Five closures or semi-closures of tidal basins in the Dutch coastal system are considered in this paper. The two closures in the Wadden Sea separate part of the corresponding basin from the Wadden Sea. The three closures in the delta area close or semi-close practically the entire corresponding tidal basin, but they differ in the exact location with respect to the coastline and in the type of closures varying from fully closed to allowing tidal flow under normal conditions. The effects of these closures have been analyzed from two management points of view, viz. the maintenance of the coast and the environmental problems in the remaining and/or the closed basin.
From the coast maintenance point of view, the ebb-tidal delta and the remaining tidal basin together form a sediment sharing system. The sediment deficits and/or surpluses in the two elements together determine if the tidal inlet / estuary will be a source or sink for the coastal system after the closure. It is concluded that the position of the closure structure with respect to the coastline is important for this aspect. When only a part of the tidal basin is closed, as in the case of the two closures in the Wadden Sea, also the tidal wavelength is a relevant length scale to consider. The Texel Inlet became a sink of sediment after the closure of the Zuiderzee causing serious erosion in the adjacent coasts. The sediment deficit in the basin of the Zoutkamperlaag and the sediment surplus in its ebb-tidal delta keep practically in balance. When almost the entire basin is closed as in the cases of the closures in the delta area, the area outside the closure becomes a sink of sediment when the closure is relatively landwards located and vice versa.
The type of environmental problems in the remaining and/or closed basin caused by the closure mainly depends on the type of the closure. When the closed basin becomes a fresh water lake/reservoir, accumulation of fluvial sediment can cause pollution of the bottom, as in the case of the closures of Zuiderzee and Haringvliet. If the refresh- ment of the water in the closed basin is too limited, as in the case of Grevelingen, water quality problem can occur on the long-term. When the tidal flow is weakened, as in the case Zoutkamperlaag after closure of Lauwerszee and the Eastern Scheldt Estuary, serious erosion of the inter-tidal flats can take place.