Title: Seepage-induced streambank erosion and instability: in-situ constant-head experiments Authors
Submitted to: Journal Hydrologic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 5, 2012
Publication Date: N/A
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 many streams and rivers and can make up as much as 80% of the total sediment entering these waters. Compared to erosion by the flowing stream, less 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 and are made up of a loose, highly conductive loamy sand layer between layers of soil that are less conductive and tend to adhere together. 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 rate of seepage flow and sediment content in the flow. 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 seepage eroded the bank creating an undercut area in the bank face that grew rapidly in size as flow continued. These undercuts also showed tendencies to heal themselves when upper layers of soil that adheres together fell into the void caused by seepage erosion. The cohesive material blocked further erosion of the sandy layer by seepage. When stream water levels rise, the flowing water can remove these blocks of soil that have failed. One experiment tested the effect of streamflow in removing the failed soil blocks. 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: Streambank erosion is known to be a major source of sediment in streams, contributing the majority of the total sediment load in some watersheds. Compared to fluvial processes, less work has been performed on the effects of seepage on streambank erosion and failure, especially the linkage between surface water and groundwater mechanisms. Prior research in the laboratory has examined seepage mechanisms, but field data is needed to validate these conclusions. This research hypothesizes that seepage plays a major role in streambank erosion and stability, especially when acting in concert with fluvial processes. This research instrumented a streambank on Dry Creek, a tributary to Little Topashaw Creek, a deeply incised stream with near vertical banks, located in Chickasaw County, Mississippi. Bank stratigraphy consisted of a conductive loamy sand layer between loam topsoil and an underlying cohesive layer. The site was instrumented with a network of tensiometers and observation wells to measure soil-water pressures and water table elevations. A trench installed 2.8 m from the bank and approximately 2 m below ground surface provided a constant head on the near-bank groundwater system when filled with stream water. Groundwater conditions were monitored during a series of induced seepage experiments. The bank face was outfitted with a seepage collection device to measure seep flow rates and sediment transport. During the injections, seep flow rates were as high as 0.4 L/min at one location with sediment transport rates of approximately 0.86 kg/min. The seeps experienced ‘self-healing’ in which small-scale bank failures due to undercutting blocked further particle mobilization. One experiment simulated concurrent fluvial erosion by removing failed material from the undercut and resulted in erosion rates near 6.0 kg/min. Seepage erosion was shown to be an important mechanism of streambank failure, especially when acting in concert with fluvial erosion processes that prevent self-healing.