Submitted to: Earth Surface Processes and Landforms
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/12/2006
Publication Date: 1/30/2007
Citation: Fox, G.A., Wilson, G.V., Simon, A., Langendoen, E.J., Akay, O., Fuchs, J.W. 2007. Measuring Streambank Erosion Due to Groundwater Seepage: Correlation to Bank Pore Water Pressure, Precipitation and Stream Stage. Earth Surface Processes and Landforms. DOI:10.1002/esp. 1490. Interpretive Summary: One of the major sources of sediment input to streams is streambank material. One cause of streambank failure is subsurface flow of water to the bank where seepage erodes bank layers and weakens the bank material. The objective of this research was to characterize a streambank undergoing seepage erosion and determine seepage flow rates and erosion rates. The streambank site was located in Northern Mississippi in the Goodwin Creek experimental watershed. Soil samples from streambank layers revealed less than a 10-fold difference in vertical permeability with depth but differences between vertical and lateral permeability of a coarse textured layer contributed to the tendency for lateral flow through this layer. Subsurface flows and sediment loads were dependent on the type of seep. The timing of "low-flow" seeps was related to river stage and precipitation. "High-flow" seeps began after rainfall events resulted in high stream stages and streambanks wetting up to near-saturation. Flow rates from these "high-flow" seeps were as high as 16 gallons per hour and sediment concentrations commonly approached three-fourths of a pound per gallon. "Buried" seeps would start and stop between events but exhibited the most significant erosion rates reaching as high as 88 pounds of sediment per hour and sediment concentrations as high as 7.5 pounds per gal. In cases where saturated conditions temporarily develop and persistent "high-flow" seeps occur, the seepage erosion and collapse of streambanks may be significant, especially in cases with "buried" seeps.
Technical Abstract: There exists a lack of information on one of the basic mechanisms governing sediment input to streams: streambank erosion by ground water flow or seepage erosion. The objective of this research was to characterize a streambank undergoing seepage erosion and quantify subsurface flows and seepage erosion of bank sediment. The streambank site was located in Northern Mississippi in the Goodwin Creek (GC) experimental watershed. Data on seepage erosion at GC was compared to limited seepage erosion measurements reported in the literature. Undisturbed soil cores along with disturbed soil samples from soil layers on the streambank face suggested less than an order of magnitude difference in vertical Ks with depth but differences between lateral Ks of a concretion layer and the vertical Ks of the underlying layers contribute to the propensity for lateral flow at these locations. GC seeps were not similar to other seeps reported in the literature in that eroded sediment did not originate from the seepage layer but rather from layers underneath the primary seepage layer. Subsurface flow and sediment load, quantified using 50-cm wide lateral flow collection pans placed against exposed faces of the stream bank, were dependent on the type of seep: intermittent "low-flow" seeps (flow rates typically less than 0.05 L/min) primarily active following large rainstorm events, persistent "high-flow" seeps (average flow rate of 0.39 L/min) which developed the most significant headcuts, and "buried" seeps which eroded unconsolidated bank material from previous bank failures. The timing of "low-flow" seeps correlated to river stage and precipitation. "High-flow" seeps at GC began after rainfall events resulted in the adjacent streambank reaching near-saturation (i.e., soil pore water pressures less than 5 kPa) conditions. Seep discharge from these "high-flow" seeps was measured as high as 1.0 L min-1 and sediment concentrations commonly approached 100 g L-1. "Buried" seeps were intermittent but exhibited the most significant erosion rates and sediment concentrations, reaching as high as 738 g min-1 and 989 g L-1, respectively. In cases where perched water table conditions exist and persistent "high-flow" seeps occur, the subsequent erosion and bank collapse of streambank sediment may be significant, especially in cases with "buried" seeps.