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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 Soil Type Minimum <br /> trench lengt <br /> (ft) per 1,000 ft2 <br /> of roof are- <br /> "Coarse sand"(more than 50%of sand fraction remains on#4 sieve) 20 <br /> "Medium sand"(more than 50%of sand fraction remains on#40 sieve) 30 <br /> Loamy sand 75 <br /> Sandy loam 125 <br /> Loam 190 <br /> Silt loam 275 <br /> Fill(only per SWMMWW requirements) 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 />