Morphodynamic adjustments in the Yichang-Chenglingji Reach of the Middle Yangtze River since the operation of the Three Gorges Project

摘要

To investigate the evolution characteristics of the Yichang-Chenglingji Reach (YCR) of the Middle Yangtze River and develop simulation methods, the temporal and spatial morphodynamic adjustments of the YCR were analysed comprehensively on the basis of runoff, sediment load, and cross-sectional profiles data at 220 cross sections from 2002 to 2016. Results showed that significant channel degradation has occurred in the YCR, especially in the low-flow channels, due to a drastic reduction in sediment yield since the operation of the Three Gorges Project (TGP). The variations of upstream flow and sediment regimes would drive equilibrium channels to become increasingly narrow and deep, and the current bankfull width and depth were less than their equilibrium counterparts in the recent period. However, the recent channel evolution was mainly characterized by the prominent channel deepening via a one-way deformation due to the large-scale bank revetments within the YCR, where the reach-scale bankfull depth increased by 1.6 m in the span of 15 years. A semi-empirical method for determining the equilibrium values of channel degradation and aggradation volume at the YCR was proposed on the basis of a comprehensive analysis of the effects of flow, sediment concentration, sediment gradation, and water depth. Moreover, the equilibrium values of the channel width and depth were estimated with the Least Action Principle method. The calculation methods for the equilibrium width, depth, width/depth ratio, and aggradation/degradation volume were combined with the Delayed Response Model (DRM). Then, the methods for calculating the variations in channel geometry and aggradation/degradation volume were derived. Results showed that the proposed methods effectively simulated the channel geometry and cumulative erosion volume in the YCR from 2002 to 2016. In addition, the response of the channel evolution in the YCR was closely related to the previous four-year hydrological conditions, implying that channel evolution may lag behind the variations of flow and sediment discharges.