LID Analysis Considerations in Western Washington

LID (Low Impact Development) practices, such as, green roofs, bioretention swales, rainwater cisterns, rain gardens, compost amended soil, and permeable pavement can be used to reduce stormwater runoff and return urban creeks to more natural conditions. Continuous simulation computer modeling of LIDs has been achieved by adding the ability to represent these practices in the Western Washington Hydrology Model version 3 (WWHM3), developed for the Washington State Department of Ecology (WSDOE) by Clear Creek Solutions, Inc. WWHM3 uses HSPF continuous simulation hydrology to model LIDs. This paper and presentation focuses on analysis considerations when modeling LID permeable pavement, impervious runoff dispersion, green roof, rain gardens, and bioretention swale practices in Western Washington. Issues related to the Western Washington seasonal rainfall patterns and evapotranspiration, native soil infiltration rates, and the use of underdrains are presented. WWHM3 stormwater modeling results demonstrate the feasibility and limitations of using LIDs to meet Washington state NPDES permit requirements for new land use development.