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USGS2025

Mitigating flood risks in urban estuaries: Tidal dynamics, shoreline hardening, nature-based solutions, and floodgates in San Francisco Bay

Hydrodynamic models are valuable tools for understanding the primary factors influencing daily and peak water levels and for guiding discussions on potential adaptation strategies for managing flood risk in coastal areas. This analysis uses the Delft3D San Francisco Bay-Delta Community Model to simulate water levels and incorporates the effects of a number of adaptation measures in the urban San Francisco Bay estuary, California. In particular, we examine the influence of shoreline hardening, nature-based solutions, and subregional floodgates on regional water levels. The result shows that under present conditions, tidal amplification is responsible for generating a wide distribution of extreme water levels across San Francisco Bay. Tidal amplification is found to decrease under sea level rise, thereby producing a relative damping effect on extremes. A comparison of different shoreline scenarios demonstrates that hard frontal shorelines result in higher tidal amplification, whereas restored (soft) shorelines lower amplification. The current shoreline configuration has both hard and soft characteristics and results in an intermediate tidal response. In some areas, wetland restoration reduces extreme water levels by as much as 20 cm, whereas hard-shoreline addition elevates them by as much as 10 cm for 1.5 m of sea level rise. Furthermore, local floodgates can significantly reduce high water levels without major adverse effects elsewhere in San Francisco Bay. These findings point toward the justification for a range of adaptive measures across political boundaries, weighing hard and soft options in addressing the mounting danger of sea level rise.

California↗

USGS2021

Drivers of extreme water levels in a large, urban, high-energy coastal estuary – A case study of the San Francisco Bay

Reliable and long-term hindcast data of water levels are essential in quantifying return period and values of extreme water levels. In order to inform design decisions on a local flood control district level, process-based numerical modeling has proven an essential tool to provide the needed temporal and spatial coverage for different extreme value analysis methods. To determine the importance of different physical processes to the extreme water levels we developed a process-based numerical model (Delft3D Flexible Mesh) and applied it to simulate a large, urban, high-energy coastal estuary (the San Francisco Bay). The unstructured grid with 1D/2DH model elements, allows for efficient model simulations and therefore it was possible to simulate over 70 years between 1950 and 2019. Results show significant skill in reproducing observations for the entire modeled time period with an average root-mean-square error of 8.0 cm. A process-based modeling approach allows for the explicit in- and exclusion of different physical processes to quantify their importance to the extremes. For the 100-year still water level (SWL), tide (70%) and non-tidal residual (NTR) (25%) explain the majority of the simulated high water levels in the Bay relative to Mean Higher High Water (MHHW). However, closer to the Delta, local fluvial inflow increases in importance. For longer return periods, the importance of tide decreases and the importance of remote NTRs and fluvial inflow increases.

California↗