Sediment transport under wave and current combined flows on a tide-dominated shoreface, northern coast of France

Abstract

Hydrodynamic circulation and sediment transport patterns were investigated on a macrotidal shoreface off the southern North Sea coast of France from in situ directional wave and current measurements and through numerical modeling of sediment flux. Current meters equipped with wave gauges were deployed at two sites on the mid-upper shoreface, in approximately 5–6 m water depth, during 6 and 15 days. Sediment flux was estimated using the SEDTRANS96 numerical model (Li, M.Z., Amos, C.L, 2001. SEDTRANS96: the upgraded and better calibrated sediment transport model for continental shelves. Comp. Geosc. 27, 619–645) based on Grant and Madsen's (Grant, W.D., Madsen, O.S., 1986. The continental shelf bottom boundary layer. Ann. Rev. Fluid Mech. 18, 265–305) combined flow boundary layer theory. The hydrodynamic measurements and modeling results show that sediment on the shoreface is essentially transported alongshore in response to shore-parallel tidal currents. The direction of the net sediment transport is largely determined by the asymmetry of the tidal flows, which can be flood- or ebb-dominated, but during important wind speed events, winds blowing in the same direction as tidal flow can significantly reinforce tidal currents, or conversely, can limit tidal current speeds when acting in the opposite direction. Although sediment transport on this macrotidal shoreface is strongly controlled by tidal processes, waves can significantly enhance bed shear stresses, resulting in increased sediment remobilization, and hence higher sediment flux. Wave-induced onshore sediment transport may occasionally occur, but is several orders of magnitude lower than longshore transport. This macrotidal shoreface is thus characterized by a hydraulic regime strongly dominated by shore-parallel tidal flows that favour longshore sediment dispersal rather than shore-normal transport. Our results imply that the water depths from which sediment may be transported shoreward to beaches is certainly much more restricted on a macrotidal shoreface, such as the one bordering the northern coast of France, than on micro- and mesotidal shoreface environments where weaker tidal currents do not significantly limit cross-shore sediment movements.