Flood mitigation under sea level rise

Extreme water levels in San Francisco Bay result from tides, storm surge and river flow. This study used the San Francisco Bay-Delta Community Model, run with Delft3D Flexible Mesh on a 100 m grid, to test three adaptation measures: hardening the shoreline at the edge of urban areas, restoring wetlands at 740 locations, and closing subregional floodgates. Scenarios ran at current sea level and at up to 1.5 m of sea level rise. The model reproduced observed water levels with an average error just under 12 cm. Wind waves and sediment transport were not included.

 

Under present conditions, tidal amplification produces a wide spread of extreme water levels across the Bay. Amplification decreases as sea level rises, so the 100-year water level increases by about 20 cm less than the sea level rise itself. With 1.5 m of sea level rise, wetland restoration lowered extreme water levels by as much as 20 cm in some areas, while added hard shoreline raised them by as much as 10 cm.

 

Floodgates at San Leandro Bay, the Napa River and the Dumbarton Bridge reduced high water levels under sea level rise of up to 1.0 m, without major adverse effects elsewhere in the Bay. The authors note that local adaptation may address flood risk up to about 1 m of sea level rise. Between 1.0 and 1.5 m, low-lying areas flood more often and regional measures are increasingly needed. The Alameda County Flood Control and Water Conservation District funded the work.

 

Downloads

• Paper: Nederhoff, K., Saleh, R., Barnard, P. L., and Stacey, M. T. (2025). Mitigating flood risks in urban estuaries: Tidal dynamics, shoreline hardening, nature-based solutions, and floodgates in San Francisco Bay. Journal of Waterway, Port, Coastal, and Ocean Engineering, 151(6), 04025031.

• Data: 2025 extreme still water levels for four Bay configurations, at current sea level and with 1.5 m (4.92 ft) of sea level rise.