Sensitivity of dune erosion prediction during extreme conditions

Coastal dunes play an important role in protecting low-lying hinterland from flooding. In The Netherlands, dunes form the major part of the primary sea defence, that protects half of the population and over two-third of the economy from the sea. To design and maintain coastal dunes that properly protect the hinterland, a regular safety evaluation is performed every six years and measures are taken if the prescribed safety standard is not met. The empirical dune erosion model DUROS+ (Vellinga, 1986; van Gent et al., 2008) is the core of the currently used safety evaluation method for dunes. This model does only take the nearshore bathymetry explicitly into account. It is not clear beforehand whether the offshore bathymetry has a significant influence on the dune erosion rate under extreme storm conditions. This paper aims at investigating the sensitivity of the dune erosion rate for different parts of the cross-shore profile. Three clearly distinct cross-shore profiles along the Dutch coast are used to estimate the erosion under extreme storm conditions, using XBEACH (Roelvink et al., 2009), and distinguish between the sensitivity to the lower (seaward) part and the upper (landward) part of the profile. The division between the two profile parts has been made at the MSL-4 m contour. The profiles used for this investigation are considered to cover the range of different profiles as present along the Dutch coast. It can be concluded that the landward profile part is of main importance for an accurate dune erosion estimation. A different seaward profile part can influence the erosion, under extreme storm conditions, by about 10 %. On the other hand, a different landward profile part can lead to a few hundred percent difference in erosion