The URL can be used to link to this page

Home My WebLink AboutRS_Drainage Report_210416_v1.pdf Renton Sunset Gardens City of Renton, WA Technical Information Report April 2021 | Land Use Application Renton Sunset Gardens – Renton Housing Authority i Technical Information Report April 2021 Prepared for: Renton Housing Authority 2900 Northeast 10th Street Renton, WA 98056 Prepared by: KPFF Consulting Engineers 1601 Fifth Avenue, Suite 1600 Seattle, WA 98101 Phone: (206) 622-5822 KPFF Project No. 2000705 04/16/2021 KPFF Consulting Engineers ii This page intentionally left blank. Renton Sunset Gardens – Renton Housing Authority iii Table of Contents 1. Project Overview ........................................................................................................................................... 1 2. Conditions and Requirements Summary ................................................................................................. 11 Core Requirement No. 1: Discharge at the Natural Location ...................................................................... 11 Core Requirement No. 2: Off-Site Analysis ................................................................................................. 12 Core Requirement No. 3: Flow Control ........................................................................................................ 12 Core Requirement No. 4: Conveyance System ........................................................................................... 12 Core Requirement No. 5: Construction stormwater pollution prevention .................................................... 12 Core Requirement No. 6: Maintenance and Operations ............................................................................. 12 Core Requirement No. 7: Financial Guarantees and Liability ..................................................................... 12 Core Requirement No. 8: Water Quality facilities ........................................................................................ 13 Special Requirements ................................................................................................................................... 13 Special Requirement No. 1: Other Adopted Area-Specific Requirements ............................................ 13 Special Requirement No. 2: Flood Hazard Area Delineation ................................................................ 13 Special Requirement No. 3: Flood Protection Facilities ........................................................................ 13 Special Requirement No. 4: Source Control ......................................................................................... 13 Special Requirement No. 5: Oil Control ................................................................................................ 14 Special Requirement No. 6: Aquifer Protection Area ............................................................................ 14 3. Off-Site Analysis ......................................................................................................................................... 14 Task 1: Study Area Definition and Maps ..................................................................................................... 14 Task 2: Resource Review ............................................................................................................................ 15 Basin Plan .............................................................................................................................................. 15 Floodplain and Floodway Research ....................................................................................................... 15 Off-Site Analysis Reports ....................................................................................................................... 15 Sensitive Areas Folio .............................................................................................................................. 15 Soils Survey ............................................................................................................................................ 15 Wetlands Inventory ................................................................................................................................. 15 Section 303(d) List of Polluted Waters ................................................................................................... 16 City of Renton Erosion Maps and Landslide Maps ................................................................................ 16 Task 3: Field Inspection ............................................................................................................................... 16 Task 4: Drainage System Description and Problem Descriptions ............................................................... 16 Task 5: Mitigation of Existing or Potential Problems ................................................................................... 16 4. Flow Control, LID, and Water Quality Facility Analysis and Design ..................................................... 16 Part 4.A: Existing Site Hydrology ................................................................................................................. 16 Part 4.B: Developed Site Hydrology ............................................................................................................ 17 Part 4.C: Performance Standards ................................................................................................................ 18 Part 4.D: Flow Control System .................................................................................................................... 18 Part 4.E: Water Quality System ................................................................................................................... 18 5. Conveyance System Analysis and Design............................................................................................... 19 6. Special Reports and Studies ..................................................................................................................... 19 KPFF Consulting Engineers iv 7. Other Permits ............................................................................................................................................. 19 8. Construction Stormwater Pollution Prevention Plan (CSWPPP) Analysis and Design...................... 19 Part 8.A: Erosion and Sediment Control Plan Analysis and Design ........................................................... 19 Part 8.B: Stormwater Pollution Prevention and Spill Control Plan Design ................................................. 19 9. Bond Quantities, Facility Summaries, and Declaration of Covenant ................................................... 20 10. Operations and Maintenance Manual ...................................................................................................... 20 List of Figures Figure 1: TIR Worksheet ..................................................................................................................................... 2 Figure 2: Project Site Location Map .................................................................................................................... 8 Figure 3.1: Existing Drainage Basins and Subbasins ......................................................................................... 9 Figure 3.2: Proposed Drainage Basins and Subbasins..................................................................................... 10 Figure 4: Soils .................................................................................................................................................... 11 Figure 5: Off-Site Analysis Study Area Map ...................................................................................................... 14 List of Tables Table 4-1: Existing Basin Conditions ................................................................................................................. 17 Table 4-2: Developed Basin Conditions ............................................................................................................ 18 Table 7-1: List of Permits ................................................................................................................................... 19 Appendices Appendix A – Project Plans Appendix B – Geotechnical Report Appendix C – MGSFlood Report KPFF Consulting Engineers 1 1. Project Overview Renton Sunset Gardens is located at 2900 Northeast 10th Street, Renton, Washington (King County Parcel ID 7227900075). It is bordered by Northeast 10th Street to the south and Jefferson Avenue Northeast to the east, north, and west (see Figure 2). The proposed project site consists of a 14,341-square-foot mixed-use building with 76 residential units over commercial space and separate solid waste unit; a 13,946-square-foot parking lot; a 2,229-square-foot driveway; 6,440 square feet of pedestrian walkways and amenity areas; and 11,365 square feet of landscaping. Off-site improvements in the public right-of-way include street widening, concrete curbs, landscaping strips, concrete sidewalks, concrete driveway, ADA curb ramps, and additional property dedicated to the City of Renton (COR). New and replaced impervious surfaces in the public right-of-way total 6,278 square feet; pervious surfaces total 5,737 square feet. The site is within the Center Village (CV) zoning classification, Commercial Mixed-Use (CMU) land use designation, the Urban Design District ‘D’ overlay, and the boundaries of the Sunset Area Community Planned Action Ordinance. The site currently contains a one-story commercial building, asphalt parking lot, two driveways, concrete pedestrian walkways, and landscaping. The existing site slopes moderately at approximately 6 percent from the northeast to the southwest. There is an existing rockery wall along the eastern edge of the property that supports an approximate 9-foot elevation drop from Jefferson Avenue Northeast on the east to the site on the west. This area adjacent to the existing rockery is considered a moderate landslide hazard per COR Landslide Hazard Map. There is existing stormwater conveyance infrastructure along the western property frontage in Jefferson Avenue Northeast and along the southern property frontage in Northeast 10th Street. The majority of surface runoff on the site is collected via sheet flow in catch basins and conveyed to a detention pipe in the existing parking lot. It is then conveyed to the existing 12-inch storm drain in Jefferson Avenue Northeast and eventually discharges to Johns Creek. Area along the western and southern edges of the property line and in the right-of-way flow off site overland into the right-of-way where it is collected in catch basins and conveyed to the same public conveyance system as the rest of the site. The proposed development will discharge to the same existing stormwater conveyance systems in Jefferson Avenue Northeast and Northeast 10th Street. New pollution-generating surfaces will be treated using Best Management Practices (BMPs) that meet enhanced basic water quality standards. CITY OF RENTON SURFACE WATER DESIGN MANUAL 2017 City of Renton Surface Water Design Manual 12/12/2016 8-A-1 REFERENCE 8-A TECHNICAL INFORMATION REPORT (TIR) WORKSHEET Part 1 PROJECT OWNER AND PROJECT ENGINEER Part 2 PROJECT LOCATION AND DESCRIPTION Project Owner _____________________________ Phone ___________________________________ Address __________________________________ _________________________________________ Project Engineer ___________________________ Company _________________________________ Phone ___________________________________ Project Name __________________________ CED Permit # ________________________ Location Township ________________ Range __________________ Section _________________ Site Address __________________________ _____________________________________ Part 3 TYPE OF PERMIT APPLICATION Part 4 OTHER REVIEWS AND PERMITS  Land Use (e.g., Subdivision / Short Subd.)  Building (e.g., M/F / Commercial / SFR)  Grading  Right-of-Way Use  Other _______________________  DFW HPA  COE 404  DOE Dam Safety  FEMA Floodplain  COE Wetlands  Other ________  Shoreline Management  Structural Rockery/Vault/_____  ESA Section 7 Part 5 PLAN AND REPORT INFORMATION Technical Information Report Site Improvement Plan (Engr. Plans) Type of Drainage Review (check one): Date (include revision dates): Date of Final:  Full  Targeted  Simplified  Large Project  Directed __________________ __________________ __________________ Plan Type (check one): Date (include revision dates): Date of Final:  Full  Modified  Simplified __________________ __________________ __________________ Renton Housing Authority Thaddeus Egging KPFF Consulting Engineers (206) 622-5822 Sunset Gardens 2900 NE 10th St Renton, WA 98056 23N 05E 09 ü ü (425) 226-1850 2900 NE 10th St Renton, WA 98056 REFERENCE 8: PLAN REVIEW FORMS AND WORKSHEET TECHNICAL INFORMATION REPORT (TIR) WORKSHEET 12/12/2016 2017 City of Renton Surface Water Design Manual 8-A-2 Part 6 SWDM ADJUSTMENT APPROVALS Type (circle one): Standard / Blanket Description: (include conditions in TIR Section 2) ____________________________________________________________________________________ ____________________________________________________________________________________ ____________________________________________________________________________________ Approved Adjustment No. ______________________ Date of Approval: _______________________ Part 7 MONITORING REQUIREMENTS Monitoring Required: Yes / No Start Date: _______________________ Completion Date: _______________________ Describe: _________________________________ _________________________________________ _________________________________________ Re: SWDM Adjustment No. ________________ Part 8 SITE COMMUNITY AND DRAINAGE BASIN Community Plan: ____________________________________________________________________ Special District Overlays: ______________________________________________________________ Drainage Basin: _____________________________________________________________________ Stormwater Requirements: _____________________________________________________________ Part 9 ONSITE AND ADJACENT SENSITIVE AREAS  River/Stream ________________________  Lake ______________________________  Wetlands ____________________________  Closed Depression ____________________  Floodplain ___________________________  Other _______________________________ _______________________________  Steep Slope __________________________  Erosion Hazard _______________________  Landslide Hazard ______________________  Coal Mine Hazard ______________________  Seismic Hazard _______________________  Habitat Protection ______________________  _____________________________________ N/A ü Urban Design District 'D' Overlay Johns Creek Moderate REFERENCE 8-A: TECHNICAL INFORMATION REPORT (TIR) WORKSHEET TECHNICAL INFORMATION REPORT (TIR) WORKSHEET 2017 City of Renton Surface Water Design Manual 12/12/2016 Ref 8-A-3 Part 10 SOILS Soil Type ______________________ ______________________ ______________________ ______________________ Slopes ________________________ ________________________ ________________________ ________________________ Erosion Potential _________________________ _________________________ _________________________ _________________________  High Groundwater Table (within 5 feet)  Other ________________________________  Sole Source Aquifer  Seeps/Springs  Additional Sheets Attached Part 11 DRAINAGE DESIGN LIMITATIONS REFERENCE  Core 2 – Offsite Analysis_________________  Sensitive/Critical Areas__________________  SEPA________________________________  LID Infeasibility________________________  Other________________________________  _____________________________________ LIMITATION / SITE CONSTRAINT _______________________________________ _______________________________________ _______________________________________ _______________________________________ _______________________________________ _______________________________________  Additional Sheets Attached Part 12 TIR SUMMARY SHEET (provide one TIR Summary Sheet per Threshold Discharge Area) Threshold Discharge Area: (name or description) Core Requirements (all 8 apply): Discharge at Natural Location Number of Natural Discharge Locations: Offsite Analysis Level: 1 / 2 / 3 dated:__________________ Flow Control (include facility summary sheet) Standard: _______________________________ or Exemption Number: ____________ On-site BMPs: _______________________________ Conveyance System Spill containment located at: _____________________________ Erosion and Sediment Control / Construction Stormwater Pollution Prevention CSWPP/CESCL/ESC Site Supervisor: _____________________ Contact Phone: _________________________ After Hours Phone: _________________________ 1 TBD by contractor per Section 1.2.3.1.A of CORSWDM March 4, 2021 1 AmC (Arents, Alderwood material)8% - 15%Uncategorized SP-SM (per geotechnical report, Appendix B) ~1% - 8%Uncategorized REFERENCE 8: PLAN REVIEW FORMS AND WORKSHEET TECHNICAL INFORMATION REPORT (TIR) WORKSHEET 12/12/2016 2017 City of Renton Surface Water Design Manual 8-A-4 Part 12 TIR SUMMARY SHEET (provide one TIR Summary Sheet per Threshold Discharge Area) Maintenance and Operation Responsibility (circle one): Private / Public If Private, Maintenance Log Required: Yes / No Financial Guarantees and Liability Provided: Yes / No Water Quality (include facility summary sheet) Type (circle one): Basic / Sens. Lake / Enhanced Basic / Bog or Exemption No. _______________________ Special Requirements (as applicable): Area Specific Drainage Requirements Type: SDO / MDP / BP / Shared Fac. / None Name: ________________________ Floodplain/Floodway Delineation Type (circle one): Major / Minor / Exemption / None 100-year Base Flood Elevation (or range): _______________ Datum: Flood Protection Facilities Describe: Source Control (commercial / industrial land use) Describe land use: Describe any structural controls: Oil Control High-Use Site: Yes / No Treatment BMP: _________________________________ Maintenance Agreement: Yes / No with whom? _____________________________________ Other Drainage Structures Describe: *will be provided at construction document permitting* *will be provided at construction document permitting* REFERENCE 8-A: TECHNICAL INFORMATION REPORT (TIR) WORKSHEET TECHNICAL INFORMATION REPORT (TIR) WORKSHEET 2017 City of Renton Surface Water Design Manual 12/12/2016 Ref 8-A-5 Part 13 EROSION AND SEDIMENT CONTROL REQUIREMENTS MINIMUM ESC REQUIREMENTS DURING CONSTRUCTION  Clearing Limits  Cover Measures  Perimeter Protection  Traffic Area Stabilization  Sediment Retention  Surface Water Collection  Dewatering Control  Dust Control  Flow Control  Control Pollutants  Protect Existing and Proposed BMPs/Facilities  Maintain Protective BMPs / Manage Project MINIMUM ESC REQUIREMENTS AFTER CONSTRUCTION  Stabilize exposed surfaces  Remove and restore Temporary ESC Facilities  Clean and remove all silt and debris, ensure operation of Permanent BMPs/Facilities, restore operation of BMPs/Facilities as necessary  Flag limits of sensitive areas and open space preservation areas  Other _______________________ Part 14 STORMWATER FACILITY DESCRIPTIONS (Note: Include Facility Summary and Sketch) Flow Control Type/Description Water Quality Type/Description  Detention  Infiltration  Regional Facility  Shared Facility  On-site BMPs  Other ________________ ________________ ________________ ________________ ________________ ________________  Vegetated Flowpath  Wetpool  Filtration  Oil Control  Spill Control  On-site BMPs  Other ________________ ________________ ________________ ________________ ________________ ________________ ________________ Part 15 EASEMENTS/TRACTS Part 16 STRUCTURAL ANALYSIS  Drainage Easement  Covenant  Native Growth Protection Covenant  Tract  Other ____________________________  Cast in Place Vault  Retaining Wall  Rockery > 4′ High  Structural on Steep Slope  Other _______________________________ *will be provided at construction document permitting* bioretentionü REFERENCE 8: PLAN REVIEW FORMS AND WORKSHEET TECHNICAL INFORMATION REPORT (TIR) WORKSHEET 12/12/2016 2017 City of Renton Surface Water Design Manual 8-A-6 Part 17 SIGNATURE OF PROFESSIONAL ENGINEER I, or a civil engineer under my supervision, have visited the site. Actual site conditions as observed were incorporated into this worksheet and the attached Technical Information Report. To the best of my knowledge the information provided here is accurate. ____________________________________________________________________________________ Signed/Date KPFF Consulting Engineers 8 Figure 2: Site Location N Renton Sunset Gardens – Renton Housing Authority 9 Figure 3.1: Existing Drainage Subbasins KPFF Consulting Engineers 10 Figure 3.2: Proposed Drainage Subbasins Renton Sunset Gardens – Renton Housing Authority 11 Figure 4: Soils 2. Conditions and Requirements Summary The proposed redevelopment project results in more than 2,000 square feet but less than 50 acres of new plus replaced impervious surface; therefore, a “Full Drainage Review” is required per the 2017 City of Renton Surface Water Design Manual (CORSWDM), Section 1.1.2.4. The following is a summary of how the project will comply with the nine core requirements and six special requirements. CORE REQUIREMENT NO. 1: DISCHARGE AT THE NATURAL L OCATION The project will maintain the existing topography and drainage patterns of the site. Runoff from the project site will continue to be discharged to the existing public stormwater conveyance systems in Jefferson Avenue Northeast and Northeast 10th Street, identical to the existing site conditions; therefore, the project meets the discharge requirements in Section 1.2.1 of the CORSWDM. Stormwater runoff from the frontage improvements will not be redirected as a result of this project. The on-site drainage facilities are not anticipated to create a significant adverse impact to downstream properties or drainage systems. KPFF Consulting Engineers 12 CORE REQUIREMENT NO. 2: OFF-SITE ANALYSIS A Level 1 downstream analysis is included in Section 3 of this report, as required by Section 1.2.2 of the CORSWDM. CORE REQUIREMENT NO. 3: FLOW CONTROL The site falls within the Peak Rate Flow Control Standard area matching existing site conditions per the COR Flow Control Map. Per Section 1.2.3.1.A of the CORSWDM, the facility requirement in Peak Rate Flow Control Standard areas is waived for any threshold discharge area in which the target surfaces subject to this requirement will generate no more than a 0.15-cubic-feet-per-second (CFS) increase in the existing site conditions 100-year peak flow. Existing site conditions are defined as conditions that existed prior to May 1979 unless land cover changes have been approved by the COR since May 1979 as part of a permit or approval. Current site conditions were constructed as part of an approved permit in 1982, so the “existing site condition” to be used for flow control analysis purposes shall be the current land use. The target surfaces include new impervious and new pervious surfaces. The project includes on-site BMPs and will not generate more than a 0.15 CFS increase in the existing site conditions 100-year peak flow; therefore, Core Requirement No. 3 is not applicable. See Appendix C for hydrology calculations demonstrating compliance. CORE REQUIREMENT NO. 4: CONVEYANCE SYSTEM Proposed conveyance system elements on-site and off-site shall be analyzed, designed, and constructed per Section 1.2.4 of the CORSWDM. Conveyance system calculations will be provided during construction document permitting following entitlement. CORE REQUIREMENT NO. 5: CONSTRUCTION STORMWATER PO LLUTION PREVENTION Temporary Erosion and Sediment Control (TESC) BMPs will be implemented during construction to prevent the transport of sediment from the project site to the maximum extent practicable. TESC BMPs are anticipated to include silt fences, conveyance swales, check dams, a sediment pond with a liner, catch basin inserts, mulching of exposed areas, dust control, and other measures as necessary. A Construction Stormwater Pollution Prevention Plan (CSWPPP) will be prepared during construction document permitting following entitlement. CORE REQUIREMENT NO. 6: MAINTENANCE AND OPERATIONS The owner is responsible for the maintenance and operations of the proposed drainage facilities. A maintenance and operations manual will be provided during construction document permitting following entitlement. CORE REQUIREMENT NO. 7: FINANCIAL GUARANTEES AND L IABILITY The project will post a construction bond, assignment of funds, or a certified check before a building permit is issued in accordance with the COR Municipal Code 4-6-030. The applicant will also maintain liability insurance throughout the duration of construction. A maintenance bond will also be submitted to the COR before receiving acceptance of any newly constructed on-site private storm drain improvements. Bond quantities will be provided during construction document permitting following entitlement. Renton Sunset Gardens – Renton Housing Authority 13 CORE REQUIREMENT NO. 8: WATER QUALITY FACILITIES Fifty percent or more of the site’s runoff is from commercial land use; therefore, Enhanced Basic Water Quality is required per Section 1.2.8.1 of the CORSWDM. The project will treat runoff from target pollution-generating impervious surfaces (PGIS) using a series of infiltrating bioretention planters. See Section 4 of this report for water quality system analysis and design. CORE REQUIREMENT NO. 9: ON-SITE BMPS The site is greater than 22,000 square feet; therefore, the project is subject to Large Lot BMP Requirements per Section 1.2.9.2 of the CORSWDM. On-site BMP requirements may be satisfied through two methods: (1) application of BMPs to the maximum extent feasible, or (2) complying with the Low Impact Development (LID) Performance Standard. The project opts to satisfy Core Requirement No. 9 by applying BMPs to the maximum extent feasible for all new and replaced impervious surfaces and new pervious surfaces. Using the list for large lot BMPs in Section 1.2.9.2.2 of the CORSWDM, bioretention was determined to be the first feasible BMP for the site and will be implemented to the maximum extent feasible. The project results in an impervious surface coverage greater than 65 percent on the buildable portion of the site; therefore, on-site BMPs will be applied to at least 20 percent of the target impervious surfaces. Soil amendment will be applied to all new pervious surfaces in accordance with Appendix C.2.13 of the CORSWDM. Roof downspout connects to the public stormwater system will be via perforated pipe connection in accordance with Appendix C.2.11 of the CORSWM. SPECIAL REQUIREMENTS In addition to the nine core requirements, the project must evaluate compliance with the six special drainage requirements. Special Requirement No. 1: Other Adopted Area-Specific Requirements There are no known adopted area-specific regulations for the site; therefore, Special Requirement No. 1 is not applicable. Special Requirement No. 2: Flood Hazard Area Delineation Per the FEMA Flood Map Service Center, the project does not contain and is not adjacent to a flood hazard area; therefore, Special Requirement No. 2 is not applicable. Special Requirement No. 3: Flood Protection Facilities The project will not rely on existing or proposed flood protection facilities for protection against hazards posed by erosion or inundation; therefore, Special Requirement No. 3 is not applicable. Special Requirement No. 4: Source Control Water quality source controls applicable to this project will be applied in accordance with the King County Stormwater Pollution Prevention Manual and Renton Municipal Code, Title IV. See Section 8 of this report for the Construction Stormwater Pollution Prevention Plan Analysis and Design. KPFF Consulting Engineers 14 Special Requirement No. 5: Oil Control The site is not considered high-use because its anticipated average daily traffic count is less than 100 vehicles per 1,000 square feet of gross building area; therefore, Special Requirement No. 5 is not applicable. Special Requirement No. 6: Aquifer Protection Area The site is not located within the Aquifer Protection Areas; therefore, Special Requirement No. 6 is not applicable. 3. Off-Site Analysis TASK 1: STUDY AREA DEFINITION AND MAPS The majority of stormwater runoff from the site is collected in catch basins within the existing parking lot, passed through a detention pipe below the parking lot, and conveyed to the existing 12-inch storm drain in Jefferson Avenue Northeast to the west. It then flows through a public piped conveyance system where it reaches Johns Creek and eventually discharges to Lake Washington. The downstream flow path is shown in Figure 6 below. The 1.39-acre site receives 0.01 acre of off-site runoff upstream. See Figure 3.1 above for off-site drainage areas. Figure 5: Off-Site Analysis Study Area Map Renton Sunset Gardens – Renton Housing Authority 15 TASK 2: RESOURCE REVIEW All resources listed from the CORSWDM were reviewed for potential problems in the study area. These include information from basin plans, FEMA maps, off-site analysis reports, sensitive area map folios, soil survey data, wetland inventory maps, Washington State water quality atlas maps, and COR erosion and landslide maps. Basin Plan The site is within the Johns Creek Subbasin, as part of the East Lake Washington – Renton Basin. COR’s Surface Water System Plan includes an analysis of the East Lake Washington Basin and its subbasins. The Johns Creek Subbasin discharges directly to Lake Washington at Gene Coulon Beach Park. Johns Creek and Lake Washington at Gene Coulon Beach Park are both on Ecology’s 303(d) list for pH, temperature, bacteria, and dissolved oxygen due to their association with industrial sites and tributary discharge containing sediments. Johns Creek can also experience backwater from Lake Washington during high flows, so it is subject to sediment deposition due to its generally flat slope. The South Lake Washington Roadway Improvement Report’s hydrology analysis found localized flooding in areas near The Landing and the PACCAR/residential areas, not within the vicinity of the project. The North Renton subbasin is almost entirely developed, so no major changes in land coverage are expected to occur. Floodplain and Floodway Research The FEMA Flood Map Service Center shows the most recent flood insurance maps as of August 19, 2020. The site and its downstream areas are within Zone X, areas determined to be outside the 0.2 percent annual chance floodplain. Off-Site Analysis Reports Technical Information Reports (TIRs) from nearby developments were provided by COR Community and Economic Development staff and reviewed as part of this study. The developments include Sunset Terrace Building 1, Sunset Oaks Apartments, Solera Subdivision, and Renton Highlands Library. The off-site analyses found no existing or potential problems identified within their respective project vicinities. Sensitive Areas Folio The King County iMap shows an erosion hazard area approximately 0.8 miles downstream from the site’s point of discharge. This area runs along the eastern edge of Interstate 405 and consists of a vegetated embankment with slopes ranging from 20 percent to 60 percent. Stormwater is conveyed across the embankment in a 24-inch main owned by Washington State Department of Transportation (WSDOT). Runoff from the project site is not anticipated to impact this area as it is only traversing the area via an existing piped stormwater main. Soils Survey The Natural Resources Conservation Service’s Web Soil Survey has provided a basic understanding of existing soil conditions. The east half of the site consists of Alderwood (AmC) soil, as shown in Figure 4 above. Geotechnical borings on the west half of the site found loose to medium-dense fill over recessional outwash deposits consisting of medium to fine sand with silt. See Appendix B for the Geotechnical Engineering Report. Wetlands Inventory The U.S. Fish and Wildlife National Wetlands Inventory map does not indicate any wetlands within the off-site analysis study limits. KPFF Consulting Engineers 16 Section 303(d) List of Polluted Waters The Washington State Water Quality Atlas on the Department of Ecology’s website does not indicate any impaired waterbodies within the off-site analysis study limits. City of Renton Erosion Maps and Landslide Maps The COR sensitive areas maps indicate moderate landslide hazard in the southeast corner of the site. This area contains an existing rockery wall accounting for an approximately 9-foot elevation drop from Jefferson Avenue Northeast on the east to the property on the west. The project proposes to maintain and direct stormwater runoff away from the rockery wall to reduce landslide hazards, identical to the existing conditions. The maps also indicate a high erosion hazard and high landslide hazard approximately 0.8 miles downstream from the project’s point of discharge. This is the same area mapped as an erosion hazard on King County’s iMap, as discussed above in reviewing Sensitive Areas Folio. TASK 3: FIELD INSPECTION KPFF conducted a Level 1 investigation of the existing on-site and off-site drainage systems of the project site and its vicinity on March 4, 2021. No existing or potential constrictions or downstream drainage problems were visible during the field inspection. TASK 4: DRAINAGE SYSTEM DESCRIPTION AND PROBLEM DE SCRIPTIONS The majority of runoff from the site will be conveyed to the existing 12-inch public main in Jefferson Avenue Northeast. Roof runoff will be conveyed to the existing 12-inch public main in Northeast 10th Street. The systems combine in the intersection of Jefferson Avenue Northeast and Northeast 10th Street, and eventually discharge to Lake Washington via John’s Creek. There are no existing drainage complaints within the extents of this analysis, and no downstream drainage problems were visible in field observations. TASK 5: MITIGATION OF EXISTING OR POTENTIAL PROBLE MS No existing or potential problems were found in the off-site review; therefore, no mitigation is proposed. 4. Flow Control, LID, and Water Quality Facility Analysis and Design PART 4.A: EXISTING SITE HYDROLOGY The project consists of one threshold discharge area because the existing conveyance systems combine within 1/4 mile downstream. The existing site hydrology contains two drainage subbasins. Refer to Figure 3.1 above for Existing Drainage Subbasins. Subbasin 01 contains the majority of the site including the existing parking lot, the existing building, and minimal off-site area northeast of the property boundary. Stormwater is collected by three existing catch basins in the parking lot or by the building roof and conveyed to a detention pipe underneath the parking lot. It then flows to the existing 12-inch public main in Jefferson Avenue Northeast at the site’s discharge point Renton Sunset Gardens – Renton Housing Authority 17 Subbasin 02 contains the remaining areas on-site along the eastern and southern property lines and areas off- site that sheet flow into the public right-of-way. Stormwater generally sheet flows to the southwest, collects in existing catch basins in Jefferson Avenue Northeast and Northeast 10th Street, and discharges to the existing 12-inch public main in Northeast 10th Street. Table 4-1: Existing Basin Conditions Subbasin Area (acres) Soil Type Land Cover 01 1.10 Till Asphalt pavement; building rooftop; sidewalk; rockery wall; landscaping 02 0.29 Till Asphalt pavement; concrete driveway; sidewalk; landscaping PART 4.B: DEVELOPED SITE HYDROLOGY The developed site hydrology will contain three drainage subbasins. Refer to Figure 3.2 above for proposed drainage subbasins. Subbasin 01 contains the interior drive aisle, parking lot, trash enclosure building, and surfaces adjacent to and downstream of the PGIS, including pedestrian walkways and landscaping. Parking lot runoff flows to the southwest to be collected in trench drains immediately outside the extents of the covered parking area. It is conveyed north where it combines with runoff from the interior drive aisle and pedestrian walkways, collected via catch basins. This combined flow enters a flow splitter to be routed one of two ways. Up to 0.23 CFS is directed to a 4/10 horsepower submersible pump then daylights into a series of infiltrating bioretention planters. 0.23 CFS is the calculated flow required to result in less than a 0.15 CFS increase to the existing conditions 100-year peak flow. The two upstream planters overflow into a riser which daylights into the planter downstream. The last planter in the series directs all overflow runoff to the existing 12-inch public main in Jefferson Avenue Northeast. Flows above 0.23 CFS in the flow splitter are conveyed directly to the same existing 12-inch public main in Jefferson Avenue Northeast. Subbasin 02 contains the roof surfaces from the new mixed-use building. Runoff will be captured by roof drains and discharge directly to the existing 12-inch public main in Northeast 10th Street via perforated pipe connections. Subbasin 03 contains the remaining areas on-site and all right-of-way frontage improvements along the western, southern, and eastern property lines that sheet flow into the public right-of-way. Stormwater generally sheet flows towards the existing street centerlines, collects in existing catch basins, and discharges to the existing public mains in Jefferson Avenue Northeast and Northeast 10th Street. Refer to Appendix A for project drainage plans. KPFF Consulting Engineers 18 Table 4-2: Developed Basin Conditions Subbasin Area (acres) Soil Type Land Cover 01 0.48 Till Asphalt pavement; building rooftop; sidewalk; rockery wall; bioretention planters; landscaping 02 0.49 Till Building rooftop 03 0.42 Till Asphalt pavement; sidewalk; landscaping PART 4.C: PERFORMANCE STANDARDS The project is exempt from flow control as discussed in Section 2, Core Requirement No. 3 of this report. On-site BMPs will be implemented to the maximum extent feasible per the Large Lot BMP Requirements in Section 1.2.9.2.2 of the CORSWDM. The conveyance system will have sufficient capacity to convey and contain the 25-year peak flow per Section 1.2.4 of the CORSWDM. The project is subject to Enhanced Basic Water Quality per Section 1.2.8.1 of the CORSWDM. PART 4.D: FLOW CONTROL SYSTEM The project is exempt from flow control as discussed in Section 2, Core Requirement No. 3 of this report. Bioretention and other on-site BMPs will be implemented to the maximum extent feasible per the Large Lot BMP Requirements in Section 1.2.9.2.2 of the CORSWDM. Detailed infeasibility discussion and supporting documentation will be provided during construction document permitting following entitlement. PART 4.E: WATER QUALITY SYSTEM All new PGIS and surfaces adjacent to and downstream of the PGIS will be directed to a series of three infiltrating bioretention planters designed in accordance with Section 6.8 of the CORSWDM. MGSFlood was used to size the water quality facilities to satisfy two requirements: provide treatment for 91 percent of the total PGIS runoff volume to comply with Core Requirement No. 8 and result in less than a 0.15 CFS increase to the existing conditions 100-year peak flow to qualify for exemption from Core Requirement No. 3. Because the bioretention drainage area exceeds 5,000 square feet of PGIS, a biosoil hydraulic conductivity of 3 inches per hour is used. Native soil hydraulic conductivity was estimated at a conservative 0.5 inches per hour considering soil rates used for developments nearby. Infiltration rates determined using a Pilot Infiltration Test will be provided and used during construction document permitting following entitlement. The three infiltrating bioretention planters all contain 12 inches of ponding, 6 inches of freeboard, and an overflow riser to collect runoff that overtops the 12 inches of ponding. The cumulative bottom area of all three planters is 650 square feet. See Appendix C for the MGSFlood report demonstrating compliance. Renton Sunset Gardens – Renton Housing Authority 19 5. Conveyance System Analysis and Design No existing capacity issues were found in the off-site analysis as discussed in Section 3 of this report; therefore, existing off-site conveyance systems are not analyzed. All new on-site and off-site conveyance system elements shall be analyzed, designed, and constructed per Section 1.2.4.1 of the CORSWDM. Conveyance system calculations will be provided during construction document permitting following entitlement. 6. Special Reports and Studies A Geotechnical Engineering Report by Otto Rosenau & Associates, Inc. was prepared on September 16, 2020. The explorations found soils to consist of sand with gravel fill over medium to fine sand with silt recessional outwash deposits. It determined that on-site infiltration for surface water drainage could be feasible. See Appendix B for a copy of the Geotechnical Engineering Report. 7. Other Permits Section 1.1.3 of the CORSWDM provides other required permits for the project that are included below. Table 7-1: List of Permits Agency Permit/Approval Washington State Department of Ecology NPDES Construction Stormwater permit City of Renton Civil Construction Permit Clearing, Grading, and Building Permits 8. Construction Stormwater Pollution Prevention Plan (CSWPPP) Analysis and Design PART 8.A: EROSION AND SEDIMENT CONTROL PLAN ANALYS IS AND DESIGN Temporary and permanent erosion and sediment control (ESC) measures will be implemented per Section D.2.1 of the CORSWDM. A CSWPPP will be prepared during construction document permitting following entitlement. PART 8.B: STORMWATER POLLUTION PREVENTION AND SPIL L CONTROL PLAN DESIGN Stormwater pollution prevention and spill control (SWPPS) measures will be implemented per Section D.2.2 of the CORSWDM. A CSWPPP will be prepared during construction document permitting following entitlement. KPFF Consulting Engineers 20 9. Bond Quantities, Facility Summaries, and Declaration of Covenant The Site Improvement Bond Quantity Worksheet provided by the Community of Economic Development (CED) will be prepared during construction document permitting following entitlement. 10. Operations and Maintenance Manual The stormwater facilities located on-site generally consist of catch basins for collection; storm drainage pipes for conveyance; infiltrating bioretention planters for water quality treatment; stormwater structures including manholes, a flow splitter, and a pump; and rock splash pads for outfall protection. The Maintenance and Operations Manual is intended for the project owner once the project is complete and will be prepared during construction document permitting following entitlement in accordance with Appendix A of the CORSWDM. SEE SEPARATE ATTACHMENT FOR APPENDICES