Tidal and residual flows in the Western Dutch Wadden Sea I: Numerical model results

Abstract

A two-dimensional numerical model, which includes tidal basins in the western Dutch Wadden Sea and a part of the adjacent North Sea, is used to study tidal and tidally-driven residual flows and elevations. The model is verified by comparing observed and computed water elevations in some stations and transport rates through the tidal inlets. The consequence of topographical and geometrical differences for the terms in the governing equations is discussed by comparing the magnitude of these terms in some characteristic grid points, viz. the open sea, inlet and basin channel, in which all terms are decomposed along and perpendicular to the dominant current direction. Averaging over a tidal period shows that compared to the time-dependent equations an important shift takes place in the relative influence of the different terms, mainly caused by the increased influence of the advective term. The resulting tidally-driven residual flow field is interpreted as a combination of a constant volume transport (1 to 2% of the tidal transport in the tidal inlets) between connected tidal basins and isolated residual eddies (with velocities of 10 to 15% of the tidal velocity amplitude and a typical length scale of between 3 and 10 km). Observations confirm that the drop in residual elevations in tidal inlets and the rise in tidal basins are phenomena characteristic of all tidal basins.