Study On Numerical Simulation Of Near-bed High-concentration Sediment Suspension Of The Changjiang Estuary

The phenomenon of near-bed high-concentration sediment suspension often occurs in the turbidity maximum in estuaries,and the phenomenon of near-bed highconcentration sediment suspension is also considered to be one of the most important mechanisms for siltation in the channel and port.The deep waterway channel of the Changjiang estuary requires huge sums of money for dredging maintenance every year.It also faces serious channel siltation problems.In recent years,in the field of sediment related research,numerical simulation has become an indispensable research method.Many scholars have been committed to the research and model development of highconcentration sediment formation mechanism.In the numerical simulation study of sediment in the Changjiang estuary,the sediment simulation at the lower concentration has been developed more mature and can achieve good simulation accuracy.However,the sediment simulation at high concentration is still a difficult point,and the simulation results are often Poor.The purpose of this paper is to improve the mechanism of the sediment model of the Changjiang estuary and to make the model have a certain simulation accuracy for the near-bed high-concentration sediment suspension in the Changjiang estuary.Improvement,the main research results of this paper include:1.Through a one-dimensional model developed based on FVCOM,the effect of hindered settling on the concentration of sediment in water is quantitatively determined,but the turbulence suppression significantly changes the vertical structure of sediment concentration and enhances the stratification of water column.Thus,a relatively obvious near-bed high-concentration sediment suspension is formed.The comparison with the measured data shows that the combined effect of hindered settling and turbulence suppression has a significant improvement on the simulation results of nearbed high-concentration sediment suspension in the one-dimensional model.The simulation result match the vertical profile of near-bed high-concentration sediment suspension of the Changjiang estuary during spring tide fairly.2.Further application of the hindered settling and turbulence suppression into a three-dimensional FVCOM model of Changjiang estuary,the results show that the effect of hindered settling and turbulence suppression in the three-dimensional model is basically consistent with that in one-dimensional model.However,due to the more complicated physical mechanism in the three-dimensional model,after the addition of hindered settling and turbulence suppression,the simulation results will have a certain displacement in the high sediment concentration area.By comparing with the time series of measured data,the model with including hindered settling and turbulence suppression can describe the near-bottom high concentration process during the spring tide fairly,but it is inferior to the near-bed high-concentration sediment suspension during the neap tide.According to the analysis,during the observation time,the phenomenon of near-bottom concentration during the neap tide period has different formation mechanism from the spring tide period.Further research and analysis is needed to obtain good simulation results during the neap tide period.3.Through the reanalysis of the flume experiment results from previous people,a new expression of the coefficient ?,the ratio of the vertical turbulent diffusion coefficient of sediment and fluid eddy viscosity,is obtained.Then we applied the expression of ? into the 3D FVCOM model of the Changjiang estuary.By comparison with original model result,it is found that after adding ? the vertical diffusion of the bottom sediment is reduced and the concentration of sediment at the bottom layer is obviously improved during small current velocity period.During large current velocity period,the vertical diffusion of the bottom sediment is improved and the concentration of sediment at the bottom layer is slightly decreased after adding the coefficient ?.