Tidal and residual flows in the western Dutch Wadden Sea: III. Vorticity balances
Ridderinkhof, H. (1990). Tidal and residual flows in the western Dutch Wadden Sea: III. Vorticity balances, in: Ridderinkhof, H. Residual currents and mixing in the Wadden Sea = Reststromen en menging in de Waddenzee. pp. 43-60 In: Ridderinkhof, H. (1990). Residual currents and mixing in the Wadden Sea = Reststromen en menging in de Waddenzee. PhD Thesis. Rijksuniversiteit Utrecht: Utrecht. XIII, 91 pp., more |
| Abstract | A vorticity-dynamics approach is used to examine the origin of the small-scale residual current field in the western Dutch Wadden Sea. For a representative part of the Wadden Sea, the magnitude of vorticity and of terms in the balance equation for vorticity is determined on the basis of results from a two-dimensional numerical model. The torque from bottom friction along the side walls of the tidal channels appears to be the dominating mechanism in generating tidal relative vorticity, the magnitude of which is much larger than planetary vorticity. Especially near a tidal inlet, stretching and squeezing of fluid columns is of importance in increasing/decreasing relative vorticity. Averaging over a tidal period shows, compared to the tidal equations, an increased influence of the non-linear advective and streching/squeezing terms in the tidally-averaged balance. However, although the relative influence of these strong non-linear terms increases, the influence of the weak non-linear terms originating in bottom friction cannot be ignored. The mechanism responsible for the headland eddies near a tidal inlet and the topographical eddies in the channels of the Wadden Sea is essentially the same, viz. the transfer of vorticity from a source region where this vorticity is produced by differential bottom friction, to adjacent regions. This transfer of tidal vorticity, or advection, is most effective near a transition from straight to curved isobaths where a gradient in the production of tidal vorticity occurs. This is illustrated by showing the vorticity possessed by a particular fluid column during a tidal excursion. The dominant influence of the bathymetry on the small scale residual current pattern is used for a qualitative discussion of the residual flow field in other parts of our numerical model. |
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