A Study of Vertical Patterns and Mechanisms of Along-Channel Estuarine Circulation

• Main Author: Qin, Qubin.
• Format: Others
• Language: English
• Published: W&M ScholarWorks 2012
• Subjects: Hydrology
• Online Access: https://scholarworks.wm.edu/etd/1539617913; https://scholarworks.wm.edu/cgi/viewcontent.cgi?article=3043&context=etd

The classical theory of estuarine dynamics has been challenged by a number of recent studies regarding the important contributions of tidal, lateral, and Coriolis effects to the subtidal dynamics at leading order. Although the classical theory excluded these processes, Hansen and Rattray’ ( R65) b subtidal circulation. However, the success of the predictive skill is not well understood. In this thesis, a perturbation approach is introduced to derive a “ ” (GS) of subtidal along-channel velocity profile for estuarine circulation incorporating the effects of tidal and lateral processes, which allows us to evaluate the possible circulation patterns, the underlying mechanisms, and to diagnose the contributions of these nonlinear processes. The GS applies to the entire estuary at any location without including presumptions of constant baroclinic pressure gradient and constant eddy viscosity. The GS was tested against a numerical model simulation case. A non-dimensional parameter  is introduced for determining the structure of estuarine circulation. The success of HR65 has been evaluated and compared with GS. It is found that HR65 is a particular case of GS when estuarine circulation is dominated by horizontal pressure gradients and it has a good predictive skill when X is in the range between -0.53 and -0.20. However, its scaling of the strength of estuarine circulation could deviate from the true circulation when these nonlinear effects are important unless this scaling or its dependence on constant eddy viscosity is rectified by a scaling factor Y . GS can be considered as the superimposition of flow uc that is only related to depth-averaged velocity C and exchange flow ur. The exchange flow consists of six possible patterns. Four of them have two-layer structure, and the remaining two have xii three-layer structure. The pattern of the estuarine circulations will be altered and some patterns cannot be observed when the magnitude of C is comparable to exchange flow. The circulation exhibits a total inflow (or outflow) when C is much large than exchange flow. The typical pattern of circulation is the two-layer circulation with seaward flow in the upper layer and landward flow in the lower layer. Both the lateral processes and tides could significantly change the pattern through its effect on mixing and barotropic forcing. Although terms higher than 4th-order can reduce the deviation of model results from the real circulation, their inclusion does not have clear physical meaning. Therefore, the general solution only retains a 4th-order approximation of the circulation that can explain both two-layer and three-layer circulation patterns being observed in estuaries. Besides the density-driven circulation, other possible circulations such as tideinduced and laterally-induced circulation are discussed. It is found that they could create two-layer circulation that has structure similar to observations. However, the results suggest that baroclinic forcing is the primary fundamental mechanism of estuarine circulation.