Sediment concentration and vertical mixing under breaking waves

Field measurements of near-bed sediment concentrations and sediment diffusivity in the breaker zone and in the inner surf zone of natural beaches were obtained using stacks of optical and fiber-optical backscatter sensors under conditions with strongly plunging breakers as well as with spilling surf bores. The field data suggest that significant differences exist between these wave types in terms of near-bed sediment concentration levels and vertical mixing of sediment from the bed upward into the water column. When plotting mean sediment concentration profiles in log C-z space, profiles are upward concave for spilling surf bores, and sediment diffusivity increases approximately linearly upward from the bed. For plunging waves, however, mean sediment concentration profiles are linear in a log C-z plot indicating vertically constant sediment diffusivity. This suggests that under surf bores, sediment suspension from the bed occurs through diffusion processes while vertical mixing is mainly a convective process under plunging waves. Under surf bores, sediment is largely confined to the lower layers of the water column while it is lifted to significantly higher elevations under plunging breakers.