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Modélisation numérique des transferts sédimentaires de l'estuaire de la Gironde au plateau continental

Abstract : Estuaries are major vectors of sediment (i.e. mud and sand) transfer between terrestrial environments and the seas. Contaminants, mostly arising from human activities along the shores, are potentially adsorbed on fine particles and transported to the sea through estuaries. Therefore, studying sediment dynamics in these zones is crucial for marine resource protection. However, the complexity of physical, biochemical and sedimentary processes taking place along the interfacial areas represent a highly challenging topic. The PhD project takes place in the framework of the ANR AMORAD project, which aims at improving understanding and numerical modeling of potentially contaminated sediment fluxes from continental areas to the sea. Thus, based on the Gironde Estuary study site and its adjacent continental shelf, this PhD thesis focuses on: (i) investigating the influence of hydro- meteorological events on sediment fluxes and (ii) determining the sediment accumulation and dispersion areas on the continental shelf and especially the role of the West Gironde Mud Patch on continental sediment trapping. To this end, a realistic 3D numerical model, based on the MARS3D hydrodynamic model, the WAVE WATCH 3® wave model and the MUSTANG sediment module, has been implemented on the Gironde Estuary and the adjacent continental shelf. Based on a sensitivity analysis to sediment parameterization, the uncertainties on sediment fluxes at the estuarine mouth have been quantified. Moreover, based on the previous sensitivity analysis and on the study of the spin-up period required for model stabilization, the latter has been calibrated from the estuary to the shelf. It has demonstrated a satisfactory validation state with good performance compared to observed SSC, and a remarkable robustness of sediment flux and distribution over 6 years. The study of residual sediment dynamics revealed a strong influence of baroclinic circulation on sediment exchanges at the estuary mouth, with enhanced sediment import into the estuary during wet season. Further analyses made it possible to study the areas of sediment accumulation and, more particularly, the storage conditions associated with hydro-meteorological forcing in the West Gironde mudflat. Trapping efficiency is negatively correlated with wave action (i.e. erosion) and positively correlated with river flow (i.e. accretion). On average, about 25% of the mud exported from the estuary is stored in the West Gironde Mud Patch.
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Mélanie Diaz. Modélisation numérique des transferts sédimentaires de l'estuaire de la Gironde au plateau continental. Océanographie. Université de Bretagne occidentale - Brest, 2019. Français. ⟨NNT : 2019BRES0095⟩. ⟨tel-02981004⟩

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