Morphological Evolution Of The South And North Passage Diffluence Pass In The Yangtze Estuary And Its Response To The Deepwater Channel Regulation Project

This thesis is based on the South and North Passage diffluence pass's historical and morphological data collected since 1959, also the data of the sediment diversion and South and North passage's ratio since 1964; meanwhile on the basis of a field observation(fixed station and cross section) of the South and North Passage diffluence pass's water-sediment characteristics, the field observed data collected from the drift current experiment through the concerned methods and theory of geomorphology, river dynamics, and coastal engineering. We do research on the South North Passage diffluence pass's landform and the evolution of the braided sandbank joint's shape, the influence done by Yangtze estuary deepwater channel regulation project, we come to the conclusion as following:1. The South and North Passage diffluence pass is connected with the south channel's downstream river section. Since the transfixion of the north Passage, the talweg position of the south channel's downstream river section has swung from south to north and then to south again. When the position of the talweg is further south, the deep trench and the North Passage will be connected smoothly, which will benefit the diffluence of the North Passage, the lower segment talweg's position of south port is now steady in the south part of the river channel.2. The south and north Passage diffluence pass sandbar head has shown the moving regularity, that is upward and downward strain in the natural state. Simultaneously the shape of the sandbar head change regularly. The change usually begins from the up stage of the sandbar head, the bifurcation angle changes from blunt to acute, while opposite in the downward part, which basically obey the rules of the shoal-in-center-river's evolvement of Yangtze estuary.3. Before the diffluence pass project started, the position of the stream sandbar head will have a series of changes, but meanwhile the sandbar head's azimuth will be stable at about 305 degrees, which is of the same direction with the external ocean tide's afference. After Jiangya Nansha shoal'incorporating into the Jiuduansha shoal, the diffluence pass of the south north passage has risen by 10.43 km, making the sandbar azimuth fluctuate at nearly 310 degree, yet is still to stabilize before the diversion port project.4. After the completion of the diffluence pass project, the entrances of the north passage and south passage experienced erosion and deposition in many places. There are erosion and deposition in the north passage's entrance, while general erosion in south passage's entrance. In the north part of the submerged breakwater project, the deposition continued. After finishing the project, the north riverbed of north passage has experienced a stage of deposition, and then became into erosion (maybe because of the activity of human beings). The-5-meter-isobath rose after the transient fall. The rising trend in 2005 retarded, and then occurred to further north side at the azimuth of 314.2 degree. Of the same time, the cross section of the north passage entrance continued to decrease in this period, mainly in the north part of the submerged breakwater and the north passage of the river bed.5. On the cross section of the south and north passage diffluence pass and the distribution of the velocity profile, the ebb flow dynamic of the south passage is stronger than that of the north. The average distribution of sediment concentration on the cross section in the south passage is larger than that in the north. About the north side of the submerged dyke contains a higher sediment concentration. At the same time, drifting experiment demonstrates that the diversion point of the south and north passage lies not in the extension line of the submerged breakwater but in its north side.6. The ratio is related with landform of the south and north passage diffluence pass, especially with the shape of the head of shoal, head azimuth. When Jiuduansha shoal is still stream sediment and the sandbar head azimuth being steady at 305 degree, the north passage distributing ratio will be at a increasing stage. In the north part of the submerged dyke, the existence of sustained flow area makes a large amount of suspended sediment deposited, bringing about the flow section's continued reduction of the north passage's entrance. In line with river fluvial phase, the reduction of the flow sections will inevitably reduce the ebb water flow entering the north passage. The north passage diffluence ratio then enters a stage of continuous falling.7. The result of the model of current in The South and North Passage diffluence pass using Mike21 shows that when the head azimuth of the submerged breakwater project is changed, the flow pattern will be improved., and the diffluence ratio will be adjusted.