Three-dimensional hydrodynamic modeling of Tomales Bay, California

The TRIM model is applied to simulate hydrodynamics and salinity in Tomales Bay during winter conditions. The model is calibrated to current meter data. Then it is applied, without modification of model parameters, to simulate the seasonal variability of salinity along the axis of Tomales Bay. The salinity is accurately predicted during a six month winter and spring period with multiple flow events that cause stratification. The gradual vertical mixing of a surface lens of freshwater is predicted accurately. The model is then applied with idealized forcing in order to study dispersion mechanisms in Tomales Bay. Appropriate dispersion coefficients for a 1D tidally-averaged model are estimated from the three-dimensional model results for different steady freshwater (creek) flow rates. The flux through several cross-sections is analyzed in detail by decomposition of velocity and salinity profiles. Tomales Bay is frequently divided into two regions, the "Inner Bay" and the "Outer Bay," distinguished primarily by differences in bathymetry. This study indicates that different transport mechanisms are important in the two regions. in the outer portion of the bay, which is characterized by a channel-shoal structure and strong tidal currents, tidal dispersion mechanisms dominate. In the "Inner Bay," which has a more uniform cross-section and reduced tidal currents, baroclinic flow is the dominant flux mechanism. In addition to this spatial variability, the strength of some transport mechanisms is tied strongly to freshwater flow entering Tomales Bay.