The Effect of the Wave Period on Dune Erosion

Abstract

Dune erosion is a relatively fast developing cross-shore transport process which occurs during severe wave attack combined with high water levels. Large quantities of sand coming from the dunes are transported seaward. If no seaward or lateral losses occur, the sand balance will be closed in cross-shore direction. The eroded sand will deposit on the beach and foreshore in such a way that a so-called erosion profile appears. For the judgement of the safety of the Dutch dune coast a method is used to compute, for given design conditions, the amount of dune erosion. The core of the method is the so-called DUROS-model. This model does not take into account the effect of the wave period on the amount of dune erosion and was developed for the at that time supposed reach of peak wave periods for the Dutch coast (Tp <= 12 s). According to present understanding one has to take longer wave periods into account (Tp up to 16 or 18 s). In addition, preliminary explorations supposed the dune erosion process to be dependent on the wave period. However, the used model was not validated for these high wave periods. Recently, a series of small scale physical model tests, with peak wave periods varying from about 10 to 19 s (prototype), have been carried out by WL | Delft Hydraulics in the Schelde flume. According to that research dune erosion is clearly dependent on the wave period. The aim of this study was to analyse the above mentioned test series and to verify the model DUROSTA (and briefly UNIBEST-TC) concerning the wave period influence on dune erosion (based on recent small scale tests). (These time-dependent cross-shore transport models take the wave period into account.) Most important conclusion about the physical model research was that assuming the applied scale relations and a relatively long storm duration an increase of dune erosion volume above storm surge level of 2535 % is found for a prototype peak wave period of Tp = 18.4 s with respect to Tp = 12 s. The conclusions about the model DUROSTA are more complicated. Since the wave heights were measured at several locations along the cross-shore profile, it became clear that the model (with default settings) did not completely reproduce the measured cross-shore wave height development. The wave height has been tuned for each test to the measured (incident) wave height near the wave board. This resulted in a good reproduction of the measured relative increase of dune erosion with increasing wave period. From several ways of tuning more near the dune appeared that DUROSTA only computes consistently more dune erosion for a longer wave period if the wave height near the dune is larger. This in contrast to a number of the tests where, although approximately equal measured wave heights near the dune (including reflection and long waves), the larger the wave period clearly results in more dune erosion.