Observations and modeling of alongshore variation in dune erosion

Abstract

The dunes form the primary protection against the sea in the Netherlands, but they can only withstand the sea during storms when they are high and wide enough. It is therefore of vital importance for the Dutch population that the safety of the dunes is assessed regularly. At present the DUROS+ model is used for this assessment, which gives good results for approximately straight and uniform coasts. However, a process-based model could give better results for more complex areas. The process-based model Xbeach has shown potential to predict dune erosion under an extensive amount of controlled laboratory experiments, but only few validations have been performed based on field data. Therefore we performed a hydrodynamic and morphodynamic validation, based on a field data set that was obtained near Egmond aan Zee. The hydrodynamic validation showed that XBeach predicts the Hm0 infragravity wave height reasonably well, with a εrms of 0.086 and a r2 of 0.92, but that the short wave height is overestimated, with an εrms of 0.043 and a r2 of 0.29. The morphologic validation showed that the dunefoot retreat predicted by XBeach was 37% higher than the average observed dunefoot retreat and the eroded volume was over predicted by 76 %, but XBeach was capable to reproduce the alongshore trends in the observed dune erosion reasonably well. Furthermore, XBeach predicted alongshore variation in the eroded volume for both a topography with a uniform bathymetry and a topography with a uniform dune topography, which proved that both alongshore variation in the dune and beach topography and alongshore variation in the bathymetry lead to alongshore variation in the eroded volume.