Modelling and observation of oscillatory sheet-flow sediment transport

Sand transport beneath large waves occurs in the plane-bed, sheet-flow regime. Comparisons between two sedimenttransport models and oscillatory sheet-flow experiments conducted in a flow tunnel are presented here. The experiments represent field-scale asymmetric (velocity-skewed) wave conditions over fine, medium and coarse sands, with median grain diameters of 0.13, 0.27 and 0.46 mm, respectively. The two numerical models used in the study are a two-phase flow model and a simpler two-layer, turbulence-closure model, both of which are one-dimensional vertical (1DV). The two-phase model takes account of the complete fluid–particle interactions, and the two-layer model uses an empirical description of the processes within the sheet-flow layer. The measured and predicted time-varying velocity, concentration and flux profiles, as well as the erosion depth and the net transport rates are compared and analysed. Overall, the predictions of both models are shown to be in good agreement with the measurements. The models predict the changing characteristics of the sheet-flow layer with grain size including the increasing importance of phase-lag effects for finer sands and the change in the net transport rate direction from onshore for coarse and medium sands to offshore for fine sands, with important implications for sediment sorting in the nearshore zone