Evolution Processes Of Mid-channel Bars In The Middle And Lower Yangtze River And Xiang River

The evolution of the mid-channel bars in the branching rivers is affected by the conditions of water and sediments and human activities.With the construction of cascade reservoirs and water conservancies in the upper rivers,large amounts of sediment are intercepted,resulting in drastic changes in the water and sediment conditions in the lower of the water conservancies.Riverbed and the mid-channel bars in the middle and lower rivers have been washed by clear water for a long time,and their natural evolution has also taken place.The change will inevitably shift from the upstream siltation to the downstream erosion.Therefor,the study of the new evolution process in the mid-channel bars in the lower of water conservancies has a reference value for dike safety,waterway regulation and protection of wetlands on the beach,which in turn will help deepen understanding of the evolution of the branching rivers.The distribution and basic characteristics of alluvial rivers in China have been analyzed,and the formation reasons and formation conditions have been discussed briefly.Selecting the branching reach in the middle and lower reaches of the Yangtze River and the middle and lower reaches of the Xiang River as the research object,the water and sediment changes,the morphological changes and the erosion and deposition process in the mid-channel bars have been analyzed.The hydrodynamic conditions of the mid-channel bars have also been simulated by MIKE 21 FM.The main research results are as follows.(1)The branching rivers in China are mainly distributed between 23°-34°N,and a small amount is between 45°-54°N,while does not exist between 34-45°N.A preliminary analysis of the cause of the branching rivers indicates that when the sediment transport capacity of flow is similar to the amount of incoming sediment,the flow will have a certain scouring effect on the riverbed and the riverbank in flood season while silting occurs during the non-flood season.At the same time,river valleys such as nodes,etc.?are used to control and produce the different intensity of water and sediment transport.At this time,it is easy to form The branching river type.(2)Since the construction of the upstream reservoirs such as the Three Gorges Reservoir,the annual runoff has not changed much,but after 1985 the annual sediment in the middle and lower of the Yangtze River began to decrease,and then decreased rapidly after 2003.The Baisha bar in the middle of the Yangtze River has shrunk by more than 44.37%.The Yudai bar in the lower of the Yangtze River has shrunk by more than 5%and the Tieban bar in the lower of the Yangtze River has shrunk by more than 36.14%.Scouring mainly occurs during the flood season with high flow and high water levels.The shrinkage of the downstream mid-channel bars is lagging behind that of the midstream mid-channel bars,but the erosion rate is all getting faster and faster.(3)Since the 1970s,the annual runoff in the middle and lower of the Xiang River has changed little,but the annual sediment has been decreasing year by year.Before 1995,the mid-channel bars in the middle of the Xiang River was still under development,but after 1995 they began to undergo substantial erosion and shrinkage.The mid-channel bars in the lower of the Xiang River began to shrink after 2008 because of been supplied with water and sediment from tributaries.