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ROOF DOWNSPOUT BMPS <br />Roof downspout BMPs include infiltration trenches, dry wells, and partial dispersion systems <br />BMP T5.10A Roof Downspout Full Infiltration <br />Downspout full infiltration systems are trench or drywell designs intended for use in infiltrating runoff from <br />roof downspout drains. <br />Infiltration Trench <br />The minimum trench length is based upon the soil type in the location of the infiltration trench. <br />The soils report will provide the USDA Soil Type. The table below provides the minimum trench <br />length per 1,000 ft2 of contributing roof area. Roof downspout infiltration trenches shall be <br />designed in accordance with the SWMMWW. <br />USDA pe <br />sand" (more than 50% of sand fraction remains on #4 sieve) <br />Minimum <br />trench length <br />(ft) per 1,000 <br />of roof area <br />20 <br />"Medium sand" (more than 50% of sand fraction remains on #40 sieve) <br />30 <br />Loamysand <br />75 <br />Sandyloam <br />125 <br />Loam <br />190 <br />Silt loam <br />275 <br />Fill (only per SWMMWW requirements) <br />60 <br />Trench length required (ft) = Roof area (ft2) *((minimum trench length) (ft)) <br />1000 (ft2) <br />• Roof area requiring mitigation (ft2): <br />• USDA Soil Type per soils report: __ <br />• Trench length required: <br />(Trench length cannot exceed 100 ft. If length exceeds 100 ft, multiple trenches will be required.) <br />• Number of trenches: <br />• Length of each trench: <br />City of Everett Public Works <br />Small Project Stormwater Site Plan Report -Dec 2019 38 <br />