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Home My WebLink AboutRS_Preliminary_TIR_10232020_v1 Western Washington Division Eastern Washington Division 165 NE Juniper St., Ste 201, Issaquah, WA 98027 407 Swiftwater Blvd, Cle Elum, WA 98922 Phone: (425) 392-0250 Fax: (425) 391-3055 Phone: (509) 674-7433 Fax: (509) 674-7419 www.EncompassES.net PRELIMINARY TECHNICAL INFORMATION REPORT For Mangat Short Plat 1415 N 24th St Renton, WA 98056 October 23, 2020 10/23/2020 Prepared by: Ian Dahl Encompass Engineering Job No. 20547 Prepared For: Sukhjinder Mangat 10623 SE 279th Street Kent, WA 98030 Mangat Short Plat Preliminary Technical Information Report 10/23/2020 P a g e | i Table of Contents I. PROJECT OVERVIEW ................................................................................................................ 1 II. CONDITIONS AND REQUIREMENTS SUMMARY ...................................................................... 7 III. OFFSITE ANALYSIS .................................................................................................................. 12 IV. FLOW CONTROL, LOW IMPACT DEVELOPMENT AND WATER QUALITY FACILITY ANALYSIS AND DESIGN ........................................................................................................................... 19 V. CONVEYANCE SYSTEM ANALYSIS AND DESIGN ..................................................................... 23 VI. SPECIAL REPORTS AND STUDIES ............................................................................................ 23 VII. OTHER PERMITS ..................................................................................................................... 23 VIII. CSWPP ANALYSIS AND DESIGN .............................................................................................. 23 IX. BOND QUANTITIES, FACILITY SUMMARIES AND DECLARATION of COVENANT ................... 23 X. OPERATION AND MAINTENANCE MANUAL .......................................................................... 23 List of Figures Figure 1 – TIR Worksheet Figure 2 – Vicinity Map Figure 3 – Soils Map and Legend Figure 4 – Existing Conditions Map Figure 5 – Developed Conditions Map Figure 6 – Drainage Review Flow Chart Figure 7 – Downstream Map Appendix A Limited Geotechnical Evaluation by Cobalt Geosciences dated August 20, 2020. Appendix B WWHM Output 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 __________________ __________________ __________________ 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 ______________________  _____________________________________ 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: _________________________ 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: 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 _______________________________ 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 Mangat Short Plat Preliminary Technical Information Report 10/23/2020 P a g e | 1 I. PROJECT OVERVIEW Project: Mangat Short Plat Site Address: 1415 N. 24th Street, Renton, WA 98056 (See Vicinity Map) Tax Parcel #: 334450-0234 Zoning District: R-6, Single Family Residential Site Area: 21,412 SF (0.49 Acres) Site Location: The site is in the City of Renton within the NW quarter of Section 5, Township 23 North, Range 5 East, W.M, King County, Washington. The site is located on the south side of N 24th St, west of the intersection of Meadow Ave. N and N 24th St. Figure 2: Vicinity Map Mangat Short Plat Preliminary Technical Information Report 10/23/2020 P a g e | 2 Pre-developed Site Conditions The project site is located in the City of Renton on a 21,412 SF (0.49 Acres) lot that is zoned R-6 (single-family residential). The site is accessed from N 24th St, east of the intersection of N 24th St and Meadow Ave N. The site is bordered to the east, west and south by single-family residences and to the north by City of Renton ROW. The site is currently developed with a single-family residence accessed from N 24th St via a concrete driveway at the northeast corner of the site. South of the residence, a grassy hill slopes off-site to the southwest at slopes of 4-8%. The property is located within the East Lake Washington drainage basin, however it has two separate discharge areas. Runoff from the northern portion of the existing site generally sheet flows towards N 24th St where it flows downhill to west and enters the City of Renton storm system through a type 1 catch basin. Runoff from the southern portion of the site sheet flows to the southwest towards an undeveloped forested hillside, before entering the Bella Vista Apartment complex storm system, where it converges with the stormwater from the northern discharge area at just under a ¼ mile from the project site. Stormwater is ultimately discharged to Lake Washington, located approximately ½ mile west of the site. See full downstream analysis in Section III of this Technical Information Report (TIR). An Existing Conditions Map is included as Figure 4 at the end of this Section. Critical Areas A Wellhead Protection Area, Zone 1 Modified and an Aquifer Protection Area, Zone 2 are mapped on the project site according to City of Renton Maps and the RSWDM Reference 15-B. No other critical areas have been identified on the project site. Soils Per the US Department of Agriculture (USDA), Natural Resources Conservation Service (NR CS) Web Soil Survey (WSS) information, the entire project site is underlain with Indianola loamy sand, 5 to 15 percent slopes (See Figure 3 on the following page). A Limited Geotechnical Evaluation prepared by Cobalt Geosciences (Appendix A) confirms this soil classification across the site, with the discovery of weathered and unweathered glacial till. An in-situ infiltration test was performed and an infiltration rate of 0.22 inches per hour was found which is unsuitable for stormwater infiltration. Mangat Short Plat Preliminary Technical Information Report 10/23/2020 P a g e | 3 Figure 3: Soil Map and Legend Developed Site Conditions The project proposes the development of two (2) single-family lots within the 21,412 SF (0.49 Acres) parcel, as well as an 11.5-foot right-of-way dedication with frontage improvements, and road widening at the intersection of Meadow Ave N and N 24th St for an emergency vehicle turnaround. Lot 1 is 10,277 SF (0.24 Acres) and is located along the eastern portion of the site with driveway access off of N 24th St. Lot 2 is 9,536 SF (0.22 Acres) and is located on the western portion of the site with driveway access off of N 24th St. Frontage improvements will include the addition of a 5-foot sidewalk, 8-foot planter strip, as well as curb and gutter along the edge of asphalt roadway. At the intersection of N 24th St and Meadow Ave N, the existing asphalt roadway Mangat Short Plat Preliminary Technical Information Report 10/23/2020 P a g e | 4 will be widened by approximately 7 feet to accommodate a hammerhead Emergency Vehicle Turnaround. The parcel is zoned R-6, which allows for a maximum building coverage of 40% and a maximum impervious surface coverage of 55%. The maximum allowable building/impervious surface for each lot is discussed below: • Lot 1: The maximum allowable building coverage for Lot 1 is 10,277 SF*0.4 = 4,111 SF, and the maximum impervious surface coverage is 10,277 SF*0.55 = 5,652 SF. The final site layout of Lot 1 has not been determined at this stage; therefore, this report assumes that the future impervious surfaces required to construct the residence on Lot 1 will use the maximum coverages stated above. • Lot 2: The maximum allowable building coverage for Lot 2 is 9,536 SF*0.4 = 3,814 SF, and the maximum impervious surface coverage is 9,536 SF*0.55 = 5,245 SF. The final site layout of Lot 2 has not been determined at this stage; therefore, this report assumes that the future impervious surfaces required to construct the residence on Lot 2 will use the maximum coverages stated above. Stormwater runoff from the proposed development of Lots 1 and 2 will be managed as follows: • Lot 1: Stormwater BMP’s satisfying Core Requirement #9 will be used to the maximum extent feasible to mitigate new runoff created by the proposed single family residence and other impervious surfaces on-site. A 50-foot basic dispersion trench with notched board and 25-foot native vegetated flowpath is proposed on Lot 1 south of the residence to mitigate stormwater from rooftop areas. This report assumes that 3,500 SF of rooftop area will be conveyed to this dispersion trench, the maximum allowed for its size. • Lot 2: Stormwater BMP’s satisfying Core Requirement #9 will be used to the maximum extent feasible to mitigate new runoff created by the proposed single family residence and other impervious surfaces on-site. A 50-foot basic dispersion trench with notched board and 25-foot native vegetated flowpath is proposed on Lot 1 south of the residence to mitigate stormwater from rooftop areas. This report assumes that 3,500 SF of rooftop area will be conveyed to this dispersion trench, the maximum allowed for its size. Please refer to Core Requirement # 9 in Section II and Section IV of this TIR for additional discussion on stormwater BMPs. A Developed Conditions Map is provided as Figure 5 at this end of this Section. Mangat Short Plat Preliminary Technical Information Report 10/23/2020 P a g e | 7 II. CONDITIONS AND REQUIREMENTS SUMMARY The 2017 City of Renton Surface Water Design Manual (RSWDM) was utilized to determine and address all core and special requirements. Based on the criteria specified in Figure 1.1.2.A of the RSWDM, the project falls under Full Drainage Review. Per Section 1.1.2.4 of the RSWDM, the project must meet all nine (9) core and all six (6) special requirements. See Figure 6 below for more information on how the type of drainage review was d etermined. Figure 6: Drainage Review Flow Chart Mangat Short Plat Preliminary Technical Information Report 10/23/2020 P a g e | 8 Core Requirements Core Requirement #1: Discharge at the Natural Location The proposed development runoff will follow existing drainage patterns that flow to the north into N 24th St and the City of Renton storm system as well as southwest towards an undeveloped, forested hillside. Refer to the Level 1 Downstream Analysis in Section III of this TIR for a complete description of the existing drainage path s. Core Requirement #2: Downstream Analysis A Level 1 Downstream analysis has been completed for the site and no existing or potential problems have been identified. This analysis is included in Section III of this TIR. Core Requirement #3: Flow Control Facilities Based on the City of Renton’s flow control application map, the project site is located within the Peak Rate Flow Control Standard (Existing Site Conditions). Flow control facilities are required to match the developed peak discharge rates to existing site conditions peak discharge rates for 2-, 10- and 100-year return periods. The proposed stormwater BMPs result in a 0.0109 CFS decrease in the 100-year flow using 15-minutes time steps. This is below the allowable threshold of a 0.15 CFS maximum increase per Section 1.2.3.1.A of the RSWDM; therefore, the project meets flow control exemption 2. No formal flow control facilities are proposed at this time; however, flow control BMPs will be implemented as described in Core Requirement #9. Please refer to Section IV of this TIR for additional discussion. Core Requirement #4: Conveyance System Conveyance in compliance with the requirements detailed in Section 1.2.4.1 of the City of Renton 2017 SWDM will be provided with final engineering. Core Requirement #5: Construction Stormwater Pollution Prevention The limits of construction for the project are less than 1 acre. A temporary erosion and sediment control (TESC) plan providing details on best management practices (BMPs) to be implemented during construction is included in the engineering plan set. A Construction Stormwater Pollution Prevention Plan (CSWPPP) will be provided with final engineering. Please refer to Section VIII of this TIR for additional discussion. Core Requirement #6: Maintenance and Operations An Operation and Maintenance Manual will be provided with final engineering. Core Requirement #7: Financial Guarantees and Liability The owner will arrange for any financial guarantees and liabilities required by the permit. Mangat Short Plat Preliminary Technical Information Report 10/23/2020 P a g e | 9 Core Requirement #8: Water Quality Facilities In accordance with Section 1.2.8.1.A of the RSWDM, Basic Water Quality Treatment is not required for this project as new plus replaced pollution generating impervious surfaces (PGIS) do not exceed 5,000 SF, meeting exemption 1. 1,949 SF of asphalt road repair and widening is proposed on N 24th St, 494 SF of concrete driveway is proposed within the ROW dedication area, and 750 SF of asphalt driveway is estimated within Lots 1 & 2 (at a 15-foot width). These quantities total to 3,193 SF of PGIS that will be created or replaced as a result of this project. Core Requirement #9: Flow Control BMPs This project is classified as a small subdivision; therefore, it is subject to the Small Subdivision Project BMP Requirements detailed in Section 1.2.9.3.1 in the RSWDM. Although implementation of individual lot BMPs is not required until building permit application, BMPs have been considered for the future improvements on Lots 1 and 2 based on Section 1.2.9.2 and of the RSWDM. See Section IV of this TIR for further discussion and flow control analysis. Impervious Surface BMPs Full Dispersion: Infeasible for Lots 1 and 2. • Lots 1 and 2: The space required for a 100-foot native vegetated flowpath segment is not available on these sites. Full Infiltration: Infeasible for Lots 1 and 2. • Lots 1 and 2: The Limited Geotechnical Evaluation prepared by Cobalt Geosciences dated August 20, 2020 (Appendix A) states that infiltration is infeasible at the site due to underlain glacial till and confirmed by an in-situ infiltration test. Limited Infiltration: Infeasible for Lots 1 & 2. • Lots 1 and 2: BMP’s relying on infiltration are not feasible as described above. Rain Gardens/Bioretention: Infeasible for Lots 1 and 2. • Lots 1 and 2: BMP’s relying on infiltration are not feasible as described above. Permeable Pavement: Infeasible for Lots 1 and 2. • Lots 1 and 2: BMP’s relying on infiltration are not feasible as described above. Mangat Short Plat Preliminary Technical Information Report 10/23/2020 P a g e | 10 Basic Dispersion: Feasible for Lots 1 and 2 • Lot 1: A 50-foot Basic Dispersion trench with notched board and 25 -foot native vegetated flowpath is proposed for Lot 1. The maximum area of 3,500 SF will be conveyed to this trench from the rooftop of the residence. • Lot 2: A 50-foot Basic Dispersion trench with notched board and 25 -foot native vegetated flowpath is proposed for Lot 1. The maximum area of 3,500 SF will be conveyed to this trench from the rooftop of the residence. Reduced Impervious Surface Credit: Infeasible for Lots 1 and 2 • Lots 1 and 2: The maximum impervious surfaces assumed for Lots 1 and 2 of 5,652 SF and 5,245 SF respectively, exceed the threshold of 4,000 SF, which is required for eligibility for a reduced impervious surface credit. Native Growth Retention Credit: Infeasible for Lots 1 and 2 • Lots 1 and 2: The necessary area to allow for 3.5 SF of native vegetated surface for every SF of impervious surface is not available on either lot. Tree Retention Credit: Feasible for Lots 1 and 2 • Lots 1 and 2: At least 30% of significant trees will be retained per Department of Community and Economic Development notes in the Pre-Application Meeting. Soil Amendment: Feasible for Lots 1 and 2 • Lots 1 and 2: All disturbed, pervious areas of the project will meet soil amendment requirements as detailed in Section C.2.13 of the RSWDM. Mangat Short Plat Preliminary Technical Information Report 10/23/2020 P a g e | 11 Special Requirements Special Requirement #1: Other Adopted Area-Specific Requirements Critical Drainage Area – N/A Master Drainage Plan – N/A Basin Plan – N/A Lake management Plan – N/A Shared Facility Drainage Plan – N/A Special Requirement #2: Flood Hazard Area Delineation The limits of this project do not lie within a delineated FEMA 100-year floodplain. Special Requirement #3: Flood Protection Facilities This project does not rely on or propose to modify/construct a new flood protection facility. Special Requirement #4: Source controls The project is a single-family residential development; therefore, this requirement is not applicable. Special Requirement #5: Oil Control This project is not considered high-use in need of oil control. Special Requirement #6: Aquifer Protection Area The site is located within Zone 2 of the City’s Aquifer Protection Area, and complies with the requirements listed in section 1.3.6 of the RSWDM. No flow control facilities or open conveyance systems are proposed on the site. Mangat Short Plat Preliminary Technical Information Report 10/23/2020 P a g e | 12 III. DOWNSTREAM ANALYSIS A Level 1 Downstream analysis has been conducted per the requirements in Section 1.2.2.1 of the RSWDM. Please see Tasks 1 through 4 below for a summary of the results. Task 1: Define and Map the Study Area The area of analysis extends from the site discharge points along N 24th St and the southern limit of the site to approximately a quarter-mile downstream where the two discharge areas converge. A Downstream Map is provided in Figure 7 below. Figure 7: Downstream Map Task 2: Review All Available Information on the Study Area Per King county resources, there have been no significant drainage complaints within a quarter - mile downstream of the site. Task 3: Field Inspect the Study Area A field inspection was performed by Encompass Engineering & Surveying on October 6, 2020. Please refer to Task 4 for a detailed description of the downstream drainage system and analysis. Mangat Short Plat Preliminary Technical Information Report 10/23/2020 P a g e | 13 Task 4: Describe the Drainage System Runoff from the site is split into two Natural Discharge Areas (A & B), however these flowpaths converge within a ¼ mile downstream of the site, creating one Threshold Discharge Area for the site. Runoff from Discharge Area A sheet flows into N 24 th St and flows to the west along the south side of the crowned roadway along the edge of pavement (A1). A raised asphalt bump directs the flow northwest towards a type 1 catch basin (A2) which conveys the stormwater north through an 8” CMP pipe on Garden Ct N. After approximately 100 LF, the stormwater enters another type 1 catch basin (A3) and flows west onto private property o n parcel 0523059065. According to City of Renton Maps, the stormwater enters a type 2 CB (A4) on parcel 0523059065 and moves west through two type 1 CB’s and two type 2 CB’s via a 12” PVC stormwater main, before entering a type 2 CB (A5) in the Bella Vista Apartment Complex. From here, stormwater runs in a counterclockwise loop through a series of type 1 CB’s (A6) around the complex with stormwater main starting at 12” PVC and sizing up to 15” and then 18” PVC before entering a type 2 CB (A7) at the entrance of the apartment complex where it joins the stormwater from Discharge Area B (described below). This is where the analysis was completed, just over ¼ mile downstream of the site, but less than ¼ mile from Discharge Area B, the shorter of the two flowpaths. Please refer to Figure 7 above, for the approximate location of identified drainage features. Runoff from Discharge Area B sheet flows off-site to the southwest onto grass and forested areas (B1) on adjacent parcels 3344500235 & 3344500250. The slope increases to approximately 30% as runoff continues to sheet flow down the undeveloped, forested hillside until it reaches a depression (B2) behind three townhouse buildings in the Bella Vista Apartment complex. This low point runs to the south behind these units in an overgrown vegetated depression until it reaches a grass lawn area (B3) and sheet flows to the west onto a paved parking lot. A type 1 CB (B4) picks up this storm water and coveys it to the north via a 12” storm pipe into a Type 2 CB (B5) where it joins the stormwater from Discharge Area B at just under ¼ mile from the project site. Stormwater from both discharge areas continue through the City of Kent stormwater system and discharge into Lake Washington (A8/B6) at Coulon Beach Park at app roximately a ½ mile from the project site. If conveyance system nuisance, severe erosion, severe flooding, or wetland hydrology problems are identified downstream of the site under Core Requirement #2, additional impact analysis and/or mitigation may be required. As discussed in Task 2, there have been no recent significant drainage complaints within a quarter-mile downstream of the site. As no existing drainage or conveyance issues have been recently identified, further impact analysis and mitigation of t he downstream stormwater system is not required. Please refer to the conveyance analysis in Section V of this TIR for additional discussion and evaluation of the on -site conveyance system capacity. Photographs from the site visit are included below. Mangat Short Plat Preliminary Technical Information Report 10/23/2020 P a g e | 14 Element A1- Sheet flow on N 24th St at site frontage downhill to the West Element A2- Type 1 CB with raised asphalt bump to direct stormwater Mangat Short Plat Preliminary Technical Information Report 10/23/2020 P a g e | 15 Element A3- Type 1 CB conveying stormwater onto private property to the West Element A5- Type 2 CB in Bella Vista Apartment Complex Mangat Short Plat Preliminary Technical Information Report 10/23/2020 P a g e | 16 Element A6- Series of type 1 CB’s in counterclockwise loop around Bella Vista Complex Element A7/B5- Type 2 CB (w/rectangular lid) where Discharge Area A & B converge Mangat Short Plat Preliminary Technical Information Report 10/23/2020 P a g e | 17 Element B1- Sheet flow off-site to southwest onto undeveloped grass & forest Element B2- Low point at bottom of hillside behind Bella Vista townhomes Mangat Short Plat Preliminary Technical Information Report 10/23/2020 P a g e | 18 Element B3- Sheet flow down grass lawn towards asphalt parking area and Bella Vista storm system Element A8/B6- Final Discharge into Lake Washington Mangat Short Plat Preliminary Technical Information Report 10/23/2020 P a g e | 19 IV. FLOW CONTROL AND WATER QUALITY FACILITY ANALYSIS AND DESIGN Part A: Existing Site Hydrology The 21,412 SF (0.49 AC) site is currently developed with a single-family residence accessed from N 24th St via a concrete driveway at the northeast corner of the site. South of the residence, a grassy hill slopes off-site to the southwest at slopes of 4-8%. The property is located within the East Lake Washington drainage basin, however it has two separate discharge areas. Runoff from the northern portion of the existing site generally sheet flows towards N 24th St where it flows downhill to west and enters the City of Renton storm system through a type 1 catch basin . Runoff from the southern portion of the site sheet flows to the southwest towards an undeveloped forested hillside, before entering the Bella Vista Apartment complex storm system, where it converges with the stormwater from the northern discharge area at just under a ¼ mile from the project site. Stormwater is ultimately discharged to Lake Washington, located approximately ½ mile west of the site. See full downstream analysis in Section III of this Technical Information Report (TIR). The Limited Geotechnical Evaluation prepared by Cobalt Geosciences (Appendix A) confirms the NRCS Indianola loamy sand soil classification on the site, and has determined infiltration to be infeasible. WWHM 2012 was used to model the existing condition within the 22,870 SF (0.53 AC) limits of disturbance using existing surfaces found on site including rooftop, driveway, lawn and walkways. Part B: Developed Site Hydrology The project proposes the development of two (2) single-family lots within the 21,412 SF (0.49 Acres) parcel, as well as an 11.5-foot right-of-way dedication with frontage improvements, and road widening at the intersection of Meadow Ave N and N 24th St for an emergency vehicle turnaround. Lot 1 is 10,277 SF (0.24 Acres) and is located along the eastern portion of the site with driveway access off of N 24th St. Lot 2 is 9,536 SF (0.22 Acres) and is located on the western portion of the site with driveway access off of N 24th St. Frontage improvements will include the addition of a 5-foot sidewalk, 8-foot planter strip, as well as curb and gutter along the edge of asphalt roadway. At the intersection of N 24th St and Meadow Ave N, the existing asphalt roadway will be widened by approximately 7 feet to accommodate a hammerhead Emergency Vehicle Turnaround. Stormwater runoff from the proposed development of Lots 1 and 2 will be managed as follows: • Lot 1: Stormwater BMP’s satisfying Core Requirement #9 will be used to the maximum extent feasible to mitigate new runoff created by the proposed single family residence and other impervious surfaces on-site. A 50-foot basic dispersion trench with notched board and 25-foot native vegetated flowpath is proposed on Lot 1 south of the residence to mitigate stormwater from rooftop areas. This report assumes that 3,500 Mangat Short Plat Preliminary Technical Information Report 10/23/2020 P a g e | 20 SF of rooftop area will be conveyed to this dispersion trench, the maximum allowe d for its size. • Lot 2: Stormwater BMP’s satisfying Core Requirement #9 will be used to the maximum extent feasible to mitigate new runoff created by the proposed single family residence and other impervious surfaces on-site. A 50-foot basic dispersion trench with notched board and 25-foot native vegetated flowpath is proposed on Lot 1 south of the residence to mitigate stormwater from rooftop areas. This report assumes that 3,500 SF of rooftop area will be conveyed to this dispersion trench, the maximum a llowed for its size. WWHM 2012 was used to model the proposed condition as follows: • Lot 1: The modeled limits of disturbance for Lot 1 is 10,277 SF. The maximum impervious surface per R-6 zoning of 5,652 SF was assumed with 3,500 SF of this area being conveyed to the Basic Dispersion trench. Per Table 1.2.9.A in the RSWDM, 90% of the area going to basic dispersion can be modeled as impervious, and 10% as lawn. The remaining 4,625 SF of pervious surface on Lot 1 will meet soil amendment requirement as detailed in Section C.2.13 of the RSWDM; therefore, it has been modeled as 100% pasture with moderate slopes in type C soils. • Lot 2: The modeled limits of disturbance for Lot 2 is 9,526 SF. The maximum impervious surface per R-6 zoning of 5,245 SF was assumed with 3,500 SF of this area being conveyed to the Basic Dispersion trench. Per Table 1.2.9.A in the RSWDM, 90% of the area going to basic dispersion can be modeled as impervious, and 10% as lawn. The remaining 4,291 SF of pervious surface on Lot 1 will meet soil amendment requirement as detailed in Section C.2.13 of the RSWDM; therefore, it has been modeled as 100% pasture with moderate slopes in type C soils. • ROW Dedication & Road Widening: The modeled limits of disturbance for the ROW dedication area and road widening is 3,829 SF. This will include 603 SF of sidewalk, 495 SF of driveway as well as 1,954 SF of asphalt road repair and widening. The remaining 777 SF of pervious surface will meet soil amendment requirement as detailed in Section C.2.13 of the RSWDM; therefore, it has been modeled as 100% pasture with moderate slopes in type C soils. Please refer to Core Requirement #9 in Section II of the TIR for additional discussion. A Developed Conditions Map is provided as Figure 5. A summary of the existing and developed analyses is provided in the table below: Mangat Short Plat Preliminary Technical Information Report 10/23/2020 P a g e | 21 WWHM Model (Mitigated) Area Description Size Developed Condition ROW Dedication Sidewalk 603 SF (0.01 AC) Modeled as Sidewalk, Mod ROW Dedication Driveway 495 SF (0.01 AC) Modeled as Driveway, Flat ROW Dedication Roadway 1,959 SF (0.05 AC) Modeled as Roads, Mod ROW Dedication Lawn 777 SF (0.02 AC) Modeled as Pasture, C, Mod Lot 1 Rooftop 4,927 SF (0.11 AC) Modeled as Rooftops, Flat Lot 1 Driveway 375 SF (0.01 AC) Modeled as Driveways, Flat Lot 1 Lawn 4,975 SF (0.11 AC) Modeled as Pasture, C, Mod Lot 2 Rooftop 4,520 SF (0.10 AC) Modeled as Rooftops, Flat Lot 2 Driveway 375 SF (0.01 AC) Modeled as Driveways, Flat Lot 2 Lawn 4,641 SF (0.11 AC) Modeled as Pasture, C, Mod TOTAL LIMITS OF DISTURBANCE 23,642 SF (0.54 AC) See Above WWHM Model (Pre-Developed) Area Description Size Existing Condition Rooftop 2,899 SF (0.07 AC) Modeled as Rooftops, Flat Driveway 1,009 SF (0.02 AC) Modeled as Driveways, Mod Walkways/Decks 808 SF (0.02 AC) Modeled as Sidewalks, Mod Roadway 580 SF (0.01 AC) Modeled as Roads, Mod Lawn 18,346 SF (0.42 AC) Modeled as Lawn, C, Mod TOTAL LIMITS OF DISTURBANCE 23,642 SF (0.54 AC) See Above Mangat Short Plat Preliminary Technical Information Report 10/23/2020 P a g e | 22 Part C: Performance Standards Based on the City of Renton’s flow control map, the project site is located within the Peak Rate Flow Control Standard (Existing Site Conditions). Flow control facilities are required to match the developed peak discharge rates to existing site conditions peak discharge rates for 2-, 10- and 100-year return periods. However, the site meets flow control exemption 2 per Section 1.2.3.1.A of the RSWDM, and flow control facilities are not required. In addition, the site is smaller than 22,000 SF. Therefore, the site is subject to Small Lot BMP Requirements detailed in Section 1.2.9.2.2 of the RSWDM. The site falls within a Basic Water Quality treatment area in accordance with Section 1.2.8.1.A of the RSWDM, however new plus replaced impervious pollution generating areas within the project site is under 5,000 SF and therefore water quality treatment is not required . Part D: Flow Control System As shown in the table on the following page, the proposed stormwater BMPs result in a 0.0109 CFS decrease in the 100-year flow using 15-minutes time steps. This is below the allowable threshold of a 0.15 CFS maximum increase per Section 1.2.3.1.B of the RSWDM; therefore, the project meets flow control exemption 2. No formal flow control facilities are proposed at this time; however, flow control BMPs will be implemented as described in Core Requirement #9 in Section II of this TIR. Please refer to Appendix B for a copy of the full WWHM data output. Part E: Water Quality System In accordance with Section 1.2.8.1.A of the RSWDM, Basic Water Quality Treatment is not required for this project as new plus replaced pollution generating impervious surfaces (PGIS) do not exceed 5,000 SF, meeting exemption 1. 1,949 SF of asphalt road repair and widening is proposed on N 24th St, 494 SF of concrete driveway is proposed within the ROW dedication area, and 750 SF of asphalt driveway is estimated within Lots 1 & 2 (at a 15-foot width). These quantities total to 3,193 SF of PGIS that will be created or replaced as a result of this project. Mangat Short Plat Preliminary Technical Information Report 10/23/2020 P a g e | 23 V. CONVEYANCE SYSTEM ANALYSIS AND DESIGN A conveyance system analysis will be provided with final engineering VI. SPECIAL REPORTS AND STUDIES • Limited Geotechnical Evaluation by Cobalt Geosciences dated August 20, 2020 VII. OTHER PERMITS • Civil Construction Permit • Final Short Plat • Building Permits • Right-of-Way Use Permit VIII. CSWPP ANALYSIS AND DESIGN A CSWPPP will be provided with final engineering. IX. BOND QUANTITIES, FACILITY SUMMARIES AND DECLARATION of COVENANT Bond Quantities, Facility Summary and Declaration of Covenant to be provided with Final Engineering. X. OPERATION AND MAINTENANCE MANUAL An Operation and Maintenance Manual to be provided with Final Engineering. Mangat Short Plat Preliminary Technical Information Report Appendix A Limited Geotechnical Evaluation by Cobalt Geosciences dated August 20, 2020 Mangat Short Plat Preliminary Technical Information Report Appendix B WWHM Output WWHM2012 PROJECT REPORT 20547 Magnat 10/15/2020 10:40:37 AM Page 2 General Model Information Project Name:20547 Magnat Site Name: Site Address: City: Report Date:10/15/2020 Gage:Seatac Data Start:1948/10/01 Data End:2009/09/30 Timestep:15 Minute Precip Scale:1.000 Version Date:2019/09/13 Version:4.2.17 POC Thresholds Low Flow Threshold for POC1:50 Percent of the 2 Year High Flow Threshold for POC1:50 Year 20547 Magnat 10/15/2020 10:40:37 AM Page 3 Landuse Basin Data Predeveloped Land Use Basin 1 Bypass:No GroundWater:No Pervious Land Use acre C, Lawn, Mod 0.42 Pervious Total 0.42 Impervious Land Use acre ROADS MOD 0.01 ROOF TOPS FLAT 0.07 DRIVEWAYS FLAT 0.02 SIDEWALKS MOD 0.02 Impervious Total 0.12 Basin Total 0.54 Element Flows To: Surface Interflow Groundwater 20547 Magnat 10/15/2020 10:40:37 AM Page 4 Mitigated Land Use Basin 1 Bypass:No GroundWater:No Pervious Land Use acre C, Pasture, Mod 0.24 Pervious Total 0.24 Impervious Land Use acre ROADS MOD 0.05 ROOF TOPS FLAT 0.21 DRIVEWAYS FLAT 0.03 SIDEWALKS MOD 0.01 Impervious Total 0.3 Basin Total 0.54 Element Flows To: Surface Interflow Groundwater 20547 Magnat 10/15/2020 10:40:37 AM Page 5 Routing Elements Predeveloped Routing 20547 Magnat 10/15/2020 10:40:37 AM Page 6 Mitigated Routing 20547 Magnat 10/15/2020 10:40:37 AM Page 7 Analysis Results POC 1 + Predeveloped x Mitigated Predeveloped Landuse Totals for POC #1 Total Pervious Area:0.42 Total Impervious Area:0.12 Mitigated Landuse Totals for POC #1 Total Pervious Area:0.24 Total Impervious Area:0.3 Flow Frequency Method:Log Pearson Type III 17B Flow Frequency Return Periods for Predeveloped. POC #1 Return Period Flow(cfs) 2 year 0.083567 5 year 0.123869 10 year 0.154157 25 year 0.196622 50 year 0.231377 100 year 0.268872 Flow Frequency Return Periods for Mitigated. POC #1 Return Period Flow(cfs) 2 year 0.123102 5 year 0.157048 10 year 0.18029 25 year 0.21062 50 year 0.233957 100 year 0.257959 Annual Peaks Annual Peaks for Predeveloped and Mitigated. POC #1 Year Predeveloped Mitigated 1949 0.139 0.167 1950 0.138 0.163 1951 0.081 0.105 1952 0.047 0.082 1953 0.043 0.092 1954 0.067 0.101 1955 0.070 0.112 1956 0.071 0.108 1957 0.096 0.128 1958 0.058 0.101 20547 Magnat 10/15/2020 10:41:12 AM Page 8 1959 0.048 0.102 1960 0.086 0.113 1961 0.073 0.107 1962 0.047 0.090 1963 0.077 0.107 1964 0.066 0.100 1965 0.102 0.129 1966 0.052 0.086 1967 0.135 0.144 1968 0.116 0.168 1969 0.092 0.116 1970 0.077 0.116 1971 0.094 0.137 1972 0.128 0.143 1973 0.043 0.083 1974 0.097 0.126 1975 0.101 0.136 1976 0.073 0.102 1977 0.066 0.100 1978 0.081 0.128 1979 0.079 0.170 1980 0.166 0.180 1981 0.078 0.126 1982 0.150 0.178 1983 0.085 0.140 1984 0.057 0.090 1985 0.078 0.120 1986 0.079 0.109 1987 0.081 0.162 1988 0.040 0.099 1989 0.054 0.133 1990 0.256 0.253 1991 0.181 0.187 1992 0.062 0.091 1993 0.041 0.082 1994 0.035 0.085 1995 0.067 0.111 1996 0.125 0.136 1997 0.093 0.122 1998 0.076 0.115 1999 0.202 0.236 2000 0.085 0.121 2001 0.063 0.130 2002 0.129 0.154 2003 0.113 0.138 2004 0.181 0.230 2005 0.077 0.107 2006 0.076 0.097 2007 0.236 0.223 2008 0.174 0.182 2009 0.104 0.154 Ranked Annual Peaks Ranked Annual Peaks for Predeveloped and Mitigated. POC #1 Rank Predeveloped Mitigated 1 0.2562 0.2531 2 0.2362 0.2359 3 0.2022 0.2297 20547 Magnat 10/15/2020 10:41:12 AM Page 9 4 0.1810 0.2234 5 0.1810 0.1875 6 0.1736 0.1816 7 0.1656 0.1802 8 0.1500 0.1782 9 0.1390 0.1697 10 0.1379 0.1685 11 0.1345 0.1668 12 0.1286 0.1633 13 0.1276 0.1620 14 0.1255 0.1543 15 0.1165 0.1536 16 0.1131 0.1444 17 0.1035 0.1432 18 0.1017 0.1402 19 0.1009 0.1383 20 0.0974 0.1366 21 0.0963 0.1364 22 0.0940 0.1362 23 0.0932 0.1328 24 0.0915 0.1302 25 0.0861 0.1293 26 0.0852 0.1282 27 0.0849 0.1279 28 0.0813 0.1262 29 0.0812 0.1259 30 0.0806 0.1218 31 0.0791 0.1211 32 0.0787 0.1204 33 0.0784 0.1160 34 0.0782 0.1160 35 0.0773 0.1145 36 0.0769 0.1131 37 0.0767 0.1122 38 0.0761 0.1115 39 0.0755 0.1094 40 0.0729 0.1082 41 0.0726 0.1072 42 0.0714 0.1069 43 0.0700 0.1066 44 0.0673 0.1050 45 0.0667 0.1020 46 0.0662 0.1016 47 0.0661 0.1011 48 0.0633 0.1010 49 0.0624 0.1000 50 0.0580 0.0996 51 0.0573 0.0986 52 0.0544 0.0965 53 0.0525 0.0920 54 0.0485 0.0908 55 0.0470 0.0904 56 0.0466 0.0903 57 0.0432 0.0863 58 0.0429 0.0848 59 0.0411 0.0829 60 0.0397 0.0824 61 0.0346 0.0821