Morphodynamic evolution in a meandering reach of the Middle Yangtze River under upstream and downstream controls

Abstract

Morphodynamic evolution in an alluvial river is usually controlled by various boundary conditions. During the past nearly 50 years, remarkable morphodynamic evolution occurred in the Jianli reach of the Middle Yangtze River owing to the combined effects of an artificial cutoff, the upstream operation of the Three Gorges Project (TGP), and the downstream confluence of the Dongting Lake. To better understand the characteristics of morphodynamic changes in the whole study reach, variations in thalweg shifting and bankfull channel geometry were quantified using a reach-averaged approach, and the effects of upstream and downstream controls were investigated on channel geometry adjustments. Calculated results indicate that: (i) events of an artificial cutoff and high flows caused the average rate of reach-scale thalweg migration to be greater than 35 m/yr, but there was a 22% reduction in the mean migration rate after the TGP operation; (ii) channel geometry adjusted mostly in the aspect of bankfull depth under various river regulation engineering, with the reach-scale bankfull depth increasing by 0.95 m from 2002 to 2016; (iii) the reach-scale bankfull dimensions are closely associated with these accumulated effects of both the altered flow-sediment regime because of upstream dam construction, and the local base-level variation owing to the downstream confluence of the Dongting Lake. Furthermore, these bankfull variables were expressed by power functions of two key parameters, covering the previous five-year average fluvial erosion intensity during flood seasons at Jianli (upstream control), and the corresponding water level difference between Jianli and Lianhuatang (downstream control). The proposed relations were calibrated by the observed data in 2002–2014, and were further verified by measurements in 2015–2016. The proposed methodology can also be applicable to estimate channel geometry adjustments of other similar alluvial rivers controlled by both upstream and downstream boundary conditions.