Beach-dune dynamics and coastal management along the North-Médoc coastline, southwest France: a decade of morphological change and erosion mitigation

Abstract

The North-Médoc coastline, exposed to energetic waves and meso-macro tides and exhibiting major erosion hot-spots, has experienced notable changes in beach and dune volumes over the last decades due to a combination of natural forcings and human interventions, including coastal defenses and beach nourishment. Annual LiDAR data (2011−2023) combined with interspersed annual GNSS surveys were used to quantify spatial and temporal variations in beach-dune topography along approximately 14.5 km of coastline. Beach-dune changes along the North-Médoc coast over the past decade were influenced by multiple factors at varying scales. (1) Long-term changes, such as chronic erosion or accretion of up to 7 m/yr, are mainly inherited from large-scale shoal welding originating from the estuary mouth (internal forcing). (2) Coastal structures have localized impacts on shoreline dynamics, sometimes altering trajectories. While alongshore structures stabilize the shoreline, they may cause issues at their ends. Cross-shore structures, like groynes, have variable effects; for instance, extending one of the groyne after winter 2013–14 led to rapid accretion, widening beach, and raising elevations updrift. This accreting area is now used as a sediment extraction area to nourish the eroding southern beaches. (3) Interannual shoreline variability, linked to winter wave height fluctuations driven by climate modes of atmospheric variability, overlaps with trends from (1) and (2). However, this external forcing is more pronounced farther from the estuary, indicating a continuum from external forcing on open coasts to internal processes near the estuary.