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Contributing <br />Impervious.- <br />Area <br />200 to 400 <br />Minimum .. of <br />Poncling Area <br />54 <br />Minimum <br />Bottom <br />Poncling <br />19 <br />400 to 600 <br />81 <br />36 <br />600to 800 <br />108 <br />55 <br />800 to 1000 <br />135 <br />74 <br />1000 to 1200 <br />162 <br />95 <br />1200 to 1400 <br />189 <br />116 <br />1400 to 1600 <br />216 <br />137 <br />1fi00 to 1800 <br />243 <br />158 <br />1800to 2000 <br />270 <br />180 <br />Option 2B Rain Garden Design Worksheet <br />The top of the ponded surface area below the overflow shall beat least 13.5% of the net new impervious <br />surface area draining to the rain garden. Projects that have a greater contributing impervious area to <br />the BMP shall use the larger of the two areas, or as an alternative may have the BMP sized by an <br />Engineer using a continuous runoff model to meet the flow control requirements of Section 6.10 of the <br />DCSS. The table below provides design geometry based upon contributing impervious area. Refer to the <br />Rain Garden Handbookfor Western Washington: A Guide for Design, Installation, and Maintenance <br />(Hinman et.al. 2013) for construction guidance. Bioretention shall be designed in accordance with <br />section 6.10 of the DCSS and the SWMMWW. Use the following criteria for design of the <br />bioretention pond. <br />SEE BIORETENTION EXAMPLE AT THE BACK OF THIS PACKET FOR REFERENCE <br />The applicable Setback requirements in Section 4-4.I (3) have been met, including: <br />• 50 feet setback from slopes greater than 15 percent and greater than 10 feet in height. <br />Minimum Side Slope = 3:1 Minimum Infiltration Rate = 0.3 in/hr <br />Minimum depth of Biosoil = 12-inches Ponding depth = 6-inches <br />Freeboard = 6-inches <br />Table 2B — Small Project Rain Garden Flow Control Design Sizing <br />• Net new impervious area requiring mitigation (ft2): <br />• Contributing impervious area to BMP: <br />Total contributing area including landscaped area (ft2): <br />Design Top of Ponded Surface Area (ft2) = 0.135*(contributing impervious area (ft2)) <br />— ft2 <br />City of Everett Public Works <br />Small Project Stormwater Site Plan Report for Projects in 15 <br />the Combined Sewer Area -September 2022 <br />