Water Resource Inventory Area 9 Stormwater Retrofit Project: Estimating cost-effective stormwater infrastructure solutions to meet flow and water quality targets
Stormwater from a disturbed landscape is one of the biggest threats to water quality and ecological health of Puget Sound, both fresh and marine waters. We developed a cost estimate for implementing stormwater Best Management Practices (BMPs) and Low Impact Development techniques in existing and future developed areas within the Water Resource Inventory Area (WRIA) 9 to meet flow and water quality targets. This project couples the Hydrologic Simulation Program-FORTRAN (HSPF) watershed hydrology model with a relatively new stormwater BMP modeling and planning tool developed by the U.S. EPA, SUSTAIN (System for Urban Stormwater Treatment and Analysis INtegration), to assess BMP strategies. The study area covers 278 square miles of the Green/Duwamish watershed and portions of the Central Puget Sound watershed that comprise WRIA 9, excluding the areas upstream of the Howard Hanson Dam and the city of Seattle. 100-acre hypothetical catchments representing the land use land cover of the study area were modeled with SUSTAIN. The BMP solutions and associated 30-year life cycle cost-effectiveness were scaled to the future land use land cover (2040) of the study area taking into account mitigation required with projected new development and redevelopment. Statistical models were used to extrapolate improvements in hydrologic indicators of the study area to improvements in Benthic Index of Biological Integrity (B-IBI) scores. Study area costs assume construction of the modeled BMP units occur over the 30-year period, with annual O&M and I&E costs increasing with installation of additional BMPs
