Effect of wave breaking on sediment eddy diffusivity, suspended-sediment and longshore sediment flux profiles in the surf zone

Turbulence intensity estimates from a wave basin experiment of prototype scale provide description of an eddy viscosity profile above the bottom-boundary layer under unbroken and broken waves. Detailed suspended-sediment concentration data from the DUCK94 surf zone experiment show that the sediment eddy diffusion profile (Ks) in the nearbed is consistent with a linear nearbed profile whose slope depends on the friction velocity. A full water column Ks profile that reflects the wave-breaking state is formulated from the wave basin and DUCK94 data. A simple model shows that the vertical structure of the Ks profile above the nearbed region is an important factor in predicting suspended-sediment profiles in the surf zone, in contrast to locations outside the surf zone where bottom-boundary layer friction is the primary mechanism in controlling suspension and distribution of suspended sediment. Under broken wave conditions, the difference between the predictions of suspended-sediment load in the lower 50 cm using a Ks profile that reflects broken wave influences and one that does not, averages 28 percent in this study. The prediction of longshore sediment flux is even more strongly influenced by the water column Ks profile because even small quantities of sediment that are suspended into the upper water column are subjected to higher velocities than in the nearbed. In this study, the ratios of measured longshore flux to prediction average 0.67 when the wave breaking state is not taken into account in the Ks profile.