Suspended sediment transport mechanisms in high-energy swash

Abstract

Measurements of suspended sediment concentration at three heights above the bed, and cross-shore velocity at a single height were obtained using miniature optical backscatter sensors and a miniature electromagnetic current meter, from the swash-zone of a high-energy macrotidal dissipative beach. Time-series from relatively low ( H s≈0.8 m) and high ( H s≈2.2 m) energy conditions were chosen for analysis. Possible onshore and offshore sediment transport mechanisms are identified as (a) sudden offshore to onshore velocity transition and turbulence in the swash-front leading to subsequent onshore advection by the uprush, and (b) low-frequency high-velocity backwashes exceeding a certain threshold for sediment suspension. These mechanisms are related to the velocity field through skewness and asymmetry. An increase in negative infragravity skewness in high-energy conditions suggests a potential shift towards offshore net transport. Time-averaged vertical profiles of cross-shore suspended sediment flux were calculated from the velocity and concentration time-series. These show a tendency towards offshore transport in high-energy conditions, particularly at low levels in the water column. The actual sediment fluxes are believed to be more offshore than those calculated. One reason for this discrepancy is the inability of the instruments to detect swash activity at very low levels in the water column.