Residual transport model in correlation with sedimentary dynamics over an elongate tidal sandbar in the Gironde Estuary (Southwestern France)

This study qualitatively describes by a multidisciplinary approach hydrodyn amic and residual sediment transport over a sandbar in a macrotidal environ ment. The sandbar is elongated and attached to a headland that is located i n the Gironde estuary (southwestern Prance). Bathymetric surveys, side scan -sonar sonograms, grain-size trends of surficial sediment, and 2DH numerica l simulations of current patterns were used to define a conceptual model fo r the dynamics of this sandbar, The sandbar is oriented anticlockwise with respect to the main flow, The channel-oriented bar face is ebb-dominated, w hereas the opposite face is flood-dominated, resulting in a clockwise resid ual transport, The line of sediment flu convergence corresponds to the cres t of the bar. The residual flow over the bar appears to originate from the regional estuarine flow. The dominant process responsible for the maintenan ce of the sandbar seems to be the depth-averaged main tidal flow rather tha n secondary circulation due to headlands or the interaction between the tid al flow and the seabed, To generalize this regional conceptual model a qual itative comparison is made with the theories of sandbar dynamics due to tid al currents (Pingree and Maddock 1979, Zimmerman 1981, Huthnance 1982, and others), The seabed stability approach developed by Huthnance (1982) seems to be the most reliable theory to explain the Saint-Georges Bank dynamics, even though many natural factors are not taken into account.