Effect of surface wave skewness on near-bed sediment transport velocity

The paper presents a two-dimensional model, based on the Lagrangian description of sand grain dynamics, for the estimation of mean velocities of sediment particles driven by the oscillatory motion of water generated by non-linear gravitational surface waves of the Stokes type. The model was verified against experimental results obtained in a wave flume, by comparing its predictions with the sediment velocities measured by applying a PIV (particle image velocimetry) technique. The model was used to calculate near-bed sediment transport velocities generated by surface waves of the parameters encountered in real sea conditions, with the purpose to investigate the effect of the wave velocity skewness on the near-bed sediment net transport rates. The results of numerical simulations, carried out for a range of wave parameters and sand grain sizes, showed that normalized sediment transport velocities vary approximately linearly with an index defining the wave skewness, and are practically independent of sediment grain diameters. Based on the laboratory measurements, also approximate formulae relating a bed ripple asymmetry index with the surface wave skewness index were derived.