As discussed in Chapter 2, dunes are attacked during storm surges (see Fig. 111 and Fig. 112). As a result, sand is moved from the dunes towards lower parts of the coastal profile (see Fig. 49). Normally the eroded dunes are restored during the calmer seasons. The process can be considered as an adjustment of the coastal profile to the changing hydraulic conditions.

The row of dunes is thin along some parts of the Dutch coast and the erosion during a storm surge may lead to a breach in the dune defence system. Since very large parts of the country are below mean sea level, such a situation is unacceptable. Therefore, reinforcement by dune nourishment is necessary (see Chapter 4). This requires an accurate prediction of the erosion profile of dunes during storm surges. In order to improve knowledge on this subject and to provide accurate calculation methods for dune erosion, the Dutch Technical Advisory Committee on Water Defences initiated a research program, started in 1985 (see Steetzel, 1993). The existing calculation method for the erosion profile provided accurate results for erosion of "standard" dunes. Yet, in the case of "non-standard" coasts, such as where there was a tidal gully near the beach or a curved coast, the results of the existing calculation method were not always reliable. Whereas the previous method concentrated on the analysis of erosion profiles which had been measured after storms, the new approach was directed towards an understanding of the physics of the erosion process itself. Firstly, the erosion process was studied by model measurements in two wave flumes. Secondly, a cross-shore transport model was developed which predicted the resulting erosion profile by calculating the cross-shore sediment transport during the storm surge.