Estimating the Response and Uncertainty Limits of Physical Processes in the South San Francisco Bay for Extreme Water Elevation Frequency Analysis

Coastal flooding in the far south San Francisco Bay (SSFB) can be a function of astronomical tide, residual tide (i.e. water elevation deviation from computed astronomical tide that is associated with many possible physical processes), in-bay wind speed and direction and fluvial discharge. These physical processes and coastal levee failure were considered as input parameters into a Monte Carlo Simulation (MCS) to estimate extreme water elevation frequency in the SSFB. Limited data is available in the SSFB to estimate the contribution of these physical processes to extreme water elevation statistics. Over 100 years of measured water surface elevation (WSE) data is available at the San Francisco (SF) tide station. A sensitivity analysis of storm event sampling criteria was conducted to select significant events at the SF tide station for data transfer to the project site and statistical analysis. The coincidently sampled astronomical and residual tides at the San Francisco tide station were analyzed and used to develop the storm event databases. Sampling methods employed were compared with annual maximum and partial duration approaches. Additional statistical testing was performed to justify the assumption of coincident sampling. The selected database was found to be most representative of the full range of the combinations of astronomical and residual tides that contribute to extreme water elevation statistics at the project site. A look-up table of astronomical and residual tide, wind speed and direction, and levee failure in the form of WSE responses at the project site from the hydrodynamic simulations was established for the interpolation in the MCS. The hydrodynamic model simulations indicated that the astronomical tides in the SSFB amplify inversely as a function of tidal range at the SF tide station. The residual tide varies minimally as it propagates into the SSFB. In-Bay wind set-up from a significant event was found to contribute on the order of one foot to the total WSE in the SSFB; however, wind events with strong magnitudes along the primary axis of the bay occur infrequently making an insignificant contribution to the overall flood statistics. The fluvial discharges of Guadalupe River and Coyote Creek were considered in the hydrodynamic simulations as they are located within the study area. The San Francisco tide station and fluvial flow stream gauges were used to correlate the peak fluvial discharge with coincident residual tide. Statistical dependence between residual tide and peak fluvial flow was only found at the Guadalupe River. The probability distribution functions of astronomical tide, residual tide, local wind speed and direction, and levee failure were developed as input into the MCS. It was concluded that using hydrodynamic model simulations to estimate the response of the physical processes and levee failure condition at the project site in conjunction with the application of MCS is a reasonable way to estimate extreme water elevation statistics within acceptable uncertainty limits.