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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 />Inirltration 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 Soil Type. The table below provides the minimum trench length per 1,000 <br />ft2 of contributing impervious area. Infiltration trenches shall be designed in accordance with the <br />SWMMWW. <br />I able 2A-Small Yroiect lntltration I rench Uesi-gri <br />a <br />c <br />a <br />YqC <br />4 .x <br />"Coarse sand" (more than 50% of sand fraction remains on #4 sieve) <br />20 <br />40 <br />"Medium sand" (more than 50% of sand fraction remains on #40 sieve) <br />30 <br />60 <br />Loamy sand <br />75 <br />150 <br />Sandyloam <br />125 <br />250 <br />Loam <br />190 <br />380 <br />Silt loam <br />275 <br />550 <br />Trench length required (ft) = Impervious area (ft2) *((minimum trench length) (ft)) <br />1000 (ft2) <br />■ Net new impervious 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 />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 Project Stormwater Site Plan Report for Projects in 17 <br />the Combined Sewer Area -April 2021 <br />