Mixed-grain sand transport induced by turbulent oscillatory flow over coastal ripples

The objective of this numerical study is the investigation of mixed-grain sand transport driven by turbulent oscillatory flow over fixed coastal vortex ripples. A large-eddy simulation model was employed for the coupled simulation of the three-dimensional turbulent oscillatory flow and the corresponding bed and suspended sediment transport. The immersed boundary method was implemented for the imposition of fluid flow and suspended sediment transport boundary conditions on the bed surface. Mixed-grain sediment was represented by splitting the grain-size distribution curve into equal mass fractions. Bed and suspended load were considered for each sediment fraction. The results showed that finer grains generate broader, higher-reaching suspended plumes, whereas coarser grains remain concentrated near the bed with limited vertical extent. The bed load exhibited a linear increase with grain size, while the suspended load exhibited a more complex behaviour for the fine and the coarse fractions. Overall, the study shows that mixed-grain sediment transport is not always well modelled by transport based on the single mean sediment grain size.