Evaluating and Implementing Seattle's Green Stormwater Infrastructure Approaches at a Creek Watershed Scale

Seattle has expanded its green stormwater management strategy from block and subwatershed scale projects, to evaluating and implementing GSI to address watershed-scale stormwater management objectives. This EPA-funded grant project investigated watershed–wide potential stormwater management actions and predicted benefits for Piper's Creek, an urban watershed in north east Seattle. Calibrated hydrologic and hydraulic models were used to evaluate regulatory performance targets and predict the optimum mix of stormwater retrofit strategies for long-term benefits to creek ecology. Regional experts identified creek flow metrics, against which hydrology and hydraulic models estimated the effects of implementing watershed-scale stormwater performance targets (peak and duration goals). The greatest ecological benefits predicted came from matching discharge peak and duration to pre-developed pasture for the flow range from 8% of the 2-year flow to the 2-year peak flow. This information is being used to integrate green and traditional (gray) stormwater control projects, regulatory stormwater codes, and community incentives to determine the most effective stormwater control implementation plan, as well as how to synergistically achieve other City goals such as upgrading pedestrian and bike infrastructure, increasing tree canopy cover, and improving climate change resiliency.