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Firefox <br />about:blank <br />Option 2A Infiltration Trench Design Worksheet <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. They are not designed to directly infiltrate runoff from pollutant generating <br />impervious surfaces. Projects that infiltrate runoff from pollutant generating surfaces must provide <br />treatment prior to infiltration. <br />Infiltration Trench <br />The minimum trench length is based upon the soil type in the location of the infiltration trench. The soils <br />reportwill provide the USDA Soi I Type. The table below provides them i nim urn, trench length per 1,000 <br />ft2 of contributing impervious area. Infiltration trenches shall be designed in accordance with the <br />SWMMWW. <br />Table 2A-Small Project Infiltration Trench Desiin <br />FFn F <br />"Coarse sand" (more than 50% of sand fraction remains on #4 sieve) 20 <br />40 <br />"Medium sand" (more than 50% of sand traction remains on 940 sieve) 30 <br />60 <br />Loamy sand <br />75 <br />150 <br />Sandy loam <br />125 <br />250 <br />Loam <br />190 <br />380 <br />Sift loam <br />275 <br />550 <br />Trench length required (ft) = Impervious area (ft2) "((minimum trench length) (ft)) <br />1000 (ft2) <br />M, :.Net new impervious area requiring mitigation (ft2): 404_ <br />• USDA Soil Type per soils report: Aideratoq"urbanland complex <br />■ Trench length required.,! <br />(Trench length cannot exceed 100 ft. 1f length exceeds 100 ft, multiple trenches will be required.) <br />• Number of trenches: 1 <br />• Length of each trench: 10 ft'< <br />The following setback and design requirements have been met: <br />I. The maximum length of the trench shall not exceed 100 feet from the inlet sump. <br />2, The minimum spacing between trench centerlines shall be 6 feet _ <br />3. Infiltration trenches should not be built on slopes steeper than 25% (4:1). A geotechnical analysis and <br />report may be required on slopes over 15%, or if the proposed trench is located within 200 feet of the <br />top of a slope steeper than 40%, or in a landslide hazard area. <br />City of Everett Public Works <br />Small Prryecl Stormweter Site Plan Report for Projecls in 17 <br />the Combined Sewer Arca-April 2021 <br />i 16 of 23 _1 <br />6/8/2021, 10:18 AM <br />