Impacts to San Francisco’s Combined Sewer System from Climate Change Induced Stressors

Abstract Submission: San Francisco is the only coastal city in California, and one of the few on the West Coast of the United States of America, that uses a combined sewer system managing both wastewater and stormwater in a single conveyance system. Climate change-induced sea-level rise and subsequent groundwater table rise pose significant threats to this system, resulting in increased intrusion of fresh and saltwater. The system can be overwhelmed during heavy rainfall, leading to untreated discharges from 43 outfall points into San Francisco Bay. This research develops a comprehensive numerical model using PCSWMM to simulate San Francisco's storm and sewer infrastructure, including transport/storage tunnels and pump stations, assessing the impacts of sea-level rise, groundwater table rise, and intensified rainfall on drainage performance. Using geographic information systems data and climate change projections, this study estimates increased flow volumes, the frequency of combined sewer overflows under ten sea-level rise scenarios, and the associated groundwater table rise. This study builds on past modeling efforts focused on backflow prevention and inland flooding by adding groundwater intrusion, accounting for cracks in critical sewer infrastructure. Past studies, utilizing field investigations, identified vulnerable infrastructure, which are used as inflow points for groundwater intrusion for our model. Results pinpoint areas most susceptible to sewer backups and untreated discharges, providing critical insights for enhancing urban drainage resilience against future climate change impacts.

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