The Characteristics And Explanations Of Channel Change In The Datong-Jiangyin Reach Of The Lower Yangtze River:1959-2008

Research on river channel changes caused by nature and human being has attracted increasing attention worldwide due to the economic, social, and environmental significance of these changes. As the most important river in China, the Yangtze River has been tremendously influenced by human activities as well as monsoon climate variation. The changes in water and solid in the river make it an ideal study area for understanding how natural events and human actions can impact the characteristics of a river. Therefore, the research that focused on the sedimentation and erosion of the river channel, indicating the river channel change has received a large amount of attention in fluvial geomorphology research. The sedimentation-erosion changes in the Yangtze River and the adjustment of its main course in recent decades can be attributed to both natural environmental change and human activities.After the Three Gorge’s reservoir impoundment, the bed in the middle reach of the Yangtze River scoured strongly, especially the reach close to the dam. The quantity of material that goes into the sea decreased and the trend of cut-and-fill sedimentation has changed. However, the response from the lower reach of the Yangtze River is unknown, which needs to be further investigated.The hydrodynamic condition of the lower reach of the Yangzte River, from Datong to Jiangyin, is different from the one of middle reach and estuary. So the feature of channel’s response to the hydrodynamic condition should be distinctive. Consequently, this study aims to study the characteristics and causes of channel change in the lower reach of the Yangtze River, which is beneficial to understand deeply the response from big river to the natural factors and human activities. To evaluate the dynamic changes of sedimentation and erosion of the Datong-Jiangyin reach of the lower Yangtze River from1959to2008, database of six periods of underwater topography were designed and constructed, with the help of GIS technology, on the basis of the Datong-Jiangyin River Relief Map in1959,1969,1983,1992,2003and2008. Digital Elevation Model (DEM) were constructed based on the data of map in1969,2003and2008. Changes in channel width and area of channel and mid-bar were investigated to understand the planform change. Bed-level adjustments were identified both by comparing available topographic longitudinal and vertical profiles and volume changes of different years. Based on the above critical studies, the major contribution of this dissertation are summarized as follows: (1) It is different from other reaches of the Yangtze River that the channel’s response to the Three Gorge. Although the sediment supply from upstream decreased, sediments amounts to478,224.5thousand cubic metres deposited in Datong--Jiangyin reach owing to the increasing action of tide between2003to2008.(2) Inner differences exist in the filling and scouring process for the river reach between Datong and Jiangyin. Due to the reduction of sediment from the upper reach of the Yangzte River, the upper reach between Datong and Jiangyin were scoured. In the middle reach between Datong and Jiangyin, the sediment concentration increases gradually, the tidal effection moves up,so that the sediment transport capacity of flow decreases and sand falls on the river bed. In the lower reach between Datong and Jiangyin, due to the same direction of ebb-current flow and runoff, flow velocities increases and could carry more sediment, so this reach was scoured.(3) Due to the different type of channel, the beds’ filling and scouring process are also different. In bend reach, the channel change focuses on the incision and main flow’s moving from side to side. Unstable branching adjusted strongly,with one branch filled and the other scoured. In node reach,the location of main flow had no change.(4) The planform of most reaches was confined by revetments, so the filling and scouring process occoured below the elevation of-10meters and adjusted longitudinally. The main area of deposit transfered from the lower reach to the middle reach. The center of erosion changed from the middle reach to the upper reach.(5) The dominant type of adjustment that involved most of the rivers in the region consists of a narrowing of the active channel(up to11.9%from1983to1992). Mid sandbars become more and more stable. The main reason can be ascribed to the effection of river-training works.(6) In a state of nature, the channel change of adjacent reaches is relevant. The location change of main flow in node reach has a tremendous impacts on the channel change of the following reach. The shore protection engineerings play an important role on the stability of the node reach’s river bed. Recently, the relevant relationship between up and down reaches declines.