07/25/2007 Env Bd Packet - Laserfiche WebLink

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• • IV. RULE C: STORMWATER MANAGEMENT Proposed changes to the stormwater rule reflect advances in the science of and approach to stormwater management generally and the technical recommendations of the District Engineer in particular. The focus has shifted in part away from managing the impacts of development at the "end of the pipe" to rethinking project design in order to decrease the impacts that need to be managed. The emphasis on ponding and regional treatment has given way to a better understanding of infiltration systems and their ability to manage stormwater onsite. Under the proposed rule, stormwater Best Management Practices would be evaluated with more sensitivity to site - specific conditions, and specific terms for runoff volume and water quality management on public roadway work and similarly constrained linear public projects would be added. A. Introduction Under pre - settlement conditions, a portion of any precipitation event infiltrates the soil column. Water filtering into the soil recharges groundwater and feeds groundwater - dependent natural resources. As the soil becomes saturated, excess water flows over the land as runoff and collects in rivers, streams, lakes, wetlands, and other low areas. The type of soil and its vegetative cover can alter the volume of infiltration. Bare clay, for instance, typically has a higher volume of runoff than sandy soil covered with dense grass. Under developed conditions, less rainfall is able to filter into the soil column. Roadways, roofs, sidewalks, compacted lawns and parking lots all reduce the amount of water that can infiltrate into the soil. Changing the course that water travels following a precipitation event amounts to an alteration of the hydrologic cycle. Under pre - settlement conditions, only a small percentage of total rainfall enters streams and other water bodies as surface runoff. Most rainfall evaporates, is taken up by plants and released through transpiration, or infiltrates. Infiltrated water either discharges to streams or travels deeper into soils to replenish groundwater aquifers. Development activities lead to the removal of vegetation and the creation of impervious surfaces, resulting in less water moving though the system as interflow or groundwater, and reduced evaporation /transpiration. The percent of rainfall entering streams and other water bodies as surface runoff increases greatly as imperviousness within the watershed increases. Increased volumes of surface runoff can degrade the stability and function of rivers and streams, increase pollutant Toads, and lead to flooding, especially in landlocked basins.