Modeling streambank instability by seepage undercutting

Predicting bank collapse due to seepage erosion undercutting has not been fully studied or modeled, even though its role in streambank erosion may be important. The limitation originates from the lack of field measurements or laboratory experiments as well as the unavailability of discrete element models that can effectively simulate seepage erosion. The objective of this research was to demonstrate a procedure for incorporating seepage undercutting into two-dimensional variablysaturated flow and bank stability models and to investigate the role of seepage undercutting on bank instability. Integrated flow and bank stability models were used to simulate soil-water pressure variations and bank stability with and without seepage erosion with regard to input parameter uncertainty using Monte Carlo analysis. The percentage decrease in the mean factor of safety, Fs, ranged between 42 and 91% as the depth of undercutting increased, dependent upon the initial stability of the bank. For stable banks, the probability of failure reached 100% when the depth of the undercutting reached 30 to 50 mm. The propensity of streambanks to fail during the recession limb of hydrographs may be the combined result of seepage undercutting and reduced cohesion.