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Construction Industry Training Council
of Washington
Technical Information Report
Civil Permit Application
Prepared by:
Tony Dubin, P.E., Dubin Environmental
July 6, 2020
DEVELOPMENT ENGINEERING
jchavez 08/28/2020
SURFACE WATER UTILITY
jfarah 08/28/2020
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Table of Contents
Section 1: Project Overview ............................................................................................................................... 5
1.1 Purpose and Scope ................................................................................................................................... 5
1.2 Existing Conditions .................................................................................................................................... 5
1.3 Post-Developed Conditions ...................................................................................................................... 5
Section 2: Conditions and Requirements Summary ...................................................................................... 15
2.1 Core Requirements ................................................................................................................................. 15
2.1.1 CR 1 – Discharge at Natural Location ..................................................................................... 15
2.1.2 CR 2 – Offsite Analysis .............................................................................................................. 15
2.1.3 CR 3 – Flow Control .................................................................................................................. 15
2.1.4 CR 4 – Conveyance System ...................................................................................................... 17
2.1.5 CR 5 – Construction Stormwater Pollution Prevention ........................................................... 17
2.1.6 CR 6 – Maintenance and Operation ........................................................................................ 17
2.1.7 CR 7 – Financial Guarantees and Liability .............................................................................. 17
2.1.8 CR 8 – Water Quality Facilities ................................................................................................. 17
2.1.9 CR 9 – Onsite Best Management Practices (BMPs) ............................................................... 17
2.1.10 SR 1 – Other Adopted Area-Specific Requirements................................................................ 18
2.1.11 SR 2 – Flood Hazard Area Delineation .................................................................................... 18
2.1.12 SR 3 – Flood Protection Facilities ............................................................................................ 18
2.1.13 SR 4 – Source Controls ............................................................................................................. 18
2.1.14 SR 5 – Oil Control ...................................................................................................................... 18
2.1.15 SR 6 – Aquifer Protection Area................................................................................................. 18
Section 3: Offsite Analysis ............................................................................................................................... 20
3.1 Task 1 – Study Area Definition and Maps ............................................................................................. 20
3.2 Task 2 – Resource Review ..................................................................................................................... 21
3.3 Task 3 – Field Inspection ....................................................................................................................... 21
3.4 Task 4 – Drainage System Description and Problem Descriptions ..................................................... 21
Section 4: Flow Control and Water Quality Facility Analysis and Design ...................................................... 21
4.1 Flow Control ............................................................................................................................................. 21
4.1.1 Existing Site Hydrology .............................................................................................................. 22
4.1.2 Developed Site Hydrology ......................................................................................................... 22
4.1.3 Performance Standards ............................................................................................................ 22
4.2 Flow Control System ............................................................................................................................... 25
4.3 Water Quality System.............................................................................................................................. 25
Section 5: Conveyance System Analysis and Design .................................................................................... 27
Section 6: Special Reports and Studies ......................................................................................................... 28
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Section 7: Other Permits ................................................................................................................................. 28
Section 8: CSWPP Plan Analysis and Design ................................................................................................. 28
Section 9: Bond Quantities, Facility Summaries, and Declaration of Covenant .......................................... 29
Section 10: Operations and Maintenance Plan ............................................................................................. 29
Section 11: Conclusion .................................................................................................................................... 31
Appendix A: Geotech Report ............................................................................................................................ 32
Appendix B: Erosion and Sediment Control Plan Sheets .............................................................................. 33
Appendix C: MGS Flood Report ....................................................................................................................... 34
Table of Figures
Figure 1. TIR worksheet ........................................................................................................................................................ 6
Figure 2. CITC site area map .............................................................................................................................................. 12
Figure 3. CITC site drainage map ....................................................................................................................................... 13
Figure 4. CITC site soils map .............................................................................................................................................. 14
Figure 5. CITC project site flow control requirement (reference 15A) ................................................................................. 16
Figure 6. CITC project site is within Zone 1 of the wellhead protection area ...................................................................... 19
Figure 7. CITC public drainage system to outfall ................................................................................................................ 20
Figure 8. Existing site hydrology -- basins and land cover .................................................................................................. 23
Figure 9. Develop site hydrology – basins and land cover .................................................................................................. 24
Figure 10. Enhanced basic water quality treatment bioretention facility plan view .............................................................. 25
Figure 11. Enhanced basic water quality treatment bioretention facility section view ......................................................... 25
Figure 12. Preliminary bioretention inspection and operations plan .................................................................................... 30
Table of Tables
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Table 1. Large Lot BMP Feasibility/Infeasibility Review ...................................................................................................... 18
Table 2. CITC Drainage and Critical Areas Resource Review in COR Maps ..................................................................... 21
Table 3. Existing and Redeveloped Site Hydrology ............................................................................................................ 22
Table 4. Applicable Stormwater Performance Standards ................................................................................................... 22
Table 5. Water Quality Facility Dimensions and Description ............................................................................................... 26
Table 6. PGIS Areas for Bioretention Calculations ............................................................................................................. 26
Table 7. Bioretention Water Quality Treatment Performance ............................................................................................. 26
Table 8. Bioretention Curb Cut Total Required Opening Length ......................................................................................... 27
Table 9. Roof Drain Pipe Hydraulic Calculations ................................................................................................................ 27
Table 10. Trench Drain Hydraulic Calculations ................................................................................................................... 27
Table 11. New Storm Drain Pipe Hydraulic Calculations .................................................................................................... 28
Table 12. Impact of New Alley Catch Basin on Existing Alley Storm Flows ........................................................................ 28
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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 and meet the Peak Rate Flow Control Standards – Match Existing flow
control standard.
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 A) 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 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 the 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
Motors 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 non-infiltrating bioretention
facility. The building interior will consist of classrooms, laboratories, offices, and student lounge areas. Sec-
tion 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 Peak Rate Flow Control – Match Existing area (see Figure 5 on the next page).
Stormwater leaving the project site is conveyed through City of Renton storm drainage system to the Cedar
River. Flow control is discussed in further detail in Section 4.0.
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Figure 5. CITC project site flow control requirement (reference 15A)
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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. This report includes erosion and sediment control (ESC) plan sheets as Appendix B. A
more detailed Construction Stormwater Pollution Prevention Plan (CSWPPP) that identifies erosion and sedi-
ment control practices will be included with the building permit application. The public and private property
designs are proceeding in parallel with the public right-of-way utility design ahead of the building design.
However, the public and private construction will proceed together using the plan included in the building
permit’s CSWPPP.
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
non-infiltrating bioretention/water quality facility sizing and performance calculations are described in Sec-
tion 4. Detailed drawings will be included in the civil engineering design plan set.
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 CITC project will reduce runoff rates and pollutant
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. City of Renton staff recommended bioretention design measures that will prevent infiltration from
occurring onsite (e.g., fully concrete lined facility underlain by sand and an impermeable liner). This limits the
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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.
2.1.10 SR 1 – Other Adopted Area-Specific Requirements
With the exception of the aquifer protection requirements described above, 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.
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 6 shows
the project locations and wellhead protection areas. As described above, infiltrating stormwater BMPs are
not permitted on the project site. The non-infiltrating bioretention facility will be designed with specific
measures to prevent onsite infiltration.
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Figure 6. 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 7 shows the conveyance alignment from the
CITC site to the outfall.
Please note,
The CITC project site is currently, almost fully impervious and connected to the public storm drainage sys-
tem. The CITC project will reduce the site’s impervious area and thus will result in a) lower peak flows, b)
lower flow durations, and c) smaller total runoff volumes. Additionally, the project will include a non-infiltrat-
ing bioretention system that is designed for water quality treatment but will also provide further attenuation
of peak flows. Therefore, in addition to meeting the peak rate flow control standard, the CITC project will re-
duce demands on the downstream storm drainage system and the potential for flooding.
Figure 7. 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. Subsequent site visits through the fall and early-winter of 2019-20
have shown no indication that the area is subject to onsite flooding.
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. The CITC project will reduce onsite impervious area and therefore will lower peak flow rates in the
downstream public storm drainage system.
Section 4: Flow Control and Water Quality Facility Analysis and Design
4.1 Flow Control
The project site is located within a Peak Rate Flow Control – Match Existing area (see Figure 5 in Section CR
3 – Flow Control) and must meet the following criteria:
• Match the developed peak discharge rates to existing site conditions peak discharge rates for 2-,
10-, and 100-year return periods.
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4.1.1 Existing Site Hydrology
The existing site hydrology was developed for the CITC parcels, the alleyway, and the non-utility related curb,
gutter and full depth road replacement around the perimeter of the site (mostly on N 2nd Street). The site is
entirely impervious with the exception of a 0.19-acre grassy area located in the northeast parcel. Figure 8
shows an aerial photo of the current site conditions overlaid with land cover types.
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. Figure 9 shows the developed site basin areas and land cover types. Table 3 summarizes
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 3. Existing and Redeveloped Site Hydrology
Drainage Area Existing Condition (ac) Redeveloped Condition(ac)
Impervious Pervious Impervious Pervious
West Parcels (Non-Pollution Generating Surfaces)
BuildingA 0.75 0 0.75
Plaza 0.07 0 0.03 0.04
East Parcels
Parking Lot 0.53 0 0.48 0
Landscaping 0 0.19 0 0.24
Offsite
Alley 0.12 0 0.12 0
TOTAL 1.47 0.19 1.38 0.28
A. The building roof and plaza area are non-pollution generating surface and will not drain to the
bioretention system. The plaza area imperviousness is reduced by the CITC project.
The bioretention is located near the low spot on the property and captures most of the runoff from the site.
However, a small portion of the alleyway and the parking lot entrance must be sloped northward away from
the bioretention to match existing grades along N. 2nd Street.
4.1.3 Performance Standards
Table 4 lists the COR performance standards and describes how they apply to this project.
Table 4. Applicable Stormwater Performance Standards
Category Applicable Performance Standard
Area-specific flow control
facility standard
The project site is located within a Peak Rate Flow Control – Match Existing 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 chan-
nels to convey the 25-year and 100-year peak flows. However, these standards do not directly ap-
ply to our design, because the CITC site does not include onsite pipe s, culverts, or open channel
conveyance infrastructure. All stormwater will be conveyed via sheet flow from the parking lot or
alleyway to a bioretention facility. During civil design, the project team will ensure notches or
other bioretention entrance structures are adequately sized.
Area-specific water qual-
ity treatment 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
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Figure 8. Existing site hydrology -- basins and land cover
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Figure 9. Develop site hydrology – basins and land cover
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4.2 Flow Control System
The CITC project is within a Peak Rate Flow Control – Match Existing area, and the developed site will con-
tain less impervious area and therefore will produce lower peak flows. The project includes a non-infiltrating
bioretention system that will further attenuate peak flows but is designed primarily for water quality treat-
ment.
4.3 Water Quality System
The east parcels listed in Table 3 above contain pollution generating surfaces that will drain to the non-infil-
trating bioretention facility. An MGS Flood v4.49 model was constructed to evaluate the water quality treat-
ment performance of the bioretention. Figure 10 and Figure 11 show the bioretention in plan and section
view, and Table 5 lists the bioretention layer depths and configuration.
Figure 10. Enhanced basic water quality treatment bioretention facility plan view
Figure 11. Enhanced basic water quality treatment bioretention facility section view
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Table 5. Water Quality Facility Dimensions and Description
Design Element Description
Bioretention length 77 feet
Bioretention width 10.5 feet
Bioretention H;V side wall Vertical concrete side walls
Surface ponding depth 9 inches
Bioretention media depth 18 inches
Gravel subgrade depth 8 inches (for underdrain pipe)
Bioretention entrance Flow enters via piped flow or curb cuts around
perimeter of bioretention; rock inside bioreten-
tion to minimize scour at entrances
Bioretention overflow Overflow structure per COR STD 262.40
Table 6 summarizes a) the PGIS areas that contribute flow to the bioretention and b) the PGIS areas topo-
graphically lower than the bioretention that bypass the facility (e.g., full depth roadway replacement work
around the perimeter of the property). Please note, the utility-related roadway replacement work along Gar-
den and a portion of Meadow Ave N are exempt from water quality treatment requirements and are not in-
cluded in the table. The CITC building roof and plaza area are non-PGIS and are not included in the table.
Table 6. PGIS Areas for Bioretention Calculations
Location PGIS (ac) To Bioretention (ac) Bypass (ac)
East Parcel 0.483 0.468 0.015
West Parcel 0 0 0
Garden Ave. N 0 0 0
Alley 0.116 0.106 0.010
N. 2nd Street 0.019 0 0.019
Meadow Ave. N 0.003 0 0.003
Total 0.621 (100%) 0.574 (92.5%) 0.047 (7.5%)
Note: the west parcel contains the building and plaza areas, which are not PGIS. The Garden Ave N and
Meadow Ave N areas exclude the full depth road replacement that is associated with utility work.
The Renton SWDM requires that 91 percent of annual runoff receives water quality treatment. Because a
portion of the project area runoff does not drain to the bioretention (see above), the bioretention has been
oversized so that the aggregate performance meets the water quality treatment standard. Table 7 shows the
bioretention is sized the treat 99.82% of annual runoff. Because the bioretention receives flow from 92.5%
of the PGIS area, the bioretention system will treat 92.3% of the total annual PGIS runoff and meet the en-
hanced water quality treatment standard. See Appendix C for detailed calculations.
Table 7. Bioretention Water Quality Treatment Performance
Location PGIS Area % Runoff Treated
Treated by bioretention 0.574 (92.5%) 99.82%
Bypasses bioretention 0.047 (7.5%) 0%
Aggregate Treatment Total 0.621 (100%) 92.3%
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The non-infiltrating bioretention system meets the enhanced basic water quality treatment standard for re-
moving 80 percent of total suspended solids (TSS) and 50 percent of zinc. The site also meets the peak flow
standard. The MGS Flood report is included as Appendix C.
Section 5: Conveyance System Analysis and Design
Stormwater will be conveyed from the parking lot and alley to the bioretention facility via sheet flow or via
trench drains. Roof runoff will be conveyed via downspouts and a combination of existing laterals or new pip-
ing to the existing storm drainage system in the alleyway. The parking lot will be graded toward the bioreten-
tion 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 8 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 1.5 inches in depth at the curb cuts. Please note, these calculations are conservative. Most of the
runoff will enter the bioretention via piped connections. The total curb cut opening in the design exceeds the
4.3 foot minimum shown in the table.
Table 8. Bioretention Curb Cut Total Required Opening Length
Tributary 100-year Peak Flow Total Curb Cut OpeningA
Parking lot 0.474 cfs 4.3 feet
A. The total curb cut opening will be achieved by multiple smaller curb cuts that are located based on
final parking lot grading.
The roof drain conveyance piping in Garden and the alley is 6-in diameter with 1.5 percent or
greater slope. Table 9 compares the 100-year flow (compared in MGS Flood) to the full pipe hy-
draulic capacity, which was computed using Manning’s equation. The pipes have sufficient ca-
pacity. Table 10 compares the 100-year flow to the full pipe capacity for the trench drains that
convey water to the bioretention.
Table 9. Roof Drain Pipe Hydraulic Calculations
Pipe Segment Slope Diameter Full Pipe Capacity (cfs) 100-Year Flow (cfs)
Garden North Tight Line 1.5% 6-in 0.69 0.07
Garden South Tight Line 1.5% 6-in 0.69 0.08
Alley Tight Line 2.0% 6-in 0.80 0.07
Table 10. Trench Drain Hydraulic Calculations
Pipe Segment Slope Diameter Full Pipe Capacity (cfs) 100-Year Flow (cfs)
East Trench Drain 0.8% 6-in 0.50 0.25
Middle Trench Drain 0.6% 6-in 0.44 0.04
West Trench Drain 1.1% 6-in 0.59 0.12
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The project also includes new storm drain piping in Garden Ave N and Meadow Ave N. As part of
the pipe profile design, an MGS Flood model was prepared to estimate peak flows, depths, and
velocities. Table 11 lists the 100-year flow conditions for the pipes in Garden Ave N and
Meadow Ave N. Note: all pipes are 12-in diameter.
Table 11. New Storm Drain Pipe Hydraulic Calculations
Pipe Segment Slope 100-year Peak Flow (cfs) Depth (in) Velocity (ft/s)
Garden CB1 to CB2 4.8% 0.06 0.06 2.9
Garden CB2 to CB3 0.8% 0.26 2.3 2.5
Garden CB3 to SDMH 0.8% 0.51 3.3 3.0
Meadow CB1 to CB2 1.6% 0.06 0.8 1.8
The project will also install a new catch basin at the entrance to the alley at Bronson, which will
capture flow from the north side of Bronson between Meadow Ave N and the alley. Table 12
lists the existing and new tributary area to the existing storm drain system in the alley just down-
stream of the new catch basin. This new catch basin will add flow to the existing storm drainage
system in the alley although the amount will be negligible.
Table 12. Impact of New Alley Catch Basin on Existing Alley Storm Flows
Tributary Tributary Area (ac) 100-year Peak Flow Flow Depth
Existing 0.243 0.237 cfs 2.28 in
Post-Project 0.241 0.245 cfs 2.33 in
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
3. Building Permit
If additional permits are required, this section may be amended as the civil design progresses.
Section 8: CSWPP Plan Analysis and Design
Appendix B includes erosion and sediment control sheets. The construction stormwater pollution prevention
(CSWPP) plan and design will be refined as part of the building permit application. See Figure 1, Part 13 for a
CITC Renton Technical Information Report Civil Permit Application
29
list of CSWPP plan elements that will be addressed. The public right-of-way and building parcel CSWPPP will
be implemented together during construction.
Section 9: Bond Quantities, Facility Summaries, and Declaration of Covenant
The bond quantities worksheet has been submitted as part of the civil permit application. The specific quan-
tities and corresponding bond totals for erosion and sediment control, right-of-way improvements, storm-
water drainage facilities, etc., may be refined as the civil design and building design proceed, and refined
bond quantities may be provided to City of Renton staff, as appropriate. Similarly, a declaration of covenant
for inspection will be developed as part of the building permit application and submitted under separate
cover.
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 12 shows a calendar of inspection activities and corresponding maintenance activities. The
plan will be refined, as appropriate, with input from the landscape architect and building owner as the pro-
ject nears completion.
CITC Renton Technical Information Report Civil Permit Application
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Figure 12. Preliminary bioretention inspection and operations plan
1300 Bronson Way N
CITC Headquarters
CITC Renton Technical Information Report Civil Permit Application
31
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 non-infiltrating bioretention facility located adjacent to the park-
ing lot. The site meets the peak rate flow control – match existing standard by reducing the impervious area
relative to existing conditions and further attenuating peak flows in the bioretention facility.
CITC Renton Technical Information Report Civil Permit Application
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Appendix A: Geotech Report
CITC Renton Technical Information Report Civil Permit Application
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Appendix B: Erosion and Sediment Control Plan Sheets
CITC Renton Technical Information Report Civil Permit Application
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Appendix C: MGS Flood Report
—————————————————————————————————
MGS FLOOD
PROJECT REPORT
Program Version: MGSFlood 4.52
Program License Number: 201910003
Project Simulation Performed on: 07/22/2020 2:20 PM
Report Generation Date: 07/22/2020 2:20 PM
—————————————————————————————————
Input File Name: CITC_bioretention_2020-07-06.fld
Project Name: CITC Renton
Analysis Title: Final Bioretention Sizing
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) 0.814 0.795
Area of Links that Include Precip/Evap (acres) 0.000 0.019
Total (acres) 0.814 0.814
----------------------SCENARIO: EXISTING
Number of Subbasins: 1
---------- Subbasin : Alley + East Parcels ----------
-------Area (Acres) --------
Outwash Grass 0.190
Impervious 0.624
----------------------------------------------
Subbasin Total 0.814
CITC Renton Technical Information Report Civil Permit Application
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----------------------SCENARIO: PROPOSED+MITIGATED
Number of Subbasins: 1
---------- Subbasin : Alley + Parking Lot ----------
-------Area (Acres) --------
Till Pasture 0.221
Impervious 0.574
----------------------------------------------
Subbasin Total 0.795
************************* 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) : 39.65
Riser Crest Elevation (ft) : 40.40
Storage Depth (ft) : 0.75
Bottom Length (ft) : 77.0
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) : 809.
Area at Riser Crest El (sq-ft) : 809.
(acres) : 0.019
Volume at Riser Crest (cu-ft) : 1,091.
(ac-ft) : 0.025
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 : 39.65
Native Soil Hydraulic Conductivity (in/hr) : 0.00
Underdrain Present
Orifice NOT Present in Under Drain
Riser Geometry
CITC Renton Technical Information Report Civil Permit Application
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Riser Structure Type : Rectangular
Riser Length (ft) : 24.00
Riser Width (ft) : 20.00
Common Length (ft) : 0.000
Riser Crest Elevation : 40.40 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: Alley + Parking Lot **********
Flood Frequency Data(cfs)
(Recurrence Interval Computed Using Gringorten Plotting Position)
Tr (yrs) Flood Peak (cfs)
======================================
2-Year 0.229
5-Year 0.287
10-Year 0.341
25-Year 0.420
50-Year 0.498
100-Year 0.604
200-Year 0.612
500-Year 0.623
********** 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 0.229
5-Year 0.287
10-Year 0.341
25-Year 0.420
50-Year 0.498
100-Year 0.604
200-Year 0.612
500-Year 0.623
********** Link: Parking_BRB ********** Link WSEL Stats
CITC Renton Technical Information Report Civil Permit Application
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WSEL Frequency Data(ft)
(Recurrence Interval Computed Using Gringorten Plotting Position)
Tr (yrs) WSEL Peak (ft)
======================================
1.05-Year 39.744
1.11-Year 39.803
1.25-Year 39.882
2.00-Year 40.120
3.33-Year 40.400
5-Year 40.402
10-Year 40.404
25-Year 40.406
50-Year 40.408
100-Year 40.409
***********Groundwater Recharge Summary *************
Recharge is computed as input to Perlnd Groundwater Plus I nfiltration in Structures
Total Predeveloped Recharge During Simulation
Model Element Recharge Amount (ac-ft)
-----------------------------------------------------------------------------------------------
Subbasin: Alley + East Parcels 64.035
_____________________________________
Total: 64.035
Total Post Developed Recharge During Simulation
Model Element Recharge Amount (ac-ft)
-----------------------------------------------------------------------------------------------
Subbasin: Alley + Parking Lot 37.964
Link: Parking_BRB 0.000
_____________________________________
Total: 37.964
Total Predevelopment Recharge is Greater than Post Developed
Average Recharge Per Year, (Number of Years= 158)
Predeveloped: 0.405 ac-ft/year, Post Developed: 0.240 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): 294.92
Inflow Volume Including PPT-Evap (ac-ft): 302.76
CITC Renton Technical Information Report Civil Permit Application
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Total Runoff Infiltrated (ac-ft): 0.00, 0.00%
Total Runoff Filtered (ac-ft): 302.21, 99.82%
Primary Outflow To Downstream System (ac-ft): 302.76
Secondary Outflow To Downstream System (ac-ft): 0.00
Percent Treated (Infiltrated+Filtered)/Total Volume: 99.82%
***********Compliance Point Results *************
Scenario Existing Compliance Subbasin: Alley + 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 0.242 2-Year 0.112
5-Year 0.310 5-Year 0.199
10-Year 0.358 10-Year 0.266
25-Year 0.406 25-Year 0.319
50-Year 0.524 50-Year 0.403
100-Year 0.610 100-Year 0.436
200-Year 0.628 200-Year 0.558
500-Year 0.651 500-Year 0.723
** Record too Short to Compute Peak Discharge for These Recurrence Intervals
**** Flow Duration Performance ****
Excursion at Predeveloped 50%Q2 (Must be Less Than or Equal to 0%): -89.6% PASS
Maximum Excursion from 50%Q2 to Q2 (Must be Less Than or Equal to 0%): -71.6% PASS
Maximum Excursion from Q2 to Q50 (Must be less than 10%): -56.8% PASS
Percent Excursion from Q2 to Q50 (Must be less than 50%): 0.0% PASS
-------------------------------------------------------------------------------------------------
MEETS ALL FLOW DURATION DESIGN CRITERIA: PASS
-------------------------------------------------------------------------------------------------
NOTE FROM ENGINEER (DUBIN):
• 92.5% of project PGIS drains to bioreten-
tion
• Bioretention treats 99.82% of incoming
stormwater
• Therefore, 92.3% of total PGIS runoff re-
ceives water quality treatment
• These results are summarized in Table 7
of the TIR