HomeMy WebLinkAboutRS_WSADA_Preliminary_TIR_221028_v1
Washington State
Auto Dealers Association
Addition and Improvements
Technical Information Report
Prepared for:
Washington State
Auto Dealers Association
621 Southwest Grady Way
Renton, WA 98057
Contact: Vicki Fabrè
Phone: (206) 433-6300
Prepared by:
Sitts & Hill Engineers, Inc.
4815 Center Street
Tacoma, Washington 98409
Contact: Don Davis
Phone: (253) 474-9449
October 2022
Job Number 19,682
Washington State
Auto Dealers Association
Technical Information Report
Prepared for:
Washington State
Auto Dealers Association
621 Southwest Grady Way
Renton, WA 98057
Contact: Vicki Fabrè
Phone: (206) 433-6300
Prepared by:
Sitts & Hill Engineers, Inc.
4815 Center Street
Tacoma, Washington 98409
Contact: Don Davis, P.E.
Phone: (253) 474-9449
IN COMPLIANCE WITH CITY OF RENTON
STANDARDS
October, 2022 Job Number 19,682
PRELIMINARY
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TABLE OF CONTENTS
TABLE OF CONTENTS ............................................................................................................................. 1
1.0 PROJECT OVERVIEW ........................................................................................................................ 3
1.1 Scope ................................................................................................................................................. 3
1.2 Existing Conditions ......................................................................................................................... 3
1.3 Proposed Conditions ...................................................................................................................... 5
2.0 CONDITIONS AND REQUIREMENTS SUMMARY........................................................................ 7
2.1 Discussion of Core and Special Requirements .......................................................................... 7
2.1.1 Core Requirement #1: Discharge at the Natural Location .................................................. 7
2.1.2 Core Requirement #2: Off-site Analysis.................................................................................. 7
2.1.3 Core Requirement #4: Conveyance System ........................................................................... 7
2.1.4 Core Requirement #5: Erosion and Sediment Control ......................................................... 7
2.1.5 Core Requirement #6: Maintenance and Operations ........................................................... 7
2.1.6 Core Requirement #7: Financial Guarantees and Liability ................................................... 7
2.1.7 Special Requirement #4: Source Control ................................................................................ 7
2.1.8 Special Requirement #6: Aquifer Protection Areas .............................................................. 7
3.0 OFF-SITE ANALYSIS ........................................................................................................................ 12
3.1 Define and Map the Study Area ................................................................................................. 12
3.2 Resource Review ........................................................................................................................... 14
3.3 Field Inspection ............................................................................................................................. 14
3.4 Drainage System Description and Problem Descriptions ..................................................... 14
3.5 Mitigation of Existing or Potential Problems .......................................................................... 14
4.0 FLOW CONTROL, LID, AND WATER QUALITY FACILITY ANALYSIS AND DESIGN ......... 15
4.1 Existing Site Hydrology ............................................................................................................... 15
4.2 Developed Site Hydrology .......................................................................................................... 15
4.3 Performance Standards ............................................................................................................... 15
4.4 Flow Control System .................................................................................................................... 16
4.5 Water Quality ................................................................................................................................ 16
5.0 CONVEYANCE SYSTEM ANALYSIS AND DESIGN .................................................................... 18
6.0 SPECIAL REPORTS AND STUDIES ............................................................................................... 19
7.0 OTHER PERMITS .............................................................................................................................. 20
8.0 CSWPPP ANALYSIS AND DESIGN ................................................................................................ 21
9.0 BOND QUANTITIES AND R/D FACILITY SUMMARIES ............................................................ 22
9.1 Bond Quantities ............................................................................................................................ 22
9.2 Facility Summaries ........................................................................................................................ 22
10.0 OPERATIONS AND MAINTENANCE MANUAL ....................................................................... 23
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10.1 Purpose ........................................................................................................................................ 23
10.2 Permanent Facilities Description ............................................................................................ 23
10.3 Discussion of Maintenance ....................................................................................................... 23
10.4 Maintenance Frequency ............................................................................................................ 24
10.5 Annual Cost Estimate ................................................................................................................ 24
TABLE OF APPENDICES
A.1 TIR Worksheet
A.2 Bond Quantity Worksheet
A.3 Existing and Proposed Surface Coverage Exhibits
A.4 Site Historical Aerial Photography
A.5 Stormwater Engineering Calculations
A.6 Geotechnical Engineering Report
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1.0 PROJECT OVERVIEW
1.1 Scope
This Technical Information Report (TIR) addresses site improvements associated with the
proposed addition to the existing Washington State Auto Dealers Association (WSADA)
building located at 621 SW Grady Way, Renton, WA (Parcel No. 3340404730). Please see Figure
1 – Site Location Map for the location of the proposed building addition. The purpose of this
project is to improve the function of spaces within the existing WSADA building. This project
will add a stairway, elevator, restrooms, and storage for improved access to the second floor of
the building, where WSADA regularly holds training classes.
This Technical Information Report has been prepared in accordance with the 2017 City of
Renton Stormwater Design Manual (SWDM). A copy of the TIR Worksheet, which provides basic
information about the scope of this project, is included with this report in Appendix A.1.
1.2 Existing Conditions
The subject site is zoned as Commercial Arterial (CA), with an area of 0.75 acres. The site was
originally developed by WSADA in 2013 as offices for their employees, and as a training facility
for area auto dealers. The existing building, which is situated on the westerly extent of the site,
includes a first story of approximately 6,300 SF and a second story of approximately 2,700 SF.
The easterly two thirds of the parcel is asphalt paved for vehicular circulation and parking of
employees and trainees. The site can be accessed from SW Grady Way to the north, or from the
alley between Grady and 12th St SW to the south. An auxiliary parking lot is located on the
opposite side of the alley to the south of the WSADA site. This parking lot was constructed in
2019 to accommodate for additional parking of trainees, which occurs on weekdays near
midday.
Storm drainage for the majority of the site is currently collected in catch basins, and conveyed
through a Contech Stormceptor pretreatment device and Contech Stormfilter water quality
device into a modular detention facility comprised of Stormtech SC-740 units. This system is
discharged via a control structure into the public stormwater system in the alley south of the
site. The entire building is currently tributary to this system, which is tightlined directly into
the detention system from the west.
A smaller area at the westerly extent of the site, including runoff from the east side of Raymond
Avenue, is tributary via surface flow to a bioretention cell located off the southwest corner of
the existing building. After treatment in the bioretention cell, the stormwater is discharged to
a 48”Φ detention tank, where it is also released into the public stormwater system in the alley
to the south of the site.
Based upon our review of available information from the City of Renton, we are not aware of
any capacity problems with the existing drainage facilities adjacent to this area. The City of
Renton’s Public Works Maps combined with King County GIS together indicate the site is not
located within or near any critical areas. In addition, it is located within the Peak Rate Flow
Control Standard Zone.
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Figure 1 – Site Location Map – Proposed Building
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1.3 Proposed Conditions
The primary purpose of this project is to improve the spaces already existing within the WSADA
building. The upper floor is currently used for training sessions, which occur regularly for area
dealer representatives. Access to the second story is limited, and WSADA would like to improve
the accessibility to this floor by adding a supplementary stairway and an elevator. The addition
will also provide restrooms and storage within the expanded building footprint.
The building addition will encompass a 695 SF footprint (1,390 SF total additional floor area
among the two floors). The new addition will largely replace an existing concrete patio located
at the northwest corner of the existing building, resulting in minimal new hard surface.
Site improvements are strictly limited to accommodation of the proposed expansion. The roof
drainage system for the original building will need to be shifted to accommodate the building
expansion. The bioretention cell and associated detention tank located near the southwest
corner of the building will also need to be renovated to accommodate the building expansion.
See Table 1 for a description of existing and proposed surface conditions for the addition.
Over 50% of the stormwater generated on-site will be from a pollution-generating surface in a
commercial area and thus stormwater runoff generated will be subject to enhanced basic water
quality treatment before being conveyed to the conveyance system located within the City’s
right-of-way. Treatment for the proposed building will be provided by an Oldcastle BioPod
Biofilter. Stormwater from the proposed addition will follow existing flow paths to the west to
existing treatment. All proposed surfacing on-site is considered new or replaced impervious
surface. Please see Appendix A.5 for the sub-basin exhibit as it outlines the tributary area for
each catch basin.
Please see Appendix A.3 for Existing and Proposed Surface Coverage Exhibits.
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Table 1. Existing and Developed Condition Description – Proposed Addition
Description On-site Off-site Total
Existing Conditions
Total Project Area (ft2) 33,558 0 33,558
Existing hard surface (ft2) 26,024 0 26,024
Existing landscaping/lawn area (ft2) 7,534 0 7,534
Developed Conditions
Total Project Area (ft2) 33,558 0 33,558
Amount of new hard surface (ft2) 140 0 140
Amount of new pollution generating hard surface (PGHS) (ft2) 0 0 0
Amount of replaced hard surface (ft2) 0 0 0
Amount of replaced PGHS (ft2) 0 0 0
Amount of new plus replaced hard surface (ft2) 624 0 624
Amount of new plus replaced PGHS (ft2) 0 0 0
Amount of existing hard surfaces converted to vegetation/
landscaping (ft2) 14 0 14
Amount of Land Disturbed (ft2) 11,761 0 11,761
Native Vegetation to Pasture (acres) 0 0 0
Existing vegetation area to remain (ft2) 7,253 0 7,253
Existing hard surface to remain unaltered (ft2) 18,223 0 18,223
Value of proposed improvements ($) $1.23M 0 $1.23M
Amount to be Graded/Approx. FILL (C.Y.) ±40 0 ±40
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2.0 CONDITIONS AND REQUIREMENTS SUMMARY
2.1 Discussion of Core and Special Requirements
This project will be subject to a Category 2 Targeted Drainage Review. This type of drainage
review will require compliance with Core Requirements 1, 2, 4, 5, 6, and 7 and Special
Requirements 4 and 6 of the 2022 City of Renton Stormwater Design Manual (SWDM). See
Figure 2, Figure 3, and Figure 4 and at the end of this Section for determination of drainage
review requirements per the SWDM flow charts. Below is a description of each core
requirement and how the project proposes to satisfy each requirement.
2.1.1 Core Requirement #1: Discharge at the Natural Location
This requirement will be met by collecting and conveying stormwater to existing private
stormwater networks, which discharge into public stormwater networks, ultimately
terminating at Springbrook Creek and the Black River.
2.1.2 Core Requirement #2: Off-site Analysis
The Off-Site Analysis included in Section 3.0 of this TIR will satisfy this Core Requirement.
2.1.3 Core Requirement #4: Conveyance System
Please see Section 5.0 of this TIR for the conveyance system analysis and design. Calculations
are provided which demonstrate the sizing and slope of pipes to reroute the existing roof drain
pipe around the proposed addition. We have also included calculations to demonstrate the
sizing of proposed roof drain pipes to the modified detention tank located near the southwest
corner of the existing building.
2.1.4 Core Requirement #5: Erosion and Sediment Control
A temporary Erosion and Sediment Control Plan (TESC) will be prepared and included in the
construction plan set. Additionally, a CSWPPP under a separate cover, will be provided to satisfy
this requirement.
2.1.5 Core Requirement #6: Maintenance and Operations
Please see Section 10.0 of this TIR for the Operations and Maintenance Manual, prepared to
satisfy this Core Requirement. WSADA will be responsible for maintenance and operation of
the proposed on-site stormwater facilities upon completion. The City of Renton will continue
to be responsible for operation and maintenance of stormwater systems in the right-of-way.
2.1.6 Core Requirement #7: Financial Guarantees and Liability
We anticipate that WSADA will be required to provide a Construction Bond to the City of
Renton prior to issuance of the permit. The contractor/owner will be responsible for all erosion
and maintenance liabilities during construction.
2.1.7 Special Requirement #4: Source Control
WSADA shall implement Operational and Structural Source Control BMPs that include the
maintenance of stormwater drainage and treatment systems as described in the Operations
and Maintenance Manual, included in Section 10.0 of this report.
2.1.8 Special Requirement #6: Aquifer Protection Areas
This project is not located within an Aquifer Protection Area per COR Maps.
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Figure 2 – Drainage Review Flow Chart
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Figure 3 - Drainage Review Requirements
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Figure 4 - Drainage Review Requirements (cont.)
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Figure 5 – FEMA Map
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3.0 OFF-SITE ANALYSIS
Per City of Renton SWDM Section 1.2.2, this project must provide an off-site analysis report
that assesses potential off-site drainage and water quality impacts associated with the
redevelopment of the project site. The level of analysis required depends on the specific site
and downstream conditions. In accordance with Section 1.2.2.1, a Level 1 Downstream Analysis
has been performed.
3.1 Define and Map the Study Area
Based on the current topographic survey and our observations of the site, the site is mostly
tributary to existing, private conveyance systems on-site. The easterly two thirds of the site and
the building are tributary to a proprietary stormwater treatment and modular detention
system. The westerly third of the site, as well as a portion of the drainage from Raymond
Avenue, are tributary to an existing bioretention cell and detention pipe. Both of these on-site
stormwater management systems discharge via control structures into the public drainage
network in the alley south of the WSADA property. Once in the right-of-way, the stormwater
is conveyed north across Grady Way, and then westerly to Spring Brook Creek on the north side
of Grady Way. The downstream conveyance system is comprised of pipes ranging in diameter
from 12-inch to 66-inch. Springbrook Creek eventually discharges into the Black River, and then
on to the Duwamish River.
See Figure 6 for an illustration of the downstream flowpath from the project site to the Black
River.
Figure 6 - Downstream Flowpath
See Figure 7 for a partial topographic survey of the project site.
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3.2 Resource Review
A thorough review of the resources provided by the City of Renton Maps has been completed
for the study area. Research conducted on 10/25/2022 has shown that there are not any
adopted basin plans available for the study area. According to the FEMA Map presented in
Figure 5 of this report, the project sites lie within Zone X, which is defined as areas outside the
500-year floodplain. The site is located within the Duwamish – Green River Watershed, the Black
River Basin, and the Springbrook Creek Sub Basin. Based on the City of Renton GIS, the project
area has not been defined as a sensitive area. To our knowledge, there have not been any
reported road drainage problems associated with the roads surrounding the site. It was also
found that the site does not have a migrating river study, a 303d listing, or an adopted
stormwater compliance plan available. A geotechnical report completed by Migizi Group can
be found in Appendix A.6.
3.3 Field Inspection
During field inspections, the system appeared to be working sufficiently with no obvious clogs
or flooding. We do not anticipate any future problems with the proposed stormwater system.
Stormwater flows on the site of the proposed building are expected to increase marginally due
to an increase in impervious surface. The control structure for the detention pipe system will
not be altered, therefore, the renovated detention pipe system will be designed to
accommodate the slight increase in stormwater durations and volumes. Please see Section A.5
for the modeled areas in the predeveloped and mitigated scenarios for both sites.
Since it has been determined that there are no anticipated stormwater problems associated
with this project, Level 2 and Level 3 downstream analyses are not required.
No problems were found related to constrictions, capacity deficiencies, flooding, scouring,
overtopping hazards, sedimentation or erosion, aquatic organisms or their habitation.
Land use associated with the study area includes commercial/industrial land applications. In
addition, the study area includes utilities and roadways. Existing impervious surfaces located
within the study area include commercial businesses, paved parking lots, paved driveways, and
the paved public right-of-way. Based on existing site topography, stormwater runoff is
captured within the on-site stormwater system, treated, detained, and released via control
structures to the City’s right-of-way. It is ultimately tributary to the Black River via Springbrook
Creek.
3.4 Drainage System Description and Problem Descriptions
No drainage system problems were observed during the field inspection.
3.5 Mitigation of Existing or Potential Problems
The proposed project area does not have any existing or anticipated drainage problems as
outlined in Section 1.2.2.1.1 of the City of Renton SWDM.
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4.0 FLOW CONTROL, LID, AND WATER QUALITY FACILITY ANALYSIS AND
DESIGN
4.1 Existing Site Hydrology
The project site was previously developed in 2013 by WSADA, with all of the current site
features, including a two-story building, asphalt paved parking lot and driveways, concrete
walks, landscaping, and utilities.
Stormwater from the easterly two thirds of the site is collected in catch basins and conveyed
to a pretreatment device, stormwater quality device, and a series of modular detention tanks
located along the southerly margin of the site. The building is also tributary to this detention
system, although it bypasses the treatment functions. Stormwater from the westerly third of
the site, as well as a portion of the Raymond Avenue road surface, is tributary to a bioretention
cell and detention pipe located near the southwest corner of the existing building. Stormwater
from both detention facilities currently discharge through independent flow control structures
which were sized to match predeveloped conditions. The control structures discharge to the
existing public stormwater system in the alley between SW Grady Way and SW 12th Street,
immediately south of the WSADA property. This public stormwater system conveys the
stormwater across and westerly along Grady Way, discharging into Springbrook Creek, and
eventually the Black River.
4.2 Developed Site Hydrology
This project includes a two-story building addition at the northwest corner of the existing
WSADA building. The footprint of the building will be 624 SF, and will replace the concrete
plaza currently situated at this location. Additional new/replaced surfaces will include a new
concrete walkway connection to the sidewalk alongside Grady Way or Raymond Ave (TBD). It
is anticipated that the total new/replaced hard surfaces for this project will be less than 1,000
SF. Areas surrounding the new building and sidewalk, as well as all disturbed surfaces adjacent
to the bioretention cell and detention tank improvements, will be restored with landscaping
similar to existing conditions at the perimeter of the building.
The existing concrete plaza at the northwest corner of the building is graded drain via surface
flow to the existing bioretention cell and detention pipe system. We are proposing to mimic
this drainage scheme with the proposed building addition. The new roof drains will be
conveyed directly to the detention pipe, which will be resized (shortened with an increased
diameter) to accommodate the building expansion.
4.3 Performance Standards
The project site is within the Peak Rate Flow Control Standards (Existing Site Conditions) Area,
as illustrated in Figure 8, which requires matching the existing site conditions 2-, 10-, and 100-
year peak flows. The predeveloped site is comprised of rooftop surfacing, asphalt pavement,
concrete walks, and landscaping. The proposed conditions include a slightly higher quantity of
impervious surface than the existing conditions.
In areas where conveyance features are required, they have been designed in accordance with
Section 1.2.4 of the SWDM. Stormwater from the new roof will be conveyed directly to the
detention facility. Downstream of the storage structure is a control structure that is set to
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match the 2, 10, and 100-year flow conditions prior to development to the WSADA building in
2013.
4.4 Flow Control System
Although this project is exempt from Core Requirement #3 (flow control facilities), the
stormwater from the proposed building will be routed through an existing detention system.
We are not proposing to alter the control structure, however, the downstream pipe network
will experience slightly higher durations and volumes of stormwater due to the slight increase
of impervious surface.
4.5 Water Quality
This project is exempt from Core Requirement #8 (water quality facilities).
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Figure 8- City of Renton Flow Control Application Map
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5.0 CONVEYANCE SYSTEM ANALYSIS AND DESIGN
Proposed conveyance elements include the existing 8” CPEP conveyance piping and a revised
storage pipe to accommodate the new addition. The graph below was created with the use of
WWHM, using a 15-minute time step to determine the expected 25 year peak flow that would
be experienced by the site.
Figure 9 - WWHM
Using Manning’s Equation:
1.49
Where A = 0.349ft2 for a 7” pipe, n = 13 for PVC or CPEP, and S = 1% to represent the shallowest
designed slope, yields a flow, Q, of 1.212cfs. This is above the 25 year peak flow rate of
0.0664cfs.
The design of the proposed conveyance systems is discussed in more detail in Appendix A.5.
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6.0 SPECIAL REPORTS AND STUDIES
We are not aware of any site characteristics that are particularly sensitive to stormwater runoff.
A geotechnical report by Migizi Group can be found in Appendix A.6 of this report.
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7.0 OTHER PERMITS
In addition to the Civil Construction permit, we anticipate that Site Plan Review, Environmental
(SEPA), and Building permits will be required for this project.
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8.0 CSWPPP ANALYSIS AND DESIGN
The Construction Stormwater Pollution Prevention Plan report will be included under separate
cover, at the time of application for the Civil Construction Permit.
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9.0 BOND QUANTITIES AND R/D FACILITY SUMMARIES
9.1 Bond Quantities
See Appendix A.2 for a copy of the City of Renton bond report.
9.2 Facility Summaries
The project will implement catch basins, conveyance piping, and a new detention pipe.
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10.0 OPERATIONS AND MAINTENANCE MANUAL
10.1 Purpose
The purpose of this manual is to provide guidelines for maintaining the storm drainage system
as a part of the construction for the WSADA building addition. The site address is 621 SW Grady
Way are 201 SW 12th Street and 215 SW 12th Street, Renton, WA 98055.
Storm utility improvements that have been proposed as a part of the projects include:
o Storm Drainage Piping
o Catch Basins
o Detention Pipe
Each portion of the system has to be maintained in good working condition for the system to
function properly.
After construction, operations and maintenance of the private drainage system will be the
responsibility of WSADA. Vicki Giles Fabré is the contact person. Her contact information is:
Vicki Giles Fabré
Executive Vice President
Washington State Auto Dealers Association
621 SW Grady Way, Renton, WA 98057
(206) 433-6300
The City of Renton will continue to be responsible for the Operation and Maintenance of public
improvements.
10.2 Permanent Facilities Description
Existing rooftop conveyance systems will be realigned with new catch basins and piping.
Runoff from the proposed building addition will be conveyed to a new detention pipe located
near the southwest corner of the existing building. The detention pipe must be renovated to
accommodate the new building. The new detention pipe will be shorter with an increased
diameter relative to the existing detention pipe. The control structure for the existing
detention system will not be altered, however, due to the increased impervious surface, we
expect the durations and volumes of stormwater discharged from the site to increase slightly.
The project does not propose to alter downstream flow paths and will maintain ultimate
discharge to the Black River.
10.3 Discussion of Maintenance
Any buildup of sediment, debris, vegetation, or trash that impedes the catch basins will reduce
the storm system capacity. As a result, care must be taken to keep all inlets clear of debris.
The applicable maintenance checklists and excerpts from the 2022 SWDM have been included
with the Operations and Maintenance Manual for review during routine maintenance
inspections.
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10.4 Maintenance Frequency
Following construction of the project, the storm drainage system shall be inspected and
maintained according to their respective maintenance checklists included at the end of this
manual.
Facilities will be inspected annually, or after every significant storm event where the
precipitation is greater than or equal to one inch in 24 hours
When deficiencies are noted, the problems are to be corrected as soon as possible. Any spill of
hazardous material (e.g. fuel, lubricant, herbicide, etc.) shall be cleaned up immediately and
shall be reported to the Division of Emergency Management (1-800-523-5044). Contaminated
material will be disposed of properly.
Any questions about the existence of a problem should be directed to a Professional Engineer.
10.5 Annual Cost Estimate
Annual maintenance costs for the proposed building addition stormwater conveyance system
is expected to increase the current cost private stormwater system maintenance by $1 for every
linear foot of new piping. Existing stormwater infrastructure (including the detention pipe) is
already accommodated
Annual maintenance costs for the proposed stormwater conveyance system on-site for the
proposed addition is expected to increase the current costs of maintaining the private
stormwater system by $1 for every linear foot of new piping. Approximately 80 linear feet of
new pipe will increase the annual cost of maintenance by $80.
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Cover Sheet for Inspection Forms
Name of Inspector:
_____________________________________________________________
Date of Inspection:
_____________________________________________________________
Number of Sheets Attached:
_____________________________________________________________
Inspector’s Signature:
_____________________________________________________________
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Maintenance Log
Action
Taken
Name Date How Procedure
Was Performed
Problems
Encountered
Additional Actions
Recommended
o
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A.1 TIR Worksheet
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A.2 Bond Quantity Worksheet
The Bond Quantity Worksheet will be included under separate cover at the time of application
for the civil construction permit.
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A.3 Existing and Proposed Surface Coverage Exhibits
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A.4 Site Historical Aerial Photography
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A.5 Stormwater Engineering Calculations
As previously mentioned, although this site is exempt from additional flow control
requirements, the proposed site will be draining through an existing detention pipe facility that
will need to be modified to accommodate the building.
The existing detention pipe will need to rotated counterclockwise about the existing control
structure in order to provide clearance for the proposed building addition. Also, the pipe will
need to be shortened, and increased in diameter to provide the necessary storage volume.
We are not proposing to alter the control structure of the existing detention system, as such,
the durations and volumes of stormwater released from the site will be slightly increased, due
to the increase in impervious surface tributary to the detention pipe. The following calculations
illustrate the predeveloped and developed conditions of the detention system (existing and
proposed detention pipes, respectively).
Predeveloped Conditions:
Based on aerial photographs, included in Section A.4, the site had previously been a forested
lot before the development of the WSADA office building. As this project is introducing less
than 2,000SF, and is modifying an existing stormwater feature, the present WSADA office and
coverage will be used as ‘predeveloped conditions’ to demonstrate the modified system will
meet the needs of the existing system.
See the following excerpt from WWHM indicating the assumptions used in the “Predeveloped”
Condition:
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Mitigated Conditions:
The mitigated site coverage is comprised of almost identical surfacing to the existing site
coverage, with the exception of the addition covering a slightly larger area than the concrete
patio it is replacing.
See the following excerpt from WWHM indicating the assumptions used in the “Mitigated”
Condition:
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Flow Frequency Results:
As mentioned in Section 4.3, this project does not require flow control and the control structure
is not being changed. However, the flow frequency table included below has been included to
demonstrate that the modified stormwater detention system does not greatly change the
discharge rate for the design storm events.
Detention System Modifications:
In order to accommodate the proposed building addition, while also matching the existing
stormwater discharge characteristics of the current site, we are proposing to replace the
existing 35 linear foot segment of 48” diameter detention pipe with a shorter 60” diameter
detention pipe. The proposed 24 linear foot 60” diameter detention pipe will provide an
equivalent volume. The control structure will also need to be modified since detained water
within the 60” pipe will attain a higher pressure head than the 48” pipe. The calculations on the
following page illustrate the characteristics of the existing and proposed detention pipe and
control structure. The graph indicates that the proposed detention pipe will maintain or
slightly reduce stormwater discharges from the site relative to existing conditions.
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Water Quality:
A biofiltration cell currently treats stormwater for this portion of the site. The addition will
impact its footprint, requiring the cell to be modified. After modification, the biofiltration cell
will maintain the same treatment area. While the addition is increasing the amount of
impervious surface flowing to the biofiltration cell, it is also replacing pollution generating
impervious surface coverage with non-pollution generating impervious surface coverage.
Washington State Auto Dealers Association – Technical Information Report 45
Washington State Auto Dealers Association – Technical Information Report
A.6 Geotechnical Engineering Report
The Geotechnical Report, prepared by Migizi Group, Inc, dated September 26, 2022, is
included under a separate cover.