Incorporating Bank-Toe Erosion by Hydraulic Shear into the ARS Bank-Stability Model: Missouri River, Eastern Montana

Abstract

Bank-stability concerns along the Missouri River, eastern Montana are heightened by a simulated change in flow releases from Fort Peck Dam to improve habitat conditions for Pallid Sturgeon. The effects of the simulated flow releases on streambank pore-pressures and bank-toe erosion needs to be evaluated to properly model bank-stability. The Bank-Stability Model used incorporates pore-water pressure distributions, layering, confining pressures, reinforcement effects of riparian vegetation and complex bank geometries to solve for the factor of safety. To increase the applicability and accuracy of the model for use in predicting critical conditions, the hydraulic effects of bank-toe erosion have been added. According to the simulated flow-release plan, flows of 216 m 3 /s are increased by 38.3 m 3 /s/day for 12 days to 675 m 3 /s, held for 60 days and decreased for 12 days back to 216 m 3 /s. Results show the important contribution of bank-toe erodibility in controlling mass failure. Banks at River Miles 1624, 1676 and 1716 attain F s < 1.0 indicating imminent failure. These sites contain less resistant sandy-silt material at the bank toe, and experienced simulated undercutting up to 3m. More resistant cohesive, clay bank toes at River Miles 1589 and 1762 were undercut only 0.2 m and remained stable.

9 Modelling river-bank-erosion processes and mass failure mechanisms: progress towards fully coupled simulations

“…Consequently, the three modelling components (fluvial erosion, seepage, and mass stability) are not fully coupled, but are instead performed independently. A second limitation of the Simon et al (2003) study is that they employed a series of artificial, rectangular-shaped, hydrographs of specified height and duration in their simulations and it is not clear how these relate to natural flow events observed in the field.…”

“…One of the few attempts to investigate bank-erosion dynamics combining fluvial erosion, pore water pressure changes, and mass bank stability into a single, integrated, modelling approach is the work of Simon et al (2003), who used three models (Seep/w in combination with the USDA Bank Stability and Toe Erosion (BSM) models) to simulate bank response to flow events. However, although this is undoubtedly a useful exploratory study, it is limited for the following reasons.…”

Numerical simulation of hydrodynamics and bank erosion in a river bend

“…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.…”

Prediction of channel degradation rates in urbanizing watersheds

“…However, even these simple models require information on the bulk density, cohesion and the internal angle of friction of the material, as well as assuming various water table configurations. More sophisticated models even include assumptions regarding the influence of riparian vegetation (Simon et al, 2003). The US Task Committee on River Width Adjustment (1998) point out that great scope for improvement of stability models with erodible banks remains.…”

“…Where the river is confined within a specific material, the widthdepth ratio can be assumed to be constant (Schumm, 1960;Finnegan et al, 2005). Prediction of changes in channels with down-cutting are numerous and rely on solving simple models of slope stability based on adaptations of wedge failures (Thorne, 1982;Lohnes, 1991;Darby et al, 1996Darby et al, , 2000Simon et al, 2000Simon et al, , 2003 or cylindrical failures (Miller & Quick, 1993. However, even these simple models require information on the bulk density, cohesion and the internal angle of friction of the material, as well as assuming various water table configurations.…”