Résumé
Hydrodynamic circulation and sediment (survey) patterns (in situ directional wave and current measurements, sedimentary and topographic surveys) were investigated on a macrotidal beach located on the southern North Sea coast of France. The Calais beach site is one of the rare actively accreting sectors, due to sand supply following the onshore migration and coastal welding of a nearshore sand ridge. The aims of the present study is to understand the behaviour of a dissipative beach during storm event. The role of wave and current processes was investigated to understand the dynamics of intertidal bar-trough systems and sandflat system, specifically in a macrotidal context, under high energy events, with high sediment mobility. The hydrodynamic measurements show (1) the natural role of swash bar in beach protection during storm events (onshore decrease in current speeds and in wave heights) and (2) the role of sandflat in wave mitigation, compatible with a fining trend, from sand in the lower sand flat, to sand and interbedded mud layers towards the upper flat. Our results imply that although sediment transport on this macrotidal beach is strongly controlled by tidal processes, waves can significantly enhance bed shear stresses, increasing erosion rate remobilization and hence higher sediment transport. This pattern shows that an accretionary context on sand-rich, storm-wave dominated macrotidal coasts offers conditions for the significant trapping of mud, with implications for contemporary management issues or sedimentary budget. Keywords: macrotidal, sandflat, muddy beach, hydrodynamics, storm.