Castelle, B.; Marieu, V., and Bujan S., 2019. Alongshore-variable beach and dune changes on the timescales from days (storms) to decades along the rip-dominated beaches of the Gironde Coast, SW France. In: Castelle, B. and Chaumillon, E. (eds.), Coastal Evolution under Climate Change along the Tropical Overseas and Temperate Metropolitan France. Journal of Coastal Research, Special Issue No. 88, pp. 157–171. Coconut Creek (Florida), ISSN 0749-0208.
The high-energy meso-macrotidal 110-km long Gironde coast, SW France, is primary composed of quasi-straight sandy beaches bordered by high and wide coastal dunes. Beaches are intermediate double-barred and are essentially morphologically variable alongshore with ubiquitous rip channels incising both bars. These rip channels enforce a strong alongshore variability in the morphology of the dry beach and/or of the dune, morphological patterns referred to as megacusp embayments. In this study, we use 70-year diachronic shoreline data, 3.5-year semi-annual in situ shoreline surveys since 2014, combined with 12.5-year monthly to semi-monthly topographic surveys collected since 2005 at Truc Vert beach. Results show that 2 types of megacusp can be identified: (1) accretive megacusps on the upper beach, forming through a sequence of accretionary beach states following a storm event, are enforced by inner-bar rip channels with a spacing of O(100 m) and a typical lifetime of a few months and (2) erosive megacusps cutting the dune, forming during severe-storm driven erosive events, which are primarily enforced by the outer-bar morphology with a spacing of O(1000 m). These erosive megacusps do not migrate alongshore and can persist for years to decades. The outstanding winter of 2013/2014 drove the formation of erosive megacusps all along the coast, dramatically altered the coastal landscape and also impacted the behaviour and mean spacing of the accretive megacusps during the subsequent years. Overall, the study demonstrates the complex interplay between the nearshore morphology and the alongshore-variable changes of the foreshore/backshore from the timescales of days to decades, with accessional outstanding winters having the potential to deeply affect beach morphology and rhythmicity on the time scale of a few years, at least.