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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 /> Infiltration Trench <br /> The minimum trench length is based upon the soil type in the location of the infiltration trench. The soils <br /> report will provide the USDA Soil Type.The table below provides the minimum trench length per I,000 <br /> ft2 of contributing impervious area. Infiltration trenches shall be designed in accordance with the <br /> SWMMWW. <br /> Table 2A-Small Proeect Infiltration Trench Desi:n <br /> USDA Soil T i,.'' <br /> ype � Minimum Minimum <br /> rench lengt ' bottom area of <br /> (ft)per 1,000 ft2,, trench squar <br /> •f imperviou ft per 1,000 ft <br /> area of imperviou <br /> "Coarse sand"(more than 50%of sand fraction remains on#4 sieve) 20 40 <br /> "Medium sand"(more than 50%of sand fraction remains on#40 sieve) 30 60 <br /> Loamy sand 75 150 <br /> Sandy loam 125 250 <br /> Loam 190 380 <br /> Silt loam 275 550 <br /> Trench length required(ft) = Impervious area (ft2)*((minimum trench length)(ft)) <br /> ,04 it\A. , 1000 (ft2 <br /> • Net ew imperviousiarea requiring mitigation (ft2): <br /> • USDA Soil Type per soils report: Owinsc)( ;) -1)■ 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 /> 1. 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:I). 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 />