COMPARISONS BETWEEN SEDIMENT TRANSPORT MODELS AND OBSERVATIONS MADE IN WAVE AND CURRENT FLOWS ABOVE PLANE BEDS

As a part of the MAST2 G8-M Coastal Morphodynamics project, the predic tions of four sediment transport models have been compared with detail ed laboratory data sets obtained in the bottom boundary layer beneath regular waves, asymmetrical waves, and regular waves superimposed co-l inearly on a current, Each data set was obtained in plane bed, sheet f low, conditions and each of the four untuned numerical models has prov ided a one-dimensional vertical (1DV), time-varying, representation of the various experimental situations. Comparisons have been made betwe en the model predictions and measurements of both time-dependent sedim ent concentration, and also wave-averaged horizontal velocity and conc entration. For the asymmetrical waves and for the combined wave-curren t flows, comparisons have been made with vertical profiles of the cycl e-averaged sediment flux, and also with the vertically-integrated net sediment transport rate. Each of the turbulence diffusion models gives an accurate estimate of the net transport rate (invariably well withi n a factor of 2 of the measured value). In contrast, none of the model s provides a good detailed description of the time-dependent suspended sediment concentration, due mainly to the inability of conventional t urbulence diffusion schemes to represent the entrainment of sediment i nto suspension by convective events at flow reversal. However, in the cases considered here, this has not seriously affected the model predi ctions of the net sediment flux, due to the dominance of the near-bed transport. The comparisons in this paper are aimed not only at testing the predictive capability of existing sediment transport modelling sc hemes, but also at highlighting some of their deficiencies.