Simulating Periodic Stratification in the San Francisco Estuary

Three-dimensional simulations of circulation in the San Francisco Estuary were performed with the three-dimensional hydrodynamic model, TRIM3D, using a generic length scale turbulence closure model. The model was calibrated to reproduce observed tidal elevations and tidal currents in the San Francisco Estuary and then was applied during a period of intensive data collection. The model predicts tidal currents accurately and realistically simulates variability in salinity at both the seasonal and tidal time scale. The model results are consistent with the current conceptual understanding of stratification in the San Francisco Estuary and approximately predict the magnitude and tidal phasing of observed stratification. The simulation results were analyzed to improve understanding of periodic stratification. During spring tides, compression of salinity gradients near high water and weak tidal currents at and following high water allow the formation of weak and transient stratification. During neap tides stronger stratification is observed and predicted. This stratification is strongest during ebb when tidal straining is effective in creating stratification. Vertical turbulent mixing may be adequate to diminish vertical stratification during strong ebb tides while stratification can persist or increase through weak ebb tides.