The Migration of the Erosion Center Downstream of the Three Gorges Dam, China, and the Role Played by Underlying Gravel Layer

Abstract

Rivers disrupted by sediment cutoff often experience degradation, but the migration of the erosion center, defined as the location with the greatest degradation rates, has not been thoroughly understood. This paper focuses on the streamwise migration of the erosion center along the ∼400-km-long Yichang to Chenglingji reach (YCR) downstream of the Three Gorges Dam (TGD), China. We analyzed channel morphological adjustment based on water, sediment and channel geometry data collected during 2002–2020. Based on the location and time for the occurrence of relatively large channel degradation, a clustering algorithm was used to identify the location of the erosion center. Characteristics and morphodynamic controls of the erosion centers were studied based on the migration of incisional and coarsening waves simulated by a one-dimensional morphodynamic model for nonuniform sediment. Results show that the erosion center migrated downstream along the Yichang-Zhicheng reach with gravel-sand bed during 2002–2012, the migration rate was rapid after the dam closure then decreased with time. After ∼2012, large cascade dams started to operate along the upper Yangtze River, sediment load further decreased and degradation accelerated at the YCR. Correspondingly, the erosion center migrated to the sand-bedded upper Jingjiang reach with faster rates. The erosion center migrated for a total of over 200 km with an average rate of ∼14 km/yr during 2002–2020. The underlying gravel layer was exposed due to degradation, which enhanced bed coarsening and resulted in the propagation of the erosion center downstream of the TGD.