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MEDIUM PROJECT STORMWATER SITE PLAN REPORT PSERN Education Hill 10365 172nd Ave NE Redmond, WA 98052 CG Project No.: 16015.913 250 4th Ave S Ste 200 Edmonds, WA 98020 Phone: (425) 778-8500 Fax: (425) 778-5536 08/08/2016 ATTACHMENT 15 ---PAGE BREAK--- 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ph. [PHONE REDACTED] I f. [PHONE REDACTED] www.cgengineering.com Table of Contents Section I – Project Overview Section II – Minimum Requirements Section III –Existing Conditions Summary Section IV – Permanent Stormwater Control Plan Narrative Section V – Stormwater Pollution Prevention Plan Narrative ATTACHMENT 15 ---PAGE BREAK--- PSERN Education Hill 16015.913 August 8, 2016 Medium Project Stormwater Site Plan Report Section I, Page 1 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ph. [PHONE REDACTED] I f. [PHONE REDACTED] www.cgengineering.com Section I – Project Overview Section I Summary Narrative Vicinity Map Aerial Photograph The proposed project is a tower site for a new King County Puget Sound Emergency Ratio Network (PSERN) 170 foot tower and associated equipment to provide emergency radio communications to reach and coordinate with emergency responders. The site is adjacent to a water tower site on a 4.79 acre (208,652 square feet) parcel owned by the City of Redmond. Address: 10365 172nd Ave NE, Redmond, WA 98052 Tax Parcel Number: [PHONE REDACTED] In the existing condition, the majority of the parcel is undeveloped and is vegetated with trees. The proposed lease area will be south of two water towers on a portion of the vegetated area. The site slopes towards the east into 172nd Ave NE. The parcel is within a developed area with catch basins along this street. The average slope is about 4% with about a 4 foot change in elevation across the site. By reviewing the Redmond Watershed Map, stormwater eventually discharges to the Sammamish River via Bear Creek. Underlying soils are Alderwood gravelly sandy loam according to the Natural Resources Conservation Service (NRCS) Soil Survey Map, corresponding to hydrologic soil group In the proposed condition, the project will add 3,829 square feet of impervious area (a 50 foot by 50 foot lease area with a 1,329 square foot access driveway). The following on-site stormwater management BMPs are proposed and are further discussed in Section IV – Permanent Stormwater Control Plan Narrative: 1) Post-Construction Soil Quality and Depth (BMP T5.13) for disturbed pervious areas 2) Sheet Flow Dispersion (BMPT5.12) for all proposed impervious areas ATTACHMENT 15 ---PAGE BREAK--- PSERN Education Hill 16015.913 August 8, 2016 Medium Project Stormwater Site Plan Report Section I, Page 2 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ph. [PHONE REDACTED] I f. [PHONE REDACTED] www.cgengineering.com Figure 1.1 - Vicinity Map PROJECT SITE ATTACHMENT 15 ---PAGE BREAK--- PSERN Education Hill 16015.913 August 8, 2016 Medium Project Stormwater Site Plan Report Section I, Page 3 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ph. [PHONE REDACTED] I f. [PHONE REDACTED] www.cgengineering.com Figure 1.2 - Aerial Photograph PROJECT SITE ATTACHMENT 15 ---PAGE BREAK--- PSERN Education Hill 16015.913 August 8, 2016 Medium Project Stormwater Site Plan Report Section II, Page 1 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ph. [PHONE REDACTED] I f. [PHONE REDACTED] www.cgengineering.com Section II – Minimum Requirements The 2012 City of Redmond Clearing, Grading, and Stormwater Management Technical Notebook, referred to as the Stormwater Notebook, was used to determine minimum requirements for this project. Following Section 2.4.1 for New Development, the project must address Minimum Requirements Figure 1.3 – Flow Chart for Determining Requirements for New Development ATTACHMENT 15 ---PAGE BREAK--- PSERN Education Hill 16015.913 August 8, 2016 Medium Project Stormwater Site Plan Report Section II, Page 2 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ph. [PHONE REDACTED] I f. [PHONE REDACTED] www.cgengineering.com These requirements are addressed as follows: Minimum Requirement Preparation of Stormwater Site Plans A stormwater site plan following Chapter 3 of Volume I of the 2005 Ecology Manual and the City of Redmond Stormwater Technical Manual has been prepared, which includes civil plans and this report. The proposed project is classified as medium. Minimum Requirement Construction Stormwater Pollution Prevention Plan The includes the narrative provided in Section V – Stormwater Pollution Prevention Plan Narrative, and the erosion control plan. Minimum Requirement Source Control of Pollution Source control BMPs will be implemented during construction according to the erosion control plan. Permanent source control BMPs is not required for the site. Minimum Requirement Preservation of Natural Drainage Systems and Outfalls The natural drainage pattern will be maintained. Minimum Requirement On-Site Stormwater Management Stormwater runoff from impervious areas will be managed with sheet flow dispersion (BMP T5.12). Refer to Section IV – Permanent Stormwater Control Plan Narrative. ATTACHMENT 15 ---PAGE BREAK--- PSERN Education Hill 16015.913 August 8, 2016 Medium Project Stormwater Site Plan Report Section III, Page 1 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ph. [PHONE REDACTED] I f. [PHONE REDACTED] www.cgengineering.com Section III –Existing Conditions Summary Section II Summary Narrative NRCS Soil Map The proposed tower lease area site is adjacent to the site of two water tanks at 10365 172nd Ave NE on a 4.79 acre parcel. The majority of the parcel is undeveloped and is vegetated with trees. The proposed lease area will be south of the water towers on a portion of the vegetated area. Adjacent uses to the parcel include sports fields to the east and northeast, residential lots to the north, an elementary school to the west, and a church parking lot to the south. In its existing condition, the site slopes towards the east across the vegetated area into 172nd Ave NE. The parcel is within a developed area with catch basins along this street. The average slope is about 4% with about a 4 feet change in elevation across the site. By reviewing the Redmond Watershed Map, stormwater eventually discharges to the Sammamish River via Bear Creek (Watershed #490140). The underlying soil is Alderwood gravelly sandy loam according to the Natural Resources Conservation Service (NRCS) Soil Survey Map. This corresponds to hydrologic soil group In the developed condition, the natural drainage pattern is to remain. The project will add 3,829 square feet of impervious surface for an access road and tower site. ATTACHMENT 15 ---PAGE BREAK--- Soil Map—King County Area, Washington (Education Hill) Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 6/7/2016 Page 1 of 3 5282350 5282380 5282410 5282440 5282470 5282500 5282530 5282560 5282350 5282380 5282410 5282440 5282470 5282500 5282530 5282560 566570 566600 566630 566660 566690 566720 566750 566600 566630 566660 566690 566720 566750 47° 41' 35'' N 122° 6' 46'' W 47° 41' 35'' N 122° 6' 37'' W 47° 41' 27'' N 122° 6' 46'' W 47° 41' 27'' N 122° 6' 37'' W N Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 10N WGS84 0 50 100 200 300 Feet 0 15 30 60 90 Meters Map Scale: 1:1,170 if printed on A portrait (8.5" x 11") sheet. ATTACHMENT 15 ---PAGE BREAK--- MAP LEGEND MAP INFORMATION Area of Interest (AOI) Area of Interest (AOI) Soils Soil Map Unit Polygons Soil Map Unit Lines Soil Map Unit Points Special Point Features Blowout Borrow Pit Clay Spot Closed Depression Gravel Pit Gravelly Spot Landfill Lava Flow Marsh or swamp Mine or Quarry Miscellaneous Water Perennial Water Rock Outcrop Saline Spot Sandy Spot Severely Eroded Spot Sinkhole Slide or Slip Sodic Spot Spoil Area Stony Spot Very Stony Spot Wet Spot Other Special Line Features Water Features Streams and Canals Transportation Rails Interstate Highways US Routes Major Roads Local Roads Background Aerial Photography The soil surveys that comprise your AOI were mapped at 1:24,000. Warning: Soil Map may not be valid at this scale. Enlargement of maps beyond the scale of mapping can cause misunderstanding of the detail of mapping and accuracy of soil line placement. The maps do not show the small areas of contrasting soils that could have been shown at a more detailed scale. Please rely on the bar scale on each map sheet for map measurements. Source of Map: Natural Resources Conservation Service Web Soil Survey URL: http://websoilsurvey.nrcs.usda.gov Coordinate System: Web Mercator (EPSG:3857) Maps from the Web Soil Survey are based on the Web Mercator projection, which preserves direction and shape but distorts distance and area. A projection that preserves area, such as the Albers equal-area conic projection, should be used if more accurate calculations of distance or area are required. This product is generated from the USDA-NRCS certified data as of the version date(s) listed below. Soil Survey Area: King County Area, Washington Survey Area Data: Version 11, Sep 14, 2015 Soil map units are labeled (as space allows) for map scales 1:50,000 or larger. Date(s) aerial images were photographed: Aug 31, 2013—Oct 6, 2013 The orthophoto or other base map on which the soil lines were compiled and digitized probably differs from the background imagery displayed on these maps. As a result, some minor shifting of map unit boundaries may be evident. Soil Map—King County Area, Washington (Education Hill) Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 6/7/2016 Page 2 of 3 ATTACHMENT 15 ---PAGE BREAK--- Map Unit Legend King County Area, Washington (WA633) Map Unit Symbol Map Unit Name Acres in AOI Percent of AOI AgC Alderwood gravelly sandy loam, 8 to 15 percent slopes 5.9 94.1% AmC Arents, Alderwood material, 6 to 15 percent slopes 0.4 5.9% Totals for Area of Interest 6.3 100.0% Soil Map—King County Area, Washington Education Hill Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 6/7/2016 Page 3 of 3 ATTACHMENT 15 ---PAGE BREAK--- PSERN Education Hill 16015.913 August 8, 2016 Medium Project Stormwater Site Plan Report Section IV, Page 1 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ph. [PHONE REDACTED] I f. [PHONE REDACTED] www.cgengineering.com Section IV – Permanent Stormwater Control Plan Narrative Section IV Summary Narrative Grading and Drainage Plan (11 x 17) Best Management Practices (BMPs) were evaluated according to the 2005 Department of Ecology Stormwater Manual for Western Washington. Source control BMPs during construction are described in the which addresses Minimum Requirement 2. It is not anticipated that structural source controls or other permanent measures not described in this report will be applicable. The on-site stormwater management BMPs that will be implemented are: 1. BMP T5.12 Sheet Flow Dispersion – All impervious areas on the site will be managed with Sheet Flow Dispersion (BMP T5.12), including the equipment shelter, leased area, and access road. Cross slopes along the access road and leased area will be a minimum of and runoff will be dispersed into vegetated buffers as shown in the attached C3.1 Grading and Drainage plan. The vegetated buffers were sized according to the design guidelines from Chapter 5 of Volume V of the 2005 Department of Ecology Manual. A vegetated buffer width of 10 feet must be provided for up to 20 feet of width of paved or impervious surface and an additional 5 feet of width must be added for each additional 20 feet of width or fraction thereof. Therefore, a 10 foot buffer is provided for the 15 foot driveway, and a 17.5 foot buffer is provided for the 50 foot wide lease area. Natural drainage patterns will be maintained with the dominant flow direction being towards the east towards 172nd Ave NE. 2. BMP T5.13 Post-Construction Soil Quality and Depth – All disturbed pervious areas will be amended with compost to meet specifications of BMP T5.13. ATTACHMENT 15 ---PAGE BREAK--- PSERN Education Hill 16015.913 August 8, 2016 Medium Project Stormwater Site Plan Report Section V, Page 1 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ph. [PHONE REDACTED] I f. [PHONE REDACTED] www.cgengineering.com Section V – Stormwater Pollution Prevention Plan Narrative Section III Summary Narrative Erosion Control Plan (11x17) The project proposes the construction of a new King County Puget Sound Emergency Radio Network 170 feet tall tower, its associated equipment, and access driveway. The site is currently undeveloped and is vegetated with trees. There is a water tank site to the north of the proposed lease area. The parcel is within an area that is well developed. Existing stormwater flow direction is to the east towards 172nd Ave NE which contains catch basins. The parcel is 4.79 acres with underlying Alderwood soils. The project will add 3,829 square feet of impervious surfaces. Erosion control details will be provided consistent with the 2005 Department of Ecology Stormwater Management Manual for Western Washington. An erosion control plan sheet is provided in full size as a part of the civil drawing set, and in an attached 11x17 format. As shown on the plan, disturbance will primarily affect the south portion of the site. The 12 Best Management Practices (BMPs) are addressed as follows: Element 1: Mark clearing limits To protect adjacent properties and to reduce the area of soil exposed to construction, the limits of construction will be clearly marked before land-disturbing activities begin. Clearing limits will be to the extents of the lease area and proposed gravel driveway. The BMPs relevant to marking the clearing limits that will be applied for this project include: High Visibility Plastic or Metal Fence (BMP C103) Element 2: Establish construction access A stabilized construction entrance will be established where the proposed gravel driveway will be located with an access to 172nd Ave NE. The specific BMPs related to establishing construction access that will be used on this project include: Stabilized Construction Entrance (BMP C105) Element 3: Control Flow Rates The site is generally flat slope) and is upstream of a road with catch basins. Flow rates of the site are not expected to increase during construction. Element 4: Install sediment controls All stormwater runoff from disturbed areas shall pass through an appropriate sediment removal BMP before leaving the construction site or prior to being discharged. Silt fences will be installed on the sides of the construction area. Pollution prevention facilities on the erosion control plan must be constructed prior to or in conjunction with all clearing and grading so as to ensure that the ATTACHMENT 15 ---PAGE BREAK--- PSERN Education Hill 16015.913 August 8, 2016 Medium Project Stormwater Site Plan Report Section V, Page 2 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ph. [PHONE REDACTED] I f. [PHONE REDACTED] www.cgengineering.com transport of sediment to surface waters and adjacent properties is minimized. The specific BMPs to be used for controlling sediment on this project include: Silt Fence (BMP C233) Element 5: Stabilize Soils Exposed and unworked soils shall be stabilized with the application of effective BMPs to prevent erosion throughout the life of the project. The specific BMPs for soil stabilization that shall be used on this project include: Temporary and Permanent Seeding (BMP C120) Mulching (BMP C121) Nets and Blankets (BMP C122) Dust Control (BMP C140) Element 6: Protect Slopes The site does not contain major slopes—erosion is not anticipated from increased runoff velocities. Element 7: Protect Drain Inlets Storm drain inlet protection will be installed in adjacent catch basins. The specific BMPs that will be used is: Storm Drain Inlet Protection (BMP C220) Element 8: Stabilize Channels and Outlets Conveyance channels are not located on or in the immediate vicinity of the site. Element 9: Control Pollutants Waste materials will be handled and disposed of appropriately. The specific BMPs that will be used include: Concrete Handling (BMP C151) Element 10: Control De-watering De-watering will not be necessary. Element 11: Maintain BMPs All temporary and permanent erosion and sediment control BMPs shall be maintained and repaired as needed to assure continued performance of their intended function. Element 12: Manage the Project Erosion and sediment control BMPs for this project have been designed based on the following principles: Design the project to fit the existing topography, soils, and drainage patterns. Emphasize erosion control rather than sediment control. ATTACHMENT 15 ---PAGE BREAK--- PSERN Education Hill 16015.913 August 8, 2016 Medium Project Stormwater Site Plan Report Section V, Page 3 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ph. [PHONE REDACTED] I f. [PHONE REDACTED] www.cgengineering.com Minimize the extent and duration of the area exposed. Keep runoff velocities low. Retain sediment on site. Thoroughly monitor site and maintain all ESC measures. Schedule major earthwork during the dry season or during dry times in the winter. Maintaining an Updated Construction This shall be retained on-site or within reasonable access to the site. The shall be modified whenever there is a change in the design, construction, operation, or maintenance at the construction site that has, or could have, a significant effect on the discharge of pollutants to waters of the state. The shall be modified if, during inspections or investigations conducted by the owner/operator, or the applicable local or state regulatory authority, it is determined that the is ineffective in eliminating or significantly minimizing pollutants in stormwater discharges from the site. The shall be modified as necessary to include additional or modified BMPs designed to correct problems identified. Revisions to the shall be completed within seven days following the inspection. ATTACHMENT 15