| In recent years, with the construction of large-scale projects in the Yangtze River catchment, such as soil and water conservation engineering, Three Gorges Project and South-North Water Diversion Project, which will surely make the runoff and sediment of Yangtze River catchment and its seasonal distribution changed significantly. The construction of land reclamation, deepwater waterway, fresh water reservoirs and bridge projects in the Yangtze estuary will change the boundary conditions of the estuarine channels. So that each branch split ratio of the Yangtze estuary will change; and will change the velocity and direction of tidal current, the transport, deposition, and temporal and spatial distribution of suspended sediment; and then lead to the changes in estuarine channel geomorphology; while the change in estuarine channel geomorphology in turn influences the variation in the tidal dynamics and suspended sediment concentration (SSC). When the tidal dynamics, SSC and geomorphology changes, it will put the safety of estuarine channel boundary and the stability of estuaries large-scale projects have some impact; and directly related to the safety of the estuarine engineering, waterway transportation and people’s life and property.The study based on the above factors and the previous research. And it takes the upper of North Channel (NC), the upper middle of North Passage (NP) and Nanhui’s south shoal in the Yangtze estuary as the research objective. Variations are studied in tidal dynamics, SSC and geomorphology of the estuarine channels by utilizing the measured data of tidal currents, suspended sediment, geomorphology and water level at some adjacent tidal gauge stations in the Yangtze estuary for recent years. And then research on the mechanism of sediment transport. Finally, hydrodynamic, depositional and geomorphologic process response mechanisms to the major projects are explored. This will help to understand the hydrodynamic, depositional and geomorphologic process of estuarine channel in the recent years, and provide a theoretical basis for the rational development and utilization of estuarine and coastal resources; and for the maintenance of estuarine engineering, dredging and management of estuarine channel to provide a reference, and having important theoretical and practical significance.The main findings are as follows:1. Variation in tidal patterns. The tidal patterns are irregular semidiurnal tide in the upper of NC, the upper middle of NP and Nanhui’s south shoal. The influences of the construction of the projects make the non-linear characteristics of tidal dynamic obvious. And the characters of shallow water tidal tend to be strengthened in the upper of NC and NP and unchanged in the middle of NP and Nanhui’s south shoal.2. Variation in tidal current velocity (TCV). From 2003 to 2007, the TCV is reduced 0.19-0.28 m/s, and preferential flow tends to be reduced during flood season and increased during dry season in the upper of NC. From 2007 to 2012, the TCV is decreased during flood season and increased during dry season, and preferential flow tends to be reduced during dry season and increased during flood season. After the construction of the third stage of Yangtze estuary deepwater channel, the TCV is reduced slightly, and preferential flow tends to be reduced slightly in the south of the middle of NP. From 2003 to 2012, the TCV is increased in flood season and reduced in dry season, and preferential flow tends to be increased during flood season and reduced during dry season in the Nanhui’s south shoal.3. Variation in SSC. From 2003 to 2007, the SSC is decreased about 0.10 kg/m3, and preferential sand tends to be decreased during flood season and increased during dry season in the upper of NC. From 2007 to 2012, the SSC is decreased during flood season and increased during dry season, and preferential sand tends to be reduced during dry season and increased during flood season. After the construction of the third stage of Yangtze estuary deepwater channel, the SSC is increased 0.31-0.48 kg/m3, and preferential sand tends to be increased in the south of the middle of NP. From 2003-2012, the SSC is increased 0.19-1.11 kg/m3, and preferential sand tends to be increased during flood season and reduced during dry season in the Nanhui’s south shoal;4. Variation in micro-geomorphology during flood and dry season. From the flood season to dry season micro-geomorphology changes evident in the upper of NC, and sand waves’ development areas during flood season more than during dry season. Xinqiao channel within superimposed type ripples sand waves during flood season, while during dry season it becomes single large-scale sand waves. In the nearby north entrance of Hengsha which develop curved shape and straight-linear sand waves during flood season; while curved shape sand waves are disappeared and developing many elliptic pockmarks during dry season, and the wavelength of straight-linear sand waves got longer. The similarities and differences of riverbed micro-geomorphology are related to the hydrodynamics, particle and sorting behaviour of sediment during flood and dry season.5. Variation in shallow profile geomorphological. From January 2010 to October 2014, the suface morphology and layered structure of profile almost no significant changes in the Xinqiao channel, which is a 2 m thickness single layer. In the section near of the narrowest cross section, the substantial erosion of downstream section continued long distance in October 2014, and the maximum erosion depth is about 7 m. From January 2010 to October 2014, layered structure changed significantly on the west of the Yangtze River Bridge, upstream from the double layered into a single, middle stratification disappeared and downstream part stratified thickened, partly by double layered into a single layer.6. The mechanism of sediment transport. Euler residual flow (seaward transport), tidal pumping, Stokes effect (landward transport) and vertical-longitudinal circulation (landward transport) are main forcing agents leading to residual sediment transport. Euler flow plays a vital role in the non-tidal steady advection transport, and tidal pumping plays a vital role in the tidal transport. Sediment transport due to tidal pumping varied with season in the outside of SP and NP, while seaward transport in other channels. Euler residual flow are main forcing agents leading to residual sediment seaward transport in Xuliujing; in the north of SB are Euler residual flow t and tidal pumping seaward transpor; in the upper of NC and middle of SC are Euler residual flow, tidal pumping seaward transport and vertical-longitudinal circulation landward transport; in the upper and middle of NB, the middle and lower of NC, the middle of NP and south beach of SP are Euler residual flow, tidal pumping seaward transport and vertical-longitudinal circulation, Stokes effect landward transport; in the outside of the Yangtze Estuary are Euler residual flow seaward transport and vertical-longitudinal circulation landward transport.7. Hydrodynamics, depositional and geomorphological processes response mechanism for major projects. After the completion of Qingcaosha fresh water reservoir, the channel section will be narrowed, hydrodynamics increased; and coupled with the reduction of sediment from Yangtze River catchment resulting in reduction of SSC in the upper of NC; and then tidal current will be brought up sediment at the bottom of the estuarine channels, so that the estuarine channel erosion occurs. South jetty in the third stage of Yangtze estuary deepwater channel impedance flow, sediment resuspension and the runoff and sediment of upstream make suspended sediment become high concentration in the middle, and non-linear characteristics of tidal dynamics in the upper estuarine channel increased are stronger than the middle, which are the main reason of causing siltation in the middle of NP. Decrease of sediment from upstream, the construction of siltation reclamation project in Nanhui’s shoal and the migration of sand body make the SSC reduced. And the constructions of siltation reclamation project and East China Sea Bridge make TCV increases during flood season, flood tide dominated strengthed during dry season. Increase of tidal dynamics and decrease of SSC resulted in riverbed scour obvious in Nanhui’s south shoal. Hydrodynamic, depositional and geomorphology process is not only concerned with the reduction of sediment from catchment, but also with the construction of local estuarine engineering having a direct connection. |
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