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Home My WebLink AboutADJUSTMENT REQUEST 2016.11.21 BOUN SHORT PLAT SITE ADDRESS: 1024 Duvall Ave NE, Renton, Washington SECTION 10/TOWNSHIP 23 NORTH, RANGE 5 EAST, W.M. Surface Water Drainage Adjustment (________________________) Associated Permit Number Prepared For: Warring Properties Contact: Socheat “Kent” Khnor 845 106th Ave, Suite 200 Bellevue, WA 98003 Date Prepared: November 21, 2016 Prepared By: Drew T. Young, EIT Reviewed By: Brandon M. Loucks, P.E. Beyler Consulting LLC 7602 Bridgeport Way W, #3D Lakewood, WA 98499 253.301.4157 CONTACT phone: 253-301-4157 fax: 253-336-3950 info@beylerconsulting.com beylerconsulting.com LAKEWOOD OFFICE 7602 Bridgeport Way W #3D Lakewood, WA 98499 ISSAQUAH OFFICE 455 Rainier Blvd N Issaquah, WA 98027 Plan. Design. Manage. CIVIL & STRUCTURAL ENGINEERING | LAND SURVEYING | PLANNING PROJECT MANAGEMENT | FEASIBILITY | PERMIT EXPEDITING Page 1 November 21, 2016 TABLE OF CONTENTS SURFACE WATER ADJUSTMENT REQUEST ......................................... 2 APPENDIX A – LEVEL 1 DOWNSTREAM ANALYSIS ............................ 5 Page 2 November 21, 2016 SURFACE WATER ADJUSTMENT REQUEST Project Description The proponent of the Boun Short Plat proposes to subdivide an undeveloped parcel located in Renton, King County, Washington. The King County parcel number for the property is 102305- 9139. The project parcel is approximately 57,677 sf, 1.32 acres in size. The parcel is zoned Residential 8 (R-8) according to the City of Renton Zoning Map effective as of July 1, 2015. The allowed density range in the R-8 zone is a minimum of 4.0 dwelling units per net acre (DUA) to a maximum of 8.0 DUA. The Boun Short Plat proposes to subdivide the current parcel into 7 single-family lots equating to a proposed density of 5.29 DUA. The proposed lots range in size from the smallest being 5,098 sf in size and the largest being 6,453 sf in size. A single stormwater tract will be created with this subdivision for the purpose of stormwater treatment and mitigation for the development. The remaining area will be dedicated to the City of Renton for public right-of-way use. It has been determined that the site consists of two separate threshold discharges areas. Threshold Discharge Area #1 (West Basin) consists of a majority of the project site area that naturally discharges stormwater to the southwest corner of the parcel. Threshold Discharge Area #2 (A-East Basin, B-North basin) consists of a small portion of land along the east boundary which naturally discharges stormwater east to an existing swale within Duvall Ave NE (TDA 2.A) and the 18’ wide panhandle section of land located north of the project site that naturally discharges stormwater to the northwest (TDA 2.B). In summary, the East Basin (TDA 2.A) requests an exception from core requirement #1, as described in Section 1.2.1 of the 2009 City of Renton Surface Water Design Manual Amendment, to discharge stormwater in a location other than its natural discharge location. An overall map has been included of the Boun Short Plat Project, as well as a Level 1 downstream analysis of each basin. This is located in Appendix A of this report. Threshold Discharge Area #2.A (East Basin, TDA 2.A) Request Per Core Requirement #1: Discharge at the Natural Location of Section 1.2.1 of the 2009 City of Renton Surface Water Design Manual Amendments to the King County Surface Water Design Manual, “All surface and storm water runoff from a project must be discharged at the natural location so as not to be diverted onto or away from downstream properties. The manner in which runoff is discharged from a project site must not create a significant adverse impact to downhill properties or drainage systems.” This project requests an adjustment to Core Requirement #1 to allow a small portion of the East Basin (TDA 2.A) to discharge storm water runoff away from the natural discharge location to the West Basin (TDA 1). Existing Conditions The East Basin, which is approximately 8,771 sf (0.20 acres) in size, is currently undeveloped with densely vegetated landcover consisting of a variety of shrubs, brush and tall grasses. Small clusters of insignificant deciduous trees (< 6 inches in diameter) and a single 16” fir are located within the limits of the East Basin. The basin generally slopes northeast towards Page 3 November 21, 2016 an existing vegetated roadside drainage swale along the western edge of Duvall Avenue Northeast. See the Level 1 downstream analysis in Appendix A for further detail of the natural/existing downstream analysis of Threshold Discharge Areas #2, noted as TDA #2.A Developed Conditions The developed East Basin will be reduced to approximately 409 sf (0.009 acres) in size and will be dedicated to the City of Renton as public right of way. This area will consist of approximately 297 sf of impervious sidewalk and 112 sf of pervious lawn/landscape. Stormwater will sheet flow east towards the newly constructed closed conveyance system within Duvall Ave NE. Flow will be collected and conveyed through a series catch basins and pipes that discharge into the existing vegetated roadside swale just north of the project site, meeting the natural discharge location. The remaining East Basin area (8,362 sf) will be sloped to drain southwest towards the on- site stormwater detention and water quality vault. The vault will provide basic water quality treatment and flow control meeting the Flow Control Duration Standard matching Forested Conditions. The pervious landscape area proposed within the diverted East Basin area will sheet flow towards Chelan Place NE where accumulated stormwater runoff, if any, will be collected and conveyed to the onsite stormwater vault. Flows will then be treated and released at a controlled rate and discharged away from the natural discharge location into an existing conveyance system within Chelan Ave NE located at the southwest corner of the project parcel. Downstream Description East Basin (TDA 2.A) The downstream system of the East Basin is composed of an existing open conveyance system. The open conveyance swale allows stormwater to flow north of the project site for approximately 0.13 miles where it is collected within a 12” concrete pipe. The closed conveyance system continues to convey flows north through a series of catch basins and pipes to a point a quarter mile downstream. See Appendix B for a detailed analysis of the downstream corridor from the Boun Short Plat to the existing conveyance system within Duvall Ave NE. West Basin (TDA 1) The downstream system is composed of an existing conveyance system that was developed for the Vuong Short plat (Project R-332511). The conveyance system is a series of catch basins and 12” pipes that direct flow to a water quality tank. The water quality tank releases treated flows into a conveyance system along NE 10th Street where it continues to convey flow West to a regional combined water quality and detention pond. To allow for the project parcel to connect to the existing stormwater infrastructure within Chelan Ave NE, a downstream drainage correction will be made to allow for increased stormwater flow to bypass the existing water quality tank that was originally designed and constructed for the Vuong Short Plat. See Appendix B for a detailed analysis of the downstream corridor. Page 4 November 21, 2016 JUSTIFICATION The intent of this drainage adjustment is to collect the minor increased runoff from this proposed development within the East Basin and discharge the runoff in a controlled manner such not to disturb neighboring properties to the north and east. If the project were to mimic the natural drainage course along the northeast boundary, then released runoff could potentially cause a stormwater nuisance to the existing homes to the east and north. Releasing stormwater to this corridor could also create a higher potential for erosion potential within the existing vegetated swale. Thus, stormwater is proposed to be routed away from the neighboring properties where it will discharge to a closed conveyance system within the Boun Short Plat. Stormwater runoff within the West Basin will be treated meeting the Basic Treatment requirement and released at a Historic (forested) controlled flow rate. The amount of onsite basin area to be diverted to the West drainage basin is approximately 8,362 sf (0.19 acres). CONCLUSION It is concluded that, with slight modifications to the downstream drainage system as discussed in this request, and as deemed necessary by the City of Renton, this diversion in flow and adjustment of Core Requirement #1 of the City of Renton Amendments to the 2009 King County Surface Water Design Manual meets the objectives of safety, appearance, environmental protection, and maintainability based on sound engineering judgment. MBMBSS~SHEET OFJOB NUMBERNO. DESCRIPTION INIT. DATE DRAWN BY:CHECKED BY: SCALE: HORZ:VERT: DATE:CIVIL & STRUCTURAL ENGINEERING | LAND SURVEYING | PLANNING PROJECT MANAGEMENT | FEASIBILITY | PERMIT EXPEDITING LAKEWOOD OFFICE7602 Bridgeport Way W #3D Lakewood, WA 98499 phone: 253-301-4157 fax: 253-336-3950 ISSAQUAH OFFICE 455 Rainier Blvd NIssaquah, WA 98027phone: 425-392-8055fax: 425-392-0108 beylerconsulting.com BEYLER CONSULTING Plan. Design. ManageFIGURE 3.11EXISTING BASIN MAP BOUN SHORT PLAT BASIN MAPS CITY OF RENTON,WASHINGTON16-2061A PORTION OF SECTION 10, TOWNSHIP 23 NORTH, 5 WEST, W.M.DUVALL AVE NE CHELAN AVE NE THRESHOLD DISCHARGEAREA #1(WEST BASIN)(46,236 SF)(1.06 ACRES)THRESHOLD DISCHARGEAREA #2.A(EAST BASIN)(8,771 SF)(0.20 ACRES)BOUN SHORT PLATEXISTING BASIN MAPSCALE: 1" = 40'2040040THRESHOLD DISCHARGE AREA #2.B(NORTH BASIN)(2,669 SF)(0.061 ACRES)TO REMAIN UNDEVELOPED ANDDEDICATED TO CITY OF RENTONAS PUBLIC RIGHT OF WAY(MODIFICATION REQUEST #NATURAL DISCHARGELOCATION #1(EXISTING CATCH BASIN)NATURAL DISCHARGELOCATION #2.A(EXISTING SWALE)NATURAL DISCHARGELOCATION #2.B(SHEET FLOW TO NATIVEVEGETATION- NO APPARENTDISCHARGE LOCATION)BASIN AREA CALCULATIONSTHRESHOLD DISCHARGE AREA #1ONSITE (WEST BASIN)TOTAL BASIN AREA: 48,706 SF (1.12 ACRES)IMPERVIOUS (EXISTING SHED): 330 SFPERVIOUS (LAWN/GRASS): 48,376 SFOFFSITE (CHELAN AVE NE)TOTAL BASIN AREA: 4,148 SF (0.95 ACRES)IMPERVIOUS (ASPHALT): 1,577 SFPERVIOUS (LAWN/GRASS): 2,571 SFTHRESHOLD DISCHARGE AREA #2.AONSITE (EAST BASIN)TOTAL BASIN AREA: 6,302 SF (0.14 ACRES)PERVIOUS (LAWN/GRASS): 6,302 SFOFFSITE (DUVALL AVE NE)TOTAL BASIN AREA: 8,432 SF (0.19 ACRES)IMPERVIOUS (ASPHALT): 6,375 SFPERVIOUS (LAWN/SWALE): 2,057 SFTHRESHOLD DISCHARGE AREA #2.BONSITE (NORTH BASIN)TOTAL BASIN AREA: 2,669 SF (0.06 ACRES)IMPERVIOUS (GRAVEL DRIVEWAY): 789 SFPERVIOUS (LAWN/GRASS): 1,880 SFTHRESHOLD DISCHARGE AREA #1(OFFSITE)(4,148 SF)(0.095 ACRES)THRESHOLD DISCHARGE AREA #2.A(OFFSITE)(8,432 SF)(0.19 ACRES) LOT 45,966 SFLOT 55,658 SFLOT 65,638 SFTRACT A7,253 SFLOT 76,453 SFLOT 25,751 SFLOT 15,098 SFLOT 35,893 SFMBMBSS~434436438438 438438434 SHEET OFJOB NUMBERNO. DESCRIPTION INIT. DATE DRAWN BY:CHECKED BY: SCALE: HORZ:VERT: DATE:CIVIL & STRUCTURAL ENGINEERING | LAND SURVEYING | PLANNING PROJECT MANAGEMENT | FEASIBILITY | PERMIT EXPEDITING LAKEWOOD OFFICE7602 Bridgeport Way W #3D Lakewood, WA 98499 phone: 253-301-4157 fax: 253-336-3950 ISSAQUAH OFFICE 455 Rainier Blvd NIssaquah, WA 98027phone: 425-392-8055fax: 425-392-0108 beylerconsulting.com BEYLER CONSULTING Plan. Design. ManageFIGURE 3.21PROPOSED BASIN MAP BOUN SHORT PLAT BASIN MAPS CITY OF RENTON,WASHINGTON16-2061A PORTION OF SECTION 10, TOWNSHIP 23 NORTH, 5 WEST, W.M.DUVALL AVE NE CHELAN AVE NE THRESHOLDDISCHARGE AREA #1(WEST BASIN)(50,746 SF)(1.16 ACRES)BOUN SHORT PLATPROPOSED BASIN MAPSCALE: 1" = 40'2040040THRESHOLD DISCHARGE AREA #2.B(NORTH BASIN)(2,665 SF)(0.061 ACRES)TO REMAIN UNDEVELOPED ANDDEDICATED TO CITY OF RENTONAS PUBLIC RIGHT OF WAY(MODIFICATION REQUEST #NATURAL DISCHARGELOCATION #1(RELOCATED CATCHBASIN)NATURAL DISCHARGELOCATION #2.A(EXISTING SWALE)THRESHOLD DISCHARGEAREA #2.A(EAST BASIN)(409 SF)(0.009 ACRES)BASIN AREA CALCULATIONSTHRESHOLD DISCHARGE AREA #1ONSITE (WEST BASIN)TOTAL BASIN AREA: 50,746 SF (1.16 ACRES)IMPERVIOUS (PGIS: ASPHALT/DWAY): 8,944 SFIMPERVIOUS (ROOFTOPS): 18,802 SFPERVIOUS (LAWN/GRASS): 23,000 SFONSITE (VAULT BY-PASS)TOTAL BASIN AREA: 3,559 SF (0.082 ACRES)IMPERVIOUS (PGIS: ASPHALT/SWALK): 1,973 SFPERVIOUS (PLANTER STRIP): 1,586 SFOFFSITE (VAULT BY-PASS)TOTAL BASIN AREA: 2,528 SF (0.058 ACRES)IMPERVIOUS (PGIS: ASPHALT): 2,049 SFPERVIOUS (PLANTER STRIP): 479 SFOFFSITE (CHELAN AVE NE)TOTAL BASIN AREA: 1,620 SF (0.037 ACRES)IMPERVIOUS (ASPHALT): 1,576 SFPERVIOUS (LAWN/GRASS): 44 SFVAULT SIZING CALCULATIONS:TO DETENTION/WQ VAULT:TOTAL IMPERVIOUS: 27,746 SF (0.637 ACRES)TOTAL PERVIOUS: 23,000 SF (0.528 ACRES)VAULT BYPASS AREA CALCULATIONS:TO BYPASS DETENTION/WQ VAULT:TOTAL IMPERVIOUS: 4,022 SF (0.09 ACRES)TOTAL PERVIOUS: 2,065 SF (0.05 ACRES)THRESHOLD DISCHARGE AREA #2.AONSITE (EAST BASIN)TOTAL BASIN AREA: 409 SF (0.009 ACRES)IMPERVIOUS (SIDEWALK): 297 SFPERVIOUS (MAINTENANCE STRIP): 112 SFOFFSITE (DUVALL AVE NE)TOTAL BASIN AREA: 8,432 SF (0.19 ACRES)IMPERVIOUS (ASPHALT): 6,869 SFPERVIOUS (PLANTER STRIP): 1,563 SFTHRESHOLD DISCHARGE AREA #2.BONSITE (NORTH BASIN)TOTAL BASIN AREA: 2,669 SF (0.06 ACRES)IMPERVIOUS (GRAVEL DRIVEWAY): 789 SFPERVIOUS (LAWN/GRASS): 1,880 SFNATURAL DISCHARGELOCATION #2.B(SHEET FLOW TO NATIVEVEGETATION- NO APPARENTDISCHARGE LOCATION)THRESHOLD DISCHARGE AREA #1(OFFSITE)(1,620 SF)(0.037 ACRES)THRESHOLD DISCHARGEAREA #2.A(OFFSITE)(8,432 SF)(0.19 ACRES)THRESHOLD DISCHARGE AREA #1(ONSITE- VAULT BYPASS)(3,559 SF)(0.082 ACRES)THRESHOLD DISCHARGE AREA #1(OFFSITE-VAULT BYPASS)(2,528 SF)(0.058 ACRES) APPENDIX A – LEVEL 1 DOWNSTREAM ANALYSIS III. OFFSITE ANALYSIS An offsite analysis report has been prepared per Section 1.2.2, Core Requirement #2. This is to identify and evaluate offsite flooding, erosion, and water quality problems that may be created or aggravated by the proposed project. The primary component of this offsite analysis report is the downstream analysis. The second component of the report is to evaluate the upstream drainage system to verify that significant flooding and erosion impact will not occur as a result of the project. 3.1 DOWNSTREAM ANALYSIS The following Level 1 downstream analysis is a review of the drainage system up to a mile downstream of the site. The four tasks outlined under this review are: Task 1 – Define and map the study area Task 2 – Review all available information on the study area Task 3 - Field inspect the study area Task 4 - Drainage System Description and Problem Descriptions Task 5 – Mitigation of Existing or Potential Problems 3.1.1 Task 1 - Study Area Definition and Maps The project is located in the May Creek Drainage Sub-Basin within the Lake Washington/Cedar River Watershed. The drainage study area is approximately a mile-long path encompassing the site's downstream corridor. See Appendix A for maps of the basic study area. The site is currently unimproved, with the exception of an abandoned shed, surrounded by single family residential uses on all sides. 3.1.2 Task 2 - Resource Review The following resources have been reviewed for the downstream analysis. This is a review of available information on the downstream area at least a mile downstream. Sources include the City of Renton GIS maps, King County GIS maps, and geotechnical studies. Adopted Basin Plan  The site is located in the May Creek Drainage Sub-Basin within the Cedar River/Lake Washington Watershed (WRIA Cedar Sammamish (8)) Sensitive Area (See Appendix A for Sensitive Area Maps)  Erosion- None Mapped  Seismic – None Mapped  Landslide – None Mapped  Coal Mine – None Mapped  Streams and Wetlands Map – None Mapped  Susceptible to Groundwater Contamination – None Mapped  100-year flood plain – According to King County iMAP and the FEMA Flood Maps, the 100-year flood plain is not located near the project property. Drainage Complaints and Studies King County –Relevant DNRP drainage complaints within 1 mile of the downstream corridor of each threshold discharge area within the last 10 years were searched. TDA #1 There are currently no open complaints within the downstream corridor nor are there any relevant drainage complaints that were made within the last 10 years. Going beyond a Level 1 analysis (within the last 15 years), a single complaint pertaining specifically to a water quality cross connection was made in regards to the regional stormwater pond in 2004. See Appendix A for a drainage complaint map of the downstream corridor. TDA #2 There are currently no open complaints within the downstream corridor nor are there any relevant drainage complaints that were made within the last 10 years. Going beyond a Level 1 analysis (within the last 15 years), a single complaint was made in 2003 in regards to a potential septic tank dumping that was leaching into the roadside swale along Duvall Ave NE, just north of the site. See Appendix A for a drainage complaint map of the downstream corridor. 3.1.3 Task 3 - Field Inspection A site visit was performed on July 18, 2016 for the purpose of analyzing the proposed project site and its upstream and downstream corridor. The weather and conditions were light rain and cloudy. A description of the drainage path is described below. See Appendix B for downstream reach locations. THRESHOLD DISCHARGE AREA #1 Upstream The subject site sits on a high point with very little runoff crossing the site from adjacent properties. The site’s topography suggests that stormwater runoff from the site is divided and sheets flows to the southwest and to the northeast. The westerly portion of the site drains to the southwest into an existing catch basin within public right of way (Chelan Ave NE). Downstream Runoff contributing to the downstream corridor from the project site consists of the generated runoff from the improved roadway and rooftop of the project site. The following is a discussion of the downstream corridor from threshold discharge area #1’s natural discharge location. See Appendix B for a Summary of each reach including: pipe size, length, and material as well as a map of each reach’s location. Downstream Legend - Surface Flow Arrow - Closed Conveyance Flow Arrow - Closed Conveyance Flow Arrow (Tributary) Reach 1 0’ The runoff generated from the project parcel sheet flows southeast and is collected within a Type 1 Catch Basin (Facility ID# 178992) within Chelan Ave NE. Photo 1: Site topography facing northeast from Chelan Ave NE towards Project Site. Reach 2 0’ – 31’ Stormwater is collected within an existing Type 1 Catch Basin (Facility ID# 178992) and conveyed west within a 12-inch CPEP Storm Main sloped at 3.21% (R-332511). At time of site visit, the pipe did not seem to be flowing at capacity nor were flooding problems observed. Photo 2: Facing North on Chelan Ave NE. Stormwater flows south within closed conveyance system. Project Site Ex. CB - Type 1 ID: 178992 Ex. CB - Type 1 ID: 178992 Ex. CB - Type 1 ID: 116933 N Reach 3 31’ – 180’ Stormwater is collected within an existing Type 1 Catch Basin (Facility ID# 116931) and conveyed southwest within a 12-inch CPEP Storm Main sloped at 0.52% (R-332511). At time of site visit, the pipe did not seem to be flowing at capacity nor were flooding problems observed. Photo 3: Facing northwest on Chelan Ave NE. Stormwater flows southwest within closed conveyance system. Reach 4 180’ – 275’ Stormwater is collected within an existing Type 2-54” Catch Basin (Facility ID# 175753) and conveyed west within a 60-inch CMP WQ Tank (R-332511). This WQ facility is privately owned and operated. Photo 4: Facing north on NE 10th Street. Stormwater flows west within closed conveyance system/WQ Facility. Ex. CB - Type 1 ID: 116931 N Ex. WQ Facility ID: 175753 Reach 5 275’ – 395’ Stormwater is discharged to an existing Type 2 Catch Basin (Facility ID# 116930) and conveyed west within a 12-inch of unknown material Storm Main (R-332511). It should be noted that per as-built R-332511, the storm main connection is incorrectly shown to connect to the existing sanitary sewer within NE 10th Street. Photo 5: Facing West on NE 10th Street. Stormwater flows west within closed conveyance system. Reach 6 395’ – 477’ Stormwater is collected within a Type 2 Catch Basin (Facility ID# 116927) and conveyed north within a 12-inch CPEP Storm Main (D-22661A) sloped at 4.86%. This portion of pipe discharges into a regional stormwater facility which was developed as part of the NE 10th Street/Anacortes Ave NE Detention Pond and Storm System Improvement Project. Photo 6: Facing West on NE 10th Street. Stormwater is collected within Type II Catch Basin and conveyed north within closed conveyance system. Ex. CB - Type 2 ID: 116930 Ex. CB - Type 2 ID: 116930 Reach 7 477’ – 825’ Stormwater is detained within a combined water quality and detention pond. Stormwater is then released at a controlled flow rate from an existing Type II-54” Catch Basin (Facility ID# 115504) and conveyed south within an 18- inch CPEP Storm Main (D-22661A) sloped at 0.20%. Stormwater is collected within a Type 2-48” Catch Basin (Facility ID# 115505) and conveyed southwest within an 18-inch CPEP Storm Main (D-22661A) sloped at 0.20%. From this location, stormwater is convey back into NE 10th Street public right of way to a Type 2-48” Catch Basin (Facility ID# 116923) (D-226618). Reach 8 825’ – 1,003’ Stormwater is conveyed west from the Type 2 Catch Basin within an 18-inch CPEP Storm Main (D-226618) sloped at 0.24%. Photo 7: Facing Northwest on NE 10th Street. Stormwater is collected in Type II catch basin and conveyed west within closed conveyance system. Ex. CB - Type 2 ID: 116923 Reach 9 1,003’ – 1,038’ Stormwater is collected within a Type 2 Catch Basin and conveyed west within an 18-inch CPEP Storm Main (D-226618) sloped at 0.14%. Photo 8: Facing Northeast on NE 10th Street. Stormwater is collected in Type II catch basin and conveyed west within closed conveyance system. Additional flow enters the conveyance system from a type I catch basin located to the north of the type 2 catch basin. Reach 10 1,038’ – 1,138’ Stormwater is collected within a Type 2 Catch Basin located at the intersection of NE 10th Street and Anacortes Ave NE and conveyed north within a 30-inch CPEP Storm Main (D-226617) sloped at 0.39%. Photo 9: Facing Northwest at intersection of NE 10th Street and Anacortes Ave NE. Stormwater is collected in Type II catch basin and conveyed north within closed conveyance system. Additional flow enters the conveyance system from storm main east of this location. Ex. CB - Type 2 ID: 116903 Ex. CB - Type 2 ID: 116902 Reach 11 1,138’ – 1,258’ Stormwater is collected within a Type 2 Catch Basin located approximately 100 feet north of the intersection of NE 10th Street and Anacortes Ave NE. Stormwater is conveyed north within a 30-inch CPEP Storm Main (D-226617) sloped at 0.40%. Photo 10: Facing North along Anacortes Ave NE. Stormwater is collected in Type II catch basin and conveyed north within closed conveyance system. Reach 12 1,258’ – 1,333’ Stormwater is collected within a Type 2 Catch Basin (Facility ID# 116900) within Anacortes Ave NE and conveyed north within a 30-inch CPEP Storm Main (D-226617) sloped at 0.41%. Completing the downstream analysis ¼ of a mile downstream from the project site natural discharge location. Photo 11: Facing North along Anacortes Ave NE. Stormwater is collected in Type II catch basin and conveyed north within closed conveyance system. Ex. CB - Type 2 ID: 116901 Ex. CB - Type 2 ID: 116900 N N Ex. CB - Type 2 ID: 116895 ¼ mile downstream THRESHOLD DISCHARGE AREA #2.A Upstream The subject site sits on a high point with very little runoff crossing the site from adjacent properties. The site’s topography suggests that stormwater runoff from the site is divided and sheets flows to the southwest and to the northeast. The easterly portion of the site drains to the northeast into an existing conveyance swale within public right of way (Duvall Ave NE). Downstream Runoff contributing to the downstream corridor from the project site consists of the generated runoff from the existing asphalt within Duvall Ave NE and proposed sidewalk. The following is a discussion of the downstream corridor from threshold discharge area #2.A’s natural discharge location. See Appendix B for a Summary of each reach including: pipe size, length, and material as well as a map of each reach’s location. Reach 1 0’ The runoff generated from the project parcel sheet flows northwest and is collected within an existing roadside conveyance swale (Facility ID# 450085) located along the east boundary of the project parcel. Stormwater is conveyed north. Photo 12: Facing northwest along Duvall Ave NE. Stormwater sheet flows east within project parcel and enters existing conveyance swale along the east boundary of the site. Project Site Ex. Conveyance Swale ID: 450085 Reach 2 0’- 695’ Stormwater is conveyed north within the conveyance swale along Duvall Ave NE. Stormwater passes through a series of CMP culverts that were installed under various driveways along this segment of flow path. The swale is well established with various land covers. No evidence erosion or flooding were observed at time of site visit. Garbage and other types of debris were found at various locations along the swale which could cause potential issues if not cleaned. Photo 13: Facing northwest along Duvall Ave NE. Stormwater shallow channel flows within the swale. Stormwater flows north. Reach 3 695’- 753’ Stormwater is collected within a 12-inch concrete culvert (Facility ID# 700093). The inlet to concrete culvert had a small amount of sedimentation build up and leaf debris. Photo 14: Facing west along Duvall Ave NE. Shallow channel flow enters concrete culvert. Debris that could potential disrupt channel flow Ex. Conc. Culvert (Facility ID #700093) Reach 3 753’- 835’ Stormwater is collected within a Type 1 Catch Basin (Facility ID# 130734). Stormwater is convey north within a 12-inch concrete pipe. Photo 15: Facing west along Duvall Ave NE. Stormwater is conveyed north within a Type 1 CB. Reach 4 835’- 883’ Stormwater is collected within a Type 1 Catch Basin (Facility ID# 130735). Stormwater is convey north within a 12-inch concrete pipe. Photo 16: Facing west along Duvall Ave NE. Stormwater is conveyed north within a Type 1 CB. Ex. CB - Type 1 ID: 130734 Ex. CB - Type 1 ID: 130735 Reach 5 883’- 1103’ Stormwater is collected within a Type 1 Catch Basin (Facility ID# 130736). Stormwater is convey north within a 12-inch concrete pipe. The 12-inch concrete pipe is sloped at 3.4% (R-147402). Photo 17: Facing northwest along Duvall Ave NE. Stormwater is conveyed north within a Type 1 CB. Reach 6 1103’- 1402’ Stormwater is collected within a Type 1 Catch Basin (Facility ID# 130794). Stormwater is convey north within a 12-inch concrete pipe. The 12-inch concrete pipe is sloped at 1.3% (R-147402). Completing the downstream analysis ¼ of a mile downstream from the project site natural discharge location. Photo 18: Facing southwest along Duvall Ave NE. Stormwater is conveyed north within a Type 1 CB. Ex. CB - Type 1 ID: 130736 Ex. CB - Type 1 ID: 130794 3.1.4 Task 4 - Drainage System Description and Problem Descriptions There are no apparent drainage complaints in the vicinity of the project site. However, during the downstream analysis, a problem was identified with releasing stormwater within Threshold Discharge Area #1 to an existing water quality facility that was installed to treat stormwater for the Vuong Short Plat (Project R-332511). Any increase in stormwater above the design water quality flow rate would cause the facility to improperly function. Mitigation of this downstream problem is necessary to allow the project vault to discharge to the downstream network. 3.1.5 Task 5 - Mitigation of Existing or Potential Problems To mitigate the downstream issue identified in Task 4, a flow splitter is proposed to bypass flows higher than the water quality flow rate of the existing water quality facility to the downstream network. See Section IV of this report for further details. 3,983 332 COR Critical Areas Map - Regulated Slopes This map is a user generated static output from an Internet mapping site and is for reference only. Data layers that appear on this map may or may not be accurate, current, or otherwise reliable. None 8/29/2016 Legend 2260 113 THIS MAP IS NOT TO BE USED FOR NAVIGATION Feet Notes 226 WGS_1984_Web_Mercator_Auxiliary_Sphere Information Technology - GIS RentonMapSupport@Rentonwa.gov City and County Boundary Other City of Renton Addresses Parcels Slope City of Renton >15% & <=25% >25% & <=40% (Sensitive) >40% & <=90% (Protected) >90% (Protected) Slope King County >15% & <=25% >25% & <=40% >40% & <=90% >90% 3,983 332 COR Critical Areas Map - Seismic This map is a user generated static output from an Internet mapping site and is for reference only. Data layers that appear on this map may or may not be accurate, current, or otherwise reliable. None 8/29/2016 Legend 2260 113 THIS MAP IS NOT TO BE USED FOR NAVIGATION Feet Notes 226 WGS_1984_Web_Mercator_Auxiliary_Sphere Information Technology - GIS RentonMapSupport@Rentonwa.gov City and County Boundary Other City of Renton Addresses Parcels Seismic Hazard Areas 3,983 332 COR Critical Areas Map - Wellhead Protection Areas This map is a user generated static output from an Internet mapping site and is for reference only. Data layers that appear on this map may or may not be accurate, current, or otherwise reliable. None 8/29/2016 Legend 2260 113 THIS MAP IS NOT TO BE USED FOR NAVIGATION Feet Notes 226 WGS_1984_Web_Mercator_Auxiliary_Sphere Information Technology - GIS RentonMapSupport@Rentonwa.gov City and County Boundary Other City of Renton Addresses Parcels Wellhead Protection Area Zones Zone 1 Zone 1 Modified Zone 2 3,983 332 COR Critical Areas Map - Wetland This map is a user generated static output from an Internet mapping site and is for reference only. Data layers that appear on this map may or may not be accurate, current, or otherwise reliable. None 8/29/2016 Legend 2260 113 THIS MAP IS NOT TO BE USED FOR NAVIGATION Feet Notes 226 WGS_1984_Web_Mercator_Auxiliary_Sphere Information Technology - GIS RentonMapSupport@Rentonwa.gov City and County Boundary Other City of Renton Addresses Parcels Wetlands Date: 8/29/2016 ±The information included on this map has been compiled by King County staff from a variety of sources and issubject to change without notice. King County makes no representations or warranties, express or implied,as to accuracy, completeness, timeliness, or rights to the use of such information. This document is not intendedfor use as a survey product. King County shall not be liable for any general, special, indirect, incidental, orconsequential damages including, but not limited to, lost revenues or lost profits resulting from the use or misuseof the information contained on this map. Any sale of this map or information on this map is prohibited except bywritten permission of King County. Legend Parcels Erosion hazard(1990 SAO) Seismic hazard(1990 SAO) Landslide hazard(1990 SAO) Coal mine hazard(1990 SAO) class 1 class 2 perennial class 2 salmonid class 3 unclassified Wetland (1990SAO) Sensitive areanotice on title Chinookdistribution Wildlife network severe moderate King County Critical Areas Map Date: 8/29/2016 ±The information included on this map has been compiled by King County staff from a variety of sources and issubject to change without notice. King County makes no representations or warranties, express or implied,as to accuracy, completeness, timeliness, or rights to the use of such information. This document is not intendedfor use as a survey product. King County shall not be liable for any general, special, indirect, incidental, orconsequential damages including, but not limited to, lost revenues or lost profits resulting from the use or misuseof the information contained on this map. Any sale of this map or information on this map is prohibited except bywritten permission of King County. Legend Parcels Bonded Commercial-MF Commercial-SF Construction DOT FMD Regional Residential Drainagecomplaints King County Drainage Complaints Map 1,991 166 Threshold Discharge Area #1 - Downstream Map This map is a user generated static output from an Internet mapping site and is for reference only. Data layers that appear on this map may or may not be accurate, current, or otherwise reliable. 8/29/2016 Legend 1130 56 THIS MAP IS NOT TO BE USED FOR NAVIGATION Feet Notes 113 WGS_1984_Web_Mercator_Auxiliary_Sphere Information Technology - GIS RentonMapSupport@Rentonwa.gov City and County Boundary Other City of Renton Addresses Parcels Wetlands Network Structures Inlet Manhole Utility Vault Unknown Structure Control Structure Pump Station Discharge Point Water Quality Detention Facilities Pond Tank Vault Wetland Pipe Culvert Open Drains Virtual Drainline Facility Outline Private Network Structures 1,991 166 Threshold Discharge Area #1 - Downstream Map 2 This map is a user generated static output from an Internet mapping site and is for reference only. Data layers that appear on this map may or may not be accurate, current, or otherwise reliable. 8/29/2016 Legend 1130 56 THIS MAP IS NOT TO BE USED FOR NAVIGATION Feet Notes 113 WGS_1984_Web_Mercator_Auxiliary_Sphere Information Technology - GIS RentonMapSupport@Rentonwa.gov City and County Boundary Other City of Renton Addresses Parcels Wetlands Network Structures Inlet Manhole Utility Vault Unknown Structure Control Structure Pump Station Discharge Point Water Quality Detention Facilities Pond Tank Vault Wetland Pipe Culvert Open Drains Virtual Drainline Facility Outline Private Network Structures OFF-SITEANALYSISDRAINAGE SYSTEMTABLESURFACEWATER DESIGNMANUAL,COREREQUIREMENT#2Basin:Subbasin Name: Subbasin Number:Symbol DrainageComponent Type, Name, and Size DrainageComponentDescriptionSlopeDistancefrom site dischargeExistingProblemsPotentialProblemsObservations of field inspector, resource reviewer, or resident see map Type: sheet flow, swale, stream, channel, pipe, pond; Size: diameter, surface area drainage basin, vegetation, cover, depth, type of sensitive area, volume %¼ ml = 1,320 ft. constrictions, under capacity, ponding, overtopping, flooding, habitat or organism destruction, scouring, bank sloughing, sedimentation, incision, other erosion tributary area, likelihood of problem, overflow pathways, potential impacts 3,983 332 Threshold Discharge Area #2.A - Downstream Map 1 This map is a user generated static output from an Internet mapping site and is for reference only. Data layers that appear on this map may or may not be accurate, current, or otherwise reliable. 9/1/2016 Legend 2260 113 THIS MAP IS NOT TO BE USED FOR NAVIGATION Feet Notes 226 WGS_1984_Web_Mercator_Auxiliary_Sphere Information Technology - GIS RentonMapSupport@Rentonwa.gov City and County Boundary Other City of Renton Addresses Parcels Network Structures Inlet Manhole Utility Vault Unknown Structure Control Structure Pump Station Discharge Point Water Quality Detention Facilities Pond Tank Vault Wetland Pipe Culvert Open Drains Facility Outline Private Network Structures Inlet Manhole 3,983 332 Threshold Discharge Area #2.A - Downstream Map 2 This map is a user generated static output from an Internet mapping site and is for reference only. Data layers that appear on this map may or may not be accurate, current, or otherwise reliable. 9/1/2016 Legend 2260 113 THIS MAP IS NOT TO BE USED FOR NAVIGATION Feet Notes 226 WGS_1984_Web_Mercator_Auxiliary_Sphere Information Technology - GIS RentonMapSupport@Rentonwa.gov City and County Boundary Other City of Renton Addresses Parcels Network Structures Inlet Manhole Utility Vault Unknown Structure Control Structure Pump Station Discharge Point Water Quality Detention Facilities Pond Tank Vault Wetland Pipe Culvert Open Drains Facility Outline Private Network Structures Inlet Manhole OFF-SITEANALYSISDRAINAGE SYSTEMTABLESURFACEWATER DESIGNMANUAL,COREREQUIREMENT#2Basin:Subbasin Name: Subbasin Number:Symbol DrainageComponent Type, Name, and Size DrainageComponentDescriptionSlopeDistancefrom site dischargeExistingProblemsPotentialProblemsObservations of field inspector, resource reviewer, or resident see map Type: sheet flow, swale, stream, channel, pipe, pond; Size: diameter, surface area drainage basin, vegetation, cover, depth, type of sensitive area, volume %¼ ml = 1,320 ft. constrictions, under capacity, ponding, overtopping, flooding, habitat or organism destruction, scouring, bank sloughing, sedimentation, incision, other erosion tributary area, likelihood of problem, overflow pathways, potential impacts