Modelling wave dissipation and mean water level over salt marshes

Besides its well-known capacity to dissipate wave energy, salt marsh vegetation can also affect wave setup, although this mechanism has been much less studied and quantified so far. This study reports on a field experiment conducted under moderate energy conditions across a French Atlantic salt marsh. The data analysis is complemented with numerical simulations performed with the 3D fully-coupled wave–current modelling system SCHISM. While the model could already resolve vegetation-induced drag on mean currents and turbulence, it was here extended to account for vegetation intrawave drag effects and the wave force associated with vegetation-induced dissipation. Using published lab data, we first verify the model’s capacity to reproduce wave dissipation by vegetation and its effect on mean water levels, namely a reduction in wave setup, which is controlled by wave–current-vegetation interactions including intrawave processes. In the field, the model also demonstrates good predictive skills in simulating wave parameters across vegetation and suggests that vegetation can decrease the wave setup. However, this last process was too modest to be measured with pressure transducers, calling for future field experiments under storm conditions. This capacity of vegetation to reduce nearshore mean water levels should be thoroughly considered when evaluating the potential of salt marshes as nature-based coastal protection. This study places the SCHISM model as a state-of-the-art, efficient tool to simulate 3D multi-scale wave–current processes over vegetation ecosystems. Our results finally highlight that vegetation and depth-induced breaking induce a frequency-dependent dissipation, whose representation in phase-averaged models is presently limited and will require future research. • A field data set is collected under moderate energy conditions across a salt marsh. • Wave–current-vegetation interaction processes control wave setup and can reduce it. • SCHISM modelling system efficiently simulates 3D wave–current-vegetation interactions.