Seepage and piping: Solitary and integrated mechanisms of streambank erosion and failure

Authors: FOX, GAREY - Oklahoma State University; Wilson, Glenn

Submitted to: American Society of Agricultural and Biological Engineers
Publication Type: Popular Publication
Publication Acceptance Date: 12/15/2011
Publication Date: 3/1/2012
Citation: Fox, G., and Wilson, G.V. 2012. Seepage and piping: Solitary and integrated mechanisms of streambank erosion and failure. American Society of Agricultural and Biological Engineers. 13-14.

Interpretive Summary: Recent work has shown that a majority of the sediment entering streams and rivers now comes from streambanks. We currently lack the tools and knowledge of the streambank erosion processes to be able to analyze for all mechanisms. Research underway at Oklahoma State University, in collaboration with the USDA-ARS National Sedimentation Laboratory in Oxford, MS, is investigating the processes of groundwater in streambank erosion and failure, both as individual mechanisms and in conjunction with fluvial processes. Subsurface flow affects erosion directly by seepage and pipe flow processes and indirectly by the relationship of soil strength and erodibility properties with soil water pressure. Ground water forces can act over to destabilize banks for extended periods following flow events. Seepage contributes to erosion through several interrelated mechanisms. The presence of water in soil reduces the contact between sediment particles and reduces a soil’s strength or resistance to collapse while at the same time increasing the weight of the soil. Groundwater flowing though soil exerts a destabilizing force on that soil proportional to the hydraulic gradient which can lead to what has been termed “pop-out” streambank failures by some researchers. Pipe flow refers to preferential water flow through soil pipes or large openings in the soil created by biological or physical mechanisms. Flow through the pipe results in internal erosion of the pipe which may produce gullies by tunnel collapse. The eroded material can clog soil pipes, causing pore water pressure buildup inside the pipes that can result in streambank failures and reestablishment of ephemeral gullies. Numerical models have been applied to describe flow through soil pipes but incorporation of internal erosion into such models has proven complicated. Seepage undercuts form in streambanks when particles are mobilized in seepage flow exiting from the bank face. Through numerical modeling with bank stability software, the research team has demonstrated that these undercuts exponentially reduce the stability of a bank as the depth of the undercut increases, leading to cantilever failures of upper bank material. Also, seepage may reduce the resistance of bank sediment to be mobilized by fluvial forces. Using jet erosion tests, the research team has demonstrated that a soil’s resistance to fluvial erosion decreases exponentially when influenced by groundwater seepage forces. What becomes very interesting is the link between groundwater mechanisms and other processes! As seepage erosion undercutting initiates and progresses, blocks of cohesive soil above the undercut fail due to the increased weight and reduced support. These cohesive soil masses can act as blockages to prevent further seepage erosion and therefore heal the seep in what has been termed a “self-healing” process. For seepage erosion to continue apart from fluvial erosion there must be enough force from the seepage flow to remove the cohesive ‘capping’ material. Fluvial processes may be critical to remove the displaced sediment and continue the undercutting processes.

Technical Abstract: Recent work has shown that a majority of the sediment entering streams and rivers now comes from streambanks. We lack the understanding of the processes controlling streambank failure to be able to predict how erosion control methods will work for all conditions. Research underway at Oklahoma State University, in collaboration with the USDA-ARS National Sedimentation Laboratory in Oxford, MS, is investigating the processes of groundwater in streambank failure, both as individual processes and combined with streamflow processes. Subsurface flow affects erosion directly by seepage and flow through individual pores and indirectly by the relationship of soil water content with soil strength and erosion properties. Ground water can act over to destabilize banks for extended periods following flow events. Seepage contributes to erosion through several related mechanisms. The presence of water in soil reduces the contact between sediment particles and reduces a soil’s strength or resistance to collapse while at the same time increasing the weight of the soil. Groundwater flowing though soil exerts a destabilizing force on that soil which can lead to what has been termed “pop-out” streambank failures. Flow through large pores, called soil pipes, in the soil created by plant roots, animal burrows, or natural soil structure. Flow through the pipe results in internal erosion of the pipe which can produce gullies when the pipe collapses. The eroded material can clog soil pipes, causing water pressures to buildup inside the pipe that can result in streambank failures. Mathematical models have been applied to describe flow through soil pipes but adding internal erosion into such models has not been done yet. Undercuts or voids form in streambanks when particles flow with the seepage exiting from the bank face. Using mathematical modele, the research team has demonstrated that these undercuts reduce the stability of a bank as the depth of the undercut increases, leading to failures of the upper bank material. Also, seepage may reduce the resistance of bank sediment to be eroded by streamflow forces. Using a device called a jet erosion tests, the research team has demonstrated that a soil’s resistance to streamflow erosion decreases when influenced by groundwater seepage forces. What becomes very interesting is the link between groundwater flow processes and other processes! As seepage erosion undercutting continues, blocks of soil above the undercut fail due to the increased weight and reduced support. These blocks of soil can act as small dams that prevent further seepage erosion and therefore heal the seep in what has been termed a “self-healing” process. For seepage erosion to continue apart from streamfow erosion there must be enough force from the seepage flow to remove the failed soil block material. Steamflow processes may be critical to remove the failed sediment and continue the undercutting processes.