threshold under ambient spring conditions, and infiltration-induced failures are likely to be triggered by any increase in stage. The Pipal site is conditionally stable under ambient spring conditions, and is vulnerable both to infiltration and bank erosion.
Results indicate that the slow drawdown incorporated in the simulated flow regime permits pore-water pressure to dissipate sufficiently, and is not a factor in potential instability. In a wider context the work shows how site vulnerability, and potential remedies, can be identified relatively quickly using a combination of a comparatively sophisticated, seepage model coupled to two simple and widely accessible bank erosion and stability models. It also highlights the need to account for the three processes simulated here; comparison of the Factor of Safety data reveals the extent to which it is combinations of infiltration, hydraulic erosion and geotechnical failure that lead to bank failure. Simpler modeling approaches run the risk of overestimating bank stability.
The authors would like to thank Andy Collison and Tony Layzell for their assistance with the development of the Bank Stability Model interface.
Casagli, N.N., M. Rinaldi, A. Gargini, and A. Curini. 1999. Monitoring of pore-water pressure and stability of streambanks: Results from an experimental site on the Sieve River, Italy. Earth Surface Processes and Landforms 24:1095-1114.
Fredlund, D.G., N.R. Morgenstern, and R.A. Widger. 1978. The shear strength of unsaturated soils. Canadian Geotechnical Journal 15:313-321.
Fredlund, D.G., and H. Rahardjo. 1993. Soil Mechanics of Unsaturated Soils. John Wiley & Sons, New York.
GeoSlope International Ltd. 1998. SEEP/W Users Guide Version 4, GeoSlope International, Calgary, Canada.
Hanson, G.J. 1990. Surface erodibility of earthen channels at high stresses. Part II - Developing an in- situ testing device. Transactions of the American Society of Agricultural Engineers 33(1):132-137.
Hanson, G.J. 1991. Development of a jet index to characterize erosion resistance of soils in earthen spillways. Transactions of the American Society of Agricultural Engineers 36(5).
Hanson, G.J. and A. Simon. 2001. Erodibility of cohesive streambeds in the loess area of the midwestern USA. Hydrological Processes 15(1): 23- 38.
Langendoen, E., R. Thomas, and A. Simon. 2001. Modelling bank and near-bank processes using the CONCEPTS model. In Proceedings of the BGRG Conference, Nottingham, UK.
Partheniades, E. 1965. Erosion and deposition of cohesive soils. Journal of Hydraulics Division of the American Society of Agricultural Engineers 91:105- 139.
Osman, A.M., and C. R. Thorne. 1988. Riverbank stability analysis. I: Theory. Journal of Hydraulic Engineering 114(2):134-150.
Rinaldi, M., and N. Casagli. 1999. Monitoring of streambanks in partially saturated soils and effects of negative pore-water pressures: The Sieve River (Italy). Geomorphology 26:253-277.
Simon, A., and A. Curini. 1998. Pore pressure and bank stability: The influence of matric suction. In S. Abt, ed., Hydraulic Engineering ’98, American Society of Agricultural Engineers.
Simon, A., W. J. Wolfe, and A. Molinas. 1991. Mass wasting algorithms in an alluvial channel model. In Proceedings of the Fifth Federal Interagency Sedimentation Conference, Las Vegas, NV, pp. 2(8)22-29.
Simon, A., F.D. Shields, R. Ettema, C. Alonso, M.M. Garsjo, A. Curini, and L. Steffen. 1999a. Channel erosion on the Missouri River, Montana, between Fort Peck Dam and the North Dakota border. USDA- ARS, National Sedimentation Laboratory Research Report, Oxford, MS.
Simon, A., A. Curini, S.E. Darby, and E.J. Langendoen. 1999b. Streambank Mechanics and the Role of Bank and Near-Bank Processes in Incised Channels. In S.E. Darby, and A. Simon, eds., Incised River Channels: Processes, Forms, Engineering, and Management, pp. 123-152. John Wiley & Sons, London.