INVESTIGATIONS OF THE TIDES AND STORM SURGES FOR THE DELTAWORKS IN THE SOUTHWESTERN PART OF THE NETHERLANDS

J.J. Dronkers

Abstract


In order to protect the southwestern part of the Netherlands against inundation by storm surges, the "Delta project" has been undertaken. This entails the closure of three large sea arms situated between Western Scheldt and Rotterdam Waterway and will bring about radical changes in the tidal movement and stormflood levels of the estuaries and tidal rivers. The contours of the project are shown in fig. 1. It includes three big dams to be built in the mouths of Eastern Scheldt, Brouwershavense Gat and Haringvliet, as well as two smaller ones to be constructed further inland. An idea of the extent of these works may be gained by knowing the tidal volumes of the estuaries: Veerse Gat 2.5.109 cu. ft; Grevelingen 4.109 cu. ft? Haringvliet 9.109 cu. ft; Brouwershavense Gat 12,5.109 cu. ft and Eastern Scheldt 39>.109 ou.ft. The waters of the Delta area will then be divided into two separate basins by means of a dam in the Volkerak. The southern basin will be entirely cut off from the sea, becoming a fresh water lake. The northern, comprising the mouths of the Rhine and the Meuse will remain in communication with the sea, because the Rotterdam Waterway must stay open to shipping. Consequently, the tides and storm surges will still be able to penetrate inland via this mouth, but they can cause high water levels in the Waterway only; in the rest of the basin their effect will be considerably weakened. In the situation at present, however, the upland flow of the rivers Rhine and Meuse is mainly into the Haringvliet estuary and not the Rotterdam Waterway. As the Haringvliet estuary will be closed, large sluices are to be built in the enclosure dam as a substitute for the existing free discharge of the river water.
Until this project is completed the inhabitants of the area which it will affect are insufficiently safe against storm surges. It is, of course, always possible that floods too high for existing dike systems will occur, but in the present situation the risk is too great. This was demonstrated in February 1953 when the southwestern part of the Netherlands was suddenly hit by an exceptionally high storm surge which caused many dike breaches and vast inundation. The occurrence of a similar surge or a higher one may be estimated as once in two hundred and fifty years, as an average, which is much too high. After the realization of the Deltaplan and the heightening of the dikes of Western Scheldt, Rotterdam Waterway and the northern parts of the country, the dikes are safe tip to very high storm surges of which the occurrence is smaller than once in ten thousand years, or in other words there is only one procent chance in hundred years that a major inundation will occur.

Keywords


surge; deltaworks; tidal current

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