Cross-shore suspended sediment transport in a macro-tidal and low-sloping upper shoreface

The driving mechanisms of sediment transport across the upper shoreface have been poorly examined to date because of the scarcity of field measurements beyond the intertidal area, which is mainly due to instrumentation difficulties. Here, we investigate the cross-shore suspended sediment transport from synchronized time series of bottom pressure, current velocity and suspended sediment concentration measured by 13 m water depth, in a macro-tidal and low-sloping shoreface, under storm waves and fair weather conditions. Depending on the tidal phase and the incoming wave height, the measuring structure was alternatively located in the shoaling zone and in the outer surf zone. The analysis of field observations first revealed that steady currents dominate suspended transport, with a notable contribution of the wave-driven return current under storm wave conditions. Cross-spectral analysis further indicates that short waves globally induce an offshore-directed transport under energetic conditions associated with a crest-to-trough phase lag effect. The contribution of infragravity (IG) waves, which reaches as much as 20% of the total transport, shows contrasting directions depending on whether IG waves are bound to the short wave groups or propagate as free waves.