Fully Coupled Numerical Modelling Of Sediment Transport And Bed Change Under Currents And Waves Influences

Coastal and nearshore regions are often impacted by both tidal currents and waves,which can lead to complicated sediment transport and morphological changes.Especially during storm surge periods,waves riding on high tides will cause intense sediment movement leading to serious damages to coastal environments.During the storm surge,the flow regimes are complex and strongly unsteady.The bed surface will change rapidly in a short time and the sediment concentration is quite high,and varies considerably both temporally and spatially.The flow motion,sediment transport and morphological change influence each other and the coupled effect cannot be ignored.Thus,fully coupled models of flow motion,sediment transport and morphological change are established for numerically simulating these processes.The model first uses the non-hydrostatic wave model and the phase-averaged SWAN model to consider the wave effect.Then,the effect of bed and sediment concentration change on water flow is considered in the governing equations to realize the coupling calculation among flow motion,sediment transport and morphological changes.The models are verified by benchmark cases and are then applied to Santa Rosa Island to simulate the morphological change by storm surge during Hurricane Ivan.The main findings and conclusions are as follows:(1)The developed coupled non-hydrostatic model which considers the interaction between flow and sediment corrects the flow velocity by considering the contribution of the dynamic pressure to reflect the influence of waves on water flow and sediment transport.Turbulent dissipation caused by wave breaking is considered.Compared with the hydrostatic model,the non-hydrostatic model can better simulate wave propagation.The dynamic water pressure is mainly distributed in the scour hole,and the position where the extreme value appears corresponds to the fluctuation position of the water surface,which is the main factor for the movement of the water flow in the vertical direction.According to the computational results,the dynamic water pressure is 2-3 times of the hydrostatic pressure when water jump occurs.Comparing the calculated bed level by the hydrostatic model and non-hydrostatic model,the difference mainly exists in where the dynamic pressure is concentrated.(2)The developed coupled phase-averaged model which considers the interaction between flow and sediment considers the wave radiation stress to reflect the influence of waves on water flow and sediment movement.The developed model can effectively simulate nearshore current,sediment transport and morphological change by currents and waves.The calculation shows that the effect of waves on morphological changes is affected by the storm tide level.When the storm surge is small,the waves will cause significant changes in the bed surface.The oblique incident wave will cause obvious asymmetry in the bed change.The absolute error of morphological change calculated by the models with and without considering the wave action is 5%of the overall morphological change when there is a high storm tide and the ratio of water depth to wavelength is less than 0.15 in coastal areas,while the simulation time of the former model is doubled.(3)The overwash process in Santa Rosa Island can be divided into three stages:nearshore erosion,front dune erosion,and overwash.The erosion pattern during storm surge depends on the topography.For areas with flat terrain,sediment can be transported in large quantities to the bay.For areas with front dunes and back barriers,sediment loss is significantly less than others,which provides better protection for nearshore area.During the overwash of the Santa Rosa Island,the maximum deposition depth reaches 2 meters,and the shoreline moves backward by 300 meters.The three simulated breach areas are in line with the measured data.The bed profiles calculated by the non-hydrostatic model and phase-averaged model are basically consistent.