Courants et transports sédimentaires dans la zone littorale : le rôle des courants orbitaux et de downwelling / Currents and sediment transport in the coastal zone: the role of orbital and downwelling currents

Abstract

Extended abstract Several types of currents occur along sandy shorelines. They are responsible for significant sediment transport and represent major processes causing coastal changes. Coastal currents are generated by various forcing mechanisms of atmospheric and/or marine origin which have distinct effects on the morphology and sedimentology of the littoral zone. The aim of this paper is to examine the role of coastal currents on the sediment dynamics of sandy shorelines. Tidal currents generally have limited effects on the morphodynamics of sandy coasts where wave- and wind-induced currents appear to be the main factors contributing to sediment transport in the coastal zone. The morphological and sedimentological effects that may be caused by these different currents are exemplified by a series of wave and current measurements carried out in the nearshore zone of the Mediterranean Sea and the Beaufort Sea. Examples of measurements of hydrodynamic parameters during fair-weather and storm conditions are shown in order to illustrate typical patterns of water circulation and sediment transport generated by different forcing mechanisms. Wave-generated currents result in both longshore and offshore sediment transport through shore-parallel littoral drift and seaward-directed rip currents and undertow. Wind-generated currents may result in more significant offshore sediment dispersal, particularly during storms. Some of the results presented in this paper show that the seaward-flowing downwelling near-bottom currents that develop in the nearshore zone during storm surges may be responsible for high magnitude offshore sediment transport. These currents differ from undertow and rip currents because they are not related to wave action but are generated by onshore wind stress causing a sea -level set-up at the coast. Because the coastal zone is not solely affected by surface gravity waves and associated currents, but also by other mechanisms such as wind-driven storm currents, existing models of beach/nearshore changes that are based only on wave-energy dissipation over an equilibrium profile may be seriously inadequate.