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Construction Industry Training Council
of Washington
Technical Information Report
Land Use Permit Application Draft
Prepared by:
Tony Dubin, P.E., Dubin Environmental
September 16, 2019
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Table of Contents
Section 1: Project Overview ............................................................................................................................... 4
1.1 Purpose and Scope ................................................................................................................................... 4
1.2 Existing Conditions .................................................................................................................................... 4
1.3 Post-Developed Conditions ...................................................................................................................... 4
Section 2: Conditions and Requirements Summary ...................................................................................... 14
2.1 Core Requirements ................................................................................................................................. 14
2.1.1 CR 1 – Discharge at Natural Location ..................................................................................... 14
2.1.2 CR 2 – Offsite Analysis .............................................................................................................. 14
2.1.3 CR 3 – Flow Control .................................................................................................................. 14
2.1.4 CR 4 – Conveyance System ...................................................................................................... 14
2.1.5 CR 5 – Construction Stormwater Pollution Prevention ........................................................... 14
2.1.6 CR 6 – Maintenance and Operation ........................................................................................ 14
2.1.7 CR 7 – Financial Guarantees and Liability .............................................................................. 14
2.1.8 CR 8 – Water Quality Facilities ................................................................................................. 14
2.1.9 CR 9 – Onsite Best Management Practices (BMPs) ............................................................... 15
2.1.10 SR 1 – Other Adopted Area-Specific Requirements................................................................ 15
2.1.11 SR 2 – Flood Hazard Area Delineation .................................................................................... 15
2.1.12 SR 3 – Flood Protection Facilities ............................................................................................ 15
2.1.13 SR 4 – Source Controls ............................................................................................................. 16
2.1.14 SR 5 – Oil Control ...................................................................................................................... 16
2.1.15 SR 6 – Aquifer Protection Area................................................................................................. 16
Section 3: Offsite Analysis ............................................................................................................................... 18
3.1 Task 1 – Study Area Definition and Maps ............................................................................................. 18
3.2 Task 2 – Resource Review ..................................................................................................................... 19
3.3 Task 3 – Field Inspection ....................................................................................................................... 19
3.4 Task 4 – Drainage System Description and Problem Descriptions ..................................................... 19
Section 4: Flow Control and Water Quality Facility Analysis and Design ...................................................... 19
4.1 Flow Control ............................................................................................................................................. 19
4.1.1 Existing Site Hydrology .............................................................................................................. 20
4.1.2 Developed Site Hydrology ......................................................................................................... 20
4.1.3 Performance Standards ............................................................................................................ 20
4.2 Flow Control System ............................................................................................................................... 21
4.3 Water Quality System.............................................................................................................................. 21
Section 5: Conveyance System Analysis and Design .................................................................................... 22
Section 6: Special Reports and Studies ......................................................................................................... 22
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Section 7: Other Permits ................................................................................................................................. 22
Section 8: CSWPP Plan Analysis and Design ................................................................................................. 23
Section 9: Bond Quantities, Facility Summaries, and Declaration of Covenant .......................................... 23
Section 10: Operations and Maintenance Plan ............................................................................................. 24
Section 11: Conclusion .................................................................................................................................... 26
Appendix A: MGS Flood Report ....................................................................................................................... 27
Table of Figures
Figure 1. TIR worksheet ..................................................................................................................................... 5
Figure 2. CITC site area map ........................................................................................................................... 11
Figure 3. CITC site drainage map .................................................................................................................... 12
Figure 4. CITC site soils map ........................................................................................................................... 13
Figure 5. CITC project site is within Zone 1 of the wellhead protection area ............................................... 17
Figure 6. CITC public drainage system to outfall ........................................................................................... 18
Figure 7. Enhanced basic water quality treatment bioretention facility sketch ........................................... 21
Figure 8. CITC preliminary bond quantities worksheet (quantities to be provided under separate cover) 23
Figure 9. Bioretention facility sketch .............................................................................................................. 24
Figure 10. Preliminary bioretention inspection and operations plan ........................................................... 25
Table of Tables
Table 1. Large Lot BMP Feasibility/Infeasibility Review ................................................................................ 15
Table 2. CITC Drainage and Critical Areas Resource Review in COR Maps ................................................. 19
Table 3. CITC Direct Discharge Exemption Criteria ........................................................................................ 19
Table 4. Existing and Redeveloped Site Hydrology ........................................................................................ 20
Table 5. Applicable Stormwater Performance Standards ............................................................................. 20
Table 6. Water Quality Facility Dimensions and Description......................................................................... 21
Table 7. Bioretention Curb Cut Total Required Opening Length ................................................................... 22
Table 8. Bioretention Facility Dimensions and Description .......................................................................... 24
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Section 1: Project Overview
1.1 Purpose and Scope
This report accompanies the civil engineering plans and documents for the Construction Industry Training
Council (CITC) project site, a proposed commercial/educational project located at 1300 Bronson Way N in
the city of Renton, King County, Washington. The project site comprises Parcels No. 1352300005 and
1352300025 along Garden Avenue N and 1352300045, 1352300050, 1352300060 and 1352300071
along Meadow Avenue N. The total area of these properties is 1.5 acres. The site is bordered to the north by
N 2nd Street and to the south by Bronson Way N. A public alley bisects the project area running north-south
between Bronson Way N and N 2nd Street.
The site is located within the jurisdiction of the City of Renton, which has amended the 2016 King County
Surface Water Design Manual (KCSWDM) as the 2017 City of Renton Surface Water Design Manual (COR
Manual). Based on our review of the manual and discussions with city staff, the project must provide Basic
Enhanced Water Quality Treatment but is exempt from flow control requirements via a direct discharge ex-
emption.
Several figures are included at the end of Section 1. Figure 1 is a completed Technical Information Report
(TIR) Worksheet, which is Reference 8A in the COR Manual. Figure 2 is a site location map. Figure 3 shows
drainage basins and site characteristics. Figure 4 shows site soils.
1.2 Existing Conditions
The site currently has the former Bryant Motors building (west side of alley), paved vacant areas and a
grassy area in the northeast portion of the property. There are no existing trees on the site. There are no wet-
lands or other identified environmentally sensitive or critical areas identified by King County’s iMap. The ge-
otechnical report (Appendix B) identified the potential for soil contamination associated with the property’s
former use, but the construction team will remove any contaminated soils before CITC construction begins.
The site has gentle topography with slopes generally moving from south to north. The alley bisecting the pro-
ject area contains a 8-in diameter sanitary sewer and 12-in diameter storm drain. Stormwater is conveyed to
N 2nd Street, westward along N 2nd Street to Wells Avenue S, and then southward to an outfall into the Ce-
dar River. There are no existing flow control facilities on the project site. The existing drainage patterns
have been analyzed and are discussed in detail in the Level One Downstream Analysis (see Section 3.0).
1.3 Post-Developed Conditions
The project will redevelop the existing site into CITC school with associated parking, landscaping and a plaza
in the southwest corner of the site. The CITC building footprint will be very similar to the existing Bryant Mo-
tors building footprint and measure approximately 32,681 square feet. The alley will be reconstructed and
the eastern parcels will be converted into a parking lot, landscaping, and a bioretention facility that will be
constructed. The interior space will consist of showroom/sales area, business offices, parts and storage, ser-
vice and shop, storage area, and other supportive areas. Section 4 describes the drainage design in greater
detail.
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Figure 1. TIR worksheet
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Figure 1. TIR Worksheet (part 2 of 6)
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Figure 1. TIR Worksheet (part 3 of 6)
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Figure 1. TIR Worksheet (part 4 of 6)
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Figure 1. TIR Worksheet (part 5 of 6)
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Figure 1. TIR Worksheet (part 6 of 6)
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Figure 2. CITC site area map
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Figure 3. CITC site drainage map
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Figure 4. CITC site soils map
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Section 2: Conditions and Requirements Summary
2.1 Core Requirements
The following sections describe the core requirements and special requirements that apply to the site and
references to sections later in the report that contain detailed calculations and mapping to support our de-
scription of how these requirements are met.
2.1.1 CR 1 – Discharge at Natural Location
The project site current discharges to the city of Renton storm drainage pipe that runs northward in the alley-
way from Bronson Way N to N 2nd Street. The CITC project will continue to utilize this storm drain to convey
stormwater from the project site. The project will not change the site’s drainage discharge location.
2.1.2 CR 2 – Offsite Analysis
A downstream analysis is included in Section 3.0 below. The analysis includes:
1. Defining and mapping the study area.
2. Reviewing available information on the study area.
3. Field inspecting the study area.
2.1.3 CR 3 – Flow Control
The project is located in a Direct Discharge Exemption area. Stormwater leaving the project site is conveyed
through City of Renton storm drainage system to the Cedar River. The outfall is downstream of the Cedar
River’s confluence with Taylor Creek and meets the other direct discharge exemption criteria. Flow control is
discussed in further detail in Section 4.0.
2.1.4 CR 4 – Conveyance System
The proposed conveyance system is designed to meet the requirements outlined in Section 1.2.4 of the
CRSWDM. Refer to Section 5.0 for more information
2.1.5 CR 5 – Construction Stormwater Pollution Prevention
Onsite land disturbance will consist of clearing the site, demolishing an existing onsite building, and minor
regrading of the site. A Construction Stormwater Pollution Prevention Plan (CSWPPP) will be included with
the civil engineering design plan set.
2.1.6 CR 6 – Maintenance and Operation
Maintenance and operations of all drainage facilities are the responsibility of the owner. Refer to Section 10
for the operations and maintenance manual.
2.1.7 CR 7 – Financial Guarantees and Liability
The owner will meet all financial guarantee and liability requirements. This project will provide a Drainage
Facilities Restoration and Site Stabilization Financial Guarantee. Refer to Section 9 for bond quantity infor-
mation.
2.1.8 CR 8 – Water Quality Facilities
The project site is subject to the Enhanced Basic Water Quality Treatment Menu per the Renton Manual. The
bioretention/water quality facility sizing and performance calculations are described in Section 4. Detailed
drawings will be included in the civil engineering design plan set.
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2.1.9 CR 9 – Onsite Best Management Practices (BMPs)
The CITC project is classified as a Large Lot per Section 1.2.9.2 of the COR Manual. The proposed project
site will meet the Large Lot BMP Requirements outlined in Section 1.2.9.2.2. Below is a discussion of the
list approach for each type of surface proposed on the site.
Landscape Areas at Perimeter of Property
Post Construction Soil Quality and Depth will be used.
Plaza Area in Southwest Corner of Property
The plaza area in the southwest corner of the property will contain a mixture of hard surfaces (e.g., paver
walkway, benches, concrete for fountain) and landscaped areas. Post Construction Soil Quality and Depth
will be applied to the landscaped areas. The hard surfaces will be non-pollution generating, because they will
not be subject to vehicle traffic, subject to stormwater run-on, and will not contain erodible materials. This
portion of the property is currently fully impervious, so the redevelopment will reduce runoff rates and pollu-
tant loading. As feasible, the hard surfaces will be graded to drain into the landscaping.
Impervious Areas: Parking Lot
The project is located in an aquifer protection zone and no onsite infiltration of treated stormwater runoff is
permitted. This limits the available best management practices (BMPs) for stormwater control. Table 1 lists
the feasibility/infeasibility review.
Table 1. Large Lot BMP Feasibility/Infeasibility Review
Onsite BMP Feasible? Note
Full dispersion No Insufficient land/native vegetation
Full infiltration of roof runoff No No stormwater infiltration permitted onsite
Full infiltration No No stormwater infiltration permitted onsite
Limited infiltration No No stormwater infiltration permitted onsite
Bioretention Yes Bioretention sized with MGS Flood model
Permeable pavement No No stormwater infiltration permitted onsite
Basic dispersion N/A Does not apply because bioretention is feasible
Impervious Areas: Roof
Based on discussions with Renton staff, the Thermoplastic Polyolefin (TPO) roofing system is not considered
a pollution-generating surface and therefore does not require water quality treatment. The project is within a
direct discharge exemption area (see Section 4.1.3), so roof runoff will be routed to the alleyway and public
storm drainage system.
2.1.10 SR 1 – Other Adopted Area-Specific Requirements
To our knowledge, there are no adopted area-specific requirements that are applicable to the project site.
2.1.11 SR 2 – Flood Hazard Area Delineation
King County iMap does not indicate that the project is within a flood hazard area.
2.1.12 SR 3 – Flood Protection Facilities
The project site does not contain, nor is it adjacent to, any existing flood protection facilities. Project improve-
ments do not include flood protection measures.
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2.1.13 SR 4 – Source Controls
The proposed project is classified as a commercial site. Water quality source controls applicable to the pro-
ject site shall be evaluated and applied as described in the King County Stormwater Pollution Prevention
Manual (KCSWPPM) and Renton Municipal Code IV.
2.1.14 SR 5 – Oil Control
The project is not a high-use site; therefore, it is not subject to oil control requirements.
2.1.15 SR 6 – Aquifer Protection Area
According to the City of Renton Public Works Department Groundwater Protection Areas map (see Reference
15-B of the COR Manual), the CITC is located within Zone 1 of the wellhead protection area. Figure 5 shows
the project locations and wellhead protection areas. As described above, infiltrating stormwater BMPs are
not permitted on the project site.
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Figure 5. CITC project site is within Zone 1 of the wellhead protection area
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Section 3: Offsite Analysis
3.1 Task 1 – Study Area Definition and Maps
The existing drainage system at the CITC project site discharges water to a COR storm drainage pipe located
in the alley between the east and west project parcels. The public storm drain conveys water northward to N
2nd Street, then westward in N 2nd Street to Wells Avenue. Stormwater is then conveyed southward within
piping in Wells Avenue to an outfall in the Cedar River. Figure 6 shows the conveyance alignment from the
CITC site to the outfall.
Figure 6. CITC public drainage system to outfall
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3.2 Task 2 – Resource Review
The City of Renton’s online COR Maps application was the primary resource for evaluating the potential for
critical areas and existing drainage problems that could be affected by the CITC project. Table 2 lists the
data sources that were reviewed.
Table 2. CITC Drainage and Critical Areas Resource Review in COR Maps
No. Issues/Critical Area Type CITC Applicability
1 Floodplains and floodways
• 100-year floodplain is contained within Cedar River bank or near vicinity
• 500-year floodplain extends beyond riverbank but not to the CITC site
• No floodways in vicinity of CITC site
2 Steep slopes, landslide, and
erosion hazard areas • No steep slopes, landslide, or erosion hazard zones indicated
3 Wetlands • No new piped conveyance infrastructure (see Section 4.1.3). No known downstream
conveyance problem areas (see Section 3.4)
4 Wellhead protection areas • The project is within wellhead protection zone 1. Based on guidance from COR staff,
bioretention facilities will be non-infiltrating
5 Known drainage problem areas • No problems areas indicated by the “drainage complaints” layer within COR maps
3.3 Task 3 – Field Inspection
A windshield survey was conducted on 9/9/2019 in the early afternoon. The storm drainage alignment from
the CITC project site to the outfall was driven. The morning had been rainy and puddles were visible near the
CITC site and along portions of N. 2nd Street. However, there was no evidence of flooding in project vicinity.
Storm drain inlets were visible along the roadway, which helped confirm the public storm drainage system is
consistent with Renton’s GIS mapping.
3.4 Task 4 – Drainage System Description and Problem Descriptions
The public storm drainage system from the CITC site to the Cedar River outfall contains 1,722 feet of pipe
that ranges from 8 to 22 inches in diameter. The system is designed for gravity flow. COR maps listed no
drainage complaints along the pipe alignment between the CITC site and the Cedar River outfall. Additionally,
no drainage complaints were noted for any of the pipes and blocks that are tributary to the Wells Avenue
outfall.
Section 4: Flow Control and Water Quality Facility Analysis and Design
4.1 Flow Control
The project site is located within a Direct Discharge Exemption area. Table 3 describes how the projects
meets each of the exemption criteria.
Table 3. CITC Direct Discharge Exemption Criteria
No. Criterion CITC Applicability
1 Flow path from project to discharge point
less than 0.5 miles
• Site stormwater is conveyed in public storm drainage system along N 2nd
Street and Wells Ave. Total storm pipe length is 1,722 feet (0.33 miles)
2
From project site to river, conveyance system
will extend to OHW mark and consist of
manmade elements
• Both elements true; all conveyance will be piping and conveyance system
extends all the way to the Cedar River
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3 Conveyance system will have adequate ca-
pacity for 25-year flow
• No new piped conveyance infrastructure (see Section 4.1.3). No known
downstream conveyance problem areas (see Section 3.4)
4 Conveyance system will be adequately stabi-
lized to prevent erosion
• All onsite conveyance will be via sheet flow on parking lot; offsite flow will
be conveyed in pipe; these systems are no subject to erosion
5 Flow diversions that would increase flows to
an existing wetland or stream • The CITC project will contain no flow diversions
4.1.1 Existing Site Hydrology
The existing site hydrology was developed for the CITC parcels and the alleyway. The site is entirely impervi-
ous with the exception of a 0.19 acre grassy area located in the northeast parcel. Note: the existing surfaces
within the public right-of-way pavement replacement limits were excluded from the modeling analysis. Based
on discussions with Renton staff, water quality treatment will not be required for replaced impervious in the
public right of way for this project.
4.1.2 Developed Site Hydrology
The developed site hydrology will contain a mixture of impervious surfaces (e.g., building roof, parking area)
and landscaping. Table 4 lists the existing and redeveloped land uses. These areas were used to develop an
MGS Flood model to compute peak flows and water quality treatment performance.
Table 4. Existing and Redeveloped Site Hydrology
Drainage Area Existing Condition (ac) Redeveloped Condition(ac)
Impervious Pervious Impervious Pervious
West Parcels
BuildingA 0.75 0 0.75
Plaza 0.073 0 0.030 0.05
East Parcels
Parking Lot 0.53 0 0.49 0
Landscaping 0 0.19 0 0.24
Offsite
Alley 0.12 0 0.12 0
TOTAL 1.47 0.19 1.38 0.29
A. The building roof is a non-pollution generating surface and will not drain to the bioretention
system.
4.1.3 Performance Standards
Table 5 lists the COR performance standards and describes how they apply to this project.
Table 5. Applicable Stormwater Performance Standards
Category Applicable Performance Standard
Area-specific flow control facility
standard
The project site is located within a Direct Discharge Exemption area. See Section 4.1.3
for additional detail.
Conveyance system standard Section 1.2.4 of the COR Manual describes the requirement for new pipe, culvert and
open channels to convey the 25-year and 100-year peak flows. However, these standards
do not directly apply to our design, because the CITC site does not include onsite pipes,
culverts, or open channel conveyance infrastructure. All stormwater will be conveyed via
sheet flow from the parking lot or roof downspout to a bioretention facility. During civil de-
sign, the project team will ensure notches or other bioretention entrance structures are
adequately sized.
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Area-specific water quality treat-
ment menu
The CITC project is required to meet the enhanced water quality treatment standard, which
calls for 80% reduction in TSS and 50% reduction in zinc concentrations. According to the
BMP menu, bioretention is an acceptable technology for meeting these water quality
treatment requirements.
Source controls No stormwater infiltration permitted onsite
Oil control Not applicable to commercial/vocational site parking lot
4.2 Flow Control System
The CITC project is within a direct discharge exemption area. Therefore, the bioretention system was not
sized for flow control.
4.3 Water Quality System
The east parcels listed in Table 4 above contain pollution generating surfaces that will drain to the bioreten-
tion facility. An MGS Flood v4.49 model was constructed to evaluate the water quality treatment perfor-
mance of the bioretention. Figure 7 shows the bioretention in plan view and Table 6 lists the bioretention
layer depths and configuration.
Figure 7. Enhanced basic water quality treatment bioretention facility sketch
Table 6. Water Quality Facility Dimensions and Description
Design Element Description
Bioretention length 173 feet
Bioretention width 10.5 feet
Bioretention H;V side wall Vertical concrete side walls
Surface ponding depth 6 inches
Bioretention media depth 18 inches
Gravel subgrade depth 6 inches (sufficient for underdrain pipe)
Bioretention entrance Flow enters via curb cuts; rock inside bioreten-
tion to minimize scour at entrances
Bioretention overflow Curb cut on alley side of facility to allow over-
flows into existing public storm drain system
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The bioretention facility will filter and treat an estimated 100 percent of annual runoff. This meets the en-
hanced basic water quality treatment standard for removing 80 percent of total suspended solids (TSS) and
50 percent of zinc. The MGS Flood report is included as Appendix A.
Section 5: Conveyance System Analysis and Design
Stormwater will be conveyed from the parking lot to the bioretention facility via sheet flow. Roof runoff will be
conveyed via downspouts and sheet flow to the existing storm drainage system in the alleyway. No pipe, cul-
vert or ditch/swale conveyance facilities are part of the design. The parking lot will be graded toward the bio-
retention facility at about a 1 percent slope. Curb cuts will allow water to flow into the bioretention facility.
The curb cuts will be sized during the civil final design using a broad-crested weir equation:
𝑄=𝐶𝑤𝐿𝐻
3
2
Where Q is flow in cfs; Cw is the weir coefficient and a value of 2.5 will be used for this analysis; L is the curb
cut opening length; and H is the depth of in feet at the weir.
Table 7 lists the modeled 100-year flow rate to the bioretention from the building and alley and from the
parking lot. The table also lists the total curb cut opening length necessary to so that water does not reach
more than 1 inch in depth at the curb cuts.
Table 7. Bioretention Curb Cut Total Required Opening Length
Tributary 100-year Peak Flow Total Curb Cut OpeningA
Parking lot 0.474 cfs 7.9 feet
A. The total curb cut opening will be achieved by multiple smaller curb cuts that are located based on
final parking lot grading.
Section 6: Special Reports and Studies
The geotechnical report by Geotech Consultants, Inc., and dated April 26, 2019, was submitted by Rhodes
Architecture and Light as part of the Land Use Permit Application. Please refer to this report for information
about subsurface information. Note: the project stormwater design does not include infiltrating BMPs.
Section 7: Other Permits
The following list of permits is required for the CITC site:
1. Land Use Permit
a. Conditional Use Permit
b. Environmental Review
c. Site Plan Review
d. Lot Line Adjustment
2. Civil Permit (you guys are more knowledgeable about the ins and outs of this submittal but I would
think the clearing and grading are included in this overall permit)
3. Building Permit (I believe the Demo Permit is a sub-permit of this).
If additional permits are required, this section may be amended as the civil design progresses.
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Section 8: CSWPP Plan Analysis and Design
The construction stormwater pollution prevention (CSWPP) plan and design will be prepared and submitted
as the civil design progresses. See Figure 1, Part 13 for a list of CSWPP plan elements that will be ad-
dressed.
Section 9: Bond Quantities, Facility Summaries, and Declaration of Covenant
The preliminary bond quantities worksheet is included below as Figure 8. The specific quantities and corre-
sponding bond totals for erosion and sediment control, right-of-way improvements, stormwater drainage fa-
cilities, etc., will be updated as the civil design proceeds, and an updated bond quantities worksheet will be
provided to City of Renton staff under separate cover. Similarly, a declaration of covenant for inspection will
be developed and submitted under separate cover.
Figure 8. CITC preliminary bond quantities worksheet (quantities to be provided under separate cover)
Figure 9 shows a sketch of the proposed bioretention facility. Table 8 lists the facility dimensions. Based on
MGS Flood modeling analysis, the proposed bioretention dimensions and configuration will filter more than
Quantities and costs to be provided
under cover as civil design proceeds
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91 percent of annual runoff and therefore will meet the enhanced basic water quality treatment require-
ments.
Figure 9. Bioretention facility sketch
Table 8. Bioretention Facility Dimensions and Description
Design Element Description
Bioretention length 173 feet
Bioretention width 10.5 feet
Bioretention H;V side wall Vertical concrete side walls
Surface ponding depth 6 inches
Bioretention media depth 18 inches
Gravel subgrade depth 6 inches (sufficient for underdrain pipe)
Bioretention entrance Flow enters via curb cuts; rock inside bioreten-
tion to minimize scour at entrances
Bioretention overflow Curb cut on alley side of facility to allow over-
flows into existing public storm drain system
Section 10: Operations and Maintenance Plan
The property owner will take responsibility for operation and maintenance of the private stormwater system.
Bioretention is the only infrastructure and the operation and maintenance of this system will be straightfor-
ward. Figure 10 shows a preliminary calendar of inspection activities and corresponding maintenance activi-
ties. The plan will be refined, as appropriate, with input from the landscape architect and building owner as
the project nears completion.
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Figure 10. Preliminary bioretention inspection and operations plan
1300 Bronson Way N
CITC Headquarters
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In addition to the frequent activities listed above, the owner should CCTV the underdrain pipe periodically.
We recommend inspecting the underdrain when the bioretention media is replaced or refreshed (i.e., once
per 5 to 10 years).
Section 11: Conclusion
The CITC property stormwater management approach has been developed to meet the requirements of the
2017 Renton Surface Water Design Manual (dated 12/12/2016). The stormwater system will meet the en-
hanced basic water quality standard with a bioretention facilities located adjacent to the parking lot. The site
is located within a direct discharge exemption area.
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Appendix A: MGS Flood Report
—————————————————————————————————
MGS FLOOD
PROJECT REPORT
Program Version: MGSFlood 4.49
Program License Number: 201910003
Project Simulation Performed on: 09/13/2019 12:02 PM
Report Generation Date: 09/13/2019 12:02 PM
—————————————————————————————————
Input File Name: CITC_bioretention_2019-09-09.fld
Project Name: CITC Renton
Analysis Title: Land Use Permit Application
Comments:
———————————————— PRECIPITATION INPUT ————————————————
Computational Time Step (Minutes): 15
Extended Precipitation Time Series Selected
Climatic Region Number: 3
Full Period of Record Available used for Routing
Precipitation Station : 95004005 Puget West 40 in_5min 10/01/1939-10/01/2097
Evaporation Station : 951040 Puget West 40 in MAP
Evaporation Scale Factor : 0.750
HSPF Parameter Region Number: 1
HSPF Parameter Region Name : USGS Default
********** Default HSPF Parameters Used (Not Modified by User) ***************
********************** WATERSHED DEFINITION ***********************
Predevelopment/Post Development Tributary Area Summary
Predeveloped Post Developed
Total Subbasin Area (acres) 0.720 0.720
Area of Links that Include Precip/Evap (acres) 0.000 0.042
Total (acres) 0.720 0.762
----------------------SCENARIO: EXISTING
Number of Subbasins: 1
---------- Subbasin : East Parcels ----------
-------Area (Acres) --------
Outwash Grass 0.189
Impervious 0.531
----------------------------------------------
Subbasin Total 0.720
----------------------SCENARIO: PROPOSED+MITIGATED
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Number of Subbasins: 1
---------- Subbasin : Parking Lot ----------
-------Area (Acres) --------
Outwash Pasture 0.235
Impervious 0.485
----------------------------------------------
Subbasin Total 0.720
************************* LINK DATA *******************************
----------------------SCENARIO: EXISTING
Number of Links: 0
************************* LINK DATA *******************************
----------------------SCENARIO: PROPOSED+MITIGATED
Number of Links: 1
------------------------------------------
Link Name: Parking_BRB
Link Type: Bioretention Facility
Downstream Link: None
Base Elevation (ft) : 40.50
Riser Crest Elevation (ft) : 40.80
Storage Depth (ft) : 0.30
Bottom Length (ft) : 173.0
Bottom Width (ft) : 10.5
Side Slopes (ft/ft) : L1= 0.00 L2= 0.00 W1= 0.00 W2= 0.00
Bottom Area (sq-ft) : 1817.
Area at Riser Crest El (sq-ft) : 1,817.
(acres) : 0.042
Volume at Riser Crest (cu-ft) : 1,635.
(ac-ft) : 0.038
Infiltration on Bottom only Selected
Soil Properties
Biosoil Thickness (ft) : 1.50
Biosoil Saturated Hydraulic Conductivity (in/hr) : 6.00
Biosoil Porosity (Percent) : 40.00
Maximum Elevation of Bioretention Soil : 40.50
Native Soil Hydraulic Conductivity (in/hr) : 0.00
Underdrain Present
Orifice NOT Present in Under Drain
Riser Geometry
Riser Structure Type : Circular
Riser Diameter (in) : 6.00
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Common Length (ft) : 0.000
Riser Crest Elevation : 40.80 ft
Hydraulic Structure Geometry
Number of Devices: 0
**********************FLOOD FREQUENCY AND DURATION STATISTICS*******************
----------------------SCENARIO: EXISTING
Number of Subbasins: 1
Number of Links: 0
----------------------SCENARIO: PROPOSED+MITIGATED
Number of Subbasins: 1
Number of Links: 1
********** Subbasin: Parking Lot **********
Flood Frequency Data(cfs)
(Recurrence Interval Computed Using Gringorten Plotting Position)
Tr (yrs) Flood Peak (cfs)
======================================
2-Year 0.187
5-Year 0.241
10-Year 0.272
25-Year 0.315
50-Year 0.407
100-Year 0.474
200-Year 0.488
500-Year 0.506
********** Link: Parking_BRB ********** Link Inflow Frequency Stats
Flood Frequency Data(cfs)
(Recurrence Interval Computed Using Gringorten Plotting Position)
Tr (yrs) Flood Peak (cfs)
======================================
2-Year 0.187
5-Year 0.241
10-Year 0.272
25-Year 0.315
50-Year 0.407
100-Year 0.474
200-Year 0.488
500-Year 0.506
********** Link: Parking_BRB ********** Link WSEL Stats
WSEL Frequency Data(ft)
(Recurrence Interval Computed Using Gringorten Plotting Position)
Tr (yrs) WSEL Peak (ft)
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======================================
1.05-Year 40.507
1.11-Year 40.508
1.25-Year 40.509
2.00-Year 40.512
3.33-Year 40.514
5-Year 40.515
10-Year 40.532
25-Year 40.568
50-Year 40.689
100-Year 40.717
***********Groundwater Recharge Summary *************
Recharge is computed as input to Perlnd Groundwater Plus Infiltration in Structures
Total Predeveloped Recharge During Simulation
Model Element Recharge Amount (ac-ft)
-----------------------------------------------------------------------------------------------
Subbasin: East Parcels 63.698
_____________________________________
Total: 63.698
Total Post Developed Recharge During Simulation
Model Element Recharge Amount (ac-ft)
-----------------------------------------------------------------------------------------------
Subbasin: Parking Lot 80.256
Link: Parking_BRB 0.000
_____________________________________
Total: 80.256
Total Predevelopment Recharge is Less than Post Developed
Average Recharge Per Year, (Number of Years= 158)
Predeveloped: 0.403 ac-ft/year, Post Developed: 0.508 ac-ft/year
***********Water Quality Facility Data *************
----------------------SCENARIO: EXISTING
Number of Links: 0
----------------------SCENARIO: PROPOSED+MITIGATED
Number of Links: 1
********** Link: Parking_BRB **********
Infiltration/Filtration Statistics--------------------
Inflow Volume (ac-ft): 219.34
Inflow Volume Including PPT-Evap (ac-ft): 238.51
Total Runoff Infiltrated (ac-ft): 0.00, 0.00%
Total Runoff Filtered (ac-ft): 238.51, 100.00%
Primary Outflow To Downstream System (ac-ft): 238.62
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Secondary Outflow To Downstream System (ac-ft): 0.00
Percent Treated (Infiltrated+Filtered)/Total Volume: 100.00%
***********Compliance Point Results *************
Scenario Existing Compliance Subbasin: East Parcels
Scenario Proposed+Mitigated Compliance Link: Parking_BRB
*** Point of Compliance Flow Frequency Data ***
Recurrence Interval Computed Using Gringorten Plotting Position
Predevelopment Runoff Postdevelopment Runoff
Tr (Years) Discharge (cfs) Tr (Years) Discharge (cfs)
----------------------------------------------------------------------------------------------------------------------
2-Year 0.207 2-Year 0.201
5-Year 0.264 5-Year 0.252
10-Year 0.305 10-Year 0.252
25-Year 0.345 25-Year 0.252
50-Year 0.446 50-Year 0.252
100-Year 0.519 100-Year 0.252
200-Year 0.534 200-Year 0.270
500-Year 0.554 500-Year 0.293
** Record too Short to Compute Peak Discharge for These Recurrence Intervals