Secular shoreline response to large-scale estuarine shoal migration and welding

Abstract

The 14.5-km North-Médoc coast, southwest France, is a high-energy meso- to macro-tidal environment adjacent to the largest estuary in Europe. Over the last centuries, this coastline has locally suffered periods of severe erosion, threatening coastal infrastructures and requiring the progressive implementation of coastal structures and, more recently, localized beach nourishments. This contribution combines 84 years (1937–2021) of shoreline data from various sources, 118 years (1903–2021) of shallow water bathymetric surveys and historical photographs. Results show that, averaged in both time and space, the coast eroded by −0.6 m/yr over the last 84 years, but with a large alongshore and temporal variability. Erosion is locally peaking at −5.2 m/yr, while accretion is restricted to a remote 2.5-km and locally peaks at 5.4 m/yr. A salient characteristic of shoreline evolution is the alternation of rapid erosion (< −5 m/yr) and dramatic accretion (> 20 m/yr) periods over relatively short intervals (≈ 10 years) and across limited alongshore distances (e.g. couple of kilometers). We show that shoreline change is mainly driven by offshore shoal dynamics originating from the estuary mouth further migrating in both the cross-shore and longshore direction. Two major events occurring at different times and locations, leading to volumetric changes of the order of millions to tens of millions cubic meters are analysed. The first attachment, completed around the 1920s, supplied nearly 5-million m3 of sediment, widening and rising the beach by hundreds of meters and meters, respectively. This shoreline bulge subsequently diffused and migrated downdrift, resulting in a quasi-steady erosion rate of approximately −3.3 m/yr. The second shoal attachment, which started around the 1950s, first stopped the chronic erosion observed in the northern sectors, and subsequently drove dramatic (> 500 m) shoreline accretion in the 1970s. Current yearly small-scale beach nourishments only temporally buffer erosion, while a mega-nourishment with similar scales as the first shoal attachment could provide a more sustainable solution. We anticipate that this study, together with future modelling work will help the coastal managers and decision makers to optimize their coastal management strategy.