Channel Degradation Downstream from the Three Gorges Project and Its Impacts on Flood Level

The effects of the sediment regime on the flood level in the middle reach of the Yangtze River before and after the construction of the Three Gorges Dam (TGD) are investigated. Before the dam construction, the sediment regime has driven the flood level higher and higher over recent decades in the middle reach of the Yangtze River, which has reflected changes in the location and amount of sediment deposition. After dam completion, the magnitude and rate of channel degradation determines the process of flood stage lowering but they are difficult to estimate owing to insufficient understanding of the sediment discharge recovery process. To make a rational prediction of channel degradation of the Yangtze River downstream from the TGD, the sediment transport rate during channel degradation downstream from other dams is examined. It is found that, for any grain size, postdam sediment transport rates cannot exceed the predam level at any location along the downstream channel. Erosion amounts predicted for the reach downstream from the TGD before its closure are too high. In light of this, a numerical simulation of the channel degradation process is carried out. The results indicate that, although degradation takes place immediately after the TGD closure, the flood level in the middle reach of the Yangtze River will still remain at its predam condition in the following 20 years. This is determined not only by the regional characteristics of the middle reach of the Yangtze River but also by the common law of sediment transportation downstream from dams.