Assessing Rain Garden Placement through Hydrological Modeling in the Puget Sound Region

Green Stormwater Infrastructure (GSI) is built to intercept stormwater runoff and mitigate peak flows and stormwater pollutants before reaching surface waters. A rain garden is a type of GSI comprising a plant-soil system in which water retention and pollutant mitigation are maximized through infiltration and storage. Proper placement of rain gardens within the watershed is crucial to maximizing their effectiveness. The Lower Puyallup River Watershed in South Puget Sound, Washington, United States, consists of the primarily residential areas of Puyallup, Washington, United States, and Tacoma, Washington, United States. Preventing water quality impairment is essential because the streams and rivers in the watershed are critical habitat for salmon that return for spawning. The study's objective was to develop a framework to identify suitable sites for rain gardens in an urbanizing watershed and assess the adequacy of the method through hydrological modeling. An indexing approach to identify Hydrologically Sensitive Areas (HSA) was adopted, in which we considered the topography, runoff contributing area, soil depth, and hydraulic conductivity. The Topographic Wetness Index (TWI) and Soil Water Storage Capacity (SWSC) were computed to obtain the Hydrologic Sensitivity Index (HSI). Hydrological modeling of various conceptual hillslopes was conducted using a physically based Water Erosion Prediction Project (WEPP) model, and simulated runoff was computed under varying slope and soil property combinations. HSI values were calculated based on the simulated hillslope properties, and the relationship between HSA and runoff generation was assessed. Areas considered infeasible per criteria specified by state and county regulations were removed, and the HSI was classified based on suitability for the construction of rain gardens. The moderate HSI range (9.1 – 15.6) was deemed most suitable for rain garden placement in the study area. Most suitable sites were identified, providing a practical, scalable, and transferrable tool for prioritizing rain garden placement for stormwater runoff management at the watershed scale.