SEAWARD COASTAL DEFENCE FOR THE DUTCH COAST

Abstract

The Dutch Delta-works are nearing their completion. Decades of construction of dikes and dunes come to an end. It is obvious that after the completion of these Delta-works, this infrastructure of coastal defence works needs its year to year maintenance as well. Therefore the Dutch Goverment asked for the so called "Coastal Defence study" and mobilized all the available kwowledge with respect to this item, in order to draft a report with which the political decisionmakers could come to the selection of the most suitable maintenance-strategy. Four different possibilities were investigated. It appeared that the so-called full maintenance strategy will be the most cost effective and profitable solution and was therefore chosen. According to this strategy all retreat of the coastline will always be compensated (in general by beach-nourishment). However, some parts of the Dutch coast are facing extreme erosion as well as very poor reserves and for those locations the application of a somewhat different approach can be profitable or (in time) even necessary (for instance with repect to sealevel rise). This approach was roughly investigated as the "Seaward Coastal Defence" strategy. According to this strategy additional measures will be taken to influence the morphological system in such a way, that the coastline will not erode any longer, but be kept on its place or even grow in seaward direction. The effectiveness of this approach has to be compared with maintenance by beach-nourishment as such as well as the effects it will have elswhere on the morphological system. A pilot-study has been defined at the isle of Texel. The north-point of this island suffers severe erosion. Several beach-nourishments have been carried out here. Unfortunately the effectiveness was not always as planned.