Moving Green Stormwater Infrastructure into Seattle's CSO Control Program

Seattle is surrounded by Puget Sound, Lake Washington, and Lake Union all which receive wet-weather combined sewer overflows (CSOs). Seattle Public Utilities (SPU) has been working with King County for 40 years on a phased CSO reduction plan. To date, our efforts have resulted in a 93% reduction of CSO volume per year. However, we still have 90 SPU permitted CSO outfalls with 430 million gallons discharged annually. We are currently working to accomplish the goal of no more than one overflow per year for each uncontrolled CSO outfall. In the past, we have exclusively relied on intensive traditional approaches to manage these events, however for our CSO Plan update, we are incorporating green stormwater infrastructure (GSI) strategies as part of the toolbox to meet our goals. These efforts include: (1) comparing cost effectiveness of GSI alongside traditional strategies for our highest priority contributor basin; (2) rolling out a community-based incentive program for homeowners to install rain gardens and cisterns in CSO basins; (3) basin monitoring of parcel, right-of-way, and alley GSI projects to reduce sizing of traditional storage facilities; and (4) coordinating with King County to implement a pilot basin to maximize GSI for CSO control.