Modelling of the retreat process of composite riverbank in the Jingjiang Reach using the improved BSTEM

Bank retreat in the Jingjiang Reach is closely related not only to the near-bank intensity of fluvial erosion, but also to the composition and mechanical properties of bank soils. Therefore, it is necessary to correctly simulate bank retreat to determine the characteristics of fluvial processes in the Jingjiang Reach. The current version of BSTEM (5.4) was improved to predict riverbank retreat, by inputting a dynamic water table, and calculating the approximation of the distribution of dynamic pore water pressure in the soil near the river bank face, and considering the depositional form of the failed blocks, which is assumedly based on a triangular distribution, with the slope approximately equalling the stable submerged bank slope and half of collapsed volume deposited in the bank-toe region. The degrees of riverbank stability at Jing34 were calculated using the improved BSTEM. The results indicate the following trends: (i) the degrees of riverbank stability were high during the dry season and the rising stage, which led to minimal bank failure; and (ii) the stability degrees were low during the flood season and the recession stage, with the events of bank collapse occurring frequently, which belonged to a stage of intensive bank erosion. Considering the effects of bank-toe erosion, water table lag and the depositional form of the collapsed bank soil, the bank-retreat process was simulated at the right riverbank of Jing34. The model-predicted results exhibit close agreement with the measured data, including the total bank-retreat width and the collapsed bank profile. A sensitivity analysis was conducted to determine the quantitative effects of toe erosion and water table lag on the degree of bank stability. The calculated results for toe erosion indicate that the amount of toe erosion was largest during the flood season, which was a primary reason for bank failure. The influence of water table lag on the degree of stability demonstrates that water table lag was an important cause of bank failure during the recession stage.