Characteristics Of Mixing And Stratification And Its Influence On Suspended Sediment Transport In The North Passage Of The Yangtze Estuary

Based on the measured hydrological and sediment data of spring and neap tides in dry and flood seasons of 2016 in the North Passage of the Changjiang Estuary,the gradient Richardson number,Simpson number and potential energy anomaly were calculated to analyze the tidal cycle,spring-neap variation and seasonal variation of mixing and stratification.Taking into account the vertical difference of horizontal density gradient,the contribution terms of time derivative of potential energy anomaly was derived,including depth-mean straining,advection,non-mean straining and tidal stirring.The driving mechanisms of mixing and stratification was investigated by calculating and comparing these four contribution terms.The main conclusions of the above analysis are as follows:(1)Seasonal variation of freshwater discharge mainly affected the distance of saltwater intrusion,the saline wedge moved upstream in dry season,while the tidal dynamics had significant impacts on mixing and stratification.In the reaches with saline wedge tip migration,the water column was periodically stratified,and the process was dominated by advection term.In the main reaches of saltwater intrusion,during spring tide,the water column was periodically stratified,which was stratified during flood tidal current,and well mixed during ebb tidal current,and the stratification during flood tidal current was promoted by advection and depth-mean straining,and the mixing during ebb tidal current was promoted by tidal stirring and advection;during neap tide,the water column was permanently stratified,the stratification was enhanced during flood tidal current and weakened during ebb tidal current,and the stratification was mainly maintained by depth-mean straining.(2)The influence of mixing and stratification on the vertical structure of suspended sediment is mainly related to the intensity and location of salt water intrusion,and the stratification turbulence determines the fitting result of the vertical structure of suspended sediment.For the reach without salt water intrusion,the vertical structure of suspended sediment is mainly linear.In the reaches with saline wedge tip migration,when the vertical gradient of salinity is large,the vertical structure of sediment concentration is exponential.In the stage of no salt water intrusion,the vertical structure of suspended sediment is linear.In the main reaches of saltwater intrusion,the vertical structure of suspended sediment has a significant correlation with the vertical gradient of salinity in the lower water column.When the vertical gradient of salinity in the lower water column is large,the vertical structure of suspended sediment is exponential,and when the vertical gradient of salinity in the lower water column is small,the vertical structure of suspended sediment is linear.The time when the vertical gradient of suspended sediment is larger is basically consistent with the time when the vertical gradient of salinity is larger and the time when the water column is stratified.The vertical gradient of suspended sediment is larger when the water column is stratified and smaller when the water column is mixed.The vertical diffusion coefficient of suspended sediment can be well fitted by the vertical diffusion equation of turbulent stratification.The fitting effect of suspended sediment diffusion balance equation in dry season is better than that in flood season,and the sediment source is mainly resuspension.(3)The influence of mixing and stratification on tidal sediment transport is mainly related to the phase difference between turbulent diffusion coefficient and velocity and the variation amplitude of turbulent diffusion coefficient in a tidal cycle.The former can affect the sediment transport direction,and the two jointly determine the sediment transport intensity.The mixing and stratification in the main reaches of saltwater intrusion,is characterized by stratification in the flood tide period-mixing during the ebb tide period or stratification enhancement during the flood tide period-stratification weakening during the ebb tide period.The influence of mixing and stratification on tidal sediment transport is to produce net sediment transport in the seaward direction.In outlet and the offshore area outside the mouth of the north passage,the mixing and stratification is characterized by the stratification and mixing during ebb tide period and flood tide period mainly controlled by tidal straining.The influence of mixing and stratification on tidal sediment transport is to produce net sediment transport in the landward direction;The variation of characteristics of mixing and stratification along the north channel is conducive to the accumulation of suspended sediment in the mouth.