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Brotherton Dealership
Dealership Construction and Expansion
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
March 2022
S&H Job Number 19,480
Exhibit 11
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Brotherton Dealership
Dealership Construction and Expansion
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
March 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 ..................................................................................................................................... 6
2.0 Conditions and Requirements Summary .............................................................................. 9
2.1 Discussion of Core and Special Requirements ......................................................................................... 9
2.1.1 Core Requirement #1: Discharge at the Natural Location ................................................................ 9
2.1.2 Core Requirement #2: Off-site Analysis .............................................................................................. 9
2.1.3 Core Requirement #3: Flow Control ..................................................................................................... 9
2.1.4 Core Requirement #4: Conveyance System ....................................................................................... 9
2.1.5 Core Requirement #5: Erosion and Sediment Control ...................................................................... 9
2.1.6 Core Requirement #6: Maintenance and Operations ........................................................................ 9
2.1.7 Core Requirement #7: Financial Guarantees and Liability ............................................................. 10
2.1.8 Core Requirement #8: Water Quality ................................................................................................. 10
2.1.9 Core Requirement #9: On-site BMPs ................................................................................................. 10
2.1.10 Special Requirement #1: Other Adopted Area-Specific Requirements ...................................... 11
2.1.11 Special Requirement #2: Flood Hazard Area Delineation ............................................................ 12
2.1.12 Special Requirement #3: Flood Protection Facilities ..................................................................... 12
2.1.13 Special Requirement #4: Source Control ........................................................................................ 12
2.1.14 Special Requirement #5: Oil Control ................................................................................................ 12
2.1.15 Special Requirement #6: Aquifer Protection Areas ....................................................................... 12
3.0 OFF-SITE ANALYSIS .............................................................................................................. 17
3.1 Define and Map the Study Area ................................................................................................................. 17
3.2 Resource Review ......................................................................................................................................... 23
3.3 Field Inspection ............................................................................................................................................ 23
3.4 Drainage System Description and Problem Descriptions ...................................................................... 23
3.5 Mitigation of Existing or Potential Problems ............................................................................................. 23
4.0 FLOW CONTROL, LID, AND WATER QUALITY FACILITY ANALYSIS AND DESIGN ..... 24
4.1 Existing Site Hydrology ............................................................................................................................... 24
4.2 Developed Site Hydrology .......................................................................................................................... 24
4.3 Performance Standards .............................................................................................................................. 24
4.4 Flow Control System .................................................................................................................................... 25
4.4.1 Flow Control Exemptions Check ......................................................................................................... 25
4.4.2 Flow Control Area .................................................................................................................................. 25
4.4.3 Flow Control Area Requirements and Exceptions ........................................................................... 25
4.4.4 Identify Applicable Flow Control BMPs per Core Requirement #9 ................................................ 25
4.5 Water Quality ................................................................................................................................................ 25
5.0 CONVEYANCE SYSTEM ANALYSIS AND DESIGN ............................................................ 27
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6.0 SPECIAL REPORTS AND STUDIES ...................................................................................... 28
7.0 OTHER PERMITS .................................................................................................................... 29
8.0 CSWPPP ANALYSIS AND DESIGN ....................................................................................... 30
9.0 BOND QUANTITIES and R/D FACILITY SUMMARIES ........................................................ 31
9.1 Bond Quantities ............................................................................................................................................ 31
9.2 Facility Summaries ....................................................................................................................................... 31
10.0 OPERATIONS AND MAINTENANCE MANUAL .................................................................. 32
10.1 Purpose ....................................................................................................................................................... 32
10.2 Permanent Facilities Description ............................................................................................................. 32
10.3 Discussion of Maintenance....................................................................................................................... 32
10.4 Maintenance Frequency ........................................................................................................................... 33
10.5 Annual Cost Estimate ................................................................................................................................ 33
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) and the proposed building addition at 215 SW 12th Steet, Renton, WA (Parcel Nos.
3340402820, 3340402850, 3340402860, 3340402870, and 3340402885). Please see Figure 1 for the Site
Location Map of the proposed building and Figure 2 for the Site Location Map of the proposed building
addition. 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
connections 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 – Proposed Building
1.2 Existing Conditions
The subject sites are located on nine parcels, zoned as Commercial Arterial (CA), with a combined area of
2.84 acres of developed land. The sites, as depicted on the topographical survey, are 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 Brotherton automotive dealership.
The existing sites consist of residential development, a Cadillac dealership, and associated parking. 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
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July of 2014. Another was replaced with parking 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 wet vault water quality and detention vault that discharges into the public stormwater
system on the southwest corner of the dealership. Stormwater in a portion of the southwest corner of the
existing dealership is collected into a system that conveys it west across Maple Ave into a neighboring
private stormwater system. 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
Figure 2 – Site Location Map – Proposed Addition
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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, and an addition for the existing automotive dealership. The project proposes
grading, paving, and drainage (water quality facilities, detention, and conveyance piping) improvements
for on-site and off-site areas. See Table 1 for a description of existing and proposed surface conditions of
the proposed building and Table 2 for a description of existing and proposed surfaces for the proposed
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 a Contech Stormfilter. 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 Building
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) (ft2) 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
(ft2)
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 (ft2) 0 0 0
Value of proposed improvements ($) TBD 0 TBD
Amount to be Graded/Approx. CUT (C.Y.) TBD 0 TBD
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Table 2. Existing and Developed Condition Description – Proposed Addition
Description On-site Off-site Total
Existing Conditions
Total Project Area (ft2) 70,995 0 70,995
Existing hard surface (ft2) 66,072 0 66,072
Existing landscaping/lawn area (ft2) 4,883 0 4,883
Developed Conditions
Total Project Area (ft2) 70,995 0 70,995
Amount of new hard surface (ft2) 0 0 0
Amount of new pollution generating hard surface (PGHS) (ft2) 0 0 0
Amount of replaced hard surface (ft2) 4,416 0 4,416
Amount of replaced PGHS (ft2) 7,345 0 7,345
Amount of new plus replaced hard surface (ft2) 11,761 0 11,761
Amount of new plus replaced PGHS (ft2) 0 0 0
Amount of existing hard surfaces converted to vegetation/landscaping
(ft2) 0 0 0
Amount of Land Disturbed (ft2) 11,761 0 11,761
Native Vegetation to Pasture (acres) 0 0 0
Existing vegetation area to remain (ft2) 4,883 0 4,883
Existing hard surface to remain unaltered (ft2) 54,351 0 54,351
Value of proposed improvements ($) 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
These are two Commercial Arterial (CA) projects that each add more than 2,000 square feet of new and
replaced impervious surface. Therefore, the projects are 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 3 at the end of this Section for the Drainage Review Flow Chart for the
proposed building and Figure 4 for the Drainage Review Flow Chart for the proposed addition. 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 projects. The projects propose 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 from the proposed building will be matched through the
use of a storage vault and variable speed pump station. The proposed addition is not expected to change
stormwater flows into the receiving system by more than 0.15cfs, therefore flow control is not required
for the proposed addition.
2.1.4 Core Requirement #4: Conveyance System
Please see Section 5.0 of this TIR for the conveyance system analysis and design. This proposed building
proposes to create a new stormwater system that will service stormwater that falls on-site. The proposed
addition will be connected to a private stormwater system to the west of the 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.
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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.
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 for the proposed building by a Contech Stormfilter using METALRX media cartridges. We propose
to use the existing stormwater system for treatment of run-off from the proposed addition.
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, reduced 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 through 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 till (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
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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.
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 soils 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 an 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.
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2.1.11 Special Requirement #2: Flood Hazard Area Delineation
N/A. Per the FEMA floodplain map at the end of this Section, Figures 5 and 6, the project sites lie 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.
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 3 – Drainage Review Flow Chart – Proposed Building
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Figure 4 – Drainage Review Flow Chart – Proposed Addition
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Figure 5 – FEMA Map – Proposed Building
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Figure 6 – FEMA Map – Proposed Addition
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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 from the proposed building site
is conveyed west, into, and through, the neighboring Brotherton Cadillac automotive dealership
stormwater system. Stormwater from the site of the proposed addition flows west into, and through, the
neighboring Good Chevrolet automotive dealership stormwater system before heading south. The
conveyance from both sites 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 Figures 7 and 8 of
this Report.
Figure 7 - Downstream Flow Path – Proposed Building
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Figure 8 – Downstream Flow Path – Proposed Addition
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BROTHERTON DEALERSHIP TECHNICAL INFORMATION REPORT 19 DocuSign Envelope ID: 206613E4-C662-427B-B5B2-D00F0C51DB08
BROTHERTON DEALERSHIP TECHNICAL INFORMATION REPORT 20 DocuSign Envelope ID: 206613E4-C662-427B-B5B2-D00F0C51DB08
BROTHERTON DEALERSHIP TECHNICAL INFORMATION REPORT 21 DocuSign Envelope ID: 206613E4-C662-427B-B5B2-D00F0C51DB08
BROTHERTON DEALERSHIP TECHNICAL INFORMATION REPORT 22 Figure 9 – Topographic SurveyDocuSign Envelope ID: 206613E4-C662-427B-B5B2-D00F0C51DB08
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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
Figures 5 and 6 of this report, the project sites lie within Zone X, which is defined as areas outside the 500-
year floodplain. The sites are 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 obvious 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 on the site of the proposed building 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. The amount of area tributary to
the existing conveyance system connected to the proposed addition will remain largely the same – there
is not a substantive increase in stormwater runoff tributary to the existing conveyance system as a result
of the proposed addition. 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/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 of the proposed building currently has four residences, 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.
The site of the proposed addition currently has an automotive dealership with paved parking/display
space that drains to one of two existing stormwater facilities on-site. One is a catch basin that flows
directly into the City stormwater system from the southwest area of the site. The other is a network that
begins to the east of the site and directs stormwater into a large detention structure under the western
parking area on-site. Prior to construction of the structural addition, the project site surfacing consisted
of asphalt pavement, roofing, and landscaped islands. The site is tributary to the neighboring Good
Chevrolet’s conveyance system located on the west side of Maple Ave SW.
4.2 Developed Site Hydrology
The proposed building site will be comprised of primarily 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.
The proposed addition site will be comprised of similar surfaces to existing, though some asphalt paved
area will be converted to roofed area. In the developed condition, approximately 4,116 sf of area which
was previously asphalt pavement will now be rooftop surfacing. The project does not propose to alter the
downstream flow path and will maintain discharge to the natural location.
4.3 Performance Standards
The sites are 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 sites are
comprised of asphalt pavement, rooftop surfacing, lawn, and crushed rock surfacing. The proposed
building site conditions include a significantly higher quantity of impervious surface than the existing
conditions. The proposed addition site conditions include the same quantities of pervious and impervious
surfaces as 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 for the proposed building 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
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quality treatment is required for stormwater that falls on, or runs off of, pollution-generating surfaces on-
site such as the parking lot.
The proposed private conveyance elements for the proposed addition include conveyance piping from the
new roof to a private Type 1 catch basin that is separate from the stormwater network on site. Conveyance
piping connects this catch basin to another existing private Type 1 catch basin, which then connects to the
neighboring Good Chevrolet’s stormwater system on the west side of Maple Ave SW. It is proposed to
install a small Contech Stormfilter catch basin on-site for treatment.
4.4 Flow Control System
Flow control is required for the proposed building. Stormwater discharging from the site will be controlled
by a pump station. Flow control is not required for the proposed addition.
4.4.1 Flow Control Exemptions Check
The proposed building site is not exempt from flow control. See Section 2.1.3 of this report for further
discussion. The proposed addition site is exempt from flow control due to an increase of less than 0.15 cfs
in the 100-year storm event. 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 10 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
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 of the proposed building 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, the proposed building project proposes a pump station and supplementary detention system
to serve as a flow control facility. The proposed addition is not expected to generate a 0.15cfs or greater
increase in the 100-year peak flow, therefore flow control is not required for that project.
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, both projects propose to satisfy the enhanced water quality requirement by
installing a Contech Stormfilter.
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Figure 10- 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 reports 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
These projects propose 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 for maintaining the storm drainage system as a part
of the construction for the new Brotherton Cadillac building and new Brotherton Buick-GMC building
addition. The neighboring site addresses are 201 SW 12th Street and 215 SW 12th Street, Renton, WA
98057.
Storm utility improvements that have been proposed as a part of the projects 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 the 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. For the proposed building, 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. The proposed
addition will be treated by a Contech Stormfilter.
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 on-site for the proposed
building 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 for the proposed building.
Annual maintenance costs for the proposed stormwater conveyance system on-site for the proposed
addition is expected to increase the costs of maintaining the private stormwater system by $1 for every
linear foot of new piping and $500 for every Stormfilter catch basin. Approximately 150 linear feet of
proposed pipe and one Stormfilter catch basin will increase the cost of maintenance by $650.
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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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Figure 11 – TIR Worksheet – Proposed Building
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Figure 12 – TIR Worksheet – Proposed Addition
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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
Proposed Building:
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” cartridges will be
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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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Proposed Addition:
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 already covered by asphalt parking areas and a
structure. The asphalt being replaced presently discharges offsite to the west, separate from the existing
wet vault. There is presently no treatment or flow control in this system. No flow control will be required,
or proposed, to be installed in this system, as the peak flow produced by the 100-year storm does not
increase by more than 0.15 cfs if the existing site conditions are assumed to be lawn.
Predeveloped Conditions:
Based on aerial photographs, included in Section A.4, the site had previously been a residential location
before the construction of the existing Brotherton Cadillac automotive dealership. Any reference we have
access to showing this residential development are too blurry to acquire usable areas for stormwater
calculations. As such, it will be assumed that the existing site condition is flat lawn covering, as that is the
most restrictive from a stormwater design perspective that would be possible.
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.
See the following excerpt from WWHM indicating the assumptions used in the “Mitigated” Condition:
Flow Frequency Results:
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The flow frequency table included on the following page indicates that the peak stormwater runoff from
the developed site will increase by less than 0.15cfs for the 100-year storm condition. As the flow is
expected to change by less than 0.15cfs, a flow control structure will not be necessary.
Water Quality
More than 5,000 square feet of impervious surface is being replaced during this project. While the net
pollution-generating surface is decreasing, 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. A two-cartridge Contech ZPG Stormfilter with 18” cartridges will be used as an on-line treatment
structure. Two 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
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