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Seepage and piping: solitary and integrated mechanisms of streambank erosion and failure.

Efforts to prevent sediment entry into streams and rivers have historically been focused on controlling upland erosion. The resulting conservation practices and soil management efforts have been effective in reducing overland sediment entry into receiving waterways. However, recent work has shown that most of the sediment entering streams and rivers in many watersheds in the United States now comes from streambanks, and sediment in streams, rivers, and reservoirs remains a significant issue (fig. 1).

[FIGURE 1 OMITTED]

Most of the research on streambank erosion and failure has looked at surface flow processes. When the stream stage rises, the flow exerts greater stress on the bank and mobilizes bank sediment. We also know that groundwater, or subsurface flow, plays a role. When the streambank becomes wet, the strength of the bank decreases and it becomes easier to erode. However, we still lack the necessary tools and knowledge of the processes that would allow us to analyze the mechanisms of erosion by groundwater. There are techniques available that can protect streambanks from excess erosion, but we need to better understand the mechanisms that cause stream-bank erosion and failure to better understand, and improve the performance of our riparian management strategies.

Research underway at Oklahoma State University, in collaboration with the USDA-ARS National Sedimentation Laboratory in Oxford, Mississippi, is investigating the role of groundwater in streambank erosion, both as an individual mechanism and when acting in conjunction with fluvial processes. This research is supported by a multiple-year National Science Foundation (NSF) grant through NSF's Hydrological Sciences Division.

Groundwater processes contribute to erosion through several interrelated mechanisms. From a geotechnical perspective, analysis of potential streambank failures is a balance between resistive and driving forces. Seepage and pipe flow can influence both.

Seepage and pipe flow

Seepage undercuts a streambank when particles are mobilized in the seepage flow that exits from the bank face (fig. 2). Through numerical modeling with bank stability software, our 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 the upper bank material. Seepage can also reduce the resistance of bank sediment to be mobilized by fluvial forces. Using jet erosion tests, our team has demonstrated that a soil's resistance to fluvial erosion decreases exponentially when influenced by groundwater seepage forces.

[FIGURE 2 OMITTED]

Pipe flow refers to preferential water flow through passages in the soil, called soil pipes, that are created by biological or physical mechanisms (fig. 3). Pipe flow has historically been considered a critical process in dam and levee failure. Flow through soil pipes results in internal erosion of the pipes, which may produce gullies by tunnel collapse. The eroded material can clog the soil pipes, causing pore water pressure buildup inside the pipes that can result in stream-bank failure and re-establishment 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.

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Groundwater affects erosion directly by seepage and pipe flow processes and indirectly by the relationship of the soil's strength and erodibility properties with the soil water pressure. The presence of water in the soil reduces the contact between sediment particles and reduces the 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 the soil that is proportional to the hydraulic gradient, which can lead to what has been termed "pop-out" streambank failures. Groundwater forces can destabilize banks for extended periods following flow events.

What becomes very interesting is the link between groundwater mechanisms and other processes. As undercutting by seepage erosion progresses along a streambank, blocks of cohesive soil above the seepage point fail due to their increased weight and reduced support. On some streambanks, these cohesive soil masses can act as blockages to prevent further seepage erosion and thereby heal the seep in what has been termed a "self-healing" process (fig. 4). For seepage erosion to continue, apart from fluvial erosion, there must be enough force in the seepage flow to remove the cohesive blocking material. Fluvial processes may be critical in removing the displaced sediment and continuing the undercutting process. Such work highlights the need for future work investigating fluvial and seepage processes simultaneously.

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To improve our ability to rehabilitate stream channels, tools are needed for stability analyses that are capable of considering site-specific bank failure processes such as seepage and soil pipes. Current analyses of streambank erosion oversimplify the contributions of seepage and soil pipes, but the effects of these mechanisms are seen in numerous locations. Our team is pursuing laboratory research, field work, and numerical modeling to better understand and simulate these processes.

ASABE members Garey Fox, associate professor, Department of Biosystems and Agricultural Engineering, Oklahoma State University, Stillwater, USA; and Glenn Wilson, hydrologist and soil physicist, USDA-ARS National Sedimentation Laboratory, Oxford, Miss., USA; garey.fox@okstate.edu and glenn.wilson@ars.usda.gov.

NSF project information is available at: http://nsf.gov/awardsearch/showAward.do?AwardNumber=0943491&WT.z_pims_id=13684.

A video of a soil pipe experiment is available at: www.youtube.com/watch?v=td1h2Mzd5NY.

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Author:Fox, Garey; Wilson, Glenn V.
Publication:Resource: Engineering & Technology for a Sustainable World
Date:Mar 1, 2012
Words:874
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