Modeling a gravity current in a shallow fluid system
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Corpus Christi Bay in Texas is a wind driven system, and under most conditions winds over the bay mix the water column vertically. However, seasonal, episodic, bottom-water hypoxia has been observed in the bay in conjunction with vertical salinity stratification. This stratification may be caused by dense gravity currents entering the bay. Understanding and modeling the mechanisms that result in stratification in Corpus Christi Bay may help predict hypoxia, and for this reason that is the focus of this dissertation. An evaluation of existing gravity current modeling techniques shows that most currently available models are designed to capture either phenomena local to a gravity current, such as gravity current entrainment and spreading, or larger scale phenomena such as wind mixing and large-scale circulation, but not both. Because gravity current mixing in Corpus Christi Bay is enhanced by wind-induced turbulence, both local gravity current physics and wind mixing effects are critical elements governing gravity current propagation in Corpus Christi Bay. As existing models do not represent gravity current entrainment and wind mixing together, this dissertation develops a coupled model system that accounts explicitly for turbulent wind mixing of a bottom-boundary layer, in addition to representing other local features of dense gravity current propagation such as entrainment and spreading. The coupled model system consists of a 2D depth-averaged hydrodynamic model that calculates gravity current mixing and spreading, coupled with a 3D hydrodynamic model whose domain includes a lighter ambient fluid surrounding the gravity current. The coupled models have flexible boundary conditions that allow fluid exchange to represent mixing from both gravity current entrainment and wind mixing. The coupled model system’s development, verification and application in Corpus Christi Bay advances understanding of gravity current mechanisms, and contributes to our scientific understanding of hypoxia in Corpus Christi Bay. This modeling technique has the flexibility to be applied to other density-stratified systems that are shallow and potentially wind-driven, such as shallow desalination brine disposal sites.