Conceptual model of bank retreat processes in the Upper Jingjiang Reach

Abstract

Bank retreat is always an important part of channel evolution in the Upper Jingjiang Reach (UJR), which usually occurs due to a combination of bank-toe erosion caused by fluvial action and mass failure affected by non-fluvial factors, covering the variations in bank soil properties, in-channel water level and ground water level. After the operation of the Three Gorges Project, the variation in the flow and sediment regime, especially the dramatic reduction in sediment load, has caused significant channel degradation in the UJR, further leading to a higher frequency of bank retreat processes. In this study, an improved conceptual model for predicting the process of bank retreat was thus proposed, consisting of three submodules to simulate the processes of bank-toe erosion, mass failure and groundwater level variation, with the terms of hydrostatic confining pressure, pore water pressure and matrix suction of unsaturated soil being considered in the submodule of bank stability analysis. The proposed model was applied to simulate the bank retreat processes at two typical sections of Jing34 and Jing55 in the UJR in 2005. The results indicate that: (1) changes in ground water level lagged behind that in in-channel water level, with an obvious delayed time of 3–4 days being identified between the occurrence of peak values during the rising period at Jing55; (2) six and four bank collapses occurred at these two sections, respectively, with the calculated retreat widths of 27 m and 20 m, which were in close agreement with the measured data as well as the calculated bank profiles; and (3) a higher occurrence frequency of bank failure was identified during the flood peak and recession stages, due to intensive bank-toe erosion and high ground water level, while the failed soil-block volume was larger during the rising and flood-peak periods, affected by the bank-toe erosion degree at failure. Then, effects on the bank retreat were investigated of bank soil properties and delayed ground water level variation. The results demonstrate that: bank failure frequency and retreat width generally decreased with increases in soil shear strength and permeability, while change in frequency behaved a unidirectional tendency, with a more complicated change in retreat width also relating to the bank-toe erosion degree at failure or the timing of bank failure, which may change with these influencing factors. In addition, the delayed variation in groundwater level reduced the bank stability degree during the recession stage, leading to an increase in the frequency of bank failure.