Sediment transport near the break point associated with cross-shore gradients in vertical eddy diffusivity

Sediment clouds entrained and mixed vertically into the water column at the breakpoint were observed to advect offshore with the return current following wave breaking at a natural beach. An advection/diffusion model is applied to this phenomenon, providing for cross-shore gradients in vertical eddy diffusivity within and beyond the surf zone which result in differential entrainment and settlement near the break point. Results indicate that the horizontal gradient in turbulent vertical sediment mixing can provide a significant mechanism for bar generation, even if the surface gravity and infra-gravity waves remain uncoupled. Quantitative estimates of the rate of bar accretion are obtained after calibrating the sediment transport model using field measurements of currents and suspended sediment loads. In the absence of other processes, accretion rates of order 20 mm/hour associated with the horizontal gradient in vertical diffusivity are predicted on the offshore bar.