Temporal And Spatial Variations And Transport Mechanisms Of Suspended Sediment In The Middle Reach Of North Passage In The Yangtze Estuary

North Passage of the Yangtze Estuary, located in the central area of the estuary turbidity maximum zone, is the major outlet for fresh water and sediment transport to the East China Sea. It is also the place where the Deepwater Navigation Regulation Project constructed. Under the impacts of sharp reducing in sediment discharge from the Yangtze River basin and intense human activities, sediment transport processes and mechanisms in the North Passage are complicated and highly variable.Based on the observation of flow and suspended sediment concentration (SSC) in the middle reaches of North Passage in the Yangtze estuary from14to23April in2012, the temporal and spatial variations of velocity, SSC and grain size are analyzed over one neap-spring tidal cycle. The transport of suspended sediment for full grain size class and different grain size class are analyzed by using the method of mechanism decomposition.The results show that in the North Passage, tidal motion is irregular semi-diurnal. The current directions are generally parallel to the main axis of the North Passage. It also shows a significant flood-ebb asymmetry in one tidal cycle. Both the velocity and duration of the ebb tide are significantly larger than those of the flood tide. The current varies distinctly in a spring-neap tidal cycle. The tidally averaged flow velocity in spring tide is as twice larger as in neap tide.The characteristics of SSC in the middle reaches of North Passage showed that in most tidal cycles the SSC in ebb is smaller than that in flood. SSC increases remarkably from neap tide to spring tide due to the increase of hydrodynamic, mean SSC in spring is3or4times of that in neap. SSC also increases from the surface to the bottom and largest SSC gradient generally occurs at0.6H water layer (H denotes the water depth).During the observation, suspended sediment is fine grain in the middle reaches of North Passage, with the grain size ranging from4-62.5μm. The temporal and spatial variations of suspended grain size are significant. The results exhibit that the mean grain size of suspended sediment is larger in ebb than in flood, in spring tide than in neap tide, and in lower water layer than in upper water layer. The major types of the bedload are silt and sandy silt in North Passage. The surface bedload is fine sediment, with the mean grain size of19μm. The major types of the bed sediments are silt and sandy silt. The surface bedload transport in the North Passage presents a complicated trend, which is seaward in the northern and southern shoals but landward in deep channel. The relationship between suspended sediment and bedload is linked resuspension and settling processes. In the neap tide, sediment exchange between the bed and water is slight due to weak current. In the moderate tide, sediment exchange becomes stronger due to the increasing of flow velocity and sediment resuspension. In the ebb and flood peak during spring tide, sediment exchange is very intensive, and grain size of bedload and suspended sediment are similar to each other.The residual flow, magnitude of water and suspended sediment flux are not increased obviously from neap tide to spring tide. The Euler residual flow is landward, and the Stokes residual flow is seaward and with scale of about one fourth of the former current. In each tidal cycle the tidal net water flux is seaward. Advection transport term is the most important component of the suspended sediment transport; tidal pumping transport term is the secondary important constituent, but its magnitude and direction changed frequently within the tidal cycle. The magnitude of net vertical circulation transport term is very small and the transport direction is landward.Different size fractions of suspended sediment transport results show that clay-size and fine silt-size have the same transport features; in the moderate and spring tides, the transport of coarse silt-size is different from other size fractions in tidal pumping transport term; the sand-size is different from other size fractions in the advection transport term, tidal pumping transport term and vertical circulation sediment and net vertical circulation transport term, indicating that the transport property of sand-size class is different from other size fractions. These studies demonstrate that suspended sediment should be divided into different grain size classes when we analyze suspended sediment transport mechanism using the decomposition method.