Portland's CSO Program Modeling: Reflecting the City's Priorities for CSO and Stormwater Pollutants Using Real Time Controls

The City of Portland recently passed the midpoint of its 20-year, $1 billion Combined Sewer Overflow (CSO) Program. Designers of the next phase of the CSO system are incorporating ways to improve the Willamette water quality beyond just capturing combined sewer overflows for treatment. Portland's large CSO storage tunnels will be designed to eliminate CSO for the 3-year summer storm event but will be under-utilized for the more frequent, less extreme storms. Therefore, designers are incorporating real-time control structures to allow urban stormwater runoff to enter the CSO tunnels during small storms. Real-time controls will allow the City to capture and treat industrial and commercial stormwater from local separated systems while controlling both CSO and stormwater flows to meet the required CSO control objectives. The strategy for managing the additional risk in capturing both CSO and stormwater flows is based on models that simulate the flows and the real-time controls. The City applied models of the entire combined sewer system in addition to the CSO tunnels with real-time controls added to analyze their effect on the performance of the CSO facilities and maximize the volume of stormwater treated.