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Infiltration Dry Wells The minimum rock volume and dry well rock layer height are based upon the soil type in the location of the drywell. The soils report will provide the USDA Soil Type. The table below provides the volume of washed rock and height of washed rock required. Roof downspout dry wells shall be designed in accordance with the SWMMWW. V Volume of IF Drywell rock layer EDA:So:i1Type washed rock (ft3) height at 48in per 1000 ft2 of diameter per 1000 ft2 impervious of impervious "Coarse sand"(more than 50%of 60 4'9" sand fraction remains on#4 sieve) "Medium sand"(more than 50%of 90 77' sand fraction remains on#40 sieve) ■ Roof area requiring mitigation (ft2): ■ USDA Soil Type per soils report: ---- ___ ■ Rock layer height required:____________ ■ Volume of washed rock required: Volume of Washed Rock Required =Tributary Impervious Area (ft2)*Standard Volume 1000 (ft2) Note:Storm water facilities that infiltrate storm water by means of an underground facility such as an infiltration trench or dry well may need to be register as an underground injection well(UIC). For more information, visit everettwa.gov/1985/Stormwater-Technical-Resources BMP T5.10B Roof Downspout Dispersion Systems Downspout dispersion systems are splash blocks or trenches,which serve to spread roof runoff over vegetated pervious areas. Downspout Dispersion Trench Roof downspout dispersion trenches shall be designed in accordance with the SWMMWW. Trenches serving up to 700 ft2 roof area may use a Simple Dispersion Trench (2 ft wide X 10 ft length). Multiple simple dispersion trenches maybe used as long as none exceeds 700 ft2 of tributary area and the flow paths from the trenches do not overlap. ■ Roof area requiring mitigation: ■ Number of simple dispersion trenches proposed: ___ ❑ See attached sketch which identifies the roof area, including square footage draining to each dispersion trench. City of Everett Public Works Small Project Stormwater Site Plan Report-Dec 2019 39