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.