Stormwater Drainage Assessment For Single Family Residence Project Name: SEADER SFD Project Address: 2301 NE Perkins Way, Lake Forest Park, King County WA. Parcel Number(s): Tax Parcel No. 86659-00181 Name of Developer/Owner: Darin Seader Prepared By: Richard Deccio P.E. Deccio Engineering inc. 17217 7 th Ave W. Bothell, WA 98012 206-390-8374 Report Date: January 8, 2017 1/8/17
Project Drainage Assessment. This report has been prepared at the request of Mr. Darin Seader in support of a BLA and residential building permit. The subject property is located at 2301 NE Perkins Way, Lake Forest Park, in King County WA. Since the proposed BLA lot is less than 22,000 sf the project and under 5,000 sf of total impervious area, the design will use Appendix C, Section C.1.3 of the King County Surface Water Design Manual as adopted by the City of Lake Forest Park. The proposed BLA lot will be approximately 16,154 sf in size and will contain a residential home. The proposed lot is currently vacant and slopes from south to north with slopes around 5% to 20%. The house will be located on the south-east side of the site where the slopes are less than 15%. Located to the north are wetlands. These wetlands and their associated buffers will be outside of any of the proposed improvements. The Proposed Impervious Areas Consist of: Driveway: approx 12-ft x 45-ft 540 sf Parking: approx 20-ft x 25-ft 500 sf House: 1,200 sf Approximate Total: 2,240 sf of impervious surface. The total proposed clearing for the house, parking, and driveway is 4,500 sf, which complies with the maximum of 50% allowed under the code Because the proposed lot is less than 22,000 square feet, it is subject to the Small Lot BMP Requirements in Appendix C of the Surface Water Design Manual. As mandated by these requirements, all proposed impervious surface (2,240 square feet) is targeted for application of flow control BMPs. As for new pervious surface (totaling 4,500 square feet), it is less than 50% of the site, therefore, no flow control BMPs are required for this surface. To address the requirements for mitigation of target impervious surface, the applicability and feasibility of full dispersion was considered first. After subtracting out the 50% portion of the lot that may be cleared and the wetlands and buffers areas, only about 5,000 square feet of the lot will remain as unsubmerged native vegetated surface. This means that full dispersion could be applicable to up to 750 square feet of the target impervious surface (15% of 5,000 sf). However, because of the lot's topography and the location of proposed clearing, there is no way to achieve the minimum required 100 feet of native vegetated flow path segment that has a slope of 15% or flatter. Therefore, full dispersion is not feasible. 2
Full infiltration of roof runoff was considered next. According to the geotechnical report dated December 1, 2016, prepared by Geotech Consultants, the soil on the project site is identified as a dense glacial till and is not suitable for infiltration. Therefore, full infiltration is not applicable. This allowed for selection of basic dispersion as the flow control BMP of choice for application to the target impervious surfaces of this project. To implement basic dispersion, the roof downspouts of the proposed house will dispersed through the use of splash blocks and a minimum 50-foot vegetated flow path segments located on slopes no steeper than 15% as shown on the flow control BMP site plan and Figure C.2.4.A. The driveway and parking areas will be use porous pavement with a minimum of 6 of drain rock designed per Section C.2.6.1. ln order to prevent erosion and trap sediments within the project site, the following BMPs will be used approximately as shown on the ESC plan: Clearing limits will be marked by fencing or other means on the ground. The driveway will be constructed and graveled immediately. A rocked construction entrance will be placed at the end of the driveway. Cleared areas accepting sheet flow from the driveway and parking area will be seeded and mulched. Runoff will not be allowed to concentrate and no water will be allowed to point discharge onto the slopes. Silt fencing will be placed along the edge of the wetland buffers and slope contours at the downslope limit of clearing. Mulch will be spread over all cleared areas of the site when they are not being worked. Mulch will consist of air-dried straw and chipped site vegetation. And, if required sediment traps can be placed along the front of the site to control wet weather grading conditions. 3
SITE LOCATION 4
PROPOSED BLA LOT 5
FOOTING DRAIN W/ROCK PAD
SILT FENCE