Abstract: Since the operation of the Three Gorges project, the riverbed in the Middle Yangtze River (MYR) has been continuously eroded, leading to intensive bank erosion in certain local reaches and posing a threat to flood control safety. Based on field measurements, this study reveals the statistical trends of the MYR riverbank morphological parameters, and determines the threshold values for bank stability. The results indicate (1) the probability density of underwater slope is generally log-normal. However, when external factors－such as changes in water and sediment conditions or the implementation of bank protection projects－reduce or enhance bank stability, the actual distribution deviates from the log-normal. And the extent of deviation can be used as a basis for evaluating the overall stability of the river reach. (2) Channel pattern is not a factor that affects significantly the probability density distribution of bank slope, but its impact is relatively significant on the relative thalweg location. (3) The thresholds of underwater slopes are > 0.22, > 0.18, > 0.20, and > 0.28 in the Upper Jingjiang, Lower Jingjiang, Chenghan Reach, and Hanhu Reach respectively. These values are greater than the angle of repose of the lower bank soil (nearly 0.16), indicating that bank failure occurs only after the lower soil layer has been eroded to a certain degree, consistent with the dynamic process. (4) The thresholds of thalweg location are less than 10% of the bankfull width. In addition, using our bank stability thresholds, we can determine the distribution of areas prone to bank failure in MYR in 2020, and the results are in good agreement with observations.

Key words: bank profile, lognormal distribution, threshold values of bank stability, probability density, Middle Yangtze River