915 N BROADWAY 2024-07-22 - Laserfiche WebLink

Option 2A Infiltration Trench Design Worksheet BMP T5.10A Roof Downspout Full Infiltration Downspout full infiltration systems are trench or drywell designs intended for use in infiltrating runoff from roof downspout drains. They are not designed to directly infiltrate runoff from pollutant generating impervious surfaces. Projects that infiltrate runoff from pollutant generating surfaces must provide treatment prior to infiltration. Infiltration Trench The minimum trench length is based upon the soil type in the location of the infiltration trench. The soils report wi I I provide the USDA Soil Type.The table below provides the minimum trench length per 1,000 ft2 of contributing impervious area. Infiltration trenches shall be designed in accordance with the SWMMWW. Table 2A-Small Project Infiltration Trench Design "Coarse sand"(more than 50%of sand fraction remains on#4 sieve) 20 40 "Medium sand"(more than 50%of sand fraction remains on#40 sieve) 30 60 Loamy sand 75 150 Sandyloam 125 250 Loam 190 380 Silt loam 275 550 Trench length required(ft) = Impervious area (ft2) *((minimum trench length)(ft)) 1000 (ft2) ■ Net new impervious area requiring mitigation (ft2): ■ USDA Soil Type per soils report: ■ Trench length required: (Trench length cannot exceed 100 ft. If length exceeds 100 ft, multiple trenches will be required.) ■ Number of trenches: ■ Length of each trench: The following setback and design requirements have been met: I. The maximum length of the trench shall not exceed 100 feet from the inlet sump. 2. The minimum spacing between trench centerlines shall be 6 feet. 3. Infiltration trenches should not be built on slopes steeper than 25% (4:1). A geotechnical analysis and report may be required on slopes over 15%, or if the proposed trench is located within 200 feet of the top of a slope steeper than 40%, or in a landslide hazard area. City of Everett Public Works Small Project Stormwater Site Plan Report for Projects in 13 the Combined Sewer Area-September 2022