|Sand motion induced by oscillatory flows: sheet flow and vortex ripples|Ribberink, J.S.; van der Werf, J.J.; O’Donoghuet, T.; Hassan, W. (2013). Sand motion induced by oscillatory flows: sheet flow and vortex ripples. J. Turbulence 9(20). dx.doi.org/10.1080/14685240802220009
In: Journal of Turbulence. IOP Publishing: Bristol, UK. ISSN 1468-5248, more
Bed load; Ripples; Sediment transport; Sheet flow; Suspension; Marine
|Authors|| || Top |
- Ribberink, J.S.
- van der Werf, J.J., more
- O’Donoghuet, T.
- Hassan, W., more
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’).