Loading...
HomeMy WebLinkAboutRS_Stormwater_Technical_Information_Drainage_Report_190916_v1 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 CITC Renton Technical Information Report Land Use Permit Application Draft 2 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 CITC Renton Technical Information Report Land Use Permit Application Draft 3 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 CITC Renton Technical Information Report Land Use Permit Application Draft 4 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. CITC Renton Technical Information Report Land Use Permit Application Draft 5 Figure 1. TIR worksheet CITC Renton Technical Information Report Land Use Permit Application Draft 6 Figure 1. TIR Worksheet (part 2 of 6) CITC Renton Technical Information Report Land Use Permit Application Draft 7 Figure 1. TIR Worksheet (part 3 of 6) CITC Renton Technical Information Report Land Use Permit Application Draft 8 Figure 1. TIR Worksheet (part 4 of 6) CITC Renton Technical Information Report Land Use Permit Application Draft 9 Figure 1. TIR Worksheet (part 5 of 6) CITC Renton Technical Information Report Land Use Permit Application Draft 10 Figure 1. TIR Worksheet (part 6 of 6) CITC Renton Technical Information Report Land Use Permit Application Draft 11 Figure 2. CITC site area map CITC Renton Technical Information Report Land Use Permit Application Draft 12 Figure 3. CITC site drainage map CITC Renton Technical Information Report Land Use Permit Application Draft 13 Figure 4. CITC site soils map CITC Renton Technical Information Report Land Use Permit Application Draft 14 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. CITC Renton Technical Information Report Land Use Permit Application Draft 15 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. CITC Renton Technical Information Report Land Use Permit Application Draft 16 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. CITC Renton Technical Information Report Land Use Permit Application Draft 17 Figure 5. CITC project site is within Zone 1 of the wellhead protection area CITC Renton Technical Information Report Land Use Permit Application Draft 18 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 CITC Renton Technical Information Report Land Use Permit Application Draft 19 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 CITC Renton Technical Information Report Land Use Permit Application Draft 20 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. CITC Renton Technical Information Report Land Use Permit Application Draft 21 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 CITC Renton Technical Information Report Land Use Permit Application Draft 22 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. CITC Renton Technical Information Report Land Use Permit Application Draft 23 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 CITC Renton Technical Information Report Land Use Permit Application Draft 24 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. CITC Renton Technical Information Report Land Use Permit Application Draft 25 Figure 10. Preliminary bioretention inspection and operations plan 1300 Bronson Way N CITC Headquarters CITC Renton Technical Information Report Land Use Permit Application Draft 26 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. CITC Renton Technical Information Report Land Use Permit Application Draft 27 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 CITC Renton Technical Information Report Land Use Permit Application Draft 28 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 CITC Renton Technical Information Report Land Use Permit Application Draft 29 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) CITC Renton Technical Information Report Land Use Permit Application Draft 30 ====================================== 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 CITC Renton Technical Information Report Land Use Permit Application Draft 31 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