Laserfiche WebLink
3.1.1 Downspout Full Infiltration Systems (BMP T5.10A) <br />Downspout full infiltration systems are trench or drywell designs intended <br />only for use in infiltrating runoff from roof downspout drains. They are <br />not designed to directly infiltrate runoff from pollutant -generating <br />impervious surfaces. <br />Application <br />Projects subject to Minimum Requirement #5 (Section 2.5.5, Volume I) <br />must provide for individual downspout full infiltration systems or full <br />dispersion if feasible. Evaluate the feasibility, or applicability, of <br />downspout full infiltration unless full dispersion is proposed. Use the <br />evaluation procedure below to determine the feasibility of downspout full <br />infiltration. <br />Runoff Modeling <br />If roof runoff is infiltrated according to the requirements of this section, <br />for Roof <br />the roof area may be discounted from the project area used for sizing <br />Downspout Full <br />stormwater facilities. <br />Infiltration <br />Procedure for 1. Have one of the following prepare a soils report to determine if soils <br />Evaluating suitable for infiltration are present on the site: <br />Feasibility • A professional soil scientist certified by the Soil Science Society of <br />America (or an equivalent national program) <br />• A locally licensed on -site sewage designer <br />• A suitably trained person working under the supervision of a <br />professional engineer, geologist, hydrogeologist, or engineering <br />geologist registered in the State of Washington. <br />The report shall reference a sufficient number of soils logs to establish <br />the type and limits of soils on the project site. The report should at a <br />minimum identify the limits of any outwash type soils (i.e., those <br />meeting USDA soil texture classes ranging from coarse sand and <br />cobbles to medium sand) versus other soil types and include an <br />inventory of topsoil depth. <br />2. If the lots or site does not have outwash or loam soils, and full <br />dispersion is not feasible, then consider a rain garden or bioretention <br />BMPs (the next lower priority on -site stormwater management <br />system). <br />3. Complete additional site -specific testing on lots or sites containing <br />outwash (coarse sand and cobbles to medium sand) and loam type soils. <br />Individual lot or site tests must consist of at least one soils log at the <br />location of the infiltration system, a minimum of 4 feet in depth from the <br />proposed grade and at least 1 foot below the expected bottom elevation <br />of the infiltration trench or dry well. <br />IVolzime III — Hydrologic Analysis and Flow Control BMPs — December 2014 <br />3-4 <br />