Methods for determining streambank critical shear stress and soil erodibility: Implications for erosion rate predictions
According to the U.S. EPA, excess sediment is a significant cause of water quality impairment for rivers. The goal of this study was to compare different methods of determining two parameters used to estimate streambank erosion, soil critical shear stress (tau(c)) and soil erodibility (k(d)) and to determine the impact of those differences on predictions of streambank erosion. At 25 field sites, bank erosion tests were conducted using a submerged jet test device, and soil samples were collected. Critical shear stress was measured using a multi-angle submerged jet test device (JT) and estimated based on Shields' diagram (SD) and empirical relations based on the soil parameters, percent clay (P-c), plasticity index (I-w), particle size (D-50) and percent silt-clay (SC). Additionally, using a single set of tau(c) values, the kd measured by the jet test was compared to predictions from two empirical k(d) relations. Using these parameter values, streambank erosion rates were predicted for a local stream. The jet tau(c) estimates were as much as four orders of magnitude greater than the SD, P, and D50 estimates, indicating the SD and empirical methods underestimate tau(c). The two empirical k(d) equations produced similar k(d) values that were generally two orders of magnitude less than the values from the jet test measurements. Erosion predictions followed the same trend as the k(d) data, with the jet test measurements resulting in higher predictions. Field validation of these methods over a wide range of soil types is recommended to develop methods of estimating k(d) and tau(c) for fine-grained streambank soils.