Flow velocities, sediment transport and morphological change in the swash zone of two contrasting beaches

As part of a large UK research project focussing on the role of swash on shoreline change, two field experiments were conducted on contrasting sandy beaches: a flatter dissipative beach and a steeper intermediate beach. This paper describes almost 300 h of nearshore velocity data collected during this project, supplemented by suspended sediment transport, sediment tracer and beach profile data. During both field experiments, the upper part of the beach experienced progressive accretion, indicating that onshore sediment transport prevailed in the swash and inner surf zones. Suspended sediment transport and tracer dispersal were also onshore. The associated cross-shore flow field was, however, dominated by offshore-directed mean flows, especially on the steeper beach. Velocity skewness (defined as <u3> / <u2>1.5, where u is the cross-shore velocity) and kurtosis (defined as <|u|3u> / <u2>2) were mainly directed offshore, implying that energetics-based sediment transport models are unable to predict the direction of sediment transport in the swash zone. Possible explanations for the observed accretion include enhanced bed shear stress on the uprush (due to in/exfiltration effects, flow acceleration and/or bore turbulence) and advection of bore-entrained sediment into the swash zone.