On The Effects Of The Channel Change And Regulation Of Channel That Dredging Sand At The Middle And Lower Section Of The Yangtze River

The sand extraction of the Yangtze River has brought benefits to the industry. On the other hand, the confusion of sand extraction has brought enormous economic losses to the departments of flood control, navigation and others, and has caused harmful effects on ecological environment and the society. The issue has received much attention from the State Council and the leaders of the state. To solve the contradictory between economic development and extraction confusion, it is necessary to tighten up the management of sand extraction and plan the extraction area scientifically and reasonably. To lead the sand extraction to a legal, scientific and orderly road, the Changjiang Water Resources Commission has established Changjiang Sand Extraction Administration. The Administration has proposed sand extraction schemes for the major extraction area.The large-scale confusing extraction would destroy the stable river bed form. Extracting the coarse sand and throwing away the fine sand would cause the bed material finer. Thus, the bed surface is subject to degradation and rebuilt of river bed would occur. Around the mouth of sand pit, the change of flow pattern and the increase of flow velocity would enlarge the transport capacity of flow and accelerate degradation, which would expand the area of sand pit, or affect the discharge ratio of branches. However, not all sand extraction has a harmful effect on river bed change. The advantages and disadvantages of sand extraction are completely determined by man-made factors, just as that of the construction of water conservancy project or dredging. The confusion and excess of sand extraction would possibly threaten the flood control of the bank, worsen navigation condition, or stir up troubles even. Planning sand extraction scheme in accordance with potential channel change and combining channel modification with sand extraction can improve navigation condition to a certain extent. Consequently, the management of sand extraction should be tightened and the area and quantity of sand extraction should be planned according to the scientific study result. One of the goals of this paper is to provide an instruction for orderly sand extraction.Few studies at home and abroad have focused on scientific planning of the area and quantity of sand extraction, as well as effects of sand extraction on channel change and channel modification project, with no successful experiences. As an example, the situation and scheme of sand extraction within the Wuxue reach of the Middle Yangtze River were investigated. On the basis of the study of the river channel change process and its effecting factors, the impacts of illegal sand extraction on channel change and navigation arepresented. Fixed bed physical models and mathematical models are used to examine the Sand Extraction Scheme and evaluate the impacts of extraction confusion on navigation conditions and a proposed modification project. The possibility of combining sand extraction with channel modification is discussed.Through study, the potential channel change of the reach is predicted. The sand extraction scheme proposed by the Changjiang Sand Extraction Administration is revised according to the channel modification scheme. The paper puts forward that before the implementation of the Wuxue modification project, two extraction area, known as Ergansan and Ergansi, can be opened, and that after the implementation of the project, Ergansan and Ergansi should move south. The paper points out that the period of sand extraction can be changed to permit occupation of some navigable channel at middle-water and high-water. The sand extraction should be performed at the entrance to the South Pool of the reach. All the countermeasures mentioned above will solve the contradictory between extraction and navigation and improve the navigation conditions.The study methods and countermeasures involved in this paper can be applied to other sand extraction area to strengthen the cooperation between water resources department and navigation department, wh...