Time-scales of a Dune-Beach System and Implications for Shoreline Modelling

Understanding the interactions between dune systems and beaches is critical to determining the short-term shoreline response and the long-term resilience. In this study, almost 15 years of monthly beach/dune measurements were analysed for three different profiles at Vougot Beach, France to understand and predict shoreline changes from intra- to multi-annual time- scales. Four migration modes: advance/retreat (translation modes) and steepening/flattening (rotation modes) were identified through a centroid analysis. The analysis showed that translation and rotation can occur simultaneously, with long-term trends of beach retreat and profile steepening (lower beach retreating and upper beach advancing), which was interrupted by two energetic wave events causing profile flattening (lower beach advancing and upper beach retreating). These two observations are evidence of how the sediment contribution resulting from the dune erosion events temporarily caused a large advance in the shoreline position. A recent modelling approach that accounts for different time-scales is applied to predict the shoreline changes, showing significant improvements in comparison to a traditional shoreline equilibrium model when time-scales related with the dune erosion and recovery are considered. The results showed that the dune system affects the beach profile evolution both spatially, with different impacts at different elevations along the cross-shore profile, and temporally, by periodically redistributing the sediment in the system. Plain Language Summary Changes in the shoreline position are the result of a delicate balance between hydrodynamic (e.g., waves, currents, water levels) and morphology (e.g., dunes, beachface). Dunes are a natural barrier to storms and much of their ability to stand erosive events is related to their recovery after a storm. At the same time, the beachface often responds to incoming waves by either changing its slope (rotation mode) or by advancing/retreating (translation mode). Using 15 years of profile measurements at Vougot beach, France, we identified the different modes and observed if, when and how they occur. Our analysis shows that the evolution of the beach system was controlled by the sediment exchanges and redistribution between the dune and the intertidal beach. Different elevations of the intertidal beach profile showed distinct behaviour and different time-scales of change. Our work provides evidence that a multi-temporal scale approach is necessary to predict sediment redistribution along the beach profile.