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Brotherton Buick-GMC
Cadillac Dealership
Technical
Information Report
Prepared for:
Brotherton Cadillac
215 SW 12th Street
Renton, WA 98057
Contact: Brad Brotherton
Phone: (425) 336-1089
Prepared by:
Sitts & Hill Engineers, Inc.
4815 Center Street
Tacoma, Washington 98409
Contact: Don Davis, P.E.
Phone: (253) 474-9449
January 2022
S&H Job Number 19,480
Brotherton Buick-GMC
Cadillac Dealership
Technical Information Report
Prepared for:
Brotherton Cadillac
215 SW 12th Street
Renton, WA 98057
Contact: Brad Brotherton
Phone: (425) 336-1089
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
January 2022 S&H Job Number 19,480
PRELIMINARY
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TABLE OF CONTENTS
1.0 PROJECT OVERVIEW .............................................................................................................. 3
1.1 Scope ............................................................................................................................................................... 3
1.2 Existing Conditions ........................................................................................................................................ 4
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 #3: Flow Control ..................................................................................................... 7
2.1.4 Core Requirement #4: Conveyance System ....................................................................................... 7
2.1.5 Core Requirement #5: Erosion and Sediment Control ...................................................................... 7
2.1.6 Core Requirement #6: Maintenance and Operations ........................................................................ 7
2.1.7 Core Requirement #7: Financial Guarantees and Liability ............................................................... 7
2.1.8 Core Requirement #8: Water Quality ................................................................................................... 8
2.1.9 Core Requirement #9: On-site BMPs ................................................................................................... 8
2.1.10 Special Requirement #1: Other Adopted Area-Specific Requirements ........................................ 9
2.1.11 Special Requirement #2: Flood Hazard Area Delineation .............................................................. 9
2.1.12 Special Requirement #3: Flood Protection Facilities ..................................................................... 10
2.1.13 Special Requirement #4: Source Control ........................................................................................ 10
2.1.14 Special Requirement #5: Oil Control ................................................................................................ 10
2.1.15 Special Requirement #6: Aquifer Protection Areas ....................................................................... 10
3.0 OFF-SITE ANALYSIS .............................................................................................................. 13
3.1 Define and Map the Study Area ................................................................................................................. 13
3.2 Resource Review ......................................................................................................................................... 18
3.3 Field Inspection ............................................................................................................................................ 18
3.4 Drainage System Description and Problem Descriptions ...................................................................... 18
3.5 Mitigation of Existing or Potential Problems ............................................................................................. 18
4.0 FLOW CONTROL, LID, AND WATER QUALITY FACILITY ANALYSIS AND DESIGN ..... 19
4.1 Existing Site Hydrology ............................................................................................................................... 19
4.2 Developed Site Hydrology .......................................................................................................................... 19
4.3 Performance Standards .............................................................................................................................. 19
4.4 Flow Control System .................................................................................................................................... 19
4.4.1 Flow Control Exemptions Check ......................................................................................................... 19
4.4.2 Flow Control Area .................................................................................................................................. 19
4.4.3 Flow Control Area Requirements and Exceptions ........................................................................... 19
4.4.4 Identify Applicable Flow Control BMPs per Core Requirement #9 ................................................ 20
4.5 Water Quality ................................................................................................................................................ 20
5.0 CONVEYANCE SYSTEM ANALYSIS AND DESIGN ............................................................ 22
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6.0 SPECIAL REPORTS AND STUDIES ...................................................................................... 23
7.0 OTHER PERMITS .................................................................................................................... 24
8.0 CSWPPP ANALYSIS AND DESIGN ....................................................................................... 25
9.0 BOND QUANTITIES and R/D FACILITY SUMMARIES ........................................................ 26
9.1 Bond Quantities ............................................................................................................................................ 26
9.2 Facility Summaries ....................................................................................................................................... 26
10.0 OPERATIONS AND MAINTENANCE MANUAL .................................................................. 27
10.1 Purpose ....................................................................................................................................................... 27
10.2 Permanent Facilities Description ............................................................................................................. 27
10.3 Discussion of Maintenance....................................................................................................................... 27
10.4 Maintenance Frequency ........................................................................................................................... 28
10.5 Annual Cost Estimate ................................................................................................................................ 28
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
A.7 WWHM Reports
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1.0 PROJECT OVERVIEW
1.1 Scope
This Technical Information Report (TIR) addresses improvements associated with the proposed building
located at 201 SW 12th Street, Renton, WA (Parcel Nos. 3340402780, 3340402805, 3340402880, and
3340402925). Please see Figure 1 for the Site Location Map. This construction is proposed to serve as a
stand-alone Cadillac automotive dealership for Brotherton Cadillac. The new dealership will replace
existing residential development with driveway connection s to SW 12th Street and the neighboring
Brotherton Buick-GMC dealership via the SW 13th Street former right of way.
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.
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Figure 1 – Site Location Map
1.2 Existing Conditions
The subject site is located on four parcels, zoned as Commercial Arterial (CA), with a combined area of
1.21 acres of developed land. The site, as depicted on the topographical survey, is relatively flat. The
original dealership was graded with depressions to direct stormwater into on-site catch basins. The
privately owned stormwater system extends upstream to the east, collecting stormwater runoff from
two more parking areas to the east and north of the subject property before discharging into the system
of the neighboring automotive dealership .
The existing site consists of residential development. Based on historical aerial photographs and the King
County Assessor’s Website, the site appeared to be residential housing as early as 1951. One of the
residential lots was emptied between May of 2013 and July of 2014. Another was replaced with parking
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between May of 2017 and May of 2018 and replaced with a fenced, gravel parking area. See Appendix
A.4, which includes a series of aerial photographs dating back to July, 1990.
Upon a visual examination of existing site conditions, stormwater runoff is either collected in a private
catch basin located on Parcel 3340402880, flows onto the neighboring Brotherton Cadillac automotive
dealership, or flows onto SW 12th St or SW 13th St to be collected by a public catch basin. Stormwater
collected in the private catch basin, or that flows onto the neighboring dealership, is conveyed through
an existing private stormwater system that discharges into the public stormwater system on the
southwest corner of the dealership. Based upon a 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.
1.3 Proposed Conditions
The purpose of site improvements for this project is to create a new stand-alone automotive dealership
for Brotherton Cadillac. The project proposes grading, paving, and drainage (water quality facilities,
detention, and conveyance piping) improvements for on-site areas. See Table 1 for a description of
existing and proposed surface conditions.
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 will be provided by a Contech Stormfilter. 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
Description On-site Off-site Total
Existing Conditions
Total Project Area (ft2) 51,657 6,669 57,936
Existing hard surface (ft2) 22,287 4,933 27,220
Existing landscaping/lawn area (ft2) 28,980 1,736 30,716
Total Project Area (ft2) 51,657 6,669 57,936
Amount of new hard surface (ft2) 22,287 725 23,012
Amount of new pollution generating hard surface (PGHS) (ft 2) 16,872 405 17,277
Amount of replaced hard surface (ft2) 21,267 4,560 25,827
Amount of replaced PGHS (ft2) 14,480 5,186 19,666
Amount of new plus replaced hard surface (ft2) 43,554 5,285 48,839
Amount of new plus replaced PGHS (ft2) 31,352 5,591 36,943
Amount of existing hard surfaces converted to vegetation/landscaping (ft 2) 1058 373 1,431
Amount of Land Disturbed (ft2) 51,657 6,669 57,936
Native Vegetation to Pasture (acres) 0 0 0
Existing vegetation area to remain (ft2) 2,488 146 2,634
Existing hard surface to remain unaltered (ft 2) 0 0 0
Value of proposed improvements ($) TBD 0 TBD
Amount to be Graded/Approx. CUT (C.Y.) TBD 0 TBD
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2.0 CONDITIONS AND REQUIREMENTS SUMMARY
2.1 Discussion of Core and Special Requirements
This is a Commercial Arterial (CA) project that adds more than 2,000 square feet of new and replaced
impervious surface. Therefore, the project is subject to a full drainage review and must satisfy all core
requirements (1-9) and all special requirements (1-6) of the 2017 City of Renton Stormwater Design
Manual (SWDM). See “Figure 2 – Drainage Review Flow Chart” at the end of this Section. 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 discharging stormwater to the existing private stormwater network,
which ultimately discharges into 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 #3: Flow Control
Please see Section 4.0 for a discussion of flow control for the project. The project proposes work within
the Peak Rate Flow Control Standards (Existing Site Conditions) Area. The project must match the
developed peak discharge rates to the existing site conditions peak discharge rates for the 2-, 10-, and
100-year return periods. Existing flow volumes will be matched through the use of a storage vault and
variable speed pump station.
2.1.4 Core Requirement #4: Conveyance System
Please see Section 5.0 of this TIR for the conveyance system analysis and design. This project proposes
to create a new stormwater system that will service stormwater that falls on -site. Stormwater from
upstream that will be flowing through the site will bypass storage and treatment before discharging into
the existing downstream system in conjunction with stormwater treated on-site.
2.1.5 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.
2.1.6 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. Brotherton Cadillac 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.
2.1.7 Core Requirement #7: Financial Guarantees and Liability
We anticipate that Brotherton Cadillac 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.
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2.1.8 Core Requirement #8: Water Quality
Please see Section 4.0 for a discussion of water quality for the project. Water quality treatment will be
provided by a Contech Stormfilter using METALRX media cartridges.
2.1.9 Core Requirement #9: On-site BMPs
Target surfaces subject to Core Requirement #9 On-site BMPs include replaced impervious surfaces,
already mitigated with an approved on-site BMP. This site qualifies for the Large Lot BMP Requirements
per 1.2.9.2.2 of the City of Renton SWDM, since it is larger than 22,000 square feet. These requirements
state that all proposed pervious surfaces must implement the soil amendment BMP and roof downspout
connections must be perforated pipe connections. The soil amendment BMP will be implemented for
every pervious service affected by construction. The downspout BMP does not apply to the proposed
project as infiltration is not recommended on this site, per the geotechnical report in Section 6.0.
In addition, these requirements require the consideration of the following BMPs: full dispersion, full
infiltration, limited infiltration, bioretention, permeable pavement, basic dispersion, reduce d impervious
surface credit, native growth retention credit, tree retention credit. None of the listed BMPs are feasible
for implementation on the project site. Therefore, Core Requirement #9 will be not be implemented.
The remainder of this section contains a discussion of infeasibility for each BMP.
Full Dispersion – The application of this BMP depends on the sheet flow of runoff t hrough an area of
native vegetation. The project site does not contain any native vegetation area (e.g., forested) in the
existing or proposed condition. Therefore, full dispersion is infeasible for this project.
Full Infiltration – Per Section C.2.2.2 of the Manual, full infiltration requires that “Existing soils must be
coarse sands or cobbles or medium sands and cannot be comprised of fill materials where the
infiltration device will be located.” Site soils meet this requirement , except for the top layer of soil.
However, the groundwater level renders infiltration on-site infeasible, as infiltration would have to occur
in a silt material. These conditions are described in the Geotechnical Engineering Report prepared by
Migizi Group, Inc. (dated July 8, 2021) for the proposed project. This report shows test pit logs with four
distinct soil horizons, none of which provide viable infiltration due to either the groundwater level or
material. The Geotechnical Engineer does not find that site soils will support Full or Limited infiltration
(see section 3.4 Infiltration Conditions of the Migizi report). Therefore, full infiltration is infeasible for
this project.
Limited Infiltration – The infeasibility criteria for limited infiltration are the same as those for full
infiltration except that appropriate soils may be fine sands, loamy sands, sandy loams, or loams. Note
that Section C.2.3.2 of the Manual states “Silt and clay loams, and cemented t ill (hardpan) are not
suitable for limited infiltration systems.” Site soils meet this requirement, except for the top layer of soil.
However, the groundwater level renders infiltration on-site infeasible, as infiltration would have to occur
in a silt material. The Geotechnical Engineer does not find that site soils will support Full or Limited
infiltration (see section 3.4 Infiltration Conditions of the Migizi report). Therefore, limited infiltration is
infeasible for this project.
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Bioretention – This project is underlain by soil that makes infiltration impractical, as such a bioretention
facility is infeasible.
Permeable Pavement – The soil under a permeable pavement section must provide adequate treatment
for groundwater protection. The underlying soi ls should also infiltrate for this surfacing to be durable
and effective. The requirements for adequate treatment are a minimum organic content of 1.0% and a
minimum cation exchange capacity of 8 milliequivalents per 100 grams. The uppermost soil layer on the
project site is made up of fill material and does not meet these requirements. Again, the depth to
groundwater renders BMPs that rely on infiltration ineffective . Permeable Pavement is infeasible for this
project.
Basic Dispersion – The implementation of basic dispersion limits the area of impervious surface that can
run off to a dispersion structure to 700 square feet per structure. The proposed construction will be
adding over 16,500 square feet, which would require 24 separate flow dispersion structures. It is also
required for the dispersed stormwater to flow over a vegetated surface, but there are no existing or
proposed vegetated surfaces near the proposed development. Such a flowpath cannot be provided on
the project site without a drastic reduction in the number of parking stalls and vehicle paths. We see
basic dispersion as infeasible to the project.
Reduced Impervious Surface Credit – This BMP presents four possible methods of impervious surface
reduction: restricted footprint, wheel strip driveways, minimum disturbance foundation, and open grid
decking over pervious surface. The restricted footprint method requires less than 4,000 square feet of
impervious area, which is not feasible for the proposed use of the site as a n automotive dealership. A
wheel strip driveway is not applicable for a commercial parking lot. A minimum disturbance foundation
is not feasible for an automotive workshop. Open grid decking over a pervious surface is also not
applicable to a parking lot. The reduced impervious surface credit is infeasible for this project.
Native Growth Retention Credit – The existing condition of the site does not contain any native growth.
Therefore, the native growth retention credit is infeasible for this project.
Tree Retention Credit – The existing condition of the site contains approximately 20 trees that will all be
removed by the current owner as a condition of the lease agreement . Therefore, the tree retention
credit is infeasible for this project.
2.1.10 Special Requirement #1: Other Adopted Area-Specific Requirements
N/A. The project site is not in a designated critical drainage area or in an area included in an adopted
master drainage plan, basin plan, salmon conservation plan, stormwater compliance plan, flood hazard
management plan, lake management plan, or shared facility drainage plan.
2.1.11 Special Requirement #2: Flood Hazard Area Delineation
N/A. Per the FEMA floodplain map at the end of this Section, Figure 3, the project site lies within Zone X
which is defined as “areas determined to be outside 500-year floodplain” and therefore flood-related
delineation on the project site is not required.
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2.1.12 Special Requirement #3: Flood Protection Facilities
N/A. This project does not propose to rely on an existing flood protection facility nor does it modify or
construct a new flood protection facility.
2.1.13 Special Requirement #4: Source Control
Brotherton shall implement Operational and Structural Source Control BMPs that includes the
maintenance of stormwater drainage and treatment systems as described in the Operations and
Maintenance Manual, included in Section 10.0 of this report.
2.1.14 Special Requirement #5: Oil Control
This project does not propose to develop a high-use site as defined by City of Renton SWDM; therefore,
this Special Requirement is not applicable.
2.1.15 Special Requirement #6: Aquifer Protection Areas
This project is not located within an Aquifer Protection Area per King County GIS.
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Figure 2 – Drainage Review Flow Chart
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Figure 3 – 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
an existing, private conveyance system. Those portions of the site that do not discharge to the existing,
private conveyance system are tributary to an existing City conveyance system. The project does not
propose any significant improvements to the existing conveyance elements on-site. Downstream of the
project discharge locations, according to City of Renton GIS, stormwater is conveyed west, into, and
through, the neighboring Brotherton Cadillac automotive dealership stormwater system. The
conveyance continues southward until it reaches Interstate 405 (I405). Before intersecting I405,
stormwater flows into a catch basin on the north side and is diverted to the west. The pipe daylights and
discharges the stormwater to flow over the surface before being collected in another City stormwater
conveyance pipe on the south side of the existing Good Chevrolet, INC Chevrolet dealer. Stormwater
then flows in another enclosed system under Lind Ave. SW before again daylighting and being
discharged to run along the north side of I405. Stormwater continues flowing over the surface north of
I405 before sheet flowing onto SW 13th St and being collected in a City catch basin. The pipe network
downstream of this catch basin ultimately discharges into Black River. A topographic site map is featured
in Figure 5 of this Report.
Figure 4 - Downstream Flow Path
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Figure 5 – Topographic Survey
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3.2 Resource Review
A thorough review of the resources provided by the City of Renton Maps, as outlined on page 2-11 of
the SWDM, has been completed for the study area. Research conducted on 1/17/2022 has shown that
there are not any adopted basin plans available for the study area. According to the FEMA Map
presented in Figure 3 of this report, the project site lies 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. Please see
Section 4.0 for a discussion of water quality for the project. 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 obvi ous clogs or
flooding. We do not anticipate any future problems with the proposed stormwater system. The
proposed conveyance system has more catch basins: therefore each individual catch basin will have a
smaller tributary area. Stormwater flows are expected to increase due to an increase in impervious
surface, and will be handled by a pump station and supplementary detention system to match existing
discharge into the existing downstream system. Please see Section A.5 for the modeled areas in the
predeveloped and mitigated scenarios.
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/residential land applications, such as
several automotive dealerships and the residential lots to the east of the site. In addition, the study area
includes utilities and roadways. Existing impervious surfaces located within the study area include
commercial businesses, compacted gravel access, paved parking lots, paved driveways, and the paved
public right-of-way. Based on existing site topography, stormwater runoff is captured within the City’s
right-of-way where it is collected by a storm conveyance system and discharged to the Black River.
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 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 site currently has four residence, two sheds, shared lawn surfacing, and a large parking area
composed of both crushed rock surfacing and asphalt cement pavement. Stormwater that does not
infiltrate into the lawn, sheetflows off-site primarily onto either SW 12th St or SW 13th St. Run-off can
flow onto the nearby existing Brotherton Cadillac dealership, although the site is graded in such a way
that this is unlikely. From SW 12th St or SW 13th St, stormwater flows into City stormwater conveyance
facilities.
4.2 Developed Site Hydrology
The proposed site will be comprised of primaril y asphalt pavement surfacing and roof area. There will be
landscaped islands around the perimeter of the site, with approximately 4,872 sf of landscaping.
Stormwater on-site will be collected and conveyed into an on -site storage facility after treatment. The
project does not propose to alter the downstream flow path and will maintain discharge to the natural
location.
4.3 Performance Standards
The site is within a Peak Rate Flow Control Standards (Existing Site Conditions) Area, which requires
matching the existing site conditions 2-, 10-, and 100- year peak flows. The predeveloped site is
comprised of asphalt pavement, rooftop surfacing, lawn, and crushed rock surfacing. The proposed site
conditions include a significantly 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. The proposed private conveyance elements include conveyance piping from the
new asphalt surfacing to a Contech Stormfilter before it is detained. Stormwater from the new roof will
be conveyed directly to the detention facility. Enhanced basic water quality treatment is required for
stormwater that falls on, or runs off of, pollution-generating surfaces on-site such as the parking lot.
4.4 Flow Control System
Flow control is required for this project. Stormwater discharging from the site will be controlled by a
pump station.
4.4.1 Flow Control Exemptions Check
The project site is not exempt from flow control. See Section 2.1.3 of this report for further discussion.
4.4.2 Flow Control Area
The site falls within a Peak Rate Flow Control Standard – Match Existing Area, based on the Flow Control
Application Map provided by the City of Renton (Figure 6 of this TIR).
4.4.3 Flow Control Area Requirements and Exceptions
Peak Rate Flow Control Standard – Match Existing – Areas must match existing site conditions 2-,10-,
and 100-year peak rate runoff for areas draining to constructed (man -made) or highly modified drainage
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systems so as to not create a downstream flooding problem. Per Section 1.2.3.1.A, Exception 1, of the
SWDM, the target surfaces subject to the Peak Rate Flow Control Standard Areas facility requirements
will generate more than a 0.15-cfs increase in the existing site conditions peak flows. Therefore, this
project proposes a pump station and supplementary detention system to serve as a flow control facility.
4.4.4 Identify Applicable Flow Control BMPs per Core Requirement #9
Please see Section 2.1.9 of this report for an evaluation of Flow Control BMPs per Core Requirement #9.
4.5 Water Quality
As mentioned in Section 4.3, the project proposes t o satisfy the enhanced water quality requirement by
installing a Contech Stormfilter.
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Figure 6- City of Renton Flow Control Application Map
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5.0 CONVEYANCE SYSTEM ANALYSIS AND DESIGN
Proposed conveyance elements include conveyance piping to connect the new roof surface with the
existing stormwater system on-site.
The proposed conveyance systems will be developed prior to, and included with, the civil permit
construction application.
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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
We anticipate that a Right-of-Way Use permit, Land Use Master Application, Civil Construction Permit,
Building Permit, and an Environmental Review Permit (SEPA) will be required.
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8.0 CSWPPP ANALYSIS AND DESIGN
The Construction Stormwater Pollution Prevention Plan report will be included with the Civil
Construction Permit application.
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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
This project proposes to implement a Contech CDS hydrodynamic separator, Contech Stormfilter,
Oldcastle StormCapture water detention, and conveyance piping.
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10.0 OPERATIONS AND MAINTENANCE MANUAL
10.1 Purpose
The purpose of this manual is to provide guidelines fo r maintaining the storm drainage system as a part
of the construction for the new Brotherton Cadillac building. The site address is 201 SW 12th Street,
Renton, WA 98057.
Storm utility improvements that have been proposed as a part of the project include:
o Contech CDS hydrodynamic separator
o Contech Stormfilter CB
o Oldcastle StormCapture
o Storm Drainage Piping
o Catch Basins
o Pump Station
Each portion of the system has to be maintained in good working condition for th e system to function
properly.
Operations and Maintenance of the private drainage systems will be the responsibility of the Owner,
Brotherton. Brad Brotherton is the contact person at Brotherton. His contact information is:
Brad Brotherton
Brotherton Cadillac
215 Southwest 12th Street
Renton, WA 98057
Phone: (425) 336-1089
The City of Renton will continue to be responsible for the Operation and Maintenance of public
improvements.
10.2 Permanent Facilities Description
Conveyance piping will convey stormwater from the new rooftop and paved surfaces to the existing
stormwater facilities on-site. The project does not propose to alter downstream flow paths and will
maintain ultimate discharge to the Black River. A Contech CDS hydrodynamic separator and Contech
Stormfilter will provide water quality treatment. Oldcastle StormCapture will store stormwater so it can
be discharged at a controlled rate by the proposed pump station.
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.
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The applicable maintenance checklists and excerpts from the 2017 SWDM have been included with the
Operations and Maintenance Manual for review during routine maintenance inspections.
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 stormwater conveyance system is expected to increase the
costs of maintaining the private stormwater system by $1 for every linear foot of new piping, $500 for
each CDS, $500 for every Stormfilter catch basin, $500 per pump station, and $300 for each
StormCapture unit. Approximately 800 linear feet of proposed pipe, one CDS, one Stormfilter catch
basin, one pump station, and 6 StormCapture units will have an annual cost of maintenance of $4,100.
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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
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A.1 TIR Worksheet
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A.2 Bond Quantity Worksheet
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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
Flow Control
This project is in a location subject to meeting the Peak Rate Flow Control Standard, as described in
Section 4.4. Therefore, we have prepared the following calculations to determine if flow control will be
required for this project.
As previously mentioned, the site is predominantly currently covered by lawn and several small
structures. The site presently discharges offsite to the west. There is presently no treatment or flow
control in this system. Flow control will be required, to be installed in this system to match the existing
discharge rate for the 2-, 10-, and 100-year storm peak flow rates.
Predeveloped Conditions:
Based on aerial photographs, included in Section A.4, the site had previously, and is currently, a
residential location. The eastern residences appear to have been replaced with parking between May of
2013 and May of 2018. Areas of the replaced residential space used for stormwater design were
acquired from aerial photographs taken in May of 2010 as they are clear enough to use for approximate
surface areas. Spaces that appear to have remained largely unchanged had their areas gathered from
survey data of the site.
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 asphalt pavement parking and the rooftop of the proposed
structure with sidewalks and landscaping along the perimeter of the site and proposed automotive
dealership.
See the following excerpt from WWHM indicating the assumptions used in the “Mitigated” Condition:
Flow Frequency Results:
The flow frequency table included on the following page indicates that the peak stormwater runoff from
the developed site will increase by more than 0.15cfs for the 100-year storm condition. As the flow is
expected to change by more than 0.15cfs, and more than 5,000 square feet of new and replaced
impervious surface are proposed, a flow control structure will be necessary. It is proposed to use a pump
station for flow control, and to maintain connection with the existing downstream facilities. Excess flow
will be detained in Contech clamshell storage vaults.
Water Quality
More than 5,000 square feet of impervious surface is being installed or replaced during this project,
therefore water quality treatment is required. Water quality treatment flows calculated by WWHM are
shown below. The proposed rooftop was not included in the water quality calculations as it is considered
to not be a pollution-generating surface and will bypass the treatment structure. It was, however,
included in the calculations for detention requirements. Treatment flows did not change with, or
without, the inclusion of the roof surface. A five-cartridge Contech METALRX Stormfilter with 18”
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cartridges will be used as an on-line treatment structure. Five 18” cartridges will be required to handle
the on-line treatment flow, as each cartridge has a capacity of 7.5gpm, or 0.0167cfs.
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A.6 Geotechnical Engineering Report
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A.7 WWHM Reports