Abstract:

[Objectives] This paper aims to provide theoretical basis and scientific support for the prediction, early warning, and systematic prevention and control of riverbank collapse in the middle reach of mainstream Yangtze River by reviewing the characteristics and mechanisms of riverbank collapse. [Methods] By reviewing domestic and international literature, we systematically summarize current research status of riverbank collapse along the middle reach of mainstream Yangtze River, including the definition, classification, spatiotemporal distribution characteristics, and main influencing factors and their action mechanisms. Using measured data from hydrological stations including runoff, sediment load, and water level variations, we analyze the changes in water and sediment conditions before and after the operation of the Three Gorges Reservoir and their impacts on bank collapse. Particular focus is given to the mechanisms of bank collapse from both internal factors (such as the properties of riverbank soil and channel morphology) and external factors (such as hydrological and sediment conditions, water level fluctuations, vegetation coverage, and human activities). [Results] Spatiotemporal distribution characteristics: riverbank collapse along the middle reach of mainstream Yangtze River exhibits variation both longitudinally and laterally along the river; the frequency of collapse in the lower Jingjiang section is higher than that in the upper Jingjiang section, and collapses occur more often on the left bank than on the right bank. Affected by water scouring and water level fluctuations, the flood season and the recession period after the flood are peak periods for bank collapse. After the operation of the Three Gorges Reservoir, the number of river sections with strong catastrophic bank collapse decreases significantly. Internal factors of bank collapse include the composition of riverbank soil (such as the binary structure of an upper cohesive soil layer and a lower sandy soil layer), the height difference between the beach and the trough, channel sinuosity, and bank slope gradient. External factors include water and sediment conditions (such as longitudinal flow scouring, circulation erosion, and backflow scouring), water level fluctuations, vegetation coverage, and human activities (such as near-bank sand mining, sudden loading, and slope excavation). Water flow scouring is the dominant factor of bank collapse, especially the scouring effect of longitudinal flow, which directly impacts the riverbank crest and slope. Meanwhile, circulation erosion, backflow scouring, and water level fluctuations also jointly contribute to the occurrence of bank collapse. Bank protection projects achieve significant results in controlling collapse, but collapse still occasionally occurs in protected sections. Bank protection work does not significantly change the flow structure, but intensifies scouring at the unprotected toe of the slope, resulting in deep troughs approaching the bank and further aggravating riverbank erosion. [Conclusions] Bank collapse is the result of the combined effects of multiple factors. It remains necessary to conduct in-depth research on the characteristics of riverbank collapse in the middle reaches of the Yangtze River and to clarify the threshold values of influencing factors, to provide a reference for slope stability assessment and for the prediction and early warning of bank collapse. During design and construction, attention should be paid to controlling the underwater slope gradient of engineering works and to considering the impact of toe scouring on the overall stability of the riverbank. With the continued operation of hydropower stations such as Wudongde and Baihetan in the lower reaches of the Jinsha River and the ongoing implementation of soil and water conservation efforts in the basin, the “clear water scouring” in the middle and lower reaches of the Yangtze River will persist. Therefore, priority should be given to preventing bank collapse caused by strong longitudinal scouring and channel pattern adjustment in highly sinuous river sections, and to strengthen protection, routine monitoring, and early warning in these areas. Future research may further explore the effect of vegetation in stabilizing riverbanks, and enhance monitoring of river morphology, the stability of specific cross-sections, and channel evolution, in order to increase the inspection frequency and monitoring precision in high-risk reaches and areas prone to collapse.

Key words: bank collapse, bank slope stability, river management, revetment engineering, middle reach of the Yangtze River