Influence Of Wave-Induced Currents On Topographic Evolution Of Different Types Of Ebb Tidal Deltas

Ebb tidal delta is a sandy shoal formed outside the tidal branching on a sandy coast.It is a delta formed by the sediment deposited at the exit of the tidal channel due to the diffusion of the current.The ebb delta is a component of tidal branching dynamic geomorphologic system.In this paper,numerical simulation is used to explore the influence of wave current on the topographic evolution of different types of ebb tidal deltas.The numerical model used in this paper is composed of hydrodynamic model,sediment transport model and terrain evolution model,in which the tidal current and wave current action are mainly included.The difference scheme and discrete method of each model are also given.In order to ensure the accuracy of numerical simulation calculation,the hydrodynamic model was verified by comparing with experimental data and analytical solution,and the sediment transport model was verified by comparing with experimental data.Firstly,this paper takes the ebb delta of Shuidong Bay and the ebb delta of Zhanjiang Bay as the numerical simulation objects.The results of topographical evolution under wave currents and numerical simulation calculations in the Shuidong Bay area show that the new sediment transport rate formula selected in this paper has the effect of promoting nearshore sedimentation compared with the Sousbly sediment transport rate formula.The numerical simulation of the reduced topography of Zhanjiang Bay that has the same size with the Shuidong Bay shows that different topography has different effects on the evolution of ebb tide delta,as reflected in the shallow shoal on the northeast side of Zhanjiang Bay has inhibitory effect on wave erosion.Then,this paper selects the actual size terrain of Zhanjiang Bay as the numerical simulation object,and carries out numerical simulation calculation on the flow field distribution and terrain evolution under the action of single tidal current and single wave-generated current.The simulation of the flow field shows that the flow field in Zhanjiang Bay is dominated by tidal current.The wave breaks in the nearshore area,which causes the increasing of nearshore velocity.Due to the influence of topography,the development of ebb deltas on the north and south sides of the bay mouth shows obvious asymmetry.The action of tidal current alone makes a greater contribution to the formation of the delta on the south side of the bay mouth,and slightly scours the delta on the north side of the bay mouth.Incoming waves from different directions cause different degrees of erosion and deposition on the north and south side of the bay mouth.The incoming waves of EN and EEN directions contribute more to the delta on the north side,and the incoming waves of ES and ESS directions contribute more to the formation of the delta on the south side of the bay.Finally,this paper uses actual wave data from wave survey stations to simulate the formation and recent evolution of the ebb delta in Zhanjiang Bay,and analyze the mechanism of tidal currents and wave-generated currents in the formation of the ebb delta in Zhanjiang Bay.The contribution of tidal current is mainly to deposit sediment on the south side of the bay mouth,while the contribution of wave-generated current is mainly reflected in the sedimentation of sediment on both sides of the bay mouth caused by its coastal sand transport.The simulation results of the recent topographic evolution of Zhanjiang Bay are in good agreement with the situation that the Dongxin Beach of Zhanjiang Bay has been scoured recently.