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A 3D sediment transport model based on the modified environmental fluid dynamics code (EFDC) and the nearshore waves simulation model (SWAN) is developed to study the change of suspended sediment concentration and bottom shear stress under the actions of pure current and wave-current. After being validated by the field measured data, the proposed sediment transport model is applied in the Oujiang River Estuary, China. The results show that the ratios of both bottom shear stress and suspended sediment concentration of pure current to those of wave-current show a gradually increase from shallow nearshore water to deep open sea. The results also show that the proportion of wave contributions on bottom shear stress and sediment concentration are above 60%, approximately 20-30% and less than 10% for the water depth of less than 5 m, 5-10 m and more than 20 m, respectively. For the waters among islands, the proportion of wave contribution to bottom shear stress and sediment concentration is reduced to 10-20% for -5 m water depth and this is more obvious for the waves of large amplitude. The bottom stress and suspended sediment concentration between islands are mainly controlled by tidal current, and the effect of wave is not significant. [ABSTRACT FROM AUTHOR]

Abstract: The effect of streaming on the sea bed boundary layer flow beneath combined waves and current has been investigated for waves following and opposing a current, taking both linear waves and second order Stokes waves into account. This flow results from an interaction between the classical wave–current seabed boundary layer mechanism and two different streaming mechanisms. The classical wave–current seabed boundary layer mechanism leads to a reduced mean velocity relative to the current alone; the two streaming mechanisms are streaming caused by turbulence asymmetry in successive wave half-cycles (beneath asymmetric forcing) and streaming caused by the presence of a vertical wave velocity within the seabed boundary layer as earlier explained by Longuet-Higgins. The effect of wave asymmetry, wave length to water depth ratio, magnitude of current and bottom roughness have been investigated by numerical simulations for realistic physical situations. Mean Eulerian quantities as well as the mass transport (wave-averaged Lagrangian velocity) are presented. The resulting sediment dynamics near the ocean bottom has been investigated; results for both suspended load and bedload are presented. For wave dominated situations, the velocity profiles beneath opposing waves and current are substantially different from those beneath following waves and current, both for linear and second order Stokes waves. As the flow becomes current dominated, the velocity profiles beneath opposing and following waves and current approach the velocity profile beneath horizontally uniform sinusoidal forcing. The net transport of suspended sediments and bedload is in the direction of the current both for waves following and opposing the current. For wave dominated situations the net sediment transport is strongly affected by the streaming mechanisms, both for linear and second order Stokes waves and current. [Copyright &y& Elsevier]

Abstract: Appropriate descriptions of turbulence are important in predicting sediment transport and seabed evolution. We use here a three-dimensional model to investigate the impacts of sediment-induced stratification, erosion parameterization, and turbulence closures on sediment transport and morphological simulations. The model is implemented in the Proudman Oceanographic Laboratory Coastal Ocean Modelling System (POLCOMS), which couples a hydrodynamic model to the General Ocean Turbulence Model (GOTM) and to a sediment transport model. This sediment transport model calculates both suspended sediment concentrations and bed evolution. A large number of turbulence closures can be tested and we focus on the methods used to calculate the turbulent length scale and the stability functions. Different modelling scenarios are assessed against experimental data of velocity profiles, suspended sediment concentrations, and trench migration in a laboratory flume. Numerical results show that the processes investigated have little impact on the flow velocity profiles, in spite of some differences on the bed shear stress. The only exception is the one-equation turbulence closure, which cannot reproduce velocity profiles appropriately. Sediment-induced stratification, erosion parameterization, and turbulence closures can all have significant impacts on the suspended sediment concentration and the bed evolution. Model-data comparisons are found to be particularly sensitive to the method used to calculate the turbulent length scale. [Copyright &y& Elsevier]

Abstract: The three-dimensional numerical model COHERENS (COupled Hydrodynamical-Ecological model for REgioNal and Shelf seas) has been adapted to compute the rates of transport as bedload and suspended load of heterogeneous bottom sediments induced by the dominant M2 tide in the English Channel. A pre-processing of an extensive surficial sediments dataset has been performed to determine the seabed composition (grain-size distribution or presence of rocks) at the computational grid nodes. Maximum bedload and suspended load transport rates over the tidal cycle as well as the contributions of the 10 different sedimentary classes to the mean transports are computed. Highest sediment transport rates occur in fine sediments areas located in the surroundings of high shear stresses areas. Medium sand is found to be predominant in bedload, while suspension load implies mainly silts in the inner shoreface and both fine and medium sands in the outer shoreface. The offshore residual bedload transport pathways are orientated westerly in the western part of the Channel and easterly in the eastern part defining a “parting” zone which runs from the Isle of Wight to the Cotentin Peninsula. An offshore “bedload convergence” occurs in the southwest of the Dover Strait; a narrow transport pathway bypassing it along the French coastline. These features reproduce those predicted by and provide higher resolution features like inshore headland-induced gyres, particularly along the English coastline. The new predicted general pattern of residual suspended load transport is very similar to the bedload pattern. Differences arise in the central “divergence” zone which exhibits a “Y” shape with two branches ending on both sides of the Isle of Wight, in the Baie de Seine characterized by a central “convergence” and along the English coastline studded with many headland-induced recirculations. [Copyright &y& Elsevier]