediment convol facilities as directed by the project plans end specifications
and regular road maintenance and litter convol.
MIMMUM REQUIREMENT #7 -FLOW CONTROL
Design Criteria -Performance Standards
Per the 2001 DOE Manual, the project must provide flow convol to reduce the impacts of
increased stormwater runoff from new impervious surfaces end land cover conversions.
W star quantity convol requires detention for all rmoff from new impervious surfaces.
Detention must reduce the peak developed outflow to 50 percent of the runoff from the
existing site during a 2-year, 24-hour storm, and maintain the existing level of discharge
during the 10-yeaz, 24-hotu and 100-year, 24-hour storms.
Flow Control System
New storm pipes provide detention. Required storage was determined through use of the
Santa Bazbara Urban Hydrogreph (SBUH) method u outlined in the 1992 DOE Manual
and verified to meet the requiremenrs of the 2001 DO£ Manual using the W WHM.
Modeling was completed for the off-site basins incorporated into the final design. The
northwest basin has approximately 0.01 acres of off-site contribution. The project will
g Clry ofYelm
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detain stormwater flows to the preexisting conditions before discharge. The southeazt
basin has no off-site contributing area. The prof act will increase peak flows by
approximately `cfs for the 100-yeaz event. The detention system exceeds the flow
requirements and allows existing base flows to continue their existing hydrology.
Conveyance System Analysis and Design
All conveyance for the system was sized using minimum flow criteria based on
Manning's Equation for flow in pipe and minimum slopes based on the Washingtnn Srate
Department of Ecology Criteria for Sewage Works ([he Orange Book). A Maming's
number of 0.013 was used for all pipes. The minimum proposed pipe size is 12 inches in
order to meet City standazds and maintenance requirements. "Rte lazgest proposed
conveyance pipe is 12 inch.
MINIMUM REQUIREMENT k8 - WETLANDS PROTECTION
Existing surface flows feed wetlands in the vicinity of Yelm Creek. These wetlands will
no[ be impacted by the project.
MINIMUhf REQUIREMENT N9 -BASIN/VI'ATERSHED PLANNING
No basin plan exists for the Yelm Creek drainage area.
MINIMUM REQUIREMENT tF10 -OPERATION AND MAINTENANCE
The storm drainage improvements will be maintained in accordance with the City of
Yelm standards. The treatment biofiltration systems will be maintained in accordance
with the DOE Stormwater Management Manual for Western Washington (Volume 4,
Chapter 9).
OPTIONAL GUIDANCE Hl -FINANCIAL LIABILITY
Bond Quantities, Facility Summaries, sod Declaration of Covenant
Not applicable.
OPTIONAL GUIDANCE k2 -OFF-SITE ANALYSIS AND MITIGATION
Modeling was completed for the entire off-site basins an incorporated into the final
design. The northwest basin has approximately 0.01 acres ofoff-site contdbudon The
project will detain stormwatei flows to the preexisting condidons before discharge into
existing conveyance systems. The southeast basin has no off-site contributing area. The
project will increase peak flows by approximately _cfs for the 100-year event, The
detention system exceeds the flow requirements and allows existing base flows to
continue their existing hydrology.
Ciry ojYelm 9
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SPECIAL REPORTS AND STUDIES
No basin plan exisu fot the Yelm Creek drainage azea.
OTHER PERMITS
No basin plan exisu for the Yelm Creek azea. The Ciry of Yelm has permit suthoriry for
the project for general permits.
t 0 C!ry oJYelm
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APPENDIX A
STORMWATER MODELING (SBUH)
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APPENDIX B
STORMWATER MODELING (WWHM)
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APPENDIX C
DESIGN DRAWINGS
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