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 n e t , 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 n e t 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 n e t and visual observations of flow-sediment interaction. Q n e t 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 n e t 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 n e t becomes following-current. • Full-scale live/fixed-bed tests of wave–current flow over ripples were conducted. • Spanwise vortex is enlarged/reduced when the wave flow follows/opposes the current. • Flux of turbulence is analysed to interpret the direction of net sand transport. • Phase-lag effect leads to a negative sand transport rate under a weak current. • Current advection determines a positive sand transport rate under a strong current. [ABSTRACT FROM AUTHOR]