Streambank erosion and instability induced by groundwater seepage

Authors: MIDGLEY, TABER - Oklahoma State University; FOX, GAREY - Oklahoma State University; Wilson, Glenn; HEEREN, DEREK - Oklahoma State University; Simon, Andrew; Langendoen, Eddy

Submitted to: ASABE Annual International Meeting
Publication Type: Proceedings
Publication Acceptance Date: 7/21/2011
Publication Date: 8/7/2011
Citation: Midgley, T.L., Fox, G.A., Wilson, G.V., Heeren, D.M., Simon, A., Langendoen, E.J. 2011. Streambank erosion and instability induced by groundwater seepage. ASABE Annual International Meeting. August 7-10, 2011. Louisville, Kentucky. Paper No. 1110988. pp. 1-21.

Interpretive Summary: Sediment is one of the most common pollutants in streams and lakes. It decreases water quality and destroys aquatic habitat. Streambank erosion is a major source of the sediment in streams and rivers and make up as much as 80% of the total sediment entering these waters. Little work has been done to study the effects of seepage on streambank erosion and failure. Prior research, primarily in the laboratory under controlled conditions, has studied seepage as a cause of bank erosion, but more field work needs to be done to prove the conclusions drawn from laboratory studies. This project studied a streambank on Dry Creek just above where it flows into Little Topashaw Creek in Chickasaw County, Mississippi. The bank was previously observed to produce seepage even during dry summer months. This banks of this creek are steep and deep. The creek flows through flood plains that are cultivated and surrounded by forested areas. Sediment has been identified as the main water quality issue in the watershed with gullies and streambanks being the main sources. The streambank soils are characterized by silt loam and clay loam layers with a more conductive loamy sand between the loam and an underlying cohesive layer. The site was instrumented with tensiometers for measuring soil water pressures and observation wells for measuring groundwater levels. Groundwater conditions and bank erosion were monitored for several weeks, followed by experiments in which water was injected into the soil to cause groundwater flow to the streambank. A trench installed 2.8 m from the edge of the bank and approximately 2 m below the surface was used to provide groundwater flow under controlled conditions. The bank face was setup with a seepage collection device that measured seepage flow rate and sediment content in the flow. Groundwater conditions were monitored by the tensiometers and observation wells. Experiments consisted of a trench injection at a constant water level and observations of flow rates, erosion rates, soil-water pressures, and water table elevations. Flow rates varied from 0.004 L/min to 1.16 L/min at different locations on the bank. It was observed that the seeps caused rapid growth of undercuts in the bank but also showed tendencies to heal itself when upper layers of cohesive soil failed. The cohesive material blocked further erosion of the sandy layer by seepage. When stream water levels rise, the flowing water can remove blocks of soil that have eroded by seepage. One experiment tested the effect of streamflow in removing eroded material. The combined erosion rates by seepage and the stream were over 6000 g/min. Seepage erosion could be a major cause of streambank failure when self-healing does not occur or when the stream removes the eroded material.

Technical Abstract: Excessive sediment is one of the most common surface water pollutants. It diminishes water quality and destroys aquatic habitat. Streambank erosion is known to be a major source of sediment in streams and rivers, contributing as much as 80% of the total sediment load in some watersheds. Little work has been done to study the effects of seepage on streambank erosion and failure. Prior research, primarily in the laboratory under well-defined and controlled conditions, has examined seepage as a mechanism for bank erosion, but more needs to be done to validate conclusions derived from the laboratory with field data. This project studied a streambank on Dry Creek (a tributary to Little Topashaw Creek) located in Chickasaw County, Mississippi. The bank was previously observed to produce seepage even during dry summer months. This creek is a deeply incised stream in the Yalobusha Watershed with near 90 degree banks. The creek flows through alluvial plains under cultivation and surrounded by forested areas. Excess sediment has been identified as the main water quality issue in the watershed with gullies and banks being the main sources. Watershed geology is characterized by silt loam and clay loam with a more conductive loamy sand between the loam and an underlying cohesive layer. The site was initially instrumented with a network of tensiometers and observation wells. Groundwater conditions and bank erosion were monitored for several weeks, followed by an induced seepage experiment. A trench installed 2.8 m from the edge of the bank and approximately 2 m below ground surface was used to provide a constant head for groundwater flow in the near-bank area. The bank face was outfitted with a seepage collection device that measured seepage flow rate and sediment transport. Groundwater conditions were again monitored by the tensiometer and observation well network. Experiments consisted of a trench injection at a constant head and observations of flow rates, erosion rates, soil-water pressures, and water table elevations. Flow rates varied from 0.004 L/min to 1.16 L/min at different locations on the bank. It was observed that the seeps experienced ‘self-healing’ erosion in which upper layer cohesive soil failures blocked further particle mobilization. One experiment simulated fluvial erosion removing the failed material, thereby, resulting in combined erosion rates of over 6000 g/min. Seepage erosion could be a dominate mechanism of streambank failure where the self-healing process is not occurring.