Accumulation and dispersion dynamics of mud and sand particles in a continental shelf under estuarine influence: a numerical modelling analysis (Gironde, France)
Mélanie Diaz (IFREMER – DYNECO/DHYSED & University of Bordeaux)
Zoom link: https://ukri.zoom.us/j/95276445782
Coastal environments are directly influenced by terrigenous inputs coming from rivers through estuaries. The complexity of the intra-estuarine dynamics associated with the strong variability of meteorological forcing makes it difficult to quantify the sediment exchanges at the estuarine mouth and the fate of fine particles offshore. Moreover, the dynamics of fine sediment trapping areas in the adjacent continental shelf (temporary or permanent storage) remains extremely challenging to address. Based on a realistic process-based numerical model, the aim of this work is (i) to investigate the dynamics of the sediment exchanges between a macrotidal estuary and its adjacent continental shelf, and (ii) to analyze the behavior of subtidal accumulation areas on sediment trapping and potential further resuspension. To do so, this study focuses on the macrotidal Gironde Estuary (France) and its adjacent continental shelf. It is one of the largest estuary of Western Europe with the most developed ETM in Europe. The surficial sediment map established by former studies exhibited the presence of a subtidal mudflat, the West Gironde Mud Patch (WGMP) offshore the estuarine mouth, which is known to trap particles coming from the Gironde Estuary. A mixed sand/mud sediment transport model was developed and applied on this study site (Diaz et al., 2020). The model outputs exhibited a strong variability of simulated sediment fluxes associated with hydrometeorological conditions. Although more fine sediments are exported within the surface turbid plume, the residual seaward mud transport is reduced during high river discharge compared to dry conditions due to the increased baroclinic circulation. Moreover, the model reproduced the signature of a subtidal mud accumulation area located to the North East of the WGMP (Figure 2). The trapping efficiency of the mudflat is negatively correlated with the wave action. Moreover, due to the estuarine turbid plume being more concentrated and developed during high river discharges, the trapping efficiency of the mud patch is enhanced compared to lower discharges.
Simulated mud fraction in the surficial sediment (11cm). Blue contours are delimitating bathymetric contours every 10m and the blue box highlights the mud accumulation area