Sediment Distribution And Before-dam Study In Middle And Lower Yangtze River Stability

As the linkage between land and sea, river system is one of the most sensitive locations in reflecting the global change. With the demands of social development, more attention has been paid to the change of river system than before, not only because human civilization is cultivated in the river basin, but also the sustainable exploration of fluvial resources relates to the social development. Sediment, the core of fluvial geomorphology, is also the fundamental component in discussing sedimentary facies evolution in river system. To deal with sediment yield, transport and deposit in river system is an exciting, but tough task for geomorphologist, especially considering the complexity of coupling hydrogeomorphology, climate and human activity, etc.As the longest river in China and the third longest one in the world, the Yangtze River geomorphology has been of the great interest for many earth scientists, including the effect of topographic change, monsoon precipitation and intensive human activity. The construction of Three Gorges Dam being closed in 2009 will impact largely the middle and lower Yangtze River morphology. The environmental evaluation and associated impact should be based on on-site investigation and research of the basin scale before the closure of dam construction. Through several field trips from 2000-2003, our research group has established the sedimentological and hydromorphological database, including >500 sediment samples along the middle and lower Yangtze river, and ADP-surveyed flow velocity, Sonar-surveyed water depth and river-bed topography. In addition, many historical maps has been incorporated in the present database, in which, useful information was retrieved, such as river width, water depth, water surface gradient, and curvature, etc. By using this database, hydrological simulation was done to obtain a series of parameters, i.e. Froude Number, Unit Stream Power, and Shields Stress, etc. These have been applied to study sediment transport processes, in relation to longitudinal and lateral stabilization of the middle and lower Yangtze River system. Furthermore, river channel stabilization condition was forcasted under the assumption of larger discharge of 60,000 m~3/s. The present study highlights flood prevention, river channel management and future planning, and moreover, post-dam study after the construction of Three Gorges Dam in the middle and lower Yangtze River.