15.3.3: E.3.3- Discussion

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Page ID: 16466

Judith Bosboom & Marcel J.F. Stive
Delft University of Technology via TU Delft Open

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It is interesting to compare the two closures in the Wadden Sea. The closure of the Zuiderzee takes away a major part of the original tidal basin, whereas the closure of the Lauwerszee takes away a relatively smaller part of the basin. However, the closure of the Lauwerszee caused a significant decrease of the tidal prism and the closure of the Zuiderzee did not. The reason is that the Frisian Inlet is a short basin and the Zuiderzee was originally a long basin and the special location of the Afsluitdijk.

For the effect of morphological development both closures have in common that they caused sedimentation in the remaining tidal basins, and erosion outside the inlet. The difference is that the closure of the Zuiderzee caused serious erosion of the coasts adjacent to the inlet and the closure of the Lauwerszee did not. This can be explained by the fact that the equilibrium size of the ebb-tidal delta is related to the tidal prism. As the tidal prism did not decrease after the closure of the Zuiderzee, the equilibrium size of the ebb-tidal delta remains the same. This means that there is no sediment surplus in the ebb-tidal delta. The sediment deficit in the tidal basin can be easiest satisfied by eroding the coast, as the size of the ebb-tidal delta will effectively increase when the coastline retreats. In other words by eroding the coast and the ebb-tidal delta at the same time the effective size of the ebb-tidal delta can remain the same. In the case of the closure of the Lauwerszee the tidal prism decreases. This caused a sediment deficit in the basin, and at the same time a sediment surplus in the ebb-tidal delta area. The sediment deficit in the basin can then simply be satisfied by eroding the ebb-tidal delta. It is thus important to realize that the tidal basin and the ebb-tidal delta form a sediment sharing system. The closure of the Lauwerszee does not cause a sediment deficit in this sediment sharing system as a whole, whereas the closure of the Zuiderzee did cause a sediment deficit of this sediment sharing system.