A coupled simulation of lateral erosion, unsaturated seepage and bank instability due to prolonged high flow

The impacts of various hydrological processes such as infiltration, saturated/unsaturated seepage and river drawdown on bank retreat are complex, yet crucial in understanding fluvial erosion processes. The bank retreat generally occurs due to a combination of erosion and mass failure. In this study, coupled analyses of a bank retreat during prolonged high flow in Southern Thailand were performed considering these hydrological and mechanical processes, by means of finite element seepage model (SEEP/W), stability analysis (SLOPE/W) and erosion model (BSTEM). The simulation result was then validated based on field observation. A detailed characterization of unsaturated soil hydraulic properties of the bank soils, namely soil–water retention curve (SWRC) and unsaturated permeability (k-function) was carried out on undisturbed soil samples in the laboratory and used in the seepage analysis. Two modes of mass instability were considered in the analysis, namely sliding failure and cantilever failure. The analysis revealed that a prolonged antecedent rainfall lead to saturation of the bank soil which triggered sliding failure of the bank at an early stage of the flood event. As water level progressively increased, repeated cycles of cantilever failure and erosion of the lower bank were reproduced in the model, yielding the estimated total bank retreat of about 5.3 metre, which agreed well with field observation. Another series of analyses with empirically estimated unsaturated soil parameters however yielded a different bank retreat behaviour, thus highlighting the importance of accurately determining hydraulic functions. The lateral erosion was finally shown to be a prominent process that induced overhanging riverbank shape and cantilever failure, thus being a major contribution of the bank retreat in this case study.