Decadal wave attenuating capacity of foreshores under extreme wave conditions

Living near estuaries comes with flood risks from riverine and coastal sources, nevertheless the population density is high and still growing. The population and assets are generally protected by conventional coastal measures, which are challenged by increased storm intensity and sea level rise (SLR). However, those solutions are static and do not adapt to a changing climate. Foreshores, consisting of a tidal flat and salt marsh can serve as add-on to conventional coastal protection. Such a hybrid solution is more sustainable, in that the salt marsh part can cope, to a certain extent, with SLR. Previous studies proved the wave attenuating capacity of foreshores. However, all studies focused on the relative small scale only; i.e. some isolated foreshores in space and at most up to several years in time. The key question addressed in this study is: to what extent can foreshores safely act as additional coastal protection over a decadal time scale?
A unique bathymetrical dataset was analyzed, covering the whole of the Westerschelde estuary (The Netherlands) over a period of 65 years. A total of 36 transects (aligned with the dominant wind direction) were constructed over a selection of six foreshores. The wave attenuating capacity of the foreshores was assessed under daily conditions both with and without vegetation and under extreme conditions (i.e. events statistically occurring once every 10.000 years), using the numerical model SWAN (Simulating WAves Nearshore).

By assessing the wave attenuating capacity over the evolving bathymetry over a period of 65 years, it was found that foreshores always contribute to wave attenuation. Under extreme conditions, foreshores sheltered from the prevailing wind direction were more efficient in wave attenuation (i.e. decrease of wave height per meter foreshore), than foreshores located at the exposed shores. Moreover, at all foreshores, the bare tidal flat caused a baseline wave attenuation, while a linear relation was observed between the width of the salt marsh and the wave attenuation; The longer the vegetated marsh, the larger the wave attenuation. Nevertheless, the relation was different per foreshore. The relation found, provides the knowledge needed to calculate the minimum width of the salt marsh to provide the desired wave attenuation under extreme conditions.