| Shanghai Port is the most important port for China to develop its foreign trade and traffic since opened in mid-19th century. Now it's the premier commercial port and make outstanding contributions to China and the World. Under the guide of "Shanghai Four Centers Project", construction of Shanghai International Shipping Center is developing ralidly. But, the siltation in the south bank of the south channel has adverse effects to the Waigaoqiao harbor development. It is of significance for the construction of harbors and utilization of shore lines to study river regime change in south channel and find the reasons of siltation in south bank.On the basis of the former research, combined with the Terrain data and hydrology and sediment data, the author use GIS and DEM to analyse the bed-load transport, the swing of talwegs, channel morphology change and erosion-deposition variation in the south branch and the south channel of the Yangtze River in the past 50 years. The results of the study are listed as follows:1. Swing of the ebb current in the south branch is the main reason leads to the change of river regime in the south channel. The talweg has changed 3 times in the south branch from the early 20th century. Since 1980, the south waterway of Baimaosha became to be the main waterway, the talweg stoped winging and stablely directed to the head of Xinliuhesha.2. The south channel process closely related to the water and sediment discharge ratios in the South-North Channel. In the 1960s, south channel morphology changed from U-shape to W-shape due to the occurrence and decline of Xinchongming waterway, the broadened of Xinbaoshan waterway and the extended downstream of Rifengsha spit. In the beginning of this century, south channel morphology rechanged from W-shape to U-shape because of the developing of Baoshanbei waterway, swinging northward of talweg in south channel and erosion of Ruifengsha spit.3. The talweg swings northward twice in south channel in the past 50 years. In the 1970s, siltation occurred in the south channel, The talweg swinged northward because the bed resistance increase. In the end of last century, Baoshanbei waterway expanded, top spot of ebb current moved upstream caused the talweg swinged northward.4. Since 1990, south channel shows obvious erosion in the south part and deposition in the north part, but the erosion-deposition is unchanged. The talweg in south branch never swinged in recent years and the water and sediment discharge ratios are the same in the South-North Channel. So south channel keep its volume stable by adjusting channel morphology when the ebb current change direction. Now, bank protection projects have been made in the upper and lower extremes of south channel, the south and north bank line are also been artificial modified. The south channel won't show obvious erosion-deposition variation in the case of fixed boundary and decrease sendiment supply for a short time.5. There are there reasons of the siltation in Waigaoqiao harbor:First, top spot has move upstream because confluence of current flow into south channel move westwords, it made the ebb current swing northwords, and the current power turn down in the south bank. This is the main dynamical factor, which is a prerequisite for the topographical change. Second, the erosion of underpart of Ruifengsha spit led a direct result of the W-shape to U-shape change of channel morphology, thereby the sediment deposition occurred in th south bank for the sake of recovering the largened cross section area of channel. This is the immediate cause of siltation in south bank, and also an inevitable result of hydrodynamicforce change. Third, hydraulic architectures along the south bank block the near-shore current and spur the deposition of sediment. It's an important reason for near-shore deposition, but it's influence is not strong enough to affect the whole channel.
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