Low Impact Development Computations—WWHM

LIDs (Low Impact Developments) such as green roofs, bioretention, grass bioswales, rain gardens, impervious area disconnections, 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 World Wide Hydrology Model version 4 (WWHM4), developed by Clear Creek Solutions, Inc. WWHM4 uses USEPA HSPF continuous simulation hydrology to model LIDs. This paper focuses on analysis considerations when modeling LID green roofs, bioretention, grass bioswales, rain gardens, impervious area disconnections, and permeable pavement practices in Philadelphia, Pennsylvania, and Seattle, Washington. Issues related to seasonal rainfall patterns and evapotranspiration, native soil infiltration rates, and the use of underdrains are presented. WWHM4 stormwater modeling results demonstrate the feasibility and limitations of using LIDs to reduce stormwater runoff.