Recent Characteristics Of Longitudinal Suspended Sediment Transport In The North Passage Of The Changjiang Estuary

With the completion of a substantial number of water conservancy projects in the Changjiang basin and the deep-water channel project in the North Passage, the characteristics of fresh water and sediment discharged of Changjiang River and hydrodynamics and sediment dynamics of the North Passage changed remarkably, which causes that more researches are required to maintain normal navigation and stable development of deep-water channel in the North Passage. Current velocity, fresh-salt water mixing, suspended sediment concentration (SSC), water and sediment transport pattern were analyzed using the hydrological statistics analysis, harmonic analysis and transport mechanisms based on the data observed at the upper, middle and lower reach of the North Passage from October 28 to November 5 in 2013, during the dry season and the results have compared with the results calculated based on the data observed from April 14 to April 23,2012 at the upper, middle reach of the North Passage. The results represented the recent characteristics of water and sediment in the river and provided theoretical basis for the management of waterway maintenance.During the survey period, the duration of ebb and flood tide was different. Flood tidal current was northwest, ebb tidal current was southeast. 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, and the difference in current velocity between the moderate and spring tides was not significant. The maximum vertical velocity in the flood usually occurred in the middle water layer, but in the ebb the current velocity usually decreased from the surface to the bottom. In the upper and middle river channel during neap tide and lower river channel during neap, moderate and flood tides, the net water transport in the upper layer was seaward while in the lower layer the net water transport was landward.Salinity during the flood tide was usually higher than that during the ebb tide. Longitudinally the salinity was higher near the river mouth. Difference in the mean salinity between the neap tide and spring tide was insignificant. Considerable stratification appeared during the neap tide, but during the moderate and spring tides it became weak. The salt water density flow leaded to that the vertical distribution of current velocity in the North Passage different from that in non-tidal river area. During the flood tide the maximimum current velocity occurred in the middle water layer, but during the ebb tide it occurred in the water surface. Salt-induced density flow can alter the current structure and promote the formation of lognitudal estuarine circulation.SSC at each station in the North Passage increased from the neap tide to spring tide. SSC during the ebb tide was usually close or slightly higher than that during the flood tide. During the survey period, the averaged SSC in the upper reach was 1.14 kg/m3, which was 1.3 and 2.4 times higher than the SSC in the middle and lower reaches respectively.The tidal averaged SSC and velocity had a closer relationship during the study period. In the upper and middle reaches during the neap tide, and in the lower reach during the neap, moderate and spring tides, the net sediment transport in the upper layer was seaward while in the lower layer the net sediment transport was landward, implying the significant influence of the estuarine circulation.In the upper and middle reaches during the neap tide, and in the lower reach during the neap and moderate tides, seaward residual current in the upper water layer and landward residual current in the bottom water layer were observed, indicating the formation and occurrence of the longitudinal residual circulation. At each measurement station Eulerian residual current and tidal net water transport were seaward while stokes flow was landward. Suspended sediment transport was dominated by the transport term of T1, T2, T4 and T6. During the survey period, the advection term remained seaward. The tidal pumping was landward during the neap tide while the sediment transport was seaward during the moderate and spring tides. The vertical net circulation term was landward. During the neap tide, the vertical net circulation term was strong at each station, with the combination of tidal pumping, the net sediment transport was landward or seaward slightly. During the moderate and flood tides, the vertical net circulation term was weakened, and the advection and tidal pumping items were both seaward, leading to the tidal net sediment transport seaward.Compared with the water and sediment transport pattern in the Norht Passage in April 2012, significant differences were found between them, although they were both observed in the dry season. In April 2012, there was no obvious development of longitudinal residual circulation, and the residual current were all seaward from the surface to the bottom. The tidal net sediment transport was seaward. The advection item wad seaward and stronger than the tidal pumping and vertical net circulation items. The water and sediment transport mechanisms in the North Passage were influenced by multiple factors, such as runoff, tide, gravitional circulation, fresh-saline water mixing etc., and exhibited a significant temporal variation.