Effect of Seepage on River Bank Stability

Abstract

The collapse of river banks around the world has caused widespread damages to land and property. In many instances, human lives are lost as a result of such failures. A better understanding of the mechanism leading to river bank failure is necessary before engineers can arrive at a cost-effective countermeasure to prevent such a disaster. To this end, an experimental study was conducted in a laboratory flume to investigate the correlation between river bank stability and seepage under a unidirectional current. The study examined the collapse of a bank slope consisting of non-cohesive sediment with the channel and ground water flow as the only variables. The experiments were carried out on two bank slopes = 27 and 20 degrees with the horizontal. While most studies hitherto have focused on the two variables independently (i.e., seepage and current effects separately), this study investigated their combined effect on failure of the bank slope. The dimensionless Reynolds number, which is a measure of shear stresses, is used as an indicator of erosion due to the main channel flow, while the critical hydraulic gradient is used to account for the onset of collapse. Moreover, the critical hydraulic gradient is also plotted as a function of the dimensionless seepage rate, which is the ratio of the applied shear velocity and the rate of drawdown, u * /(dh/dt). The results show that an increased in channel flow velocity (hence an increase in bed shear stresses) enhances slope failure, thereby causing it to collapse at a lower hydraulic gradient than that in a quiescent condition or with very low flow velocity. Additionally, the bank slope at 20 degrees requires a higher hydraulic gradient to initiatiate collapse as compared to its 27-degree counterpart with the same channel flow velocity. The study provides an improved understanding on slope failure in river channels, particularly for cases when there is a rapid drawdown of the flow stage during the recession period of the flood hydrograph.