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Journal of Geophysical Research: Oceans








A simple method is used to separate the tidally induced and density-driven subtidal flows in a coastal plain estuary. This method is applicable to weak wind conditions and to systems with appreciable fortnightly variation of tidal amplitude. The baroclinic density-driven motion is assumed to depend on the river discharge, which generates a horizontal density gradient, and is weakened by vertical mixing, which in turn depends on tidal forcing. The barotropic tidally induced motion is assumed to be a function of the tidal amplitude. By Taylor series expansions, two equations are obtained. These equations show the dependence of the tidally induced how component on the tidal amplitude and the dependence of the density-driven flow component on the ratio between river discharge and tidal amplitude, respectively. The method is applied to water velocity data obtained in the James River, Virginia, in October-November 1996. The data cover two spring tidal cycles and two neap tidal cycles. The vertical structures, as well as the depth mean, of both tidally induced and density-driven components of the subtidal flow are obtained. Results show that the tidally induced component has a predominant seaward how in the channel and a landward flow over the shoals. The density-driven exchange how is seaward over the shoals and landward in the channel. These results are consistent with theoretical model results which show that the tidally induced component and density-driven component compete against each other. The increased tidal mixing and tidally induced exchange flow during spring tides reduce density-driven motion, which results in a weak net subtidal flow. In contrast, during neap tides, both the tidally induced flow component-of the subtidal how and tidal mixing are weak, and the tidally induced flow is overwhelmed by the density-driven flow component, which results in a stronger subtidal how. By extending the proposed method, we suggest that future studies use a least squares fitting technique to obtain an optimal estimate for the tidally induced and density-driven subtidal flow components.


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Copyright 1998 by the American Geophysical Union.

Original Publication Citation

Li, C. Y., Valle-Levinson, A., Wong, K. C., & Lwiza, K. M. M. (1998). Separating baroclinic flow from tidally induced flow in estuaries. Journal of Geophysical Research: Oceans, 103(C5), 10405-10417. doi:10.1029/98jc00582

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