Research On The Sedimentary Dynamic Processes In The Middle-lower Segment Of South And North Passage, Yangtze Estuary

Located on the narrow transition zone between the river and ocean, estuary is one of the most sensitive regions of the Land Ocean Interactions in the Coastal Zone (LOICZ). Impacted by both natural factors (such as tide, wave and runoff) and anthropogenic activities, the dynamic processes in the estuary are extremely complicated. Thereafter, the estuarine sedimentary dynamic processes have been one of the cores of the LOICZ research. Meanwhile, the researches of water-sediment movement and vertical sediments exchange in the estuary are the focus of the analysis of estuarine dynamics and deposition mechanism.As a typical bifurcated estuary, the morphodynamic patterns of Yangtze Estuary can be described as ’three-order bifurcations and four-outlet diversions into the sea’. As two main outlets of Yangtze Estuary, the North Passage and South passage are typical bifurcated channels, which play an important role in the water and sediment transportation. Meanwhile, with construction of the deep-water channel located in the North Passage and potential land resources of Naihui tidal flat situated by the South Passage, the study of water-sediment movement and vertical sediments exchange in the North Passage and South Passage are of great importance, especially for understanding of the sedimentary dynamic process in the Yangtze estuary.This thesis is based on the in-site measured data and underway measured data, including water and sediment samples, hydrological data, and suspended sediments samples collected in the regions of North Passage, South Passage and the entrance of South Passage during 2011 —2013. In this thesis, the change of water-sediment process and the spatial-temporal variations of suspended sediment and bed-sediment were studied. Furthermore, the exchange process of the suspended load near the bottom and bed sediments were also analyzed to reveal the sedimentary dynamic process in bifurcated channels based on the measured grain size parameters and associated components of sediment samples in the entrance of South Passage._The main results are summarized as follows:The tidal current in the North and South Passage can be represented as mainly reciprocated flow with dominatary ebb tide current. The lasted duration and the average velocity of the ebb tidal current in the North and South Passage are larger than those of the flood tidal one, respectively. Meanwhile, the dynamic intensity in the middle segment of the North Passage is greater than that in the lower segment, while the dynamic intensity in the lower segment of North Passage and middle segment of South Passage are almost in the same level. In the middle segment of South Passage, the average flow direction is about 317° during the flood tide and 143°during the ebb tide, respectively. In the middle segment of North Passage, the average flow direction during the flood is between 260～290°and 123°during the ebb, respectively. However, in the lower segment of North Passage, the average flow direction during the flood is between 300-320°and 135°during the ebb, respectively.The suspended sediment concentration (SSC) in the middle segment of North Passage is highest, followed by that in the lower segment of North Passage and the middle segment of South Passage. SSC during the Spring tide is much higher than that during the neap tide. Specifically, SSC during the ebb tide in the middle segment of South Passage is 2-3 times higher than that in the neap and moderate tide. Compared with the South Passage, SSC variation during the spring tide and neap tide is much higher in the North Passage. In the lower segment of North Passage, SSC during the flood in the spring tide is 3 times than that in the neap tide, while the SSC during the ebb in the spring tide is 6 times than that in the neap tide.In the middle segment of the North Passage, SSC during the flood in the spring tide is 5 and 8 folds higher than that during the flood and ebb tide in the neap tide, respectively. The sorting coefficients of suspended sediment become worse from the neap tide to the spring tide. In vertical water column, the sorting coefficients of suspended sediment are also worse from surface to bottom. The skewness values of suspended sediment during the spring tide are larger than that during the neap tide. In the surface waters, the skewness values are smaller than that in the bottom waters. The peakedness value of suspended sediment is relatively stable, which vibrates with tidal cycle, but changes little from the neaptide to the spring tide.In terms of surface sediment, the grain size of surface sediment in the middle segment of South Passage is greatest, which followed by that in the middle and lower segment of North Passage. The grain size parameter changes during the tidal cycle(from neaptide to spring tide) as follows:the mean grain size changes from coarse to fine in the middle segment of South Passage and lower segment of North Passage, while the mean grain size experiences a process of fine-coarse-finein the middle segment of North Passage. The sorting value changes little during the tidal cycle and the sorting of surface sediment grain size in the middle and lower segment of North Passage is poorer than that in the middle segment of South Passage. The skewness value of surface sediment grain size reduces slightly in the middle segment of South Passage and lower segment of North Passage. The kurtosis of surface sediment grain size shows a medium sharp form both the middle segment of South Passage and North Passage, while it presents a decrease trend in the lower segment of North Passage, and its value maintains about 1 during the spring tide.In the point of exchanging component between the suspended sediment and surface sediment, clay, fine silt and coarse silt make the greatest contribution in the sediment exchange process in the middle segment of South Passage, as well as in the middle and lower segment of North Passage. In the entrance area of South Passage, silt is the dominant component in the vertical exchange between the suspended load and the surface bed sediments. However, the coarse silt is the dominant exchange component during the exchange process between the surface bed sediments and subsurface bed sediments in the central district of the entrance area of South Passage, while fine sand exchange occurred in other regions of the entrance area of South Passage. A large amount of clay and fine silt in the surface sediment are diffused into water by resuspension, in which the fine silt accounts for the highest proportion. The characters of clay and fine silt are of similarity, as well as their response to the dynamic changes. Moreover, the ratio of fine silt to clay is relatively stable in the middle segment of South Passage and middle-lower segment of North Passage, which almost remain 1.5 during the tidal cycle. The percentage of clay and fine silt in the middle segment of North Passage is higher than that in the lower segment of North Passage and middle segment of South Passage, while the percentage of coarse silt in the middle South Passage is the highest. Moreover, the coarse silt and fine sand in the middle segment are also the most sensitive components to the velocity changes. However, the fine sand component in the middle and lower segment of North Passage only respond to the comparatively high velocities occurred in the moderate-spring tide.