Drivers of alongshore variable dune erosion during a storm event: Observations and modelling

The ability to understand and predict alongshore-variable sand dune erosion is key to better coastal management. This study utilizes detailed observations (immediately pre, during and post-storm topography, waves and water levels) collected over a 6-day period at the 3.6 km long Narrabeen-Collaroy beach in south-east Australia, to identify and explore drivers of the highly variable alongshore dune erosion caused by an East Coast Low storm in June 2011. Key characteristics of the immediately pre-storm subaerial morphology obtained by airborne Lidar (beach slope, dune toe elevation, dune height) varied considerably alongshore. Daily airborne Lidar surveys conducted at low tide indicated considerable temporal variability in the evolution of the subaerial beach profile. Despite considerable alongshore variability in the magnitude of modelled inshore wave heights during the storm, it was instead observed that the predominant determinant of maximum dune erosion was the pre-storm dune toe elevation. A simple dune impact model forced with local alongshore-variable inshore wave modelling was found to successfully predict up to 85% of the observed alongshore variability in dune erosion at this site, with this erosion tidally modulated over the 6 days to time periods when the waves were directly impacting the dune. Importantly, alongshore variation in wave height is shown to account for just 10% of the alongshore variability in dune erosion during this storm. These results reconfirm that knowledge of the pre-storm subaerial morphology, in particular the elevation of the dune toe with respect to time-varying water levels during a storm, is a key driver of alongshore variability in the erosion response along dune-backed sandy coastlines.