| Channel change is subjected to the co-action of natural and human activities, as well as the interaction of upstream water, sand, and substances from the riverbed. The Yangtze River, which is the third longest river in the world, enjoys the reputation of The Golden Waterway in our country, and acts as the strategic center of resource allocation between the north and the south of China. The city of Nanjing, which is the capital of Jiangsu Province, serves as the political, economic and cultural center. The Nanjing port of the Yangtze River has the cargo tonnage of ten thousand direct freighters, thus realizing inter-dynamic development of the Yangtze River and Yellow Sea. Though equipped with the favorable conditions above, there are also some problems to be solved of the Nanjing reach of Yangtze River. For instance, some parts of the riverbanks are in constant shift, and the retreat caused by erosion in some reaches has led detrimental consequences to levee constructions posing threat to the normal living and production of both banks. Therefore, the scientific understanding of the laws of channel change is vital in exploiting, utilizing and training river reaches properly. In addition, it further promotes The 13th Five-Year Development Plan, and acts as the model in River-Ocean combined transportation construction.This thesis takes the underwater topographic maps of 1966,1986,2000 and 2008 as the main data source, and applies GIS, RS and DEM to the analysis of channel change and variation of the Nanjing reach of the lower Yangtze River. It also sets up a database for bank and underwater research, which attains quantitative information of bank line shifts and variation of underwater topography through spatial overlay analysis on the 4th phase of riverbank and underwater topography.The study shows:(1) Recently, obvious erosional retreat, at the rate of-8.88m/a, has been discovered on northern bank of Xinjizhou and Meizizhou reach. The main reason lies in the decrease of incoming sediment and human activities. Longtan reach now is developing stably, thus laying a solid foundation for the exploitation of the upper reach, Zhenyang reach. Therefore, future work will simply focus on the control of each joint. Since the north branch of BaGuazhou reach is shrinking and silting, and the flow of south branch erosion zone is irreversible, they will be the emphasis of our future work and the solutions are to be discussed in-depth.(2) Nanjing reach of Yangtze River was in siltation from 1966 to 1986. The net siltation volume was 95×106m3, and the net siltation rate was+0.11 m/a, mainly due to the rich sediment from upstream. During the erosion period, lasting from 1986 to 2000, there was a shift from erosion to siltation of the upper stream, leading to a reduce in sediments as well as the deepening of soil and water conservation engineering. The net siltation volume was 27.2×106m3 and the net erosion rate was-0.15 m/a. In the stable period, year 2000 to 2008, the net siltation volume dropped to 8.7×106, and the net siltation rate was+0.08 m/a. The main reason lied in the upper stream erosion, which supplemented sediments and brought down the sediment capacity, thus resulting in a slight siltation of the riverbed. Accordingly, in the recent 50 years, the Nanjing tributary, Yangtze River has displayed a general characteristic of siltation-erosion-stability.(3) During 1966 to 2000, ait merging into land has caused a shift from many branches to few branches(or single). This evolution has simplified the once complicated diverting situation, and narrowed the twisting range of the mainstream, which was conducive to the stability of the river. From 2000 to 2008, under the effect of the bank protection work, the development of both the channel profile and the ait tended to be stable.(4) According to the research above, the Nanjing reach of the lower Yangtze River has evolved to a state of high stability. The bank line has crept in a much more limited range, and branches in constant stability enjoy the preliminary advantage. The south banks are universally more stable than the north banks, while the length of silted aggressive area and erosive retreating area will be in equilibration. Up till now, the Nanjing reach has undergone the evolutions from the dominance of nature, the co-action of nature and human, to the dominance of human, which has laid many uncertainties for the prediction of watercourse change. Therefore, the following work must stick to the comprehensive treatment principles of branching streams, as well as stabilizing the branch river regimes, reducing harmful branches, properly utilizing the point bar, enhance supervision cooperation, and perfecting river treatment. In this way, the river regime will keep relatively stable in the future.(5) Acknowledging and following the channel change rules is the foundation and necessity of river treatment engineering, which should be planned and carried out along with the comprehensive utilization of the river. The prime objection of the river treatment engineering is to control the river regime, whose major engineering measure lies in the bank protection engineering. |
