Cross-shore sediment transport and morphological response on a macrotidal beach with intertidal bar morphology, True Vert, France

摘要

A 3-week field investigation was conducted on a macrotidal beach with intertidal bar morphology with the aim of quantifying cross-shore sediment transport rates and beach response. Cross-shore transport rates were estimated in four different ways: (1) net transport from bed-level changes; (2) measured suspended sediment fluxes; (3) sediment fluxes associated with migrating wave ripples; and (4) total load transport predicted using the Bailard [Bailard, J.A. 1981. An energetics total load sediment transport model for a plane sloping beach. Journal of Geophysical Research, 86,10938-10954] model. Excellent qualitative agreement was found between the different measures under all recorded wave conditions, with onshore (offshore) sediment transport and beach accretion (erosion) observed under low-to- moderate (high) wave conditions. The agreement is due to the strong correspondence between the direction of mean cross-shore flows across the intertidal bar surface and that of the sediment transport. Significant onshore mean flows (u=0.1-0.2 m s~(-1)) were persistently observed under low wave conditions, and these were strongest in shallow water depths (h<1 m). These flows are attributed to the (weak) three-dimensional water circulation in the surf zone associated with the intertidal bar-rip morphology and caused onshore bar migration. Under high wave conditions, on the other hand, strong mean cross-shore flows (u=0.2-0.4 ms~(-1)), due to the bed return flow, were consistently directed offshore and resulted in bar destruction. Under low wave conditions, vertically-integrated suspended fluxes, ripple transport rates and predicted total transport fluxes were remarkably similar, but when the fluxes are integrated over a complete tidal cycle, they are considerably smaller than that derived from bed-level changes based on low tide surveys.