Theoretical Investigations to Aid in the Prediction of Combined Sewer Overflows in Coastal Urban Watersheds: Case Study of San Francisco

Abstract Combined sewer systems (CSS) function in about 700 U.S. municipalities, with over 100 near vulnerable coastal waters. Predicting changes due to rainfall, sea level rise (SLR), and groundwater rise is vital for adaptation. This study compares a hydrologic‐hydraulic model (PCSWMM) and a hybrid mass balance method for estimating combined sewer overflow (CSO) volumes. The mass‐balance approach is used as a screening tool, not a replacement for the calibrated hydraulic model. Both are applied to San Francisco's Eastside CSS, the only coastal system in California, in a replicable way. Managing wastewater and stormwater through 29 outfalls into San Francisco Bay, the model's validation shows good agreement (Nash‐Sutcliffe 0.65–0.75; square error <3.0). While individual outfalls may vary, the overall system estimates are reliable. Monthly CSO predictions range from within 1% to four times observed values, with better accuracy over multi‐year periods, suitable for long‐term planning. Future scenarios for 2030–2035 and 2090–2095 consider base rainfall, SLR, and groundwater. Saltwater inflows due to SLR could increase CSOs by up to 9.8%. Groundwater inflows raised CSOs by 22%–32% and main plant flows by up to 51%. Intensified rainfall could produce about 2.5 times the baseline CSO volume, making it a major driver, with groundwater issues worsening impacts. This dual‐method framework offers a transferable tool for assessing CSS resilience under climate stress.