3D Mixing patterns in San Francisco South Bay

Estuaries are complex systems that show a variety of hydrodynamic conditions. These conditions are influenced by bathymetry, geometry, climate characteristics, interacting fresh and salt water flows, and mixing effects caused by winds and wind-waves. The interaction between fresh and salt water flows in combination with mixing effects caused by wind and waves give rise to spatial and temporal varying salinity and temperature fields. Gradients in these fields can give rise to density driven circulation. These density driven currents play an important role in sediment circulation and water quality properties of estuaries. This research focuses on density driven circulation patterns in complex estuarine systems, in which San Francisco South Bay (SFSB) presents an interesting and excellent case study. SFSB can be considered as a unique estuarine system that is influenced by both far-field and local fresh water sources during periods of high fresh water inflow. Fresh water inflow from local sources causes a classic estuarine circulation with a lower salinity landward. In contrast, peak fresh water flows from the far-field sources enter SFSB through the same inlet as through which ocean water enters SFSB, which is exceptional in an estuarine system. As a result, SFSB changes in a reverse estuary, which is normally only found in regions where evaporation rates exceed precipitation and fresh water inflow so that the estuary becomes saltier than the ocean. The research objective of this thesis is to understand the circulation pattern and density dynamics in SFSB, caused by spatial and temporal gradients in salinity and temperature. A 3D process-based model, Delft3D FM, is set up in order to simulate the influences of fresh water, tide and wind, referred to as the driving forces, on the density driven circulation in SFSB. Model results showed that the main drivers of estuarine circulation in SFSB are the tide and the frehs water inflow. The influence of the wind on the density driven circulation is negligilbe. Additionally, it is found that the influence of salinity is dominant over the influence of water temperature on the water density in SFSB. During the majority of the year the fresh water inflow is low. As a result, SFSB is well mixed and a classic estuarine circulation can be observed. However, peaks in fresh water from far-field fresh water sources can drive a temporal reverse estuarine circulation in SFSB. The reverse estuarine circulation and stratification are found to be restricted to the main channel of SFSB. In addition, it is found that the amount of fresh water that originates from the far-field fresh water sources determines the extent of which reverse estuarine circulation can be observed in SFSB.Civil Engineering and GeosciencesHydraulic Engineerin