Sandspit evolution in macrotidal settings. A comparative study

Abstract : Coastal barriers have been intensively studied since they constitute natural protections against wave attack and marine submersion. In a context of climate change and sea-level rise acceleration, understanding their future development is critically. Here we present the main results of a research project that aimed in reconstructing and comparing the evolution of two sandy barriers (sandspits). The originality of the study was to examine sandspits located along macrotidal coasts since most barriers are considered in their context of wave-built sedimentary bodies. In order to examine when and how these sandspits developed, a stratigraphic reconstruction based on ground penetrating radar (GPR) investigations were performed, completed with vibrocore acquisitions for sediment facies analyses, GPR ground-truthing, and chronology. Historical land and sea charts were used to refine the time frame of evolution. To be able to infer the role of tides vs. wave climate in sandspit evolution, two sandspits subject to significant difference in tidal ranges were studied. The Saint-Germain spit (NW France, English Channel coast, tidal range up to 14 m); The Arcay spit (SW France, Atlantic Ocean coast, tidal range up to 7 m). Both sites are influenced by relatively similar low to moderate wave dynamics (Hs~0.5 m). The comparison of historical charts demonstrates that the formation of the two spits occurred between 1650 and 1700 AD, within the period of the Little Ice Age (LIA). Our study suggests that LIA climate conditions (enhanced storm and wind activities) were favourable for spit formation. Despite the convergence in time of formation, the depositional history of the two spits differs significantly. It appears that the Arcay spit experienced a regular elongation that is still ongoing today. On the contrary, the evolution of Saint-Germain spit was periodic and very fast. Already about 1750 AD it reached its current extension and morphology, suggesting that in half a century or maybe less, the entire spit formed. The GPR and core data reflect these contrasted evolutions of the spits. Data from Arçay demonstrate that the spit complex is mainly made of three wave-dominated units (spit platform, beach, washover) and evidence a dominant shore-parallel mode of construction. 14C ages for Arcay spit are consistent with an onset of construction during the 1600 s. Data from the Saint-Germain spit display two main units that are respectively tide-dominated (estuary, embayment) and mixed tide, wave and wind-influenced (sand spit body). 14C and OSL ages constrain the onset of sandspit formation to around 1000 years ago, and the most recent sand construction, around 400 years ago. The Saint-Germain spit complex results both from a vertical and progradational / longshore accretion. Differences in sediment supply are assumed to explain these two contrasted construction modes (quick progradational vs progressive longshore mode). This difference can be partly related to tidal ranges. Sand fluxes, both wave and wind-induced, are much higher on hypertidal sites, with large intertidal foreshore/flats, than those on macrotidal beaches. This difference was probably enhanced during the severe wind conditions of the LIA.
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https://hal.archives-ouvertes.fr/hal-01617070
Contributeur : Pierre Weill <>
Soumis le : lundi 16 octobre 2017 - 09:58:11
Dernière modification le : mardi 23 octobre 2018 - 09:54:35

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  • HAL Id : hal-01617070, version 1

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Bernadette Tessier, Clément Poirier, Mikkel Fruergaard, Dominique Mouazé, Pierre Weill, et al.. Sandspit evolution in macrotidal settings. A comparative study. 33rd IAS Meeting of Sedimentology, Oct 2017, Toulouse, France. 2017. 〈hal-01617070〉

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