Spatial and temporal evaluation of storm-induced erosion modelling based on a two-dimensional field case including an artificial unvegetated research dune

Abstract

The objective of this study was to evaluate the temporal and spatial performance of the XBeach model predicting storm surge-induced erosion and breaching on the basis of a two-dimensional field case. Within the scope of a physical experiment, a large-scale artificial unvegetated research dune was built at the German Baltic Sea Coastline and surveyed before, during and after a surge event to generate high-resolution digital elevation models of the temporal and spatial failure process. The measured data was used to set-up and calibrate two-dimensional XBeach models. The numerical results showed that the default settings of the latest XBeach-X release led to a very good agreement (BSS = 0.82) between the simulated and measured post-storm bed levels in the area of the research dune, while a higher agreement was observed on the overwash and inundation dominant eastern side than on the collision dominant western side. Evaluating the model accuracy during the storm surge using 44 digital elevation models of the seaside slope showed a spatially accurate prediction of the two observed breaches (±5 m) but also the occurrence of additional breaches. With respect to timing and breach width evolution, however, the results highly deviated from the observation and were found to be very sensitive to changes in grid resolution and to changes of default parameters. Exchanging the nearshore bathymetry with measurements one and two months prior to the physical experiment, where a 49% and 51% lower subaerial beach volume was observed, resulted in a significantly overestimated dune erosion (31% and 33%) and, hence, in a lower accuracy of the default model (BSS = 0.69). Overall, the XBeach model is capable of accurately predicting the evolution of storm-induced erosion during a sufficiently long and overwash/inundation-dominant surge event when current high-resolution input data is available.