Coupled simulations of fluvial erosion and mass wasting for cohesive riverbanks

The erosion of sediment from riverbanks affects a range of physical and ecological issues. Bank retreat often involves combinations of fluvial erosion and mass-wasting, and in recent years bank retreat models have been developed that combine hydraulic erosion and limit equilibrium stability models. In related work, finite element seepage analyses have also been used to account for the influence of pore-water pressure in controlling the onset of mass-wasting. This paper builds on these previous studies by developing a simulation modeling approach in which the hydraulic erosion, finite element seepage and limit equilibrium stability models are, for the first time, fully coupled. Simulations are presented for scenarios where (i) there is no fluvial erosion and the bank geometry profile remains constant throughout, (ii) there is no fluvial erosion but the bank profile is deformed by simulated mass-wasting, and (iii) the bank profile is allowed to freely deform in response to both simulated fluvial erosion and mass-wasting. Results demonstrate the significant role that fluvial erosion plays in steepening the bank profile, or creating overhangs, thereby triggering mass-wasting. However, feedbacks between the various processes also lead to unexpected outcomes. Specifically, fluvial erosion also affects bank stability indirectly, as deformation of the bank profile alters the hydraulic gradients driving infiltration into the bank, thereby modulating the evolution of the pore-water pressure field. Consequently, the frequency, magnitude and mode of bank erosion events in the fully coupled scenario differ from the two scenarios in which not all the relevant bank process interactions are included.