Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/11/2007
Publication Date: 11/1/2007
Citation: Fox, G.A., Chu-Agor, M., Wilson, G.V. 2007. Erosion of Noncohesive Sediment by Groundwater Seepage: Lysimeter Experiments and Stability Modeling. Soil Science Society of America Journal 71(6): 1822-1830.
Interpretive Summary: Streambanks have been found to fail due to seepage out of the bank carrying sediment with the flow in a process termed seepage erosion. This form of erosion leaves the bank undercut which weakens the bank and makes it susceptible to collapse. The role of laterial, subsurface flow on loss of stability of streambanks was further studied to evaluate a sediment transport model for seepage erosion. Laboratory experiments were performed using soil beds repacked to mimic streambanks. The experiments were conducted on two soils: a loamy sand (slightly cohesive) and a sieved sand (noncohesion). Tensiometers were used to measure soil-water pressures. Seven experiments were performed on streambanks with slopes of 90, 60, 45, 36, and 26 degrees. Flow and sediment concentrations were measured at the outflow flume. Results indicate that the existing sediment transport model adequately matched the measured behavior of the streambank experiments for both soils without modifying the parameters. This research then determined whether the bank stability models were capable of capturing both small and large scale streambank failures. The model predicted large-scale failures for bank angles greater than 45 degrees in which tension cracks formed on the soil surface. The model failed to predict collapses for bank angles less than 45 degrees in which tension cracks formed on the bank's seepage face. The failure to predict collapse was considered to be due to the assumption in the model that slip surfaces have a cylinder shape.
Technical Abstract: Seepage erosion may be a significant mechanism of streambank erosion and failure in numerous geographical locations. Previous research has investigated erosion by lateral subsurface flow and developed a sediment transport model similar to an excess shear stress equation. As a continuation of this earlier research, slope destabilization driven by lateral, subsurface flow was studied to further verify the recently proposed sediment transport model. Laboratory experiments were performed using a two-dimensional soil lysimeter. The experiments were conducted on two soils: a field soil (loamy sand) and a sieved sand with greater sand content and less cohesion. Pencil-size tensiometers were used to measure soil pore-water pressure. A series of seven lysimeter experiments were performed by varying the bank slope (90, 60, 45, 36, and 26 degrees). Flow and sediment concentrations were measured at the outflow flume. Results indicate that a slight modification of the existing seepage sediment transport model adequately simulates lysimeter experiments for both noncohesive soils without modifying the seepage parameters of the excess shear stress equation, especially for bank angles greater than 45 degrees. The research then determined whether integrated finite element and bank stability models were capable of capturing both small and large scale sapping failures. The models predicted large-scale failures for bank angles greater than 45 degrees in which tension cracks formed on the bank surface. The models failed to predict collapses for bank angles less than 45 degrees in which tension cracks formed on the seepage face. The failure to predict collapse was hypothesized to be due to the assumption of cylindrical slip surfaces.