Sediment supply to beaches: cross-shore sand transport on the lower shoreface

Many beaches have been built by an onshore supply of sand from the shoreface, and future long-term coastal evolution critically depends on cross-shore sediment exchange between the upper and the lower shorefaces. Even so, cross-shore sediment supply remains poorly known in quantitative terms and this reduces confidence in predictions of long-term shoreline change. In this paper, field measurements of suspended sediment load and cross-shore transport on the lower shoreface are used to derive a model for sediment supply from the lower to the upper shoreface at large spatial and temporal scales. Data collection took place at five different field sites that exhibit a wide range of wave conditions and sediment characteristics. Data analysis shows that both suspended sediment load and cross-shore sediment transport scale with the grain-related mobility number which ranged up to ψ ≈ 1000 in the measurements while the effect of orbital velocity skewness is more limited. A 1 year long simulation of sediment transfers between the lower and the upper shorefaces on a natural beach compares well with transport rates estimated from long-term bar migration patterns and aeolian accretion on the same beach.