Tidal exchange between the North Sea and Dutch Wadden Sea and mixing time scales of the tidal basins

Abstract

Particle trajectories in a numerical model of the western Dutch Wadden Sea and the adjacent North Sea are used to study the tidal exchange between the North Sea and the tidal basins. Tidal exchange of water masses appears to depend strongly on the tidal phase at which the computation is started. In general the volume displaced by the large-scale through-flow between connected tidal basins is much smaller than the exchanged volume. For all inlets North Sea water mainly enters the Wadden Sea from the southern side of the inlet whereas basin water leaves the Wadden Sea mainly along the northern coast. Differences in exchanged volume between the inlets are caused by differences in bathymetry at the seaward side of the different inlets. Schematizing each tidal basin as a single well-mixed box, in which an exchange coefficient parameterizes the exchange with the North Sea, allows a rough estimate of the turnover time of a tidal basin. A salt balance for a simple two-box schematization of the Marsdiep and Vlie basins gives an independent estimate of the exchange coefficient between the Marsdiep basin and the North Sea. Its value appears to be larger than the tidally exchanged volume. The revised salt balance, in which the effect of the through-flow between the Marsdiep and Vlie basins is incorporated, is used to calculate the flushing time of fresh water in the western Dutch Wadden Sea which appears to be larger than previous estimates. The tidally exchanged volume through the inlets, as calculated with the numerical model combined with the relation between the exchange coefficient and the tidally exchanged volume for the Marsdiep basin, is used to give a rough estimate of the turnover time of all Dutch Wadden Sea basins. For the Eierlandse Gat and the Borndiep the exchanged volume is calculated by means of our hydrodynamical model. For the other basins it is assumed that its value is a fixed percentage of the tidal prism entering a basin.