Submitted to: Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE)
Publication Type: Proceedings
Publication Acceptance Date: 8/10/2007
Publication Date: 10/1/2007
Citation: Chu-Agor, M., Wilson, G.V., Fox, G.A. 2007. Modeling streambank instability by seepage undercutting. ASABE paper no. 072117, Proceedings of the American Society of Agricultural and Biological Engineers International, July 2007, Minneapolis, MN. 1-12. Interpretive Summary: Seepage from streambanks can carry soil particles with the flowing water. This seepage erosion undercuts the streambank and can cause them to collapse. Predicting bank failure due to undercutting by seepage erosion has not been fully studied. There is very little field and laboratory measurements of seepage erosion and mathematical models that can simulate undercutting by seepage erosion do not exist. The objective of this research was to develop a method for including seepage erosion into bank stability models and to investigate the role of undercutting on bank failure. SEEP/W, was used to model soil-water pressures during seepage erosion observed in laboratory experiments. SLOPE/W was used to model bank stability with and without seepage erosion by comparing the factor of safety at different stages of the seepage erosion process. The decrease in the average factor of safety ranged from 42 to 91% as the degree of undercutting increased. This work demonstrated that a stable bank can quickly become unstable when undercutting by seepage erosion is included. The probability of failure reached 100% when the degree of the undercutting reached approximately 30 to 50 mm into the bank face. This research demonstrates that the tendency of streambanks to fail may be due as much, or more, to the effect of seepage erosion undercutting the streambanks as it is to the reduced soil strength of the bank as they wet-up from the seepage. This research also highlighted the need to incorporate seepage erosion into subsurface flow and streambank stability models.
Technical Abstract: Predicting bank collapse due to undercutting brought about by seepage erosion has not been fully studied or modeled, albeit its role in streambank erosion may be important. The limitation originates from the limited field measurements or laboratory experiments that successfully measure or simulate streambank seepage erosion as well as the unavailability of discrete element models that can effectively simulate seepage erosion and the corresponding mass wasting. The objective of this research was to demonstrate a procedure for incorporating seepage undercutting into bank stability models and to investigate the role of seepage undercutting on bank instability. A numerical finite element model, SEEP/W, was used to model soil-water pressure variations during seepage erosion observed in laboratory experiments with two-dimensional soil lysimeters. Model parameters were calibrated and verified using measured soil- water pressure and cumulative discharge data from the lysimeter experiments. A general limit equilibrium bank stability model called SLOPE/W was used to simulate bank stability with and without seepage erosion by comparing the computed factor of safety at different stages of the seepage erosion process with regard to input parameter uncertainty using Monte Carlo analysis. The percentage decrease in the mean factor of safety ranged between 42 and 91% as the degree of undercutting increased, dependent upon the initial stability of the bank. The factor of safety, Fs, converged to a specific value as undercutting progressed, suggesting that a stable bank (Fs>1) can quickly become unstable (Fs<1) when seepage undercutting is considered. For stable banks, the probability of failure reached 100% when the degree of the undercutting reached approximately 30 to 50 mm. This research verifies that the propensity of streambanks to fail during the recession limb of hydrographs may be the combined result of seepage erosion undercutting the streambanks and the reduced apparent cohesion of the bank. This research also highlighted the need to incorporate the dynamic process of seepage erosion into integrated subsurface flow and streambank stability models.