1600 121ST ST SE 2016-01-01 MF Import

Casper Creek Townhouses 1/27/93 Revised 6/07/93 GENERAL. SITE DESCRIPTION 88-4555 CRO IThe subject oroperty, which consists of 11.45 acres, is located on the east side of Silver Lake Road approximately 730 feet west of the Bothell-Everett Highway in the City of Ev�,•ett, Wash�ngton. Asphalt driveways and an existing structure are currently located � on the site. It will be assumed for purpose of analysis that the existing condition of the site is forest. The soil on-site consists of Aldenvood-Urban Land Complex, Custer Fine Sandy Loam, and McKerna Gravelly Silt Loam. It is proposed that the site be developed Iwith t�venty-three multi•family units. EXISTING DRAINAGE The site consists of three drainage sub-basins which area tributary to Silver Lake Creek(see Existing Basin Map). Runoff from Basin "A" is overland to the north to an existing 18 concrete culvert which conveys flow under Silver Lake Road. Runoff from easin ' B" is overland to the east to an existing stream which runs north to south along the east nroperty line. Runoff irom Basin "C" is overland to the south to an existing wetland located in the southeast property corner. A 0.41 acres wetland exists near tlie central portion of the west property line. It is proposed that approximately 0.14 acres of this area be used as wetland mitic�ation area to compensate for several small wetlands located throughout the site which will be filled upon construction. A minimum 35' buNer will be provided around the perimeter of the wetland/wetland mitigation area. No development will occur within this area. A 0.45 acre wetland is located along the south property line. A wetland buNer wil� be provided along the edge of the wetland and no deve'�pment will occur within the bufier/wetland area. PROPOSED DRAINAGE I � It is proposed that developed runoff from impervious areas be conveyed to a detention pond located in the central portion of the site. This pond will be designed to meet � performance requirements for the Department of Fisheries and the City of Everett. The pond will be designed to release 50% of the 2-year/24-hour ailowable reiease rate for storms up to and including the 2-year/24•hour developed design storm and the 10- Iyear/24-hour allowable release for storms up to and including the 25-year/24-hour developed design storm. Detained flow from the proposed pond (storms up to the 2- year/24-hour design storm) will be conveyed easterly to a baffle•type oil/water separator Ibefore being discharged into a biofiltration swale which will convey flo�v to the existing stream located along the north portion of the east property line. The sizing of the pond will be based on the existing conditions of Basin "B", �vhich is the basin currently tributary Ito this point. Impervious areas �vithin Basin "A" and Basin "C" �vill be diverted to the R/D facility located in Basin "B". Since existing runoff from the three sub•basins discharge to Silver Lake Creek a short distance do�vnstream, this is considered to be a Iocaiized Idiversion and not a major basin diversion. Calculations �-�ill sho�v that developed runott from undetained areas of Basin "A" and Basin "C" will be similar with respect to peak rate of runoff and volume of runoff to the existing peak rate of rur,off from these basins. I