Location: Watershed Physical Processes ResearchTitle: Influence of Seepage Undercutting on the Root Reinforcement of Streambanks) Author
Submitted to: Earth Surface Processes and Landforms
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/8/2007
Publication Date: 7/1/2008
Citation: Cancienne, R.M., Fox, G.A., Simon, A. 2008. Influence of Seepage Undercutting on the Stability of Root Reinforced Streambanks. Earth Surface Processes and Landforms, DOI: 10.1002/esp.1657. Interpretive Summary: Streambank erosion serves as a primary source of sediment loading to streams. Recent research has suggested that undercutting by seepage erosion may play an important role in streambank failures. The objective of this research was to utilize the Bank Stability and Toe Erosion Model (BSTEM) to determine the relative importance of seepage undercutting versus losses of bank strength due to soil water while considering the reinforcing effects of vegetation. Two streambanks in northern Mississippi with well-documented soil strength data were simulated. Results show that seepage erosion that caused undercutting of 20-55 cm was sufficient to destabilize the simulated streambanks even with reinforcement of the banks by roots.
Technical Abstract: Streambank erosion serves as a primary source of sediment loading to streams. Recent research has suggested that undercutting by seepage erosion may play an important role in streambank failure compared to increased soil pore-water pressure. The objective of this research was to utilize the Bank Stability and Toe Erosion Model (BSTEM) to determine the relative importance of seepage undercutting versus increased soil pore-water pressure as functions of shear strength, bank angle, and cohesion by roots. Two streambanks in northern Mississippi with well-documented soil strength parameters were simulated. Simulations included three bank angles, four pore-water pressure distributions (unsaturated, partially saturated, and fully saturated), six distances of undercutting (0 to 40 cm), and thirteen vegetation conditions (root cohesions (cr) from 0.0 to 15.0 kPa). The predicted factor of safety (FS) decreased as bank angle and distance of undercutting increased and as cr decreased. For both banks, the critical distance of undercutting required to reach conditionally unstable conditions (i.e., FS = 1.3) during unsaturated conditions appeared to increase logarithmically with root cohesion approaching an asymptote with increases in cr. Bank slope appeared to influence the critical distance of undercutting uniformly across the range of cr investigated. However, the influence of the water table position on this critical distance was not uniform across cr, but rather decreased as cr increased. The distance at which undercutting became equivalently important to soil-pore water pressure decreased as cr increased, with values typically ranging between 20 and 55 for cr of 0.0 to 15.0 kPa. This research depicts the conditions at which seepage undercutting needs to be considered relative to other mechanisms such as increased soil pore-water pressure in bank stability analyses.