The Dutch coastal system consists of the North Sea coast, a series of Wadden Sea tidal inlets in the north and various estuaries in the southwestern delta area. During the last century, various engineering works in the Dutch coastal system have been carried out for flood defence and / or land reclamation purposes. Among these engineering works, two of the tidal inlets in the Dutch Wadden Sea and three of the estuaries in the southwestern delta were entirely or partly closed or semi-closed (Fig. E.1). The first closure is the Afsluitdijk, a dam of 30 km finished in 1932 and separating the former Zuiderzee from the Wadden Sea. The last of this series closures is the Eastern Scheldt Storm Surge Barrier, finished in 1986 which semi-closed the Eastern Scheldt Estuary.
These closures differ in type and location within the corresponding basins. Three of the five are fully closed dams: the Afsluitdijk, the closure of Lauwerszee and the closure of the Grevelingen. Haringvliet is closed by a dam in combination with sluices which only allow discharge of fresh water from the upstream river. The Eastern Scheldt Storm Surge Barrier only closes the basin under extreme conditions and it allows tidal flow through although with reduced cross-section. The two closures in the Wadden Sea only close the landwards part of the tidal basin. The closures in the southwestern delta area practically close the entire corresponding basin, but their locations relative to the mouth of the basin also differ from each other.
These closures of the tidal basins have been impacting the development of not only the (semi-) enclosed basins themselves, but also the adjacent coast and tidal basins. They influence the large scale morphological development and the sediment budget, and thereby impacting the maintenance of the coast by sand nourishment. On smaller scale they influence the developments of the channels, inter-tidal flats and other morphological elements. Especially the influence on the morphological development of the inter-tidal flats has impacts on the ecological system.
In this paper we evaluate the effects of all these closures to the morphological development of the various parts of the Dutch coastal system. The evaluation is mainly based on analysis of field data. Bathymetric data have been collected since 1926. Further use is also made of the results of earlier modelling studies. Special attention will be paid to the influence of the location of the closure and the type of the closure. It will e.g. be shown that the total sediment deficit for establishing the new morphological equilibrium caused by a closure is very much dependent on the location of the closure relative to the mouth of the tidal basin. We will also show the different environmental problems caused by the different type closures. We believe that the lessons learned from the evaluation can also be relevant for elsewhere in the world.