Numerical study of sediment transport above rippled beds under the action of combined waves and currents.

To study sediment suspension above ripples under the combined action of waves and currents, a three-dimensional numerical model has been developed based on the use of FLUENT software, and an external sediment transport model. The computer model has been tested against laboratory measurements involving oscillatory wave motion, as well as cases of co-linear waves with following and opposing currents, with satisfactory results. Compared with the situation in which only waves are present (called waves-alone cases), the effects from the steady current on both vortex shedding and sediment suspension above ripples have clearly been revealed by the model results. In particular, the vortices generated in combined waves and currents tend to stay low in the trough area of the ripple and are ejected earlier than those in the waves-alone case at both the ripple crest and trough, which leads to concentration peaks at different phases and with different magnitudes. The model was also applied to a field case from a multi-barred, dissipative beach at Egmond-an-Zee, in the Netherlands, to investigate the influences of a long-shore current on cross-shore sediment transport. The model results show reasonable overall agreement with the field measurements, as well as the important effects of the three dimensional flow structure on the sediment entrainment process close to the ripple surface, which is very difficult to observe in such detail in field studies. [ABSTRACT FROM AUTHOR]