Optimizing Green Infrastructure Techniques to Reduce CSO Volume in Seattle

The City of Seattle boasts many successful green stormwater infrastructure (GSI) projects and policies, many of which started out as pilot programs and grew to have a much broader application and impact (United States Environmental Protection Agency [USEPA], 2010). These programs have been extended as a cost-effective tool to reduce CSO volumes, been implemented in on-the-ground pilot projects, and analyzed city-wide using basin GSI effectiveness modeling. Using refinements and lessons learned from the GSI pilot projects, GSI is being optimized to be an integral part of CSO volume reduction in the City of Seattle.The City of Seattle aims to incorporate GSI into its CSO reduction program wherever it is feasible and cost-effective. GSI consists of practices such as rain gardens, cisterns, disconnected downspouts, and permeable pavements—all designed to intercept runoff before it reaches the combined sewer system (CSS). The GSI approach can be cost-effective while also providing enhanced community and ecological benefits over more traditional solutions. The City of Seattle has completed its conceptual feasibility analysis and evaluation of GSI and found that GSI has the potential to eliminate CSOs in a few basins, and can provide a sustainable complement to, and reduction in the size of, conventional grey infrastructure controls (e.g., storage tanks) in many of the basins.Under the long-term control plan development, each of the uncontrolled CSO basins in SPU's system was modeled and calibrated using USEPA's Stormwater Management Model (SWMM) Version 5. This modeling was used to develop integrated strategies incorporating GSI, system retrofits, and conventional infrastructure to control CSOs.Through this modeling, it has been determined that GSI techniques have a varying level of effectiveness at reducing CSOs depending on the nature of CSO events in the basin (i.e. duration and frequency) and physical characteristics within the basin (e.g. infiltration capacity and basin size). Findings and refinements to the selection of the suite of GSI practices and the sizing and design of the individual practices based on basin-specific conditions are provided in this paper.