Alameda County flood assessment

The San Francisco Bay-Delta Community Model was set up, calibrated and updated in three phases for the Alameda County Flood Control and Water Conservation District, to compute extreme still water levels along the Bay shoreline and in the Delta.

Partners: Alameda County Flood Control and Water Conservation District, Deltares USA, Aquaflows, U.S. Geological Survey Coastal and Marine Geology Program

All case studies

Three phases of Bay-Delta modeling for the Alameda County Flood Control and Water Conservation District

Phase 1 (2023) set up, calibrated and validated a Delft3D FM and SWAN model of the Bay and Delta to compute still water levels. A 70-year hindcast (water years 1951 to 2019) fed a peaks-over-threshold analysis of extremes. Water levels in the Bay were reproduced with errors below 5 inches (average 3.1 inches), and the 100-year still water level ranged from about 9 to 10.7 ft NAVD88. A Bay-wide seawall raised extremes by up to 6 inches, and waves added less than 10% to extremes relative to mean higher high water at 75% of the points analyzed.

Phase 2 (January 2024) applied the model to specific District questions. Synthetic years built from the 70-year record narrowed the 95% confidence interval of the 100-year level from 35 cm to 7 cm and lowered the 100-year value by 10 cm. Doubling or removing wind drag changed water levels by 5 to 15 cm. The phase also compared 100-year levels with Delta levee crests, tested a local Eden Landing grid at four resolutions, and compared one-dimensional channel results with HEC-RAS.

Phase 3 (August 2025) updated the Community Model to a fully two-dimensional grid with 200, 100 and 50 m cells, forced for water years 1941 to 2024. Still water level errors are about 10 cm in the downstream and central Delta. Extending the record to water years 1941 to 2024 lowered median 100-year levels by about 5.5 cm and moved the 1983 event from roughly a 70-year to an 85-year return period.

Downloads

Phase 1 report: Coastal and riverine flood assessment for the Alameda County Flood Control District, version 10, September 20, 2023.
Phase 2 memo: Additional analyses with the Bay-Delta model for the District, version 1.0, January 5, 2024.
Phase 3 memo: Update of the San Francisco Bay-Delta Community Model, August 28, 2025.