Recent Characteristics Of Flow And Suspended Sediemnt And Vertical Sediment Concentration Distribution Pattern In The North Passage Of Yangtze Estuary

The North Passage is the main channel for the transmission of runoff and sediment load from the Yangtze River,and the location of the Deepwater Navigation Channel(DNC)in the North Passage.The long-term interaction between the runoff and the hydrodynamic dynamics of the channel is very complicated.Recent changes in runoff and sediment conditions,as well as the construction of the DNC,are bound to lead to the temporal and spatial change of river hydrodynamic and salinity and suspended sediment concentration(SSC)in the North Passage,Therefore,it is necessary to study the spatial and temporal change of river hydrodynamic and SSC,as well as the vertical distribution of SSC on the basis of the field measured data in the North Passage.This provides reference for the evolution of erosion and deposition and the dredging works for the Deepwater Navigation Channel,and it also has important theoretical and practical significance.Hydrodynamic characteristics,fresh-salt water mixing,SSC,water and sediment transport pattern were analyzed using the hydrological statistics analysis,harmonic analysis and sediment dynamics based on the data observed at the upper and lower reach of the North Passage from October 28 to November 5 in 2013.At the same time,the variation characteristics and forming mechanism of the vertical distribution of SSC in the tidal cycle are also discussed.The type of tidal current in the North Passage belongs to the non-normal semidiurnal current.During the survey period,the ebb current was predominated in the upper and lower reach of the North Passage.The wave propagation property is the progressive wave,and has obvious tidal wave deformation phenomenon.Current asymmetry between the flood and ebb was significant.In most cases,the current velocity was higher in ebb than in flood,in spring tide than in neap tide.Longitudinally,the salinity was higher near the river mouth.From the neap tidal to the spring tidal,the salinity was decreasing.During the period of flood and ebb tide,the average salinity of the flood tide was slightly higher than the ebb tide in the upper reach,while the lower reach of the North Passage was completely opposite.The salinity values of the measured station were gradually increased from the surface to the bottom,and the salt water was mixed in different degrees.During the neap tide,the surface layer and the bottom salinity value differ greatly,the stratification phenomenon is very apparent.During the spring tide,the salinity values gradually increased from the surface layer to the bottom,and the stratification became weak.The fresh-salt density flow leads to that the vertical distribution of current velocity in the North Passage different.During the flood tide,the maximum current velocity occurred in the middle water layer.Under the influence of salinity gradient,runoff from the upper discharge,salt water landward from the lower,and mixed with the runoff,thus formed the longitudinal estuarine circulation.From the upper to the lower reach of the North Passage,the SSC is increasing.In each station,the SSC increased from the neap tide to the spring tide.During the neap tide,the velocity is small,the vertical suspended sediment concentration in the small changes.With the increase of velocity,the bottom suspended sediment resuspension and spread to the upper,and form a high concentration suspended sediment peak.During the neap tide,the average SSC of the flood tide was slightly higher than the ebb tide at each station,while the spring tide was opposite.The SSC increases from the surface to the bottom.On the basis of the study of longitudinal water delivery,salt transport and sediment transport in the NP,the result show that:in the lower river channel during neap and flood tides,the net water transport in the upper layer was seaward while in the lower layer the net water transport was landward.The longitudinal transport of salt is similar to the water,in the upper river channel during neap tide and lower river channel during neap and flood tides,the net salt transport in the upper layer was seaward while in the lower layer the net salt transport was landward.The suspended sediment from the upper layer to the lower layer all net transport to the sea during flood tides,the net sediment transport in the upper layer was seaward while in the lower layer the net sediment transport was landward in the other period of time.The suspended sediment transport process is obviously influenced by the distribution of salinity and the stratification of salinity density.Due to the differences of fresh-salt mixing over time,when the salinity stratification and density gradient is larger,the runoff of the surface is transported to the sea,and the lower layer of high salinity water flows to the land.Under the influence of the longitudinal estuarine circulation,the sediment transport process has been changed accordingly,and the sediment in the lower layer is transported to the land.The vertical circulations of the estuary has a greater impact on the transport process of sediment in the lower reach of the North Passage,which shows that the contribution rate is larger in the upper river channel in the net landward suspended sediment transport.Affected by large salinity gradient and weak current velocity,the vertical profiles of SSC in the neap tide mainly exhibit two types,i.e.,ladder type and L type.The former mainly appears in the flood tide,the latter appears in the ebb tide.The common characteristics of these two types are high concentrations of suspended sediment in the bottom water layers.Sediment diffusion in the neap tide was strongly suppressed by the intense salinity-induced stratification,with greater SSC appeared in the lower water column and smaller SSC appeared in the upper water column.Salinity stratification and large salinity gradient have a strong effect on the vertical distribution of SSC during the neap tide.With greater SSC,the influence of strong current velocity and weak stratification,the SSC profiles in the spring tide mainly display linear distribution.Suspended sediment was well mixed in the latter half of the ebb phase,with constant concentration in the whole water column.The occurrence of this type is related to the strong mixing of the fresh-salt water and the insufficient number of eroded sediment in the bottom bed.The stratification of salinity density has little influence on the distribution of SSC during the spring tide.The statistical analysis of the measured vertical distribution of SSC showed that the linear fitting correlation coefficient changing during the spring and neap tide,only part of the time corresponds to the Rouse distribution.Due to the linear distribution in the water column,most of S SC profiles in the spring tide fit the Soulsby equation,and can be accurately predicted by it.