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Barred macrotidal beaches are affected by continuous horizontal displacements of different hydrodynamic zones associated with wave transformation (shoaling, breaker and surf zones) due to significant tide-induced water level changes. A series of wave and current meters, complemented by a video imagery system, were deployed on a barred beach of northern France during a 6-day experiment in order to characterize the spatial and temporal variability of wave-induced processes across the beach. Wave and current spectral analyses and analyses of cross-shore current direction and asymmetry resulted in the identification of distinct hydrodynamic processes, including the development of infragravity waves and offshore-directed flows in the breaker and surf zones. Our results revealed a high spatial variability in the hydrodynamic processes across the beach, related to the bar-trough topography, as well as significant variations in the directions and intensity of cross-shore currents at fixed locations due to the horizontal translation of the different hydrodynamic zones resulting from continuous changes in water level due to tides.

The vertical distribution of longshore sediment transport was measured on sandy macrotidal barred beaches of Northern France under low to moderate wave energy conditions, using streamer traps collecting sand at different elevations above the bed. Longshore sediment flux measurements and wave and current data were collected at several locations along cross-shore transects in order to analyse the relation between the vertical patterns of suspended sand transport and the forcing hydrodynamics across the bar-trough topography. Suspended sediment fluxes generally showed a typical upward decrease in longshore transport, revealing that near-bed transport represents the major transport mode on these macrotidal beaches, notably over the bars where the highest rates of sediment transport were measured. Conversely to previous studies in which the shape of the vertical distribution was either independent of the location in the surf zone or which showed an increase in suspended transport over intertidal bars, our results revealed a strong upward decrease in longshore sediment transport over bars, especially under higher wave energy conditions, whereas more uniform vertical distributions were observed in troughs. These differences in the vertical distribution of longshore sediment flux can be explained by spatially variable wave breaking and surf zone processes over the bar-trough topography and by the distribution of small-scale bedforms across the intertidal zone, which influence sand resuspension processes and control the shape of the vertical profiles of sediment transport. Larger longshore sediment fluxes over bars can be explained by the action of breaking waves that are responsible for increased sediment remobilization, but the dominance of spilling breakers on the studied beaches likely results in significant turbulence dissipation, leading to limited sand resuspension in the water column. Comparatively, landward propagating surf bores over bars favour an increase in sediment concentration at higher elevations above the bottom in the adjacent trough. [ABSTRACT FROM AUTHOR]

Barred macrotidal beaches are affected by continuous horizontal displacements of different hydrodynamic zones associated with wave transformation (shoaling, breaker and surf zones) due to significant tide-induced water level changes. A series of wave and current meters, complemented by a video imagery system, were deployed on a barred beach of northern France during a 6-day experiment in order to characterize the spatial and temporal variability of wave-induced processes across the beach. Wave and current spectral analyses and analyses of cross-shore current direction and asymmetry resulted in the identification of distinct hydrodynamic processes, including the development of infragravity waves and offshore-directed flows in the breaker and surf zones. Our results revealed a high spatial variability in the hydrodynamic processes across the beach, related to the bar-trough topography, as well as significant variations in the directions and intensity of cross-shore currents at fixed locations due to the horizontal translation of the different hydrodynamic zones resulting from continuous changes in water level due to tides. [ABSTRACT FROM AUTHOR]