HomeMy WebLinkAbout31 - CarMax #7252 Technical Information Report
CarMax Auto Superstores
3751 E. Valley Road
Renton, Washington, 98057
Technical Information Report
Commercial Redevelopment
April 6, 2015
1505 Westlake Ave. N T 206.522.9510
Suite 305 F 206.522.8344
Seattle, WA 98109 www.pacland.com
Prepared By: Willis Liu
Reviewed By: Sarah Pangelinan, P.E.
CarMax Auto Superstores Technical Information Report Renton, Washington
PACLAND Project #10298005 Page 1
Section Page
Executive Summary ....................................................................................................................... 2
Section 1- Project Overview .......................................................................................................... 4
1. TIR Worksheet ................................................................................................................... 4
2. Site Location ...................................................................................................................... 9
3. Drainage Basins, Site Characteristics, and Subbasins ......................................................... 10
4. Soils ................................................................................................................................. 12
Section 2- Conditions and Requirements Summary ..................................................................... 12
Section 3- Offsite Analysis ........................................................................................................... 15
1. Upstream Analysis ............................................................................................................ 15
2. Downstream Analysis ....................................................................................................... 15
3. Evidence of Existing or Predicted Problems ...................................................................... 18
Section 4- Flow Control and Water Quality Facility Analysis and Design ................................... 19
1. Hydrologic Analysis ......................................................................................................... 19
2. Existing Site Hydrology .................................................................................................... 19
3. Developed Site Hydrology ............................................................................................... 20
4. Flow Control Analysis ...................................................................................................... 20
5. Water Quality System ....................................................................................................... 20
6. New PGIS ........................................................................................................................ 21
Section 5- Conveyance System Analysis and Design .................................................................... 21
1. Conveyance ..................................................................................................................... 21
2. 100 Year Flood/Overflow Condition ................................................................................. 22
Section 6- Special Reports and Studies ........................................................................................ 22
Section 7- Other Permits ............................................................................................................. 23
Section 8- CSWPPP Analysis and Design ..................................................................................... 23
Section 9- Bond Quantities, Facility Summaries, and Declaration of Covenant ........................... 24
Section 10- Operations and Maintenance Manual ....................................................................... 25
Appendices: A – Exhibits / Grading and Drainage Plan
B – Act III Theaters Drainage Report
C – Design Calculations
Table of Contents
CarMax Auto Superstores Technical Information Report Renton, Washington
PACLAND Project #10298005 Page 2
Site Location:
The property is located at 3751 E. Valley Road in Renton, Washington. The site is bordered by SW
41st Street to the south, E. Valley Road to the east, a Harley-Davidson dealership to the north, and a
medical and retail mixed-use building to the west. The site is currently developed as the Act III
Movie Theater, which has a building footprint of 61,856 square feet, with associated parking and
landscaping. A wetland and a stormwater detention and water quality facility is also located on the
western 1.13 AC of the site. The entire lot is approximately 12.28 acres in size. This project
proposes to demolish the existing theater and redevelop the site to become a used car dealership.
The project site lies within the Lower Green River Subwatershed, which is part of the
Green/Duwamish Watershed. It is in the City of Renton Black River drainage basin.
The project site is located in the City of Renton Employment Area Valley Overlay District (Valley
Community Planning area). A portion of the property is zoned IM (industrial medium) and the
remainder of the property is zoned CA (commercial arterial).
Design Criteria:
The City of Renton uses the King County Surface Water Management Design Manual (KCSWDM),
2009 Edition, as adopted by the City of Renton and the City of Renton Amendments to the King
County Surface Water Design Manual, dated February 2010. The existing site is developed and
entirely impervious with the exception of landscape islands and the existing wetland and
stormwater facilities along the western property line. The overall impervious area will remain
approximately the same, and additional required detention is not anticipated for this project.
However, additional water quality treatment will be provided to meet the requirements of Core
Requirement #8 as the project will increase the pollution generating impervious surface by more
than 5,000 SF.
Table 1
Jurisdictional Requirements
Peak Runoff Control: Peak Rate Flow Control Standard
Matching Existing Site Conditions
2-year: Match existing site conditions
10-year: Match existing site conditions
100-year: Match existing site conditions
Water Quality: Enhanced Basic WQ Treatment
Preceding detention 60% of the developed 2-year peak
flow rate
Downstream of detention The full 2-year release rate from
detention facility
Proposed Drainage System:
The project proposes to utilize the existing stormwater system for conveyance, water quality,
detention, and flow control and provide improvements, as necessary, for additional conveyance
and water quality treatment. Revisions to the stormwater system are anticipated to accommodate
the new building configuration and regrading of the site.
Executive Summary
CarMax Auto Superstores Technical Information Report Renton, Washington
PACLAND Project #10298005 Page 3
Conclusion:
The proposed stormwater management system for this project has been designed in accordance
with regulatory criteria described above and is consistent with sound engineering practice. The
site has been analyzed to determine the requirements for detention and water quality treatment,
and no significant adverse impacts to the stormwater management system are expected as a result
of the proposed development.
CarMax Auto Superstores Technical Information Report Renton, Washington
PACLAND Project #10298005 Page 4
Figure 1 - Technical Information Worksheet (KCSWDM 2009)
Section 1- Project Overview
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PACLAND Project #10298005 Page 5
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PACLAND Project #10298005 Page 6
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CarMax Auto Superstores Technical Information Report Renton, Washington
PACLAND Project #10298005 Page 9
Figure 2 - Site Location
Location: 3751 E. Valley Road. Renton, WA 98057
Section/Township/Range: SW Quarter of Section 30, Township 23, Range 05
Parcel/Tax Lot(s): 1253600030
Size: 12.28 acres
City, County, State: Renton, King County, Washington
Governing Agency: City of Renton
Design Criteria: 2009 King County Surface Water Design Manual (KCSWDM), and the City of
Renton Amendments to the KCSWDM (February 2010)
CarMax Auto Superstores Technical Information Report Renton, Washington
PACLAND Project #10298005 Page 10
Drainage Basins, Subbasins, and Site Characteristics
Drainage Basin
The project site is located in the Lower Green River Subwatershed, which is part of the
Green/Duwamish Watershed. It is in the City of Renton Black River drainage basin.
Existing Subbasins
The following information is from Drainage Calculations for Act III Theaters, City of Renton,
Washington, prepared by Brant O. Mercil, P.E. of Horton Dennis and Associates, revised August 8,
1996, the full report is included in Appendix B.
“Runoff from the parking area in front of the proposed building [Act III Theaters] will be conveyed
in a storm drain pipe system to a grass-lined swale in the proposed wetland buffer at the rear of the
building. The swale has been designed with a flat slope to promote water quality through ponding
prior to discharge into the detention basin. The swale will outfall into the first of three water quality
ponds. These ponds have been designed using the methodologies for a combined water
quality/detention basin as outlined in the 1992 edition of the King County Stormwater
Management (SWM) Manual. While the above mentioned grass lined swale will provide some
water quality benefits, site constraints precluded the design of a bio-filtration swale meeting the
requirements of the King County SWM Manual. To compensate for use of a smaller swale,
additional storage has been provided in the proposed water quality ponds. Additional water quality
storage has also been provided to compensate for paved areas downstream of the proposed
facility... The detention basin has been designed to release flows at or below the pre developed
rates for the 2, 10 and 100 year storms per City of Renton standards…The control structure in the
detention system will discharge to a 15-inch storm drain pipe. The storm drain pipe will extend
approximately 450 feet west to a connection with an existing 60-inch storm drain pipe on the west
side of Lind Avenue. The 60-inch storm drain pipe continues approximately 1300 feet north, then
1,000 feet east to an outfall in Springbrook Creek.”
The design of the existing facilities used a single basin with a tributary area of 13.23 AC. This was
estimated by starting with the project site area of 12.28 AC, subtracting 0.86 AC of bypass area,
and adding 1.81 AC to include the offsite wetland mitigation area, offsite areas (former Home Base
and warehouse), and the area of Lots 4 and 6 of the Northern Railroad BSP No. 014-92. The bypass
areas include landscaped areas that drain offsite along the south and east edges of the property, a
portion of the driveway access to SW 41st Street, and the driveway access to E. Valley Road on the
north side of the property.
Although the initial drainage report was developed using a single basin, the site can be analyzed
more effectively by examining five subbasins. There are five outfalls that discharge into the grass-
lined swale in the rear of the building that lead to the water quality ponds before entering
detention basin and discharging at the control structure. Starting at the north end of the swale and
moving south, the first outfall discharges runoff from a half of the roof of the theater and
approximately a third of the parking area in front of the building (Subbasin1). The next three
outfalls each connect to single catch basins, capturing runoff from the parking area in the rear of
the building (Subbasins 2, 3, 4). The southernmost outfall discharges runoff from the remaining
portion of the roof, the southern portion of the parking area in front of the building, Lots 4 and 6,
and the driveway access to the adjacent property to the west (Subbasin 5). See Appendix A Exhibit
A- Existing Subbasin, detailing the subbasin characteristics, flow routes, and discharge points.
CarMax Auto Superstores Technical Information Report Renton, Washington
PACLAND Project #10298005 Page 11
Proposed Subbasins
The proposed storm drainage system will be composed of the same approximate subbasins as the
existing. See Appendix A Exhibit B- Proposed Subbasin, detailing the proposed subbasins. As
shown in Table 2 – Site Areas, the impervious area is decreasing as a result of this development.
Additional landscaping and a reduction in building square footage will allow for an increase in
pervious areas. The areas below are based on the 12.28 acre project site since we are not
proposing changes to the bypass, offsite wetland mitigation area, offsite areas, or the area of Lots 4
and 6 of the Northern Railroad BSP No. 014-92.
Table 2 – Site Areas
Existing Area (SF) Proposed Area (SF)
Surface
Roof 61,856 20,221
Landscaping/Wetland (Pervious) 91,985 90,245
Pavement (PGIS) 345,918 389,293
Total 534,917 534,917
Site Characteristics
The property is located at 3751 E. Valley Road in Renton, Washington. The site is bordered by SW
41st Street to the south, E. Valley Road to the east, a Harley-Davidson dealership to the north, and
a medical and retail mixed-use building to the west. The site is currently developed as the Act III
Movie Theater, which has a building footprint of 61,856 square feet, with associated parking and
landscaping. Two wetlands and a stormwater detention and water quality facility is also located on
the western 1.13 AC of the site. The entire lot is approximately 12.28 acres in size. This project
proposes to demolish the existing theater and redevelop the site to become a used car dealership.
The proposed project will include the construction of four buildings, named as follows; Sales
(11,017 SF), Service (6,150 SF), Carwash (936 SF), and Presentation (2,118 SF) for a total building
area of 20,221 SF. The overall sales display parking area will be approximately 4.07 AC
construction of the sales display area will be completed in three phases. The customer and
employee parking area will be approximately 2.00 AC. Although the change in impervious surfaces
(roof and pavement) is negligible the pollution generating impervious surface are will be increased.
Water quality treatment will be provided for an area equivalent to the increase in pollution
generating impervious surface.
CarMax Auto Superstores Technical Information Report Renton, Washington
PACLAND Project #10298005 Page 12
Soils
Figure 3 - Site Soils (USDA SCS Soil Map)
The site is mapped as Snohomish Silt Loam (So) and Tukwila Muck (Tu) according to the USDA
Natural Resource Conservation Service, Soil Survey for King County Area, Washington.
As required by the 2009 King County Surface Water Design Manual (KCSWDM) and City of
Renton Amendment, this project is subject to a full drainage review. Therefore, the storm drainage
design for this project is required to comply with all eight (8) Core Requirements, and six (6)
Special Requirements. The requirements have been met as follows:
Core Requirement #1: Discharge at Natural Location
All surface and storm water runoff from a project must be discharged at the natural location so as
not to be diverted onto or away from downstream properties. The manner in which runoff is
discharged from the project site must not create a significant adverse impact to downhill properties
or drainage systems.
Response: Onsite runoff will be collected, conveyed and, when necessary, treated. All onsite
runoff is subsequently conveyed to the City of Renton stormwater system. The city system
discharges into Springbrook Creek (Black River). No downstream impacts are anticipated as a
result of the proposed improvements as the impervious area will remain approximately the same.
Core Requirement #2: Off-site Analysis
All proposed projects must submit an offsite analysis report that assesses potential offsite drainage
and water quality impacts associated with development of the project site and propose appropriate
mitigation of those impacts. The initial permit submittal shall include, at minimum, a Level 1
downstream analysis.
Section 2- Conditions and Requirements
PROJECT
SITE
CarMax Auto Superstores Technical Information Report Renton, Washington
PACLAND Project #10298005 Page 13
Response: See Section 3 of this report for a complete off-site analysis. A Level 1 downstream
analysis will be submitted upon civil plan submittal.
Core Requirement #3: Flow Control
All proposed projects, including redevelopment projects, must provide onsite flow control facilities
or flow control BMPs or both to mitigate the impacts of storm and surface runoff generated by new
impervious surface, new pervious surface, and replaced impervious surface targeted for flow
mitigation.
Response: Per the exceptions listed in section1.2.3.1-A of the City of Renton Amendment to the
KCSWDM the project is exempt from flow control requirements as it doesn’t increase the 100-year
peak flow by more than 0.1 cfs and it is not expected to significantly impact a critical area, or
cause severe flooding or erosion problems. The proposed 12.28 acre site generates a 5.03 cfs, a
0.02 cfs increase in the 100-year peak flow rate from the existing site conditions. Therefore
additional flow control facilities, beyond the existing stormwater ponds and flow control structure,
are not proposed as part of this project. A detailed summary and analysis of the existing and
proposed conditions is provided in Section 4 of this report.
Core Requirement #4: Conveyance System
All engineered conveyance system elements for proposed projects must be analyzed, designed,
and constructed to provide a minimum level of protection against overtopping, flooding, erosion,
and structural failure.
Response: Portions of the existing conveyance system will be reused as part of this project with
additional conveyance added to accommodate the new building configuration and site layout. The
proposed conveyance system has been designed to convey the 100-year peak flow.
Core Requirement #5: Erosion and Sediment Control
All proposed projects that will clear, grade, or otherwise disturb the site must provide erosion and
sediment controls to prevent, to the maximum extent practicable, the transport of sediment from
the project site to downstream drainage facilities, water resources, and adjacent properties. Both
temporary and permanent ESC measures shall be implemented.
Response: Erosion and sediment control requirements will be an integral part of the project
construction documents. These measures will include methods to reduce erosion of onsite site
soils and to prevent sediments from inadvertently leaving the project site. Additionally the ESC
measures will prevent sediment-laden discharge from entering the wetland or existing stormwater
water quality or flow control facilities.
Core Requirement #6: Maintenance and Operations
Maintenance and operation of all drainage facilities is the responsibility of the property owner,
except those facilities for which the City of Renton assumes maintenance and operation, and must
be maintained and operated in accordance with King County or other maintenance standards as
approved by the City of Renton.
Response: An Operations and Maintenance Manual is included in this report- see Section 10.
CarMax Auto Superstores Technical Information Report Renton, Washington
PACLAND Project #10298005 Page 14
Core Requirement #7: Financial Guarantees and Liability
In accordance with RMC 4-6-030J, RDSD shall require all persons constructing any surface water
facilities (including flow control/water quality facilities, conveyance systems, erosion control, and
road drainage), to post with the City of Renton a bond, assignment of funds or certified check. The
applicant must also maintain liability insurance.
Response: See Section 9- Bond Quantities, Facility Summaries, and Declaration of Covenant. The
Site Improvement Bond Quantity Worksheet and Declarations of Covenants will be submitted
upon approval of the Building Permit plans.
Core Requirement #8: Water Quality
All proposed projects, including redevelopment projects, must provide water quality (WQ)
facilities to treat the runoff from those new and replaced pollution-generating impervious surfaces
and new pollution-generating pervious surfaces targeted for treatment.
Response: The proposed project improvements will increase the pollution generating impervious
surface (PGIS) by 1.0 AC. The site has existing water quality in place, the existing swale and water
quality ponds, for 7.94 AC of PGIS. Enhanced Basic Treatment is required if more than 50% of the
runoff that drains to the proposed treatment facility is from commercial land use. Additional
Enhanced Basic Water Quality treatment will be provided for the increase in PGIS. Additionally,
an oil/water separator, which will discharge to the sanitary sewer system, will be provided for
discharges from the service building and carwash. A detailed summary and analysis of the water
quality basin and design is provided in Section 4 of this report.
Special Requirement #1: Other Adopted Area-Specific Requirements
The City of Renton may apply requirements for controlling drainage on an area-specific basis.
Potential other adopted area-specific requirements may include: Master Drainage Plans, Basin
Plans, Stormwater Compliance Plans, etc.
Response: The project site is within Washington State Water Resource Inventory Area 9:
Green/Duwamish and Central Puget Sound Watershed Salmon Habitat Recovery. However, the
proposed project will have no effect on any salmon habitats. The project site does not lie within
any City of Renton Master Drainage or Basin Plans.
Special Requirement #2: Flood Hazard Area Delineation
If a proposed project contains or is adjacent to a flood hazard area as determined by the Renton
Department of Storm Drainage, the project must determine those components that are applicable
and delineate them on the project’s site improvement plans and recorded maps.
Response: The project site does not lie within or adjacent to a City of Renton Flood Hazard Area
(determined by FEMA FIRM Panels 0978F and 0979F dated May 16, 1995) and therefore does not
need to apply the provisions of this requirement.
Special Requirement #3: Flood Protection Facilities
Proper analysis, design, and construction are necessary to protect against the potentially
catastrophic consequences if flood protection facilities such as levees and revetments should fail.
Response: The proposed project will not rely on an existing flood protection facility, or modify or
construct a new flood protection facility.
CarMax Auto Superstores Technical Information Report Renton, Washington
PACLAND Project #10298005 Page 15
Special Requirement #4: Source Control
If a proposed project requires a commercial building or commercial site development permit then
the water quality source controls applicable to the proposed project shall be applied in accordance
with the King County Stormwater Pollution Prevention Manual and Renton Municipal Code IV.
Response: Water quality source controls will be applied in accordance with the KCSWDM and
Renton Municipal Code IV. A pollution prevention plan has been included in section 8 of this
report.
Special Requirement #5: Oil Control
Projects proposing to develop or redevelop a high-use site must provide oil control in addition to
other water quality controls required.
Response: High use sites area defined as a commercial or industrial site that (1) has an expected
average daily traffic count equal to or greater than 100 vehicles/1,000 SF of gross building area: (2)
is subject to petroleum storage in excess of 1,500 gallons per year; (3) is subject to use, storage, or
maintenance of a fleet of 25 or more diesel vehicles. The proposed development includes an
4,000 gallon, above ground fuel storage tank.
Stormwater runoff from the high-use portion of the site will be treated to capture and detain oil
and associated pollutants. Only the impervious surface associated with the high-use portion of the
site is subject to treatment requirements. The site is considered high-use due to the anticipated
storage/transfer of petroleum in excess of 1500 gallons per year. An oil water separator will be
installed to ensure that the runoff from within the proposed service building and car wash.
Special Requirement #6: Aquifer Protection Area
If a proposed project is located within the Aquifer Protection Area as identified in the RMC 4-3-
050, this special requirement requires the project to determine those components that are
applicable an delineate them on the project’s site improvement plans.
Response: The project is not located within an Aquifer Protection Area, and therefore this
requirement does not apply.
Upstream Analysis
Based on topographic survey and field reconnaissance, there is minimal tributary area to this site.
The original drainage report for the theater, completed by Horton Dennis & Associates lists a total
of 1.53 AC offsite that contributes to the existing stormwater facility located onsite. City of Renton
catch basins and inlets along E Valley Road along the eastern side of the site intercept road runoff
before it reaches the site. Additionally, the existing topography surrounding the site prevents any
significant amount of run-on.
Downstream Analysis
After the runoff passes through the existing stormwater detention/treatment facilities, runoff enters
the municipal stormwater system at Lind Avenue SW. This system continues north for
Section 3- Off-Site Analysis
CarMax Auto Superstores Technical Information Report Renton, Washington
PACLAND Project #10298005 Page 16
approximately 1300 feet, before heading west approximately 450 feet to the discharge point of
Springbrook Creek.
To meet the requirements of the Level 1 downstream analysis, the area up to one mile downstream
of the project site must be included. One mile downstream (for this project, downstream is north)
from the point where the stormwater leaves the project site is approximately at the intersection of
SW 27th street and Springbrook Creek. As previously discussed, the storm runoff leaves the site
near the southwest corner and enters in to the Renton Municipal storm system, before traveling
along Lind Ave SW and SW 34th St. to the discharge point of Springbrook Creek. Springbrook
Creek continues in a northerly direction for approximately 1.35 miles to the King County South
Wastewater Treatment Plant.
A site visit was conducted on the morning of January 10, 2014. All above ground facilities were
examined on site including the wetland/stormwater detention area, catch basins, and swales. No
drainage issues such as areas of ponding, overtopping of the detention area, and blocked inlets
could be seen onsite. The analysis continued along Lind Ave SW and 34th Street before reaching
Springbrook Creek. No drainage problems were observed. A map of the downstream analysis can
be seen in Figure 4. Photos from the site visit are shown on the next page.
Figure 4 - Downstream Area (Renton COR Maps)
DISCHARGE
POINT
PROJECT
SITE CITY OF
RENTON STORM
SYSTEM
SPRINGBROOK
CREEK
ONSITE
DETENTION/TREATMENT
FACILITIES
CarMax Auto Superstores Technical Information Report Renton, Washington
PACLAND Project #10298005 Page 17
# Photo Description
1
Drainage ditch (swale)
on the north edge of the
adjacent Lot 6
The runoff captured in
this landscaped area is
routed through a storm
pipe connecting to a
catch basin downstream,
and then to the water
quality treatment area in
the rear (to the west) of
the building.
2
In the front parking area,
near the southeast
corner of the building
A series of catch basins
located in the parking
area are connected via
underground stormwater
pipes before being
discharged into the
swale and water quality
treatment ponds in the
rear of the building.
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PACLAND Project #10298005 Page 18
Evidence of Existing or Predicted Problems
There is no visible evidence of drainage problems on or near the project site. In addition, no
drainage problems are predicted as a result of the proposed improvements.
# Photo Description
3
To the rear (west) of the
theater
One of three existing
water quality treatment
ponds. Stormwater is
discharged into the
upstream swale, before
passing through the
treatment ponds and
connecting to the City of
Renton municipal
stormwater system.
4
Crossing of Springbrook
Creek and 34th St
This is the discharge
point of the municipal
stormwater system
downstream of the site.
CarMax Auto Superstores Technical Information Report Renton, Washington
PACLAND Project #10298005 Page 19
The City of Renton Amendment to the KCSWDM applies flow control standards based on
geographic areas as determined by the Flow Control Application Map . The project site lies within
the Peak Rate Flow Control Standard – Matching Existing Conditions. This area requires peak
runoff control within the Peak Rate Flow Control Standard area such that the post-developed peak
rate matches that of the existing 2-year, 10-year and 100-year peak discharge rates. Also, if the
proposed re-development is located within the peak Flow Rate Control Standard Area, will not
increase the existing condition 100-year peak flow be greater than 0.1 cfs, and the increased runoff
(if any) will not significantly impact a critical area, severe flooding problem, or sever erosion
problem than the facility requirement is waived.
Hydrologic Analysis
The existing and proposed site conditions were evaluated using both the King County Runoff Time
Series (KCRTS) The detailed KCRTS output has been included in Appendix C – Design Calculations
The historic 1-hour duration in the Sea-Tac rainfall region (location scale factor ST-1.0) was used to
model runoff in KCRTS.
Existing Site Hydrology
The existing site includes a storm drainage system designed to collect storm water at strategically
placed catch basins throughout the site. The storm water is then conveyed through underground
pipes to a swale at the rear of the building. After traveling through the one of the two swales, runoff
enters the first of three water quality ponds. The water quality ponds discharge to the detention
basin. Stormwater runoff discharges to the municipal stormwater system in Lind Avenue SW via the
existing flow control structure which has a 15-inch outlet pipe. The municipal stormwater system
routes to a discharge point of Springbrook Creek. The existing detention basin was designed to
release flows at or below the pre-developed rates for the 2, 10, and 100 year storms. The pre-
developed conditions and existing peak runoff rates for the project site area are shown in Tables 4
and 5 below.
Table 4
Pre-Developed (Existing)
Conditions
Area Condition
7.94 AC Pavement (PGIS)
2.92 AC Landscape/Wetland
1.42 AC Roof
Table 5
Existing Peak Runoff Rate
Event Rate (cfs)
2-year 2.54
10-year 3.03
100-year 5.01
Section 4- Flow Control and Water Quality Facility Analysis and Design
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PACLAND Project #10298005 Page 20
Developed Site Hydrology
The proposed project will have similar hydrology to the existing site. It is anticipated that
the existing systems (conveyance, detention, and treatment) will be utilized as the overall
site hydrology and division of subbasins will remain approximately the same. The amount
of pervious area is increasing due to a reduction in building size and additional
landscaping, however the site PGIS will be increased as a result of the decrease in building
size. The post-developed conditions and undetained peak runoff rates for the entire
attributed basin area (12.28 acres) are shown in Tables 6 and 7 below.
Table 6
Post-Developed Conditions
Area Condition
8.94 AC Pavement (PGIS)
2.88 AC Landscape/Wetland
0.46 AC Roof
Table 7
Existing Peak Runoff Rate
Event Rate (cfs)
2-year 2.55
10-year 3.04
100-year 5.03
Flow Control Analysis
The post developed 100-year peak runoff rate for the overall site increase by 0.02 cfs from
the existing condition. The increase in the 100-year peak runoff is less than 0.1 cfs, and the
project is located in the Peak Rate Flow Control Standard area, therefore any additional
flow control facility requirement is waived for this project.
The existing flow control facilities consist of the following structures inside of a 54 inch
diameter catch basin manhole; a 1 inch diameter orifice with an outlet elevation of 10.87,
a 3.5 inch diameter orifice with an outlet elevation of 15.00, a 6 inch diameter elbow with
an outlet elevation of 15.50, and a 15 inch diameter restrictor with an overflow elevation
of 16.50. The control structure discharges to a 15-inch storm drain pipe before connecting
into the Renton Municipal storm system. See Appendix B for the Act III Theater Drainage
Report, which includes illustrations and calculations for the existing flow control system.
Water Quality System
The proposed redevelopment includes demolition of the existing theater; construction of
the sales, service, car wash, and presentation buildings, reconfigurations of landscape
islands and repaving of the existing parking area. The roofs of the proposed buildings will
be considered non pollution-generating impervious surfaces (NPGIS). The amount of
pollution-generating impervious surfaces (PGIS) will be increasing as a result of the
decrease in roof area.
CarMax Auto Superstores Technical Information Report Renton, Washington
PACLAND Project #10298005 Page 21
There are two existing grass-lined biofiltration swales which discharge to three water
quality ponds in series. The swales and water quality ponds were sized for the existing
impervious area, as detailed by the drainage report by Horton Davis and Associates. The
project will add 1.0 AC of new PGIS. This is more than 5,000 SF of new PGIS, therefore
additional water quality treatment will be provided.
A two-step treatment train including a StormFilter system with ZPG media followed by a
StormFilter with CSF media is proposed to meet the Enhanced Basic Water requirement in
King County Surface Water Design Manual. The Enhanced basic treatment will be
provided for the increased PGIS.
New PGIS Areas
The new PGIS is a result of the revisions to the site layout and the reduced building size. It
isn’t possible to collect the exact area of new PGIS however the majority of the new PGIS,
located within the existing building footprint, will be treated. The treated basin is part of
the proposed vehicle storage area and will be greater in size than the increase in the new
PGIS.
The total area in the proposed Water Quality Basin is 1.63 AC, Exhibit 3 - Water Quality
Basin in Appendix A depicts the area to be treated. Tables 8 and 9, below, contain the
developed conditions of the treatment basin as well as the water quality design flow rate
(60% of the developed 2-year peak flow rate).
Table 8
Water Quality Basin
Area Condition
1.63 AC Pavement (PGIS)
0 AC Landscape/Wetland
0 AC Roof
Table 9
Water Quality Flow Rate
Event Rate (cfs)
2-year 0.776
60% of 2-year 0.466
Conveyance
Per the 2009 King County Surface Water Design Manual, Section 4.2.1.2, the on-site
stormwater conveyance can be calculated based on gravity flow analysis of the piping
network. Peak run-off associated with and up to a 25 year storm event has been
determined to be adequate. Uniform flow analysis utilizing Manning’s equation, shown
below, was employed with a Manning’s Roughness Coefficient (n) of 0.012.
Section 5- Conveyance System Analysis and Design
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Manning’s equation - Q=n49.1 xAxR2/3xS1/2
With: Q = Flow (CFS)
n = Manning’s Roughness Coefficient (0.012)
A = Flow Area (SF)
R = Hydraulic Radius = Area /Wetted Perimeter (LF)
S = Slope of the pipe (ft/ft)
Based on the revised basin areas and existing pipe sizes, the existing conveyance system is
adequate for the 25-year storm event. All new conveyance pipes will also be designed to
convey the 25-year storm event. Additionally, the overflow from a 100-year storm event
will not create or aggravate a severe flooding problem or severe erosion problem.
100-Year Flood/Overflow Condition
Review of the current FEMA FIRM maps (Panels 0978F, 0979 F) indicates that the project
site lies within the Zone X, which is determined to be outside of the 100-year flood plain.
If the existing storm system were to experience a major storm event, the stormwater
detention area would overflow in to the rear access drive, and along the road westerly
towards Lind Ave SW.
The stormwater system for this project has been designed to address the storm events in
accordance with design criteria described previously. In the event of a larger storm, it is
unlikely that the system would fail. Based on a review of the site and immediate
surroundings, the overflow of the stormwater conveyance system would allow surface
water to sheet flow to the public storm system in Lind Avenue.
The following Special Reports and Studies were used or have been completed for this project:
Drainage Calculations for Act III Theaters, City of Renton, Washington- dated August 8,
1996 by Horton Dennis & Associates Inc.
Report for Detailed Wetland Mitigation Plan, presented to Act III Theaters, City of Renton,
Washington- dated September 16, 1996 by W & H Pacific.
Wetland Study – dated approximately December 1991 by David Evans and Associates, Inc.
Geotechnical Investigation- by Zipper Geo Associates
FEMA FIRM Panels 0978F and 0979F dated May 16, 1995
Wetland Study - by H & S Consultants
Section 6- Special Reports and Studies
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The following governmental approvals or permits will likely be required for this project:
City of Renton SEPA Determination City of Renton Site Plan Review City of Renton Building Permit City of Renton Construction Permit City of Renton Demolition Permit Washington State Department of Ecology, NOI Coverage under the NPDES Permit
These permits will require approval by the City of Renton Planning Division or the Department of
Ecology.
All erosion and sediment control measures shall be governed by the requirements of the City of
Renton and the Washington State Department of Ecology. A temporary erosion and sedimentation
control plan will be prepared to assist the contractor in complying with these requirements. The
Erosion and Sediment Control (ESC) plan will be included with the construction plans.
1. Erosion Risk Assessment
The degree of erosion risk on the proposed project site is minimal. The following factors
contribute to a low degree of erosion risk:
Slope across the site is minimal. Runoff will not travel at high velocities across the site and,
therefore, will not cause noticeable erosion impacts.
The site consists primarily of impervious surfaces.
2. Construction Sequence And Procedure
The proposed development will include an erosion/sedimentation control plan designed to prevent
sediment-laden run-off from leaving the site during construction. The erosion potential of the site
is influenced by four major factors: soil characteristics, vegetative cover, topography, and climate.
Erosion/sedimentation control is achieved by a combination of structural measures, cover
measures, and construction practices that are tailored to fit the specific site.
Prior to the start of any grading activity upon the site, all erosion control measures, including
stabilized construction entrances, shall be installed in accordance with the construction
documents.
The best construction practice will be employed to properly clear and grade the site and to
schedule construction activities. The planned construction sequence for the construction of the
site is as follows:
1. Attend a pre-construction meeting with the City of Renton.
2. Install temporary erosion control features.
3. Demolish existing structure(s).
Section 8- CSWPPP Analysis and Design
Section 7- Other Permits
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PACLAND Project #10298005 Page 24
4. Clear and grub expansion area.
5. Grade site.
6. Construct buildings.
7. Pave site and install landscaping.
8. Remove temporary erosion control features once site is fully stabilized.
3. Trapping Sediment
Structural control measures will be used to reduce erosion and retain sediment on the construction
site. The control measures will be selected to fit specific site and seasonal conditions.
The following structural items will be used to control erosion and sedimentation processes:
Stabilized construction entrances
Filter fabric fences
Catch Basin Inlet Sediment Protection
Proper Cover measures
Weekly inspection of the erosion control measures will be required during construction. Any
sediment buildup shall be removed and disposed of off-site.
A stabilized construction entrance will be installed at the site entrance to avoid vehicle tracking off
mud off-site. The entrances are a minimum requirement and may be supplemented if tracking of
mud onto public streets becomes excessive. In the event that mud is tracked off site, it shall be
swept up and disposed of off site on a daily basis. Depending on the amount of tracked mud, a
vehicle road sweeper may be required.
Because vegetative cover is the most important form of erosion control, construction practices must
adhere to stringent cover requirements. More specifically, the contractor will not be allowed to
leave soils open for more than 14 days and, in some cases, immediate seeding will be required.
4. Wet Season TESC Operating Plan
Work between October 1st and April 30th must adhere to the Wet Season Special Provisions noted
in the 2009 KCSWDM.
The following items will be submitted upon application for the civil permits:
Site Improvement Bond Quantity Worksheet
Declaration of Covenant for Privately Maintained Flow Control BMPs
Declaration of Covenant for Privately Maintained Flow Control and WQ Facilities
Section 9- Bond Quantities, Facility Summaries, and Declaration of Covenant
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A manual detailing the operations and maintenance for all privately-maintained flow control and
water quality facilities will be submitted upon application for civil permits. This manual will also
include Appendix A of the KCSWDM (Maintenance Requirements for Flow Control, Conveyance,
and WQ Facilities).
Section 10- Operations and Maintenance Manual
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Appendix A – Exhibits
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EXHIBIT A – EXISTING SUBBASIN
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EXHIBIT B – PROPOSED SUBBASIN
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EXHIBIT C – WATER QUALITY BASIN
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Appendix B – Act III Theaters Drainage Report
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Appendix C – Design Calculations
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PRE-DEVELOPED CONDITIONS
PRE-DEVELOPED PEAK FLOW RATES
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DEVELOPED CONDITIONS
DEVELOPED PEAK FLOW RATES
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WATER QUALITY BASIN CONDITIONS
WATER QUALITY BASIN PEAK FLOW RATES