Artificial nourishment

Abstract

Artificial nourishment is an often used measure along the Dutch coast. Advantages are, amongst others, the lower costs, no negative influence on recreation and in contrast to hard measure it does not have negative consequences for the adjacent coastal areas. A disadvantage is that in most cases a repetition of the nourishment will be necessary to maintain a certain required minimum position of the coastline. Along the Dutch coast a nourishment frequency with a 5 years interval is assumed. In some coastal areas the effects of erosion are very large so that nourishments have to be repeated every 2 year. Often the material of the fill is determined by the material that is available in the borrow area. The volume, the characteristics and the location of the material of the fill will have an effect on the lifetime of a nourishment. These effects will be discussed in this study. In the past some theories about the effect of the behaviour of nourishment have been developed and field and laboratory tests are performed. The theories that are formulated are the theory of the overfill factor and the renourishment factor. The overfill factor is the estimated required fill material in m3 to produce 1 m3 of beach material when the beach is in a condition compatible with the native material. The renourishment factor is the ratio between the rate at which the fill material will erode and the rate at which the native beach material is eroding. The field tests give very specific information for the local conditions. A disadvantage of the laboratory tests is the scaling problem, when the results have to be related to prototype conditions. But both, the laboratory models and the field tests can be useful for the refinement and verification of numerical models. The references in literature give inadequate data to take into account the complications of the behaviour of nourishment with borrow material, which is different from the native material. The information is insufficient to formulate a convenient relationship between nourishments and particle size effects. Nowadays sediment transport models can be used to predict the behaviour of nourishments. For this study the numerical one-line model UNIBEST_TC is used for computing the cross-shore sediment transport rate. For computing the longshore sediment transport rate the numerical one-line model UNIBEST_LT is used. The combination of the longshore and cross-shore sediment transport rate is computed with the numerical multi-line model PaNTos. An assumption has been made for the cross-shore profile, the shape and position of the nourishment, the material of the fill and bottom and the wave climate. Although nourishments are applied generally on eroding coasts, in this study the coast is designed as a straight coast that is not subject to structural erosion. Only a section of the coast is nourished. The characteristics of the material of the fill, the position of the fill and the fill volume are varied. The coast is subject to waves that approach the coast under a certain angle. The schematisation is not for a specific location; it is to clarify the principle of the processes, but as reference for determining a wave climate and crossshore profile a location near Egmond aan Zee is chosen. Furthermore the schematisation has been made to draw a general conclusion on the effect of the material of the fill, the fill volume and the position of the fill on the lifetime of a nourishment.