Coupled simulations of fluvial erosion and mass wasting for cohesive river banks

Abstract

[1] 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. Application of the model is demonstrated by undertaking simulations of a single flow event at a single study site for scenarios where (1) there is no fluvial erosion and the bank geometry profile remains constant throughout, (2) there is no fluvial erosion but the bank profile is deformed by simulated mass wasting, and (3) the bank profile is allowed to freely deform in response to both simulated fluvial erosion and mass wasting. The results are limited in scope to the specific conditions encountered at the study site, but they nevertheless 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.

Numerical simulation of hydrodynamics and bank erosion in a river bend

“…[9] In the previous study by Darby et al [2007], fluvial erosion, groundwater flow, and limit equilibrium models for bank stability were coupled to simulate bank erosion. In this paper, further progress is made by using hydrodynamic simulations to determine the flow field along the reach, and to define the near-bank shear stresses used in the fluvial erosion model more accurately.…”

“…In contrast, relatively few attempts have been made to model the combined effects of interacting bank erosion processes [e.g., Simon et al, 2003Simon et al, , 2006. Of these studies the one by Darby et al [2007], who developed a simulation modeling approach in which hydraulic erosion, finite element seepage, and limit equilibrium stability analyses are fully coupled, is perhaps the most detailed. Nevertheless, a key remaining limitation, as recognized by those authors, is that the procedure they used to calculate the near-bank shear stresses governing fluvial erosion was a gross idealization.…”

“…The objectives of this paper can therefore be summarized as follows: (1) to make a further development of the simulation approach proposed by Darby et al [2007] by coupling groundwater flow and stability modeling at the scale of a single bank with detailed hydrodynamic modeling at the reach scale in a channel bend; (2) to investigate how temporal changes in hydrody-namic patterns occurring during a flow event hydrograph can influence bank stability processes; (3) to investigate and quantify the role of fluvial erosion, driven by hydrodynamic conditions at the reach scale, versus mass failure, driven by geotechnical factors (i.e., changes in shear strength due to variable pore water pressure conditions) at the local scale.…”

“…[5] In this paper we build on the framework for the simulation of bank processes identified by Darby et al [2007] but make further progress by utilizing a more advanced hydrodynamic model to simulate the reach-scale hydraulics within a river bend, in order to account for the spatial pattern and temporal variability of shear stress and resulting impacts on bank stability. The objectives of this paper can therefore be summarized as follows: (1) to make a further development of the simulation approach proposed by Darby et al [2007] by coupling groundwater flow and stability modeling at the scale of a single bank with detailed hydrodynamic modeling at the reach scale in a channel bend; (2) to investigate how temporal changes in hydrody-namic patterns occurring during a flow event hydrograph can influence bank stability processes; (3) to investigate and quantify the role of fluvial erosion, driven by hydrodynamic conditions at the reach scale, versus mass failure, driven by geotechnical factors (i.e., changes in shear strength due to variable pore water pressure conditions) at the local scale.…”

Bank‐collapse processes in a valley‐bottom gully, western Iowa

“…Our general goal was to determine through monitoring over a 2-year period whether positive pore pressures associated with subsurface water flow were alone sufficient to instigate bank failures. The role of subsurface water in the failure of gully and river banks has received considerable attention (e.g., Bradford and Piest, 1977;Osman and Thorne, 1988;Higgins et al, 1990;Hagerty, 1991;Darby and Thorne, 1996;Casagli et al, 1999;Simon et al, 2000;Collison, 2001;Simon and Collison, 2001;Amiri-Tokaldany et al, 2003;Dapporto et al, 2001Dapporto et al, , 2003Rinaldi et al, 2004;Darby et al, 2007;Fox et al, 2007;Wilson et al, 2007), and some authors have isolated effects of reduced matric suction in the unsaturated zone (e.g., Fredlund et al, 1978;Casagli et al, 1999;Rinaldi et al, 2004). This study, motivated by observations of bank failures seated below the water table, focuses on only the saturated zone.…”