Methods for Determining Stream Bank Critical Shear Stress: Implications for Erosion Rate Predictions

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According to the US EPA, excess sediment is a significant cause of water quality impairment forrivers. The goal of this study was to compare different methods of determining two parameters used toestimate stream bank erosion, soil critical shear stress ( c ) and soil erodibility (kd,) and to determine the impactof those differences on predictions of stream bank erosion. In situ erosion tests were conducted using asubmerged jet test device at twenty-five field sites. Determination of kd and c using the jet test device requires an initial estimation of c; cwas estimated using the Shields diagram, the Blaisdell method, and by assuming itwas zero. Soil erodibility was then determined using a least squares fit of the jet test data for three cestimates. Additionally, using a single set of c values, the kd measured by the jet test was compared to twoempirical kd relationships. Using these two sets of parameter values, stream bank erosion rates were predictedfor a local stream. Critical shear stresses produced by the Blaisdell method were an order of magnitudegreater than those predicted by the Shields diagram and the zero c assumption. However, the kd values andstream bank erosion predictions based on the jet test measurements were relatively insensitive to c . The twoempirical kd equations produced similar kd and erosion predictions that were generally an order of magnitudeless than the values based on the jet test measurements. Field validation of these methods over a wide rangeof soil types is recommended to further develop methods of estimating kd and c for stream bank soils.