Pouzet P., D. Idier. A composite approach to document a century of overwash events in a high tide environment of southern Brittany, France. Estuarine, Coastal and Shelf Science, vol. 298, March 2024, 108626.

Understanding and precisely dating the sedimentological imprint of storm-induced back-barrier washover deposits is challenging, even more so in macro-tidal environments. Indeed, in such a situation, significant storms may not lead to any washover deposits, for instance if they occurred during low tide. To tackle this challenge, we propose a method based on crossing sedimentology, historical impacts from archives and weather-marine data, together with dating, statistics and modeling technique. We chose the Petite Mer de Gâvres (PMG) site, in southern Brittany, along the French Atlantic coast, as a demonstration site in a macro tidal environment. The sediment core analyses led to the identification of six washover deposits, dated back to 1905 ± 10 yr, 1931 ± 8 yr, 1946 ± 7 yr, 1973 ± 1.5 yr, 1990 ± 3 yr, and 2000 ± 2 yr. The crossing with reanalysis weather-marine data and historical (local and regional impact) sources suggested that they have been caused by the 1896, 1924, 1950, 1974, 1990 (four storms) and 1999 (two storms) events. The post-processing made on the reanalysis data leading to this event list included the computation of the joint return period of exceedance RP (accounting for the tide, surge and wave height) and the use of the Stockdon formula to estimate whether the water level (including the wave runup) may have exceeded the dune crest level and led to overwash events. The storm on January 9, 1924 seemed to have induced the most extreme conditions, with a joint return period RP estimated to 1000 years, giving way to a “vision of desolation” according to historical documents. For all the identified storms at the origin of washover deposits, the wave height was always exceptionally strong, highlighting the key effect of this parameter. However, we need to keep in mind that the tide level should be high enough to allow overwash phenomena on such a macro tidal site. Finally, we underlined the benefits and limits of our composite method to document historical storms.

Mis à jour le 20/02/2024

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