Assessing The Erodibility of Cohesive Media in Southwestern Ontario With In-Situ Mini-JET Methodology

Despite research indicating that the resistance of cohesive sediments to fluvial erosion is related to the geologic media, regulators and industry in Southern Ontario do not typically consider the diversity in geologic landscapes when assessing the erosion potential of streams with cohesive boundaries. The development of the mini-Jet Erosion Test (JET) methodology has facilitated in-situ data collection of the resistance of cohesive media to erosion, however, it has not been widely applied in Ontario and there remain knowledge gaps in the interpretation of the test results. This research collected a miniJET dataset of 245 in-situ tests spanning 13 sites and 10 distinct geologic units. To address the gradient of weathered material within JET scour holes, a method of test segmentation is proposed where each JET is separated into a segment representing a surficial (weathered) layer and an underlying (unweathered) layer. This method of analysis results in three output parameters per JET; the depth of the first segment, a critical shear stress and an erodibility coefficient. To estimate the critical shear stress and erodibility parameters the JET dataset is analyzed using three methods of JET solution techniques (Scour Depth Method (SD), Blaisdell Method (BM), and Linear Regression Method (LR)) and the sensitivity of these solution techniques to JET duration and measurement frequency is assessed. It is demonstrated that BM is the most sensitive to test duration and LR is the most sensitive to measurement frequency. The estimation of critical shear stress is robust to JET duration and measurement frequency on a site scale, however, the estimation of the erodibility coefficient can be skewed on a site scale when JETs longer than 120 minutes are compared to tests shorter than 60 minutes. \n \nUsing the depth of the first segment as a surrogate for the presence of weathered material, subaerial tests are shown to have a higher presence of weathered material compared to submerged tests, and tests higher on banks have a greater presence of weathered material compared to tests lower on banks or along the streambed. No differences in the presence of weathered material are detected between geologic units. \n \nSD and LR have close agreement on the estimates of critical shear stress imparting greater confidence in its representativeness, but they diverge substantially on estimates of the erodibility coefficient inserting uncertainty into the representativeness of that parameter. Halton Till is shown to have a statistically lower mean critical shear stress compared to the grouped results of all the geologic units investigated, however, the difference in mean of 2.8 Pa may have limited implications given the range in critical shear stress at individual sites. \n \nComparing tests from summer and early spring at the same site indicates that seasonal processes significantly increase the presence of the surficial weathered layer but do not influence the critical shear stress of the underlying unweathered material. The higher availability of readily erodible weathered material corresponding to seasonal freshet flows suggests that late winter/early spring may be responsible for an outsized amount of erosion in streams with cohesive boundaries, particularly in headwater systems with shorter, more frequent hydrographs. This suggests that regulators and industry should place higher importance on considering how modifications to watersheds alter the frequency of detrition and regeneration of the surficial weathered material. Future research should focus on relating the importance of weathering processes on fluvial erosion rates in cohesive soils to watershed size, land use, and stormwater management techniques.

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