Abstract. Field observations often show a considerable variation in mean grain size along the coastal profile. Under high waves in shallow water, bed ripples are washed out, and sheet flow becomes the dominant transport mode: large amounts of sand are transported in a thin layer close to the bed, i.e., the sheet flow layer. This paper focuses on grain size influences on transport processes in oscillatory sheet flow. Experiments were carried out in the Large Oscillating Water Tunnel (LOWT) of Delft Hydraulics, in which near-bed orbital velocities in combination with a net current can be simulated at full scale. Three uniform sands with different mean grain size were used. It was found that in contradiction to expressions found in literature, both the erosion depth and the sheet flow layer thickness are larger for fine sand (Ds0 = 0.13 mm) than for coarser sand (Ds0 -> 0.21 mm). Measured time-averaged velocity and concentration profiles indicate that the presence of a sheet flow layer leads to an increased flow resistance and to damping of turbulence and that these effects are stronger for a thicker sheet flow layer (i.e., for fine sand). These mobile-bed effects are analyzed further by comparing the measurements with the results of a one-dimensional vertical advection-diffusion boundary layer model. Simulating the mobile-bed effects in the model by introducing an increased roughness height and a reduced turbulent eddy viscosity showed that the roughness height is of the order of the sheet flow layer thickness and that turbulence damping increases for a decreasing grain size. 27,103
A large series of field-scale experiments on turbulent sand-laden flows, conducted in preceding years in the LOWT and AOFT large oscillating water tunnels are reviewed and reanalysed. Using the combined experimental data sets, new insights are obtained on the detailed sand transport processes occurring in sheet-flow and ripple regime conditions. For sheet flow (i) new equations are presented relating maximum erosion depth and sheet-flow layer thickness to the maximum Shields parameter; (ii) detailed analysis of sediment flux data shows the dominance of the current-related flux in the sheet-flow layer and the different characters of the current-related flux for fine and medium sands; (iii) a RANS-diffusion type model is shown to reproduce important trends in net transport rate related to grain size, velocity and wave period and to predict the magnitude of net transport rate to within a factor 2 of measured values. For the ripple regime it is shown that (i) asymmetric waves generate negative ('offshore') streaming and the current-related suspended sediment flux associated with this streaming appears to be of the same order of magnitude as the wave-related suspended sediment flux; (ii) time-averaged near-bed transport and time-averaged suspended transport appear to be of about equal magnitude but of opposite sign, and are concentrated on the 'onshore' flank of the ripple for asymmetric wave conditions; (iii) near-bed transport along the onshore flank is generated by sand transported over the ripple crest during the 'onshore' half-cycle. Net sand transport under asymmetric waves can be 'onshore' directed or 'offshore' directed, depending on the degree of unsteadiness in the sand flux behaviour during the wave cycle. Dimensionless phase-lag parameters are presented, for sheet flow and ripples, which can discriminate between predominantly quasi-steady behaviour (resulting in 'onshore' transport) and predominantly unsteady behaviour (resulting in 'offshore transport').
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