Influence of breaking waves on sediment concentration profiles and longshore sediment flux in the nearshore zone
Two experiments provided data for the investigation of the influence of wave breaking on suspended sediment concentration profiles and sediment flux in the nearshore zone. A prototype wave basin experiment provided velocity fluctuation profiles in the upper water column and the DUCK94 field experiment provided detailed suspended sediment concentration profiles in the nearbed region.The wave basin turbulence intensity results show wave breaking in the surf zone not only causes increased levels of turbulence near the water surface, but also causes increased levels of turbulence throughout the water column. Eddy viscosity estimates suggest wave breaking can affect mixing and diffusion throughout the entire water column. The DUCK94 sediment concentration data provided nearbed sediment eddy diffusivity estimates, and the nearbed profile was found to increase linearly with height above the bed. Also, the relationship in the nearbed between eddy diffusivity for sediment and eddy viscosity for momentum was examined, and under unbroken waves a ratio of 0.48 was found between them. Under broken waves, no relationship could be determined, suggesting methodology used to describe turbulent diffusion outside the surf zone is not necessarily applicable inside the surf zone, and bottom boundary shear may not be the primary mechanism responsible for the vertical distribution of suspended sediment.The wave basin and DUCK94 information were combined to formulate unbroken and broken wave eddy diffusivity profiles. A simple one-dimensional model was created to examine the impacts of the eddy diffusivity profile on suspended sediment and longshore sediment flux. Under unbroken waves, the eddy diffusivity form in the upper water column is not critical because such a small amount of sediment is suspended above the wave boundary layer. Under broken waves, sediment is suspended higher in the water column and thus, the eddy diffisivity form significantly impacts the predicted suspended sediment profile. For example, the integrated suspended sediment load and longshore sediment flux (for the lower 50 cm of the water column) are under predicted by at least 25 and 33 percent, respectively, when using an eddy diffusivity profile that does not reflect wave breaking.
- Oceanography