Calibration of a Sediment Transport Model for San Francisco Bay

A sediment transport model of the San Francisco Bay estuary was developed to assess the potential impacts of projects in San Francisco Bay. The estuary comprises the North Bay, which receives freshwater inflow from the Delta, the Central Bay, which connects to the Pacific Ocean, and the South Bay, which is characterized as a semi-enclosed lagoon. The MIKE 21 cohesive sediment transport model was selected for the study. This paper describes the calibration of the model under baseline conditions. The focus of the modeling effort is the South Bay where the project is located. Previous research indicated that wind-wave resuspension is a key process in the estuary especially in the shallow regions. Therefore, a crucial element of the sediment model is the inclusion of wave-induced bottom shear stress. A calibrated nearshore wave model was coupled to the sediment model by creating a database which contained the wave parameters for every grid cell as a function of the input wind speed, wind direction, and water depth. Erosion and deposition are driven by the local sediment properties. In addition to horizontal spatial variability, the resistance of sediments to erosion increases with depth owing to consolidation. As such, the Bay bottom was prescribed by a five-layered bed model to represent the vertical profile of the sediment strength. In particular, the topmost layer was modeled as a fluid mud to reflect the water and seabed interface. The model was calibrated using time series of suspended sediment concentrations collected at seven different locations throughout the Bay. Both instantaneous (15-minute interval) and tidally averaged values were compared for simulations spanning a few months. Calibration and verification were performed over a range of hydrologic and meteorological conditions, reinforcing the ability of the model to capture the estuarine sediment processes. The results revealed that suspended sediment concentrations in the South Bay are mainly controlled by local erosion and deposition, with sediment influx playing a less significant role compared to the North Bay.