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I <br /> Option 2A Infilt ation Trench Design Worksheet IVA- <br /> BMP T5.10A Roof Do nspout Full Infiltration <br /> Downspoutfull 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.Thesoils <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 /> Table 2A-Small Project Infiltration Trench Design <br /> ® A • <br /> i - <br /> • - 111 • <br /> • • -i �4� e • - C 81 <br /> V <br /> ,I <br /> e • - e <br /> "Coarse sand"(more;thanJM of sand fraction remainsion#4 sieve) 20,' 40 <br /> "Medium sand"(more than 50%of sand fractionremamson#40 sieve); 30 60- <br /> Loamy sand 75 150 <br /> Sandy loam: f 125 250, <br /> Loam 1901., 380 <br /> Silt loam . . 275' 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 le i gth 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 Work <br /> Small Project Stormwater Site Plan Report for Projects in 13 <br /> the Combined Sewer Area-September 2022 <br />