Effects of unsteadiness on the suspended sediment flux in co-linear wave-current flow

Time variations in horizontal velocity and suspended concentration can significantly influence the net sediment transport rate in an unsteady boundary layer flow. Here the “current-related” and “wave-related” contributions to the sediment flux are examined for co-linear wave-current flow using a one-equation turbulence-closure model. Two aspects of unsteadiness are investigated by means of a case study. Initially, the phase relationship between the bottom reference concentration and the bed shear stress is shown to influence both the mean (time- and depth-averaged) suspended sediment concentration and also the sediment flux. These quantities are then shown to be influenced also by variations in the value of the ratio (β) of the sediment diffusivity to the eddy viscosity (even at a fixed value of Ws/β, Ws = sediment settling velocity. It is demonstrated that the wave-related contribution may dominate the net sediment flux for grains confined to the oscillatory boundary layer, whereas, if grains are well diffused into the outer current boundary layer region, the wave-related contribution becomes unimportant. More generally, the effect of wave size in relation to the current is investigated, and estimates of the net flux for differing reference concentration phase lags are discussed. The results presented are applicable primarily to sediment transport above plane beds (i.e. sheet flow layers) above which turbulent diffusion is the dominant mechanism of entrainment. However some tentative conclusions are also reached concerning sediment transport above rippled beds.