Mechanisms and Rates of Knickpoint Migration in Cohesive Streambeds: Hydraulic Shear and Mass Failure

Abstract

This paper presents results from a three-year field study that is being conducted to determine the morphology, processes and migration rates of nine knickpoints in the cohesive clay beds of the Yalobusha River System, Mississippi. The roles of surface erosion by hydraulic shear stress and mass failure during the upstream migration of knickpoints are evaluated. Results of submerged jet tests reveal a wide variation in the erosion resistance of the streambeds. Values of critical shear stress, τc, span almost four orders of magnitude with a median of 31 Pa, while values of the erodibility coefficient, k, span about three orders of magnitude with a median of 0.022 cm 3 /N-s. To relate τc and k-values to the erosion potential of flows, an excess shear stress approach is used to estimate erosion rates using values of average boundary shear stress, τ0. Most flows are competent to erode streambeds composed of the Naheola formation (τc = 2.24 Pa), while only the deepest flows and steepest hydraulic gradients generate boundary-shear stresses great enough to erode the Porters Creek Clay formation (τc = 199 Pa). Repeated thalweg surveys indicate that knickpoints formed in Naheola clay have migrated at an average rate of 7.2 meters per year since 1997, while knickpoints in Porters Creek Clay have migrated at an average rate of around 1.2 meters per year over the same period. Comparison of calculated erosion rates and observed knickpoint retreat rates indicates a discrepancy between migration rate and available hydraulic shear stress. Additional mechanisms have hence been identified as contributing to the migration of cohesive knickpoints: a cyclical mass-failure mechanism, and detachment of aggregates by upward directed pore-water pressure on the falling limb of hydrographs. To assess the role of mass failure in knickpoint retreat a combination of finite-element hydrologic and limit equilibrium slope-stability modeling was carried out. Thalweg survey data were used to construct a series of finite-element meshes based on the geometries of the knickpoints during migration, while head and tail water elevations were taken from stage data logged every 30 minutes above and below the knickpoint. At one example location in Naheola clay, Big Creek, every observed event greater than 0.3 m has been extracted from the stage record and simulated. Where a mass failure was indicated by a factor of safety close to or below one, the predicted failed distance was compared with the observed knickpoint retreat distance at the next survey. Results at this location show a close agreement between modeled and observed retreat rates. Quantification of knickpoint migration rates and processes in the field enables river managers to accurately determine the locus of incision at a particular time, allowing estimates of sediment sources and yields. This study has also shown that knickpoints formed in resistant substrata may act as a form of natural grade control, while knickpoints formed in more erodible substrata may require direct intervention by management agencies.