Numerical Modelling of Sediment Exchanges from the Gironde Estuary to the Continental Shelf: Hydrodynamic Model Validation and Sensitivity Analysis of Sediment Fluxes to Sediment Transport Parameters

Coastal environments are directly influenced by terrigenous inputs coming from rivers through estuaries. Quantifying the amount of nutrients and contaminants brought by sediment particles from the continental regions to the sea is of major interest for marine resource protection. Estuaries form a transition zone between river and maritime environments and therefore represent dynamic areas in terms of hydrodynamics, hydrology and sediment transport. The hydrodynamics is strongly dominated by asymmetrical tidal currents, modulated by neap and spring phasing. Moreover, the interaction between fresh and saltwater induces salinity gradients, and therefore density gradients, influencing the sediment dynamics. The complexity and the spatial variability of this interface area make it difficult to quantify the actual sediment transfers between the estuary and the ocean, either by in situ measurements, remote sensing or numerical simulation. This study is based on a 3D realistic hydrodynamic model of the Gironde estuary (South–West France) and the continental shelf seaward coupled with a multi-layer process-based sediment model accounting for sand and mud mixtures. In order to better understand the sediment dynamics between the estuary and the continental shelf, the objective is to provide a quantification of suspended sediment mass uncertainties associated with sediment transport parameters. The sensitivity analysis is conducted considering changes in the key sediment transport parameters, such as the settling velocity of the mud class and the sediment initial distribution. A better understanding of the uncertainties associated with simulated sediment transport will enhance the degree of confidence that can be placed in the quantification of the estuarine sediment exchanges with the sea. Such a quantification provides knowledge about the intra-estuarine exchanges with lateral intertidal mudflats, as well as sediment export to the sea, possible trapping in the subtidal muddy areas seaward of the mouth (e.g. the West Gironde Mud Patch) and dispersion along the adjacent coasts.