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111 <br /> V. Analysis and Design of Required Runoff Control (Treatment and Stream Bank <br /> Erosion Control) BMPs <br /> Figure 5 is a reduced copy of the proposed Drainage Plan. That plan provides design criteria <br /> to satisfy required Runoff Control BMPs for the entire 18.17-acres to be developed. For the <br /> subject site, parking lot and rooftop stormwater will be collected in catch basins and directed to <br /> a wetpond (BMP RD.05) in the northwest corner of the site. Prior to reaching the pond, grit and <br /> sediment will be removed from stormwater as it passes through a 72" catch basin with 4 feet of <br /> catch. Based on land use, oil/water separation will not be required on the site. <br /> Stream Bank Erosion Control <br /> Drainage from the developed portion of the site will be directed to a Retention/Detention pond <br /> and be released at a controlled rate into Powder Mill Gulch or the existing 18" concrete stub <br /> near the northwest corner of the site. Of the 18.17 acres to be developed, 13.46 acres (74%) <br /> drains to Powder Mill Gulch and 4.71 acres (26%) drains to the adjacent plat. Therefore, there <br /> will be two control structures releasing runoff from the pond. One control structure will release <br /> to Powder Mill Gulch at 74% of the allowable release rate. The second control structure will <br /> release at 26% of the allowable release rate to the existing 18" stub in the northwest corner of <br /> the site. <br /> ' Drainage calculations were performed to meet the following requirements: <br /> 1. The peak developed discharge rate for the 2-year, 24-hour storm shall not exceed <br /> 50% of the peak runoff rate for the same storm and existing site conditions, with a <br /> correction factor applied; and <br /> 2. The peak developed discharge rates for the 10-year and 100-year, 24-hour storms <br /> shall not exceed the peak runoff rate for the same storm and existing site conditions, <br /> with a correction factor applied. <br /> The computer program Waterworks HMS release 6.1 was used to perform the calculations. <br /> The input variables and output results are located in Appendix A and are based on the <br /> following criteria: <br /> DRAINAGE CALCULATIONS: <br /> Methodology: S.C.S. /Santa Barbara Urban Hydrograph: <br /> Criteria: Washington State DOE 1992 <br /> Type-IA Rainfall Distribution <br /> Soil Type: Alderwood <br /> Hydrologic Soil Group: C <br /> Pond and Control Structure Design <br /> A rectangular shaped pond is proposed to provide retention. The base of the proposed pond is <br /> 1 480' long and varies between 28' and 47' in width. There is 23,362 sf of surface area at the <br /> bottom of storage. The east end of the pond has 7:1 side slope and will be used as an access <br /> road to the bottom of the pond. The other three sides of the pond have 3:1 side slopes. The <br /> 3 <br /> I <br />