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Renton Dental Arts
17816 108th Ave SE
Renton, Washington, 98055
Technical Information Report
Commercial Redevelopment
1505 Westlake Ave. N T 206.522.9510
Suite 305 F 206.522.8344
Seattle, WA 98109 www.pacland.com
Prepared By: Chris Dowd
Reviewed By: Luke Randles, P.E.
Prepared: May 2017
Renton Dental Arts Technical Information Report Renton, Washington
PACLAND Project #10573001 Page 1
Section Page
EXECUTIVE SUMMARY ........................................................................................................................................... 2
SECTION 1 - PROJECT OVERVIEW ........................................................................................................................... 4
SITE LOCATION ............................................................................................................................................................ 10
DRAINAGE BASINS, SUBBASINS, AND SITE CHARACTERISTICS ................................................................................................ 11
SOILS ......................................................................................................................................................................... 12
SECTION 2 - CONDITIONS AND REQUIREMENTS SUMMARY ................................................................................ 15
SECTION 3 - OFF-SITE ANALYSIS ........................................................................................................................... 19
SECTION 4 - FLOW CONTROL AND WATER QUALITY FACILITY ANALYSIS AND DESIGN ......................................... 22
HYDROLOGIC ANALYSIS ................................................................................................................................................. 22
EXISTING SITE HYDROLOGY ............................................................................................................................................ 22
DEVELOPED SITE HYDROLOGY ........................................................................................................................................ 23
FLOW CONTROL ANALYSIS ............................................................................................................................................. 23
WATER QUALITY SYSTEM .............................................................................................................................................. 24
SUMMARY .................................................................................................................................................................. 25
SECTION 5 - CONVEYANCE SYSTEM ANALYSIS AND DESIGN ................................................................................. 26
CONVEYANCE .............................................................................................................................................................. 26
100-YEAR FLOOD/OVERFLOW CONDITION ....................................................................................................................... 26
SECTION 6 - SPECIAL REPORTS AND STUDIES ....................................................................................................... 27
SECTION 7 - OTHER PERMITS ............................................................................................................................... 28
SECTION 8 - CSWPPP ANALYSIS AND DESIGN ....................................................................................................... 29
SECTION 9 - BOND QUANTITIES, FACILITY SUMMARIES, AND DECLARATION OF COVENANT ............................... 31
SECTION 10 - OPERATIONS AND MAINTENANCE MANUAL .................................................................................. 32
APPENDICES: A – EXHIBITS
B – PLANS
C – DESIGN CALCULATIONS
D – OPERATIONS AND MAINTENANCE MANUAL
E – WATER QUALITY DEPARTMENT OF ECOLOGY CERTIFICATION
F – GEOTECHNICAL REPORTS
Table of Contents
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PACLAND Project #10573001 Page 2
Site Location:
The project property is located at 17816 108th Ave SE in Renton, Washington. The site is
bordered by 108th Ave SE/WA 515 to the west, a vacant restaurant building to the north, a
restaurant to the south, and 109th Ave SE to the east. The site is currently developed as vacant
commercial retail building, which has a footprint of 2,552 square feet, with associated parking
and landscaping. The entire lot is approximately 0.90 acres in size. This project proposes to
demolish the existing building and parking lot and redevelop the site to become a dental office.
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 Commercial Mixed Use Overlay District
(Community Planning area). The property is zoned CA (Commercial Arterial).
Design Criteria:
The City of Renton has adopted the 2017 Renton Surface Water Design Manual (RSWDM) for
the design of stormwater facilities. The existing site is developed and primarily impervious with
the exception of landscape islands and an undeveloped piece of land between the parking lot
and 109th Ave SE. The proposed development will lead to a slight increase in the amount of
overall impervious area. Per the requirements of KCSWDM, both flow control and water quality
mitigation will be required as a result of the proposed development.
Table 1
Jurisdictional Requirements
Peak Runoff Control:
Flow Control Duration Standard
Matching Forested Site Conditions
Matching durations from 50% of 2-
year flow up to full 50-year events ,
and peak flow rates for the 2-year
and 10-year runoff events
Water Quality:
Enhanced Basic WQ Treatment
Preceding detention
72% of the 2-year peak flow rate
Downstream of detention The full 2-year release rate from
detention facility
On-Site BMPs: LID Performance Standard
Matching discharge durations for
pre-developed discharge rates
from 8% of the 2-year peak flow to
50% of the 2-year peak flow.
Executive Summary
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PACLAND Project #10573001 Page 3
Proposed Drainage System:
This project proposes to utilize a detention vault for storage and modular wetland system for
water quality treatment. The proposed grading will generally direct water from the east side of
the site toward the west side of the site.
Stormwater will collect into catch basins along the valley on the east and south side of the
building and be conveyed via pipe to the northwest corner of the site to the stormwater
treatment structure. After treatment, stormwater will be directed to a detention vault, which in
conjunction with a flow control structure, will provide flow control as required by the
jurisdictional requirements identified in Table 1. After detention, stormwater will be released
existing stormwater system in 108th Ave. SE.
A stormwater adjustment shall be obtained for the use of the modular wetland system and the
use of the proposed detention facility in accordance with Section 6.2 of the 2017 City of Renton
Surface Water Design Manual.
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PACLAND Project #10573001 Page 4
Figure 1 - TIR Worksheet (2017 COR SWDM)
Section 1 - Project Overview
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Site Location
Figure 2 - Site Location
Location: 17816 108th Ave SE, Renton, WA 98055
Section/Township/Range: NE Quarter of Section 32, Township 23, Range 05
Parcel/Tax Lot(s): 3223059054
Size: 0.90 acres
City, County, State: Renton, King County, Washington
Governing Agency: City of Renton
Design Criteria: 2017 City of Renton Surface Water Design Manual (SWDM)
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PACLAND Project #10573001 Page 11
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 Sub-basins
The existing site consists of a single basin with an area of 0.90 acres. The northeast portion of
the site of the highest in elevation at 410’, and the area gradually slopes west toward the
building, which has a finished floor elevation of approximately 405.5’. The east side of the site
starts at an elevation of 411’ and slopes downward toward the southwest corner of the site
where it has an elevation of 400’. Runoff is collected at a catch basin on the west side of the
property, where it is then conveyed into the municipal stormwater system. Refer to Section 3 for
existing discharge points to and from the site.
Proposed Sub-basins
The proposed storm drainage system will be comprised of one sub-basin. The sub-basin is
about 0.90 acres in size. Stormwater on the site follows the grading toward the proposed
building footprint where it is collected in catch basins and drains southwest toward the
stormwater treatment system after travelling through a flow control manhole. After treatment,
the water will enter the open bottomed detention vault for storage and infiltration. Then the
water is conveyed through a flow control device and drains into the municipal system on 108th
Ave SE. See Appendix B for plans showing the distance from the farthest upstream end to the
nearest proposed flow control facility.
Table 2 – Site Areas
Surface Existing Area (AC) Proposed Area (AC)
Roof 0.07 0.18
Landscaping
(Pervious)
0.36 0.27
Pavement
(PGIS)
0.44 0.37
Sidewalk
(NPGIS)
0.03 0.08
Total 0.90 0.90
Site Characteristics
The project property is located at 17816 108th Ave SE in Renton, Washington. The site is
bordered by 108th Ave SE/WA 515 to the west, a vacant commercial building to the north, a
restaurant to the south, and 109th Ave SE to the east. There is also a residential property to the
NE as well. The site is currently developed as vacant commercial retail building, which has a
footprint of 2,552 square feet, with associated parking and landscaping. The entire lot is
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PACLAND Project #10573001 Page 12
approximately 0.90 acres in size. This project proposes to demolish the existing restaurant and
parking lot, then redevelop the site to become dental office space.
The proposed project will include the construction of an approximately 7,796 SF Dentist office
building. The overall paved area will be approximately 0.37 AC, and the overall change in
impervious surfaces is approximately 0.09 AC. Water quality treatment will be provided for an
area equivalent to the pollution generating impervious surface on site.
Soils
Websoil Survey
Figure 3a - Site Soils (USDA Web Soil Survey Map)
The site is mapped as Alderwood gravelly sandy loam 0 to 8 percent slopes (AgB) and
Alderwood gravelly sandy loam 8 to 15 percent slopes (AgC) according to the USDA Natural
Resource Conservation Service, Web Soil Survey for King County Area, Washington.
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PACLAND Project #10573001 Page 13
Figure 3b - Downstream Site Soils (USDA Web Soil Survey Map)
The downstream area is mapped as Alderwood gravelly sandy loam 0 to 8 percent slopes (AgB),
Alderwood gravelly sandy loam 8 to 15 percent slopes (AgC), and Arents 6 to 15 percent slopes
according to the USDA Natural Resource Conservation Service, Web Soil Survey for King County
Area, Washington.
Geotechnical Report
The following information was gathered from a report created by Geotech Consultants, Inc. on
April 11, 2017. At a depth of 4 feet below the surface, testing indicated that soil was composed
of gravelly silty sand fill overlying native, medium dense, silty sand. Underneath this soil was very
dense silty sand. The bottom of the infiltration test was at 15.9 feet.
Perched groundwater seepage was observed during the infiltration test at depths of 2.5 to 3.5
feet. The seepage levels on the logs represent the location of transient water seepage and may
not indicate the static groundwater level.
An infiltration test has not yet been performed on this site, so a preliminary design infiltration
rate was assumed based on information obtained for the adjacent site to the north. That
infiltration testing yielded a measured infiltration rate of 0.21 inches/hour. Site-specific
infiltration testing will be performed later to verify this infiltration rate. Correction factors were
applied to the measured infiltration rate to produce a design infiltration rate per the guidelines
of SWDM Section 5.2.1.
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Idesign = Imeasured x Ftesting x Fgeometry x Fplugging
Imeasured = 0.21 inches/hour
Ftesting = 0.30 for small-scale testing
Fgeometry = 1.0 (no groundwater or impermeable surface encountered)
Fplugging = 0.8 for silty sand
Idesign = 0.21 x 0.30 x 1.0 x 0.8 = 0.05 inches/hour
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PACLAND Project #10573001 Page 15
As required by the 2017 City of Renton Surface Water Design Manual (SWDM) this project is
subject to a full drainage review. Therefore, the storm drainage design for this project is required
to comply with all nine (9) 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
northwest of the site into Panther Creek. 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.
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 City of Renton SWDM, flow control will be provided for Flow Control Duration Standard
Matching Forested Site Conditions. A stormwater detention system and flow control structure will
be put in place to match pre-development conditions for 50% of the 2-year up to the full 50-year
design storm event and also the 2-year and 10-year peak flows. The detention system shall have
an open bottom, thus utilizing the available infiltration capacity of on-site soils to contribute to
flow control. A detailed summary and analysis of the existing and proposed conditions is provided
in Section 4 of this report.
Section 2 - Conditions and Requirements
Summary
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PACLAND Project #10573001 Page 16
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.
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.
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PACLAND Project #10573001 Page 17
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: Enhanced Basic Treatment is required, as more than 50% of the runoff that drains to
the proposed treatment facility is from commercial land use. Additionally, an oil/water separator,
which will discharge to the sanitary sewer system, will be provided for discharges from the
building. A detailed summary and analysis of the water quality basin and design is provided in
Section 4 of this report.
Core Requirement #9: On-Site BMPs
All proposed projects, including redevelopment projects, must provide on-site BMPs to mitigate
impacts of storm and surface water runoff generated by new impervious surface, new pervious
surface, existing impervious surfaces, and replaced impervious surface targeted for mitigation as
specified in the following sections. On-site BMPs include, but are not limited to, preservation
and use of native vegetated surfaces to fully disperse runoff; use of other pervious surfaces to
disperse runoff; roof downspout infiltration; permeable pavements; bioretention; limited
infiltration systems; and reduction of development footprint.
Response: On-site BMPs are required for this site. The BMPs will be designed so that the
stormwater discharges shall match developed discharge durations to pre-developed durations for
the range of pre-developed discharge rates from 8% of the 2-year peak flow to 50% of the 2-year
peak flow.
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 Panel 53033C0979F dated May 16, 1995) and therefore does not need
to apply the provisions of this requirement.
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PACLAND Project #10573001 Page 18
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.
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 does not exceed
any of these thresholds, and is therefore not required to provide oil control.
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 and 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.
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PACLAND Project #10573001 Page 19
Upstream Analysis
The municipal storm system coming from 109th Ave SE ties into the existing private storm
system on the East side of the site. Minimal tributary area attributes water to the site.
Stormwater runoff flows from the nearby residential streets and is guided by curbs into catch
basins before it reaches this site.
Downstream Analysis
After the runoff passes through the existing stormwater detention/treatment facilities, runoff
enters the municipal stormwater system at 108th Ave SE. After crossing over 108th Ave SE, the
system travels northwest about 2500 feet until discharge into Panther Creek. As discussed
earlier in the report, the storm runoff leaves the site at the northwest corner and enters into the
Renton Municipal storm system. The system runs underneath the Fred Meyer strip mall and
across SE Carr Rd until feeding into Panther Creek. A map of the downstream analysis can be
seen in Figure 4.
Figure 4 - Downstream Area (Renton COR Maps)
Section 3 - Off-Site Analysis
DISCHARGE
POINT
PROJECT
SITE
POINT OF
ENTRY TO CITY
OF RENTON
STORM SYSTEM
PANTHER
CREEK
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PACLAND Project #10573001 Page 20
Resource Review
The following resources were reviewed in preparing an offsite analysis:
• FEMA flood maps
• Sensitive areas maps (King County DDES)
• King County Soils Survey (USDA)
• Wetlands Inventory (RDSD)
• Section 303d list of polluted waters (WSDOE)
• City of Renton Erosion and Landslide Maps
Field Inspection
The downstream system ¼ mile from the site is entirely within subsurface piping, therefore it is
not feasible for the design engineer to physically perform a field inspection. A resource review
was performed in order to fulfill the following field inspection requirements:
1. There were not any issues found during the resource review.
2. There are no anticipated constrictions or lack of capacity in the existing drainage system.
3. Problem Type 1 - There are no anticipated failures of the conveyance system downstream
from the project site.
Problem Type 2 - After reviewing the City of Renton Erosion Hazard Map, neither the
project site nor downstream area are within an erosion hazard area.
Problem Type 3 - After reviewing the City of Renton Flood Hazard Map along with the
FEMA flood map, there not any concerns regarding the potential of a severe building or
roadway flooding problem.
4. There is no anticipated or existing overtopping, scouring bank sloughing, or
sedimentation problems because the site is not within a flood or erosion hazard area.
5. After reviewing the Wetlands Inventory, there is no anticipated destruction of aquatic
habitat or organisms because there are no major streams or lakes downstream of the
project site.
6. The land uses of the project site, downstream area, and other businesses within the
vicinity are primarily commercial. Impervious surfaces within the area include paving for
108th Ave SE, the proposed dentist office building, and the associated parking lot. See
Section 1 for soil details. The topography of this project site and downstream is fairly
flat.
7. There are no major streams or wetlands downstream of this project within ¼ mile. The
project site and downstream area are not within a landslide hazard area or erosion
hazard area. The storm drainage pipes downstream of the project site are 12” and 24”.
8. The tributary basin delineated in Task 1 are confirmed.
9. After the resource review, there are no concerning existing drainage problems.
10. N/A
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Drainage System Description and Problem Descriptions
See Figure 4 above for the location of the project site with respect to the discharge points. The
downstream drainage system is comprised of 24” and 12” storm drain pipe running at
approximately 0.5% slope. The land cover just downstream of the site is primarily impervious.
There are no existing or anticipated problems because the site and downstream area are not in
an erosion or flooding hazard area, steep slope hazard area, wetland area.
Mitigation of Existing or Potential Problems
There is no necessary mitigation because there are no anticipated drainage issues downstream
of the site.
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.
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The City of Renton 2017 SWDM applies flow control standards based on geographic areas as
determined by the Flow Control Application Map. The project site lies within a Conservation
Flow Control Area. This requires a level 2 downstream analysis. This area requires peak runoff
control within the Flow Control Duration Standard Matching Forested Site Conditions area such
that it matches the flow duration of predeveloped rates for the existing site conditions over the
range of flows extending from 50% of 2-year up to the full 50-year flow. This matches the flow
duration of predeveloped rates for the existing site conditions over the range of flows extending
from 50% of 2-year up to the full 50-year flow and the 2-year and 10-year peak flows.
Hydrologic Analysis
The existing and proposed site conditions were evaluated using the Western Washington
Hydrology Model (WWHM) The detailed WWHM output has been included in Appendix C.
The historic 1-hour duration in the Sea-Tac rainfall region (location scale factor ST-1.0) was used
to model runoff in WWHM.
Existing Site Hydrology
The existing site consists primarily of a single sub-basin. The site generally flows from east to
west, ranging from an elevation of 410 in the northeast corner to 400 at the western edge,
where a catch basin collects runoff and conveys it to the existing City of Renton municipal storm
drainage system in 108th Ave. SE. The municipal stormwater system routes to a discharge point
of Panther Creek across SE Carr Rd. Adjacent properties to the north and south do not drain
toward the project, and therefore do not contribute any runoff to the drainage basin. The pre-
developed (existing) conditions and historic peak runoff rates for the project site area are shown
in Tables 4 and 5 below. An exhibit showing the existing conditions is provided in Appendix A.
Existing peak runoff rates were determined using the WWHM modeling software and the
calculations can be found in Appendix C.
Table 4
Pre-Developed (Existing) Conditions
Area Condition
0.44 AC Pavement (PGIS)
0.03 AC Sidewalk
0.36 AC Landscape
0.07 AC Roof
*For WWHM analysis, a fully forested
condition was assumed per City of Renton
requirements.
Section 4 - Flow Control and Water Quality Facility Analysis
and Design
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Table 5
Historic Peak Runoff Rate
Event Rate (cfs)
2-year 0.026
10-year 0.050
100-year 0.070
Developed Site Hydrology
The developed site will preserve the general configuration of the existing site, with runoff
generally flowing from east to west, being collected in catch basins located throughout the site
prior to being conveyed to a treatment structure and a detention facility, then released into the
existing system in 108th Ave. SE. Therefore, existing drainage pattern and discharge locations will
be maintained. The post-developed conditions and un-detained peak runoff rates for the entire
basin area are shown in Tables 6 and 7 below. An exhibit showing the developed conditions is
provided in Appendix A. The proposed grading and drainage plan is included in Appendix B.
Developed peak runoff rates were determined using the WWHM modeling software and the
calculations can be found in Appendix C.
Table 6
Post-Developed Conditions
Area Condition
0.37 AC Pavement (PGIS)
0.08 AC Sidewalk
0.27 AC Landscape
0.18 AC Roof
Table 7
Developed Peak Runoff Rate
Event Rate (cfs)
2-year 0.258
10-year 0.383
100-year 0.557
Flow Control Analysis
The proposed project includes installation of a subsurface open-bottom modular detention
vault system in order to meet the requirements associated with the Flow Control Duration
Standard. A small amount of infiltration, based on the geotechnical information provided in
Appendix F, was taken into consideration in the design calculations. The total storage provided
by the vault system is 12,863 cubic feet, which is in excess of the required volume of 12,450 cf.
Discharges from the detention system are shown in Table 8.
Table 8
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PACLAND Project #10573001 Page 24
Mitigated Peak Runoff Rate
Event Rate (cfs)
2-year 0.016
10-year 0.031
100-year 0.060
A flow control structure (per City of Renton standard plan 237.00) will be installed downstream
of detention to regulate discharge rates. The structure shall incorporate a flow control tee
consisting of two orifices and a riser pipe.
Per City of Renton SWDM Section 1.2.9.2.2, the project is subject to the Large Lot High
Impervious BMP Requirements since the lot is greater than 22,000 square feet. This section
requires evaluation of multiple BMPs to provide flow control. Since full dispersion is not feasible,
the project is proposing to utilize limited infiltration using the open-bottom detention system.
Limited infiltration, per Appendix C Section C.2.3, involves the use of infiltration devices in soils
that are not as permeable as the medium sands or coarse sands/cobbles targeted for full
infiltration. Although limited infiltration facilities are only required to be applied to 20% of the
target impervious surface, the open-bottom detention system manages runoff from all site
impervious areas, thus satisfying the requirement.
Water Quality System
In order to provide enhanced basic treatment as required by City of Renton SWDM a modular
wetland will be installed upstream of detention. The modular wetland is intended to collect and
treat runoff generated by the pollutant-generating impervious areas on the site, mimicking the
treatment effects of a wetland by utilizing a combination of pre-treatment using separation and
pre-treatment cartridges and treatment utilizing biofiltration. The modular wetland systems have
been granted General Use Level Designation (GULD) by the Washington State Department of
Ecology (DOE) for Basic, Enhanced, and Phosphorous treatment. The approval documentation
has been included in Appendix E. Since the project utilizes limited infiltration for flow control,
the water quality structure has been located upstream of detention to maximize removal of
pollutants prior to infiltration, thereby maximizing the effectiveness of the infiltration and
minimizing maintenance requirements.
Table 9
Water Quality Flow Rates and Facility Sizing
Basin Water Quality Flow Rate (cfs)
From WWHM
MWS-L-4-8-V Treatment Flow Rate
(cfs)
A 0.0731 0.115
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Summary
Flow Control
Event Historic Flow
Rate (cfs)
Developed
Flow Rate (cfs)
Mitigated Flow
Rate/Discharge (cfs)
2-year 0.026 0.258 0.016
10-year 0.050 0.383 0.031
100-year 0.070 0.557 0.060
Flow control provided by the open-bottom detention system meets the requirements for the
Flow Control Duration Standard (City of Renton 2017 SWDM, Section 1.2.3.1), and also meets
the requirements for Individual Lot Flow Control BMPs via the use of limited infiltration.
Water Quality
Water Quality Flow Rates and Facility Sizing
Basin Design WQ Flow Rate (cfs)
From WWHM
MWS-L-4-8-V Treatment
Flow Rate (cfs)
A 0.0731 0.115
Water quality provided by the modular wetland units meets the requirements for the Enhanced
Basic Water Quality standard for treatment upstream of detention.
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Conveyance
Per the 2017 City of Renton Surface Water Design Manual, 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.
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. See Appendix C for
conveyance design calculations.
100-Year Flood/Overflow Condition
Review of the current FEMA FIRM maps (Panel 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 front access and drain out toward 108th Ave SE.
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 108th Ave SE.
Section 5 - Conveyance System Analysis and Design
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The following Special Reports and Studies were used or have been completed for this project:
• Geotechnical Engineering Report - by Geotech Consultants, Inc.
• FEMA FIRM Panels 0979F dated May 16, 1995
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 These permits will require approval by the City of Renton Planning Division or the Department of Ecology.
Section 7 - Other Permits
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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. Slope across the site is
minimal. Runoff will not travel at high velocities across the site and, therefore, will not cause
noticeable erosion impacts.
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).
4. Clear and grub site.
5. Grade site.
6. Construct buildings.
7. Pave site and install landscaping.
8. Remove temporary erosion control features once site is fully stabilized.
Section 8 - CSWPPP Analysis and Design
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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.
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The following items will be submitted upon application for the civil permits:
• Declaration of Covenant for Privately Maintained Flow Control BMPs
• Bond Quantity Worksheet
• Flow Control and Water Quality Facility Summary
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 has been prepared and can be found in Appendix D.
Section 10 - Operations and Maintenance Manual
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Appendix A – Exhibits
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Appendix B – Plans
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Appendix C – Design Calculations
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Conveyance Design Calculations
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)
12” Pipe @ 0.5% slope:
Q=n49.1 xAxR2/3xS1/2 = 2.94 CFS
n = 0.012
A = 0.771 (SF)
R = 0.286 @ 95% full
S = 0.005
Pipe capacity exceeds peak flows generated by development.
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Appendix D – Operations and Maintenance Manual
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Maintain Stormwater Facilities
The owner or operator of the project shall be responsible for maintaining the stormwater
facilities in accordance with local requirements. Proper maintenance is important for adequate
functioning of the stormwater facilities. The following maintenance program is recommended
for this project:
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Appendix E – Water Quality Department of Ecology
Certification
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Appendix F – Geotechnical Reports
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