Modelling sediment transport beneath regular symmetrical and asymmetrical waves above a plane bed

The accurate prediction of sediment transport in wave-dominated coastal areas presents a continuing challenge to engineers and oceanographers. Here a one-equation turbulent kinetic energy closure numerical model is used to investigate the sediment transport beneath large symmetrical and asymmetrical waves, and also a (co-linearly) combined wave and current flow. This model resolves the height- and time-dependence of both the velocity and sediment concentration fields during the wave cycle and, hence, the potentially significant ‘wave-related’ contribution to the net sediment transport. Comparisons are made with existing, detailed, laboratory data obtained in the plane bed (i.e. sheet flow) regime. The effects of different bottom boundary conditions on sediment in suspension are considered by comparing model solutions based on a standard reference concentration formula with those based on two ‘pick-up’ functions. For the asymmetrical waves, the results for the net transport rate are shown to be strongly dependent upon the way in which small wave-induced residual currents are represented. The net transport is always predicted to within a factor of ±1.5 of the measured values, using both the reference concentration formula and also the optimum choice for the bottom pick-up condition, provided that residual current effects are accounted for in the model. The limitations of the present modelling approach, which is based on classical turbulent diffusion arguments, are discussed in the light of the ‘convective’ sediment entrainment events at flow reversal which appear in all of the data considered and which affect the ‘wave-related’ contribution to the transport.