Numerical modelling of the erosion of marsh boundaries due to wave impact

The study describes an improvement of the 1D version of the hydro-morphodynamic model XBeach to simulate the erosion of saltmarsh edges under wave attack. The effects of sand-mud mixture and characteristics of vegetation on the erosive processes are implemented, along with a nonlinear version of the Exner equation to better simulate the horizontal migration of a vertical bank profile due to wave impact. Numerical experiments reveal that the effect of soil composition is crucial for the development of bank morphology where steep scarps are associated with muddier soils while gently smoothed cross-shore profiles are associated with sandier soils. The effect of vegetation in controlling the morphodynamic evolution under wave attack is greater when the sand fraction increases, thus contributing to the development of small steep scarps. The model is further tested against field data of a saltmarsh bank profile and wind conditions collected before and after the passage of Hurricane Isaac in Lake Borgne, Louisiana. After validation of the hydrodynamics, a tuning procedure to match the final profile is carried out on the main model parameters. The results illustrate the potential utility of the model for application to geomorphic settings subjected to wave impact and characterized by high mud content and halophytic vegetation; yet, the model still has limitations and further improvements are needed. Finally, the results demonstrate the ability of saltmarshes to withstand erosion during the peak of large storms when the marsh is submerged, and highlight transient conditions as important drivers of geomorphic change during storms.