Flow resistance adjustments of channel and bars in the middle reaches of the Yangtze River in response to the operation of the Three Gorges Dam

Since the Three Gorges Dam (TGD) was put into operation, the flood water level at an identical discharge rate has not displayed a decreasing trend along the middle reaches of the Yangtze River (MYR). The flow resistance variations of the channel and bars in response to the operation of the TGD remain poorly understood, despite the importance of understanding these for water disaster mitigation and water environment regulation. Herein, the impacts of the TGD on the downstream flow resistance of the channel and bars in the MYR were analyzed using systematic surveys of hydrological datasets, cross- sectional profiles, sediment datasets, and remote sensing images, during different periods. Under the actual natural conditions in the MYR, a modified semi-empirical formula, which considered the grain, dune resistance, as well as the topographic features of the riverbed, was proposed to predict the channel resistance. Furthermore, the effect of various dam-control flow and sediment elements on the variation in different flow resistance components, and the corresponding relationships among them were investigated. Results showed a decline in the comprehensive, channel, and bar resistances as the discharge increased, whereas there was a slight increase when reaching the bank-full discharges. Notably, the bar resistance occupied 65%, while the channel resistance, in which dune resistance was much larger than grain resistance, contributed 35% to the comprehensive resistance. In addition, while flow resistance rose over time, there was a decline as the distance from the TGD increased. In conclusion, the increased dune and bar resistances, interpreted by the fluctuated channel longitudinal profile and growing vegetated area on bars, were the dominant factors preventing the flood water level from dropping.