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Evolution of a shelly beach ridge system over the last decades in a hypertidal open-coast embayment (western Mont-Saint-Michel Bay, NW France)

Abstract : The behavior of the shelly beach ridge system forming a more or less continuous barrier lying along the southern coast of the hypertidal Mont-Saint-Michel bay (NW France) is examined using a set of various data including aerial photographs since 1947, sediment cores collected in back-barrier salt marshes, ground penetrating radar profiles, and differential digital elevation models (DEMs). The latter shows that since the middle of the 20th century, the tidal flats extending in front of the ridge system experience erosion and accretion that alternate spatially according to cross-shore corridors. The shelly ridges are better developed on line with the erosional bands. The reasons for this erosion/accretion compartment pattern are not totally elucidated. It probably results from the combined influence of shellfish farms, tidal channels and reef present on the tidal flats on wave and current dynamics. At the decadal to pluri-decadal scale, the shelly beach ridge system behavior as well as its internal organization can partly be related to the fluctuations of hydrodynamic conditions controlled by the 18.6 years nodal tidal cycle. Periods of intense landward migration alternate with periods of relative stability during the peak and the trough of the cycle respectively. This general scheme is complicated due to variations in storm wave dynamics. The most morphogenic events happen potentially during coinciding nodal cycle peaks and enhanced storminess. Such conditions actually occurred around 1995-2000, resulting in the most prominent washover fan deposit preserved within the back-barrier infilling successions. These sediment successions evidence as well that no beach ridge system older than the 19th century is preserved in front of the Medieval dike built during the 11th century on a relict beach ridge, and on which the present-day system is backed. We suggest that the enhanced storminess conditions of the Little Ice Age provoked the reworking of beach ridges formed between the 11th and 19th centuries. Changes in quantity and type of mollusk shells, related to the shellfish farming development during the 20th century are also thought to have had positive consequences on the barrier building and stability.
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Submitted on : Tuesday, November 5, 2019 - 4:04:07 PM
Last modification on : Friday, September 18, 2020 - 2:34:30 PM



Bernadette Tessier, Clément Poirier, Pierre Weill, Laurent Dezileau, Alissia Rieux, et al.. Evolution of a shelly beach ridge system over the last decades in a hypertidal open-coast embayment (western Mont-Saint-Michel Bay, NW France). Journal of Coastal Research, Coastal Education and Research Foundation, 2019, Coastal Evolution under Climate Change along the Tropical Overseas and Temperate Metropolitan France, 88(sp1), pp.77-88. ⟨10.2112/SI88-007.1⟩. ⟨hal-02349337⟩



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