Three Dimensional Modelling Of Hydrodynamics And Tidal Flushing In Deep Bay

This dissertation, "Three-dimensional Modelling of Hydrodynamics and Tidal Flushing in Deep Bay" by Aiguo, Qian, 乾愛國, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled "Three-Dimensional Modelling of Hydrodynamics and Tidal Flushing in Deep Bay" submitted by QIAN Aiguo for the degree of Master of Philosophy at the University of Hong Kong in December, 2003 Deep Bay is a semi-enclosed shallow bay on the eastern side of the Pearl Estuary, between Shenzhen to the north and the New Territories of Hong Kong to the south. As a result of large-scale development in Shenzhen, Deep Bay has experienced severe water quality deterioration in recent years. Some isolated water quality studies of Deep Bay have been conducted. However, Deep Bay is a unique embayment which exhibits large seasonal salinity variation, and no three-dimensional (3D) modeling of the tidal flushing process has yet been done. The flushing time, a key parameter that governs water quality and biological processes, needs to be quantified using a robust 3D hydrodynamic model. In the present study, a 3D hydrodynamic model of Deep Bay is developed and the 3D salinity and tidal circulation of the bay are systematically discussed. The flushing and dispersion features are studied, and the tidal flats in inner Deep Bay are simulated. The Environmental Fluid Dynamic Code (EFDC) is used to develop the 3D hydrodynamic model of Deep Bay. The topographical configuration, space and time scales, boundaries and boundary conditions are selected based on physical consideration and extensive numerical tests. The model is calibrated and verified against two different field data sets respectively for the dry and wet season. The effect of salinity on velocity field is also examined via a numerical experiment by eliminating any salinity gradient from the measured wet season tidal conditions. The analysis of field measurements and numerical modeling is synthesized, and it is demonstrated that the flows within Deep Bay are characteristic of a well mixed/weakly stratified estuary. The small freshwater flows make insignificant contribution to the salinity structure of the outer Deep Bay area, which is mainly influenced by the inflow introduced from the Pearl Estuary. Based on the validated hydrodynamic model, the tidal flushing is studied using a 3D conservative mass transport model. The flushing times of different parts of Deep Bay are quantified via systematic numerical simulation including: i) real tidal forcing on Deep Bay model and ii) typical M2 tides on a local model with only one tidal open boundary. It is demonstrated that the flushing rate is approximately 0.04 per day (flushing time 25 days) in the inner bay, and increases beyond the middle bay rapidly, to 0.3-0.4 per day at the outer end. The flushing rate is significantly affected in both the dry and wet season by salinity gradients. The dispersion of a continuous pollutant source into Deep Bay is also studied. It is demonstrated that this dispersion conforms to the flushing features within the bay. The source remains in the inner bay area with a high concentration, but is rapidly diluted beyond the Shekou cape cross section of middle Deep Bay. It is therefore demonstrated that severe water quality deterioration in Deep Bay occurs mainly in its inner bay area, and that any "red tide" occurring from time to time in inner Deep Bay will not spread to the bay as a whole. DOI: 10.5353/th_b2979900 Subjects: Hydrodynamics - Computer sim

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