San Francisco Bight Coastal Processes Study

Numerical Modeling

Numerical modeling is a powerful tool for simulating the physical processes at work in a coastal region, without the expense of intensive field deployments and the limited spatial resolution of most field instruments. Numerical models are periodically verified by field measurements to ensure that each model is producing accurate results.

Process-based models are being designed to predict the hydrodynamics, sediment transport, and morphological evolution of the mouth of San Francisco Bay. The models will then be applied to investigate the basic sediment transport pathways and evaluate causes for the recent erosion hotspot at Ocean Beach.

Nearshore processes, including wave refraction, shoaling and breaking, tidal and wind driven circulation, sediment transport, and morphological evolution are being modeled for the mouth of San Francisco Bay and adjacent Ocean Beach using Delft3D, a model for circulation and morphological evolution. The wave module, SWAN, (Simulating WAves Nearshore) predicts the non-steady propagation of short-crested waves, given key boundary conditions that include wind, bottom friction, bathymetry, and water level. FLOW is the hydrodynamic module used to model current flow.
 

The curvilinear grid designed for the SWAN and FLOW modules in Delft3D, used to model the physical processes at the mouth of San Francisco Bay.
A wave direction=300°	B wave direction=220°
The significant wave height results of SWAN modeling are plotted using common storm wave parameters (3 m significant wave height, 15 sec peak period, 10 m/s wind from 270°) as boundary conditions, but with two different wave directions, 300° (A) and 220° (B). This illustrates the varying wave focusing patterns at the mouth of San Francisco Bay when waves approach from two common directions. A northwest swell predominates in this region, with winter storm waves that exceed 3 m in height arriving numerous times each year. During this typical storm swell, SWAN shows wave focusing at precisely the location of the erosion hot spot at the southern end of Ocean Beach (A). During the El Niño Southern Oscillation (ENSO), wave approach angles shift around to the southwest, resulting in a far different wave focusing and sediment transport pattern (B).