3D Numerical Modelling Of Sediment Transport And Morphological Evolution Due To Coupled Wave-Current

Sediment transport and morphological evolution on sandy beach are important research topics in coastal dynamics.A three-dimensional numerical model of sediment transport and morphological evolution,including various mechanisms of water motion and sediment transport was developed in this dissertation.The model was validated by using a large number of laboratory and field data.The research contents and conclusions are as follows.(1)A new three-dimensional radiation stress formulation including beach slope effects was derived based on the Lagrangian solutions of progressive waves.The formulation provided a better balance between radiation stress gradient and hydrostatic pressure gradient.Moreover,the derivation could precisely satisfy a zero pressure condition for the surface.(2)A three-dimensional coupled wave-current model with unstructured grids was developed based on the FVCOM hydrodynamic model and the SWAN wave model.Information exchanges between the two models were carried out through using MCT software.The coupled model included the three-dimensional radiation stress,surface roller,and the horizontal eddy viscosity coefficient and bottom shear stress under combined waves and currents.(3)The coupled wave-current model was validated through simulations of nearshore circulations.The results showed that the model was capable of simulating wave-induced currents in the nearshore.In addition,the performances of four radiation stress formulations were compared and evaluated.The simulated wave heights and surface elevations were almost identical for each formulation.The longshore velocities from the four formulations were similar overall and matched well with the observations.For the simulations of the wave-induced undertow and complex rip currents,the new formulation derived in this study showed better performances than the other formulations.(4)Based on the coupled wave-current model,a three-dimensional numerical model of non-cohesive sediment transport and morphological evolution was developed.In the model,the suspended transport was solved by a three-dimensional advectiondiffusion equation.The exchange of sediment between suspension and bed was implemented by using source/sink term acting on the bottom of the suspension.The effects of sediment concentration on fluid density and the vertical diffusion coefficient under waves and currents were included.The bedload transport was described by the empirical formulas including the effects of wave skewness,asymmetry,and bottom slope.The morphological evolution was computed following the erosion and deposition fluxes for suspended load and the bedload transport rates.The final morphological changes provided a feedback to the calculations of waves,hydrodynamics and sediment transports.Then the coupling of wave,current,sediment and morphological evolution was completed.(5)The three-dimensional sediment transport and morphological evolution model was validated by using a large number of laboratory and field data.The validation tests included trench migration,sandbar migration,shoreline changes around the breakwater,morphological changes in the vicinity of T-head groin,multi-class sediment transports,and beach morphological evolution at the field scale.The results showed that the model was capable of simulating the changes of beach profile under the intense or weak wave condition,and both cross-shore and longshore sediment transports.The model could reasonably describe the nearshore morphological evolution for the large-scale simulation of coastal area.