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7. ON-SITE STORMWATER MANAGEMENT <br /> As documented in Section 1 of this report this project is required to meet Minimum <br /> Requirements 1-5. As a result, this project is required to use the On-Site Stormwater <br /> Management BMP's from Chapter 12 of Volume III in the 2010 COE SWMM. <br /> Per the COE SWMM the BMP's must be evaluated in the order they are listed. <br /> BMP's for roofs: <br /> 1. BMP 12.30 On-site infiltration areas: <br /> As previously mentioned in this report, the USDA Websoil Survey shows the soils as <br /> being Alderwood-Urban Land complex. The water table depth ranges between 18 and <br /> 36 inches below grade. However, soil logs provided by Brent Hackney Designs, Inc. <br /> show the soils were found to be loamy fine sands underlain with medium sands to a <br /> depth of approximately 48 inches below grade, which more closely matches the <br /> properties and qualities of Everett soil type. Therefore, infiltration is feasible for this <br /> project and the selected BMP. <br /> Per the Sizing Criteria in Section 12.3.3 the infiltration area for Everett soils is 150 <br /> square feet of bottom area per 1,000 square feet of roof area. This project proposes to <br /> construct a total of approximately 2,176 sf of roof surfaces. Therefore, 326.4 square <br /> feet of trench is required. The trench designed shall be 30 feet long by 11 feet wide <br /> and will be located west of the proposed SFR. <br /> 2. Downspout Dispersion Systems: <br /> Higher priority BMP selected. <br /> 3. Perforated Stub-out Connections <br /> Higher priority BMP selected. <br /> BMP's for other hard surfaces: <br /> 1. Sheet Flow Dispersion: <br /> A total of approximately 1,004 sf of concrete PGIS is proposed for this project. The <br /> runoff generated by the concrete driveway surfaces will be dispersed on-site via a <br /> transition zone. As per BMP 12.70, a minimum 10 ft flow path over amended or <br /> existing vegetated soils, meeting BMP 12.20 will be provided for the PGIS runoff <br /> before the dispersed sheet flows leave the site, are collected by any critical area or <br /> pass over any additional hard surfaces. <br /> Finally, any area of the site not proposed to be covered with impervious surfaces but <br /> graded or compacted will be treated to meet the criteria for BMP 12.20. Please refer to <br /> the engineering plans for basic soil amendment notes. <br /> Kromer SFR 18-0303 <br /> April 2018 Page 9 <br />