Controls of Variability in Berm and Dune Storm Erosion

The erosion impact of large coastal storm events typically occurs across broad (100s of km) sections of coastline and may include significant variability both alongshore and vertically between the berm and dunes. Identifying controls of variability in storm erosion is critical to understanding the response of coastlines to present and changing storminess. This contribution analyses immediate pre- and post-storm Lidar data of over 1700 cross-shore profile transects, determined at every 100 m alongshore and spanning 400km of the southeast Australian coastline. This unique dataset allowed for a data-driven Bayesian network analysis of the key relationships between the measured storm erosion response and a range of variables describing the antecedent morphology and hydrodynamic forcing at the coastline. It was found that while erosion of the dune and berm was observed to increase with increased exposure of the local profile to incident storm waves, additional erosion controls were found to be different for these two different sections of the beach. Erosion of the berm was specifically linked to the pre-storm berm volume, with more accreted berms experiencing a greater proportion of erosion of the overall berm, regardless of variability in forcing conditions. In contrast, dune erosion was equally controlled by the exceedance of wave runup above the antecedent dune toe elevation and the width of the beach immediately fronting the dune, with wider beaches resulting in reduced dune erosion. The results of this large, data-driven analysis provide important affirmation and insights into the primary controls of berm and dune storm erosion. Plain Language Summary The erosion caused by coastal storm events is often spatially-variable - some areas of the coast may experience severe erosion of the beach and dune, while adjacent areas may be relatively unaffected. Understanding the controls of this spatial variability in erosion is important to better manage storm erosion now and in the future. This research analyses an unprecedentedly detailed dataset of coastal storm erosion spanning a 400 km region of the southeast Australian coastline to identify controls of spatial variability in storm-induced coastal erosion. It was found that the magnitude of erosion caused by the storm event at different locations along the coast was controlled by both the storm wave and water level conditions, as well as the pre-storm state of the beach. In particular, erosion was controlled by 1) the exposure of the coastline to incident storm waves; 2) the elevation of wave runup with respect to dune toe elevation; 3) the pre-storm volume of sand on the beach; and, 4) the pre-storm width of the beach. The results provide useful insight into the controls of both local and regional-scale coastal erosion and have implications for understanding and modelling coastal vulnerability to storms now and into the future.