Vortex trapping of sand grains over ripples under oscillatory flow

Sand ripples significantly impact morphodynamics in the nearshore by generating coherent vortices, which can transport suspended sediment to greater heights in the water column than above flat beds. Coherent vortices can trap sediment grains if the settling velocity of the grain is smaller than the maximum vertical fluid velocity in the vortex (Nielsen 1992). Particle image and tracking velocimetry were used to measure small-scale fluid-sediment interactions over sand ripples in a small oscillatory flow tunnel. Here we present some of the first measurements of vortex-trapped sediment grains under oscillatory flows. Results showed that the vortex-trapped sand grain traversed an orbit offcenter of the vortex near the ripple slope. Some grains then spiralled outward and settled to the bed; others were transported by the flow as the vortex was shed from the crest. Vortex trapping can delay settling and increase settling times, potentially causing inaccurate sediment transport predictions by large-scale numerical models, which do not typically account for this non-linear small-scale process.