Recent Temporal And Spatial Variations And Transport Processes Of Suspended Sediment In The North Passage Of The Yangtze River Estuary

The North Passage is the main outlet for the runoff and sediment load of the Yangtze River discharging into the East China Sea. Influenced by the effects of salt-fresh water mixing and sediment resuspension, the North Passage is the core area of the turbidity maximum in the Yangtze River Estuary (YRE). The main part of the Deepwater Navigation Channel (DNC) engineering project which is heavily invested by the Chinese government, located in the North Passage. However, since the operation of the DNC, a silting problem began to affect it. The curving segment in the middle of the North Passage is suffering intense sedimentation problem, since the completion of the DNC engineering project, the geomorphology of river channel tends to be bending type. Under the combined action of the mechanisms such as curve effect, Coriolis force and differential advection of along-channel salinities, the lateral flows occur and have important influence on the lateral transport processes of water, salinity and suspended sediment in the estuary, and then affect the evolution of erosion and deposition of the navigation channel.Based on the two-ship simultaneous observation data of hydrology and suspended sediment, which was measured on the both sides of the channel in the upper, middle and lower reach of the North Passage, YRE, during September 24 to October 4 in 2014. In order to obtain the insight of physical process of material transport in the estuarine area, the mechanisms of longitudinal (along the channel direction) and lateral (perpendicular to the channel direction) transport of water, salt and suspended sediment in the North Passage has been explored by using methods of mathematical statistics, flux mechanism decomposition and sediment dynamics. The effect of the lateral transport process on the evolution of erosion and deposition in the North Passage of the YRE also has been discussed in this study to reveal the reason for the sedimentation and provide references for the maintenance of the DNC engineering project. Hence this study has important scientific significance and application value.The measurement data indicates that there are one to four evident suspended sediment concentration peaks within every tidal cycle at each station in the North Passage. The number, the occurrence time and the suspended sediment concentration of the peaks are associated to the location of the observation stations, the dynamic process of the flood and ebb tides, the movement of the salt water wedges and the salinity-induced density stratification. The North Passage is located in the fresh-salt water mixing area. The salt water wedges can impact the temporal variation of suspended sediment concentration by altering the vertical distribution of current velocity in flood tides. The salinity-induced density stratification decreased the turbulence intensity, resulting in the decrease of the sediment carrying capacity of the flow. The primary reason for the larger difference of suspended sediment concentrations between the first and second flood-ebb tidal cycles during the neap tide at stations S3 and N3 is the turbulence suppression effect. The sediment resuspension in the periods of the high tidal current speed and the transition between flood tide and ebb tide also can influence the suspended sediment concentration process. High sediment concentrations were observed in the bottom water layer at stations S2 and N2 in the middle segment of the North Passage during the spring tide. The formation of the high sediment concentrations is mainly connected with the movement of the high sediment concentration layer which formed in the downstream of stations S2 and N2.The flux mechanism decomposition method was used to analyze the mechanisms of the longitudinal transport of water, salinity and suspended sediment in the North Passage. The results indicate that the net water fluxes at all stations are seaward. Because of the Coriolis force, the major part of the river runoff discharges through the south side of the channel, which results in the greater Euler residual current at stations in the south side of the channel than that at the corresponding stations in the north side of the channel. The difference in Stokes residual currents between corresponding stations in the both sides of the channel is insignificant. Overall, the net water fluxes, as well as the net salt and suspended sediment fluxes in a tidal cycle at stations in the south side of the channel are greater than those at the corresponding stations in the north side of the channel. The balance of salt and suspended sediment fluxes indicate the seaward salt and suspended sediment transport is dominated by the Euler residual current transport term during spring and neap tides, but the onshore transport is controlled by Stokes drift transport term and vertical gravitational circulation transport term. The magnitude and direction of the correlation term of the tidal-period variations of salinity/suspended sediment and current (T4) at different stations change significantly, thus term T4 makes obvious contributions to the seaward or landward salt/suspended sediment transport at part of the stations. The significant change in the magnitude and direction of term T4 at each station induces the difference between the salt and suspended sediment transport structures. Compare to the regular variation of the salinity, the temporal and spatial variation of the suspended sediment concentration are more complex, this is the major reason for the difference between the salt and suspended sediment transport structures.Although it is different between the salt and suspended sediment transport structures, the suspended sediment transport process is affected by the distribution of salinity and the salinity-induced density stratification. The greater salinity gradient in the bottom water layer induces the density flow of salt water wedge during flood tides, sequently changes the flow structures. The velocity and duration of the flood current in the middle and bottom water layers were increased and the estuarine vertical circulations were generated at station N3 during spring and neap tides. The vertical circulations have great influence on the suspended sediment transport process, leading to net landward suspended sediment transport in the bottom water layer at station N3. The density flows of salt water wedges in neap tides are stronger than those in spring tides, and hence the contribution rates of the vertical gravitational circulation transport term to the landward suspended sediment transport in neap tides are greater than those in spring tides. In addition, the flocculation induced by the mixing of salt-fresh water and the turbulence suppression effect due to salinity-induced density stratification also have great influence on the suspended sediment transport process.The study shows that there exist obvious lateral transport processes of water, salt and suspended sediment, and the net transport of them are from the north dike to the south dike. The lateral processes of salt and suspended sediment transport significantly differ from the lateral water transport, and the main reason for the difference is the inhomogeneity of the vertical distribution of the salinity and suspended sediment concentration. Furthermore, compared to the regular variation of the salinity, the temporal and spatial variations of the suspended sediment concentration are more complex, and the vertical distributions of the salinity and suspended sediment concentration are inconsistent. These are the major reasons for the difference between the lateral salt and suspended sediment transport processes.Although the lateral water, salt and suspended sediment transport processes are strikingly different to each other, the lateral salt and suspended sediment transport processes are affected by the lateral water transport process. Therefore, it is necessary to investigate the mechanisms of the lateral water transport process. In general, the lateral transport of water is from the south dike to the north dike, and the magnitudes of lateral water transport at the stations in the south side of the channel are bigger than those at the corresponding stations in the north side of the channel during flood tides, but it is quite contrary during ebb tides due to the impact of the Coriolis force and the flow guiding effect of dikes and groynes. The salt water intrusion is stronger in the lower segment of the North Passage. The salinity in the north side of the channel is higher than that in the south side of the channel. The lateral difference of salinity distribution causes the south dike direction baroclinic pressure gradient force, resulting in the arising of the southward lateral flow in the lower layer and northward lateral flow in the upper layer at the north side of the channel during later flood tides. Induced by the lateral variation of water depth, along-estuary velocities are stronger in the deep part of the estuary resulting in greater advection of higher-salinity water there than on the flanks. The lateral density gradient drives a lateral baroclinic circulation with counter-rotating circulation cells in the middle reach of the North Passage during the later period of flood tide in the spring tide. In addition, the wind also affects the water transport processes at stations in the middle reach of the North Passage during the neap tide.The lateral transport processes of suspended sediment in the channel have significant influence on the evolution process of the North Passage in the YRE. The net lateral suspended sediment transport is from the north dike to the south dike at both sides of the North Passage during the measurement period. The maximum net lateral suspended sediment transport is in the middle reach of the North Passage, this may be the primary reason for the intense siltation in the shoal at the south side of the channel. As for the suspended sediments trapped in the channel, the amount can reach 2.06x105kg/m in the middle reach of the North Passage during a neap and a spring diurnal tides. Therefore, the lateral transport of suspended sediment has great influence on the back-siltation in the middle segment of the North Passage.