Graduation of Silvia Pubben

13 maart 2017 | 14:30
plaats: Room 3.98, Faculty of Civil Engineering and Geosciences
door Webmaster Hydraulic Engineering

"3D Mixing patterns in San Francisco South Bay"| Professor of graduation: Prof. dr. ir W.S.J. Uijttewaal, supervisors: R.J. Labeur (TU Delft), C.A. Katsman (TU Delft), M. van der Wegen (Deltares, Unesco-IHE)

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. Since a lot of measurement data is available, it was possible to validate the model extensively with these data sets. The model performance on salinity and temperature decreased towards the south of SFSB. Based on the model validation, the model performed well. In addition, a sensitivity analysis was carried out which focused on the atmospheric forcing conditions. The research proves that a good numerical model is a qualified tool as support of measurement data and that it is able to fill gaps in these data.

To investigate the influence of the three driving forces several model simulations are performed for the period of October 1, 2012 – September 30, 2013. Water Year 2013. Model results are only assessed for the months December and July, as these months represent a period of high fresh water inflow, as well as a period of low fresh water inflow. The model results show that the main drivers of estuarine circulation in SFSB are the tide and the fresh water inflow.  The influence of the wind on the density driven circulation is negligible. 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.

For future research in SFSB it is recommended to do additional research into the atmospheric forcing conditions, in order to improve the model performance concerning the temperature field. In addition, it is recommended to research the possibilities to include precipitation and water-evaporation in Delft3D FM, since this could improve the model performance concerning the salinity field.

This research led to results that can improve the understanding of sediment dynamics in SFSB. However, to model sediment dynamics of SFSB, the influence of waves should be added to the model. It will be valuable to perform a higher resolution study to get a better and more accurate system understanding. Therefore, it is recommended to focus on time scales ranging from hours to weekly basis, instead of time scales of  a month. 

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