Assessing Storm Impact on a French Coastal Dune System Using Morphodynamic Modeling

Abstract

Muller, H.; van Rooijen, A.; Idier, D.; Pedreros, R., and Rohmer, J., 2017. Assessing storm impact on a French coastal dune system using morphodynamic modeling.Natural coastal dune systems provide protection against flooding from the ocean for many coastal communities around the world. An in-depth knowledge of their erosion mechanisms under oceanic stress can help to identify potential weak spots along the coastline and eventually prevent damage to the hinterland. The objective of this study was to assess the effects of hydro-morphodynamic phenomena on the coastal dune morphology during storm events and to evaluate their level of protection. This paper focuses on the physical processes that cause erosion and breaching of sandy dune systems, which are typically a function of the coastal morphology in combination with the hydraulic loading. An XBeach model was set up for the coast of Les Boucholeurs (France), which suffered from severe erosion, dune breaching (at six locations), and inundation of the hinterland during storm Xynthia in February 2010. The model was run for the full storm period, and it accurately reproduced the reported dune breach locations and dimensions with limited calibration. The model results were further utilized to study the temporal and spatial breaching dynamics and to understand the effect of the prestorm morphology on dune breach development. The results show that the dune behavior during the storm was strongly related to the submersion height of the dune crest and anticorrelated with its prestorm geometry (i.e. dune height and width). The governing parameters for the poststorm dune state are, in terms of alongshore-averaged erosion and in order of relative contribution: Submersion height over the dune crest, prestorm dune width, and prestorm dune crest height. Finally, additional simulations showed the limited effects of infragravity waves and the initial bathymetry used in the simulations.