Observations of net sediment transport rate and boundary layer of wave-current flows over vortex ripples

In shallow coastal regions, shoaling waves and current together determine the net sediment transport rate,Q(net), which is critical for understanding coastal morphodynamics. Moderate waves produce vortex ripples on a sandy seabed, which dramatically changes local wave-current interaction. This study aims at improving our understanding of Q(net) and boundary layer flow under collinear wave-current flow over a rippled bed. Two sets of full-scale experiments were conducted using an oscillatory water tunnel, which approximates wave as sinusoidal oscillatory flow. The live-bed tests, in which 2-dimensional sand ripples were produced over a coarse-sand bed, provided measurements of Q(net) and visual observations of flow-sediment interaction Q(net) under the same wave condition changes from against-current to following-current as the co-existing current increases, which agrees with some previous experiments. In the fixed-bed tests, which have fixed concrete model ripples covered by sandpapers, the detailed flow fields were measured using a particle image velocimetry. The results reveal that the current enlarges the spanwise coherent vortex (SCV) under the positive half cycle (wave and current velocities are co-directional), but reduces the SCV in the negative half cycle. Using turbulence intensity as a proxy for sediment concentration, how ripple-averaged sand flux changes with the current condition was discussed. Under a weak current, the two SCVs are slightly changed, and the key flow feature is still the formation-ejection process of SCVs, so an against-current Q(net) is produced due to the phase-lag effect. Under a strong current, the SCV in the positive half cycle is significantly enlarged by the current, and it brings sand to high levels before its ejection, which makes the phase-lag effect less important than the current advection, so Q(net) becomes following-current.