Suspension and near-bed load sediment transport processes above a migrating, sand-rippled bed under shoaling waves

The present study focuses on the fine-scale flow and sand transport processes above onshore migrating ripples below skewed surface gravity waves in the shoaling zone. A set of acoustic instruments was deployed in the shoaling region of the large-scale wave channel at Canal d'Investigacio i Experimatacio Maritima, Universitat Poltiecnica de Catalunya, Barcelona, Spain, in order to provide high-resolution velocity and sediment concentration profiles with an acoustic concentration and velocity profiler (ACVP). Measurements are analyzed relative to the positions of the measured nonmoving sand bed and the interface separating the suspension from the near-bed load layer. This interface is detected here by the application of a novel acoustic bed echo detection method. Furthermore, the use of the dual-frequency inversion proposed in the work of Hurther et al. (2011) allows for the calculation of the sediment concentration profile across both the suspension and near-bed load layers. The sand bed was covered by quasi-two-dimensional suborbital ripples migrating onshore. As proposed by O'Donoghue et al. (2006), the occurrence of quasi-two-dimensional ripples is attributed to the fine-size sand of D-50 = 250 mm used in the present study under full-scale forcing conditions. In order to determine the effect of shoaled wave skewness on the ripple vortex entrainment and sediment transport, the instantaneous and mean measurements of the flow, sediment concentration, and sediment flux along the ripple profile are discussed in terms of (1) the occurrence of ripple vortex entrainment on either side of the ripple crest; (2) the wave velocity phase lagging driven by the ripple vortex entrainment process and the turbulent bed friction effects in the wave boundary layer; (3) phase lagging between velocity and maximum concentration and sediment flux events; (4) the structure of bed friction and ripple-driven turbulence across the suspension and the near-bed load layers; and (5) the streaming components. The results on these aspects strongly support that the wave velocity skewness effect under shoaling waves is fairly similar to the one obtained in skewed oscillatory water tunnel flows. Furthermore, it is found that the onshore-oriented net bed load sediment transport is at the origin of the onshore ripple migration. This flux is roughly twice as much as the opposite offshore-oriented net suspension flux dominated by the ripple vortex entrainment processes.