Sand motion induced by oscillatory flows: Sheet flow and vortex ripples

Abstract

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').

Bed level motions and sheet flow processes in the swash zone: Observations with a new conductivity-based concentration measuring technique (CCM + )

“…When near-bed velocities are sufficiently high, sand transport is confined to a high-concentration (100 to 1600 g/L) layer with a typical thickness of 10 to 100 times the grain diameter (Ribberink et al, 2008).…”

Sand transport processes in the surf and swash zones

“…by setting Ta = 0.5 in Equation 4.7), the estimated crest phase decreases while the trough phase increases. In that case, as can be concluded from Figure 4.7, the data points approach the Ribberink et al (2008) predictions more closely but move away from Schretlen's (2012) predictions. …”

“…Due to the scatter of the data and the limited range of hydrodynamic forcing covered in the present study, it cannot be concluded whether the relation between s and in Figure 4.7 is linear. Although the formulation of Schretlen (2012) performs slightly better than the one of Ribberink et al (2008), we remark that neither of the two original formulations were developed for acceleration skewed flows. If the acceleration correction is dismissed (i.e.…”

“…To assess whether wave breaking affects the sheet flow layer thickness, s is quantitatively compared with predictions by two empirical formulations for maximum s that have been proposed on the basis of detailed laboratory measurements using well sorted sand and regular oscillatory and wave conditions: firstly, the formulation by Ribberink et al (2008) based on oscillatory tunnel data:…”

Laboratory Experiments for Wave-Driven Sand Transport Prediction

“…Recourse is again made to measurements from large-scale experiments to estimate δ s . For example, Ribberink et al [2008] propose δ s /d 50 = βθ max , with calibration factor β = 10.6. For velocity-skewed oscillatory flow, the phase lag parameter for the positive half-cycle is greater than that for the negative half-cycle because (a) δ s is greater due to higher Shields and (b) flow deceleration time is shorter ((T c -T cu ) < (T t -T tu )); the result is a greater proportion of positive half-cycle-entrained sand being carried into the following negative half-cycle than negative halfcycle sand being carried into the following positive half-cycle, as seen in Figure 5.…”