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Morphodynamics of intertidal dune field in a mixed wave-tide environment: Case of Baie de Somme in Eastern English Channel

Abstract : Understanding the response of the intertidal dunes to the varying hydrodynamic conditions in mixed wave-tide environments is of high complexity. Field measurement is one of the most useful approaches to investigate the dune dynamics at different time (semi-diurnal and lunar cycles) and spatial (dunes and associated superimposed bedforms) scales. High-resolution laser scanner data for five successive surveys over a full neap-spring cycle (27 January – 9 February 2014), together with hydrodynamic measurements, have been explored to investigate the morphodynamics of dunes, their superimposed bedforms and their migration in a mixed wave-tide environment (case of the Baie de Somme, Eastern of the English Channel), by the use of a stochastic-physical coupling approach. The dunes are ~12 m wavelength, 0.4 height and oriented orthogonally to the shoreline with angles varying between - 45° and + 45° corresponding to clockwise and counter-clockwise changes, respectively. A stochastic approach, based on spectral analysis, exhibits a series of morphological defects modulating the dune changes following several scale ranges in response to various energy conditions. Such defects can be classified into four categories: (1) limited structures of ~1.6 m wavelengths locally developed along the dune during flood drainage; (2) small-scale superimposed bedforms of ~3.2 m wavelengths built-up across the dunes after high hydrodynamics and proliferated when the energy deceases; (3) large-scale superimposed bedforms of ~6.4 m resulting from the morphological growth of the smaller bedforms; (4) crestline sinuosity with ~25 m wavelength and induced by changes in the orientation of the dunes when they are exposed to high tide-wave activity. The evolving dune defects over the neap-spring cycle is strongly related to the physical mechanisms of sediment transport supplying the migrating dunes and favorable for their self-organization. The physical study of the migrating dunes with the wave-tide driven sediment transport highlights that the long-dune migration components are significantly important during high-energy episodes with a mean explained variance of 65% of the total migration while they decrease to 32% for moderate wave-tide conditions. Good matches of the cross- and long-migration components with the along-shore and the cross-shore sediment transport quantities are recorded with R2 coefficients higher than 70%. These coefficients increase for the offshore dunes where their morphological response to the time-averaged sediment transport is faster while onshore dunes respond at slower timescales resulting in lower R2 coefficients.
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Submitted on : Tuesday, March 2, 2021 - 7:58:59 AM
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Imen Turki, Sophie Le Bot, Nicolas Lecoq, Hassan Shafiei, Charlotte Michel, et al.. Morphodynamics of intertidal dune field in a mixed wave-tide environment: Case of Baie de Somme in Eastern English Channel. Marine Geology, Elsevier, 2021, 431, pp.106381. ⟨10.1016/j.margeo.2020.106381⟩. ⟨hal-03041413⟩



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