Temporal And Spatial Variations Of Flow , Salt And SSC And Vertical Profiles Of Suspended Sediment In The North Channel Of Changjiang Estuary During The Dry Season

Estuary is the place where land,river and ocean meet.Land-ocean interaction is active in this area,which the sediment is repeatedly suspended,transported and settled,resulting in erosion and accumulation of the seabed.The purpose of studying the characteristics of water and sediment transport in estuaries is to reveal the mechanism of sediment transport in estuaries and the regular pattern of erosion and deposition of river channel,so as to provide guidance for the development and management of the estuary.As the secondary inlet of the Changjiang Estuary,the North Channel is one of the important channel of water and sediment transport in the estuary.The lower reach of North Channel is located in the north part of the turbidity maximum zone of the Changjiang Estuary,which is greatly affected by saltwater intrusion.In addition,the changes of upstream incoming water and sediment from basin in recent years will inevitably have an impact on the hydrodynamics,salinity,spatial and temporal distribution of suspended sediment in the area.Therefore,it is necessary to analyze and study the temporal and spatial distribution and vertical variation characteristics of water,sediment and salt in the North Channel based on the measured water,sediment and salt data,so as to provide reference for regional development and utilization.Based on the continuous field observation of flow,salinity and suspended sediment concentration?SSC?in the upper,middle and lower reaches of North Channel in the Changjiang Estuary from April 15 to April 24,2018,the paper analyzes the temporal and spatial variations of velocity,salinity and SSC,as well as the water,salt and sediment transport process using hydrological statistical analysis,quasi-harmonic analysis and dynamic sedimentological approach.The characteristics and formation mechanism of vertical distribution of SSC in the North Channel are also discussed emphatically.The tidal motion in the North Channel belongs to the irregular semidiurnal tide.During the observation period,the semi-diurnal tidal component occupies an absolute predominance in all stations of the North Channel showing that M₂>S₂>N₂.As for the tidal wave,it is a mixed wave mainly dominated by the progressive wave.The phenomenon of tidal asymmetry is reflected in the variation of duration of flood and ebb tide and current velocity during the ebb-flood and spring-neap cycle.The duration of the ebb tide is significantly longer than that of the flood tide.The average and maximum velocities of the ebb tide are larger than those of the flood tide,while the average velocity of the spring tide is greater than that of the neap tide.The average velocity shows an increasing trend from the upper to the lower reaches of the North Channel.This study shows that runoff is the main reason of the asymmetry of tidal current in the Changjiang Estuary.Runoff has a profound impact on the water transport in the North Channel,which makes every reach of the North Channel predominate by ebb flow,and net water transport is seaward in each water layer during the spring and neap tides.The effect of runoff from inside to outside the river mouth area is relatively weakened,and the advantage of ebb flow gradually becomes weak from the upper reach to the lower reach.The water volume of each layer in the spring tide is larger than that of in the neap tide,but the advantage of ebb flow in the neap tide is obviously stronger than that in the spring tide.Also the strong runoff enhances the seaward Eulerian residual currents in the North Channel.The changes of tidal properties,durations of flood and ebb tides,tidal range and runoff intensity make the water salinity change in flood-ebb cycle and spring-neap cycle.The upper reach in the North Channel is controlled by the freshwater of the Changjiang River throughout the year,its salinity is very low,the salinity difference between surface and bottom is very small,and its vertical mixing is well.There is no obvious variation during flood-ebb tide and spring-neap tide.The average salinity of the middle reach in neap tide is a little larger than that of in spring tide,and the average salinity in the flood tide is a little higher than that in the ebb tide.In the lower reach of North Channel,the average salinity in neap tide is larger than that in spring tide.In spring tide,the average salinity of flood tide is a little larger than that of ebb tide,while condition in neap tide is the opposite.Salinity stratification coefficient,Richardson number and potential energy anomaly draw merely the same conclusion.The middle and upper reaches of North Channel are mixed evenly for a long time,while strong stratification happens in the lower reach during high tide level phase of spring tide and neap tide.It is found that the increase of current velocity can promote the water mixing,and the effect of water stratification can prohibit the turbulence of water,reduce the vertical exchange of water,and lead to the increase of velocity gradient between upper and lower layer of water.The characteristics of salt transport in the tidal cycle at each observing station in the upper,middle and lower reach are basically consistent with those of water transport.Deeply influenced by the runoff from the Changjiang River,salt transport in all the stations is also dominated by ebb tide.The salt transport in the lower reach of the North Channel is obviously larger than that in the other two stations during the spring and neap tides,and the net salt transport in the upper and middle layers is obviously larger than that in the lower layer due to salt intrusion.From the upper reach to the lower reach of the North Channel,the SSC in the water at the observing station tends to increase.Vertical differences in salinity and SSC at the lower observing station lead to stratification.The average SSC in the spring tide is larger than that in the neap tide influenced by the periodic variation of hydrodynamic condition.In the upper and middle reaches of the North Channel,the average SSC changes littlely.In the lower reach,the average SSC in the spring tide is a little higher than that in the neap tide.The stronger hydrodynamic condition of the water makes it easier to resuspend bottom sediment in spring tide,thus the SSC in this period is higher.The salinity in the upper reach of the North Channel is rarely affected by salt water intrusion,while salt and fresh water mixing happens to the observing station in lower reach of the North Channel for many times.During the flood phase of spring tide,the fine sediment is resuspended and transported upon with the flood tide.In the late phase of flood tide and the transition period of flood-neap tide,current velocity is weakened and salinity gets certain value through salt-fresh water mixing,which is easy to flocculate.The comprehensive analysis shows that saltwater intrusion,velocity intensity-resuspension,and sediment spatial distribution of river bottom material affect the spatial and temporal variation of SSC.Sediment transport in the upper reach of the North Channel from spring tide to neap tide is dominated by ebb flow.The sediment discharge of all water layers during the spring tide is greater than that in the neap tide.The sediment transport amount during the tide cycle also shows that sediment discharge in spring tide is significantly greater than that in neap tide,but the advantage of sediment transport toward sea in neap tide is more obvious.The sediment transport amount of the lower observing station of the North Channel is the largest among all observing stations during the tidal period.During the spring tide,net sediment transport is seaward in all water layers.During the neap tide,net sediment transport in the upper and middle layers of water is seaward while the condition in the lower layer is opposite though the net sediment discharge is small.It is found that the sediment transport in estuary is influenced by the velocity,duration of flood-ebb tide and salt water intrusion.Influenced by different hydrodynamic and salinity condition,the vertical distribution of SSC in the North Channel mainly shows five basic types:exponential type,oblique type,vertical-line type,stepped type and two-layer type.As the largest branch channel in the Changjiang Estuary,the North Channel is largely influenced by the runoff.Water mass here maintains a single physicochemical property for a long period of time and flow velocity changes uniformly and regularly,so oblique type often appears,especially in the middle and upper reaches of the river and the flood tide period,the frequency of occurrence can basically reach more than40%.The characteristic of oblique type is that SSC increases linearly from the surface layer to the bottom layer and the concentration gradient is substantially uniform vertically.Vertical-line type mostly appears in the environment where the water is vertically mixed uniformly,and the SSC remains basically unchanged in the vertical direction.In the environment where the SSC is not high,the salinity is low,and the hydrodynamic condition is strong,the exponential type is more likely to occur.The characteristic is that the SSC increases exponentially from the surface layer to the bottom layer,the concentration gradient also increases from the surface layer to the bottom layer,and the vertical profile is similar to exponential function.The saltwater intrusion in the lower reach of North Channel is frequent,and the stratification phenomenon is remarkable.The strong tidal current causes the bottom sediment resuspension and increases the SSC in the lower part of the water.However,the saltwater intrusion and the resulting stratification inhibit the upward diffusion of the bottom sediment.The exchange of sediment in the vertical direction is difficult to achieve,so the profiles often exhibit stepped and two-layer distribution.The step type is characterized by discontinuity in the SSC in the vertical direction,and the difference in the water between the upper and lower layers is large,and the vertical distribution is similar to the letter h.The two-layer type is characterized in that the SSC is distributed in two stages in the vertical direction,and there exists a significant concentration inflection point.What's more,Soulsby equation is used to predict the SSC profiles.It is found that the methods do well in predicting linear distribution,but forecasting error of stepped type and two-layer type using the method is too big.After improving,this method can be used to treat part of latter two profiles.And Rouse equation is used to calculate the velocity,the result is merely same as the former study.