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Option 2B Rain Garden Design Worksheet The top of the ponded surface area below the overflow shall be at least 13.5%of the net new impervious surface area draining to the rain garden. Projects that have a greater contributing impervious area to the BMP shall use the larger of the two areas, or as an alternative may have the BMP sized by an Engineer using a continuous runoff model to meet the flow control requirements of Section 6.10 of the DCSS. The table below provides design geometry based upon contributing impervious area.Refer to the Rain Garden HandbookforWestern Washington:A Guide for Design,Installation,and Maintenance (Hinman et.al. 2013) for construction guidance. Bioretention shall be designed in accordance with section 6.10 of the DCSS and the SWMMWW. Use the following criteria for design of the bioretention pond. The applicable Setback requirements in Section 4-4.1 (3) have been met, including: • 50 feet setback from slopes greater than 15 percent and greater than 10 feet in height. Side Slope = 3:1 Minimum Infiltration Rate = 0.3 in/hr Minimum depth of Biosoil = 12-inches Ponding depth = 6-inches Freeboard = 6-inches Table 213— Small Project Rain Garden Flow Control Design Sizing Contributiii7l Minimum . . • deep) RIM Impervious (6" Area Ponding Area Ponding 200 to 400 54 19 400 to 600 81 36 600 to 800 108 55 800 to 1000 135 74 1000 to 1200 162 95 1200 to 1400 189 116 1400 to 1600 216 137 1600 to 1800 243 158 1800 to 2000 270 180 ■ Net new impervious area requiring mitigation (ft2): ■ Contributing impervious area to BMP: ■ Total contributing area including landscaped area(ft2): Design Top of Ponded Surface Area (ft2) = 0.135*(contributing impervious area (ft2)) = ft2 City of Everett Public Works Small Project Stormwater Site Plan Report for Projects in 19 the Combined Sewer Area-May2021