Knickpoint erosion and migration in cohesive streambeds

Unstable channel systems in areas of thick loess deposits in the midwestern United States are characterized by knickpoints that erode and migrate upstream in streambeds composed typically of 50 - 80% silt-sized material. To better predict knickpoint migration and upstream destabilization of channel systems, a study to determine streambed erodibility and rates and mechanics of knickpoint migration was established. Ten knickpoints (zones) in the Yalobusha River System, Mississippi were instrumented with a set of four pressure transducers to monitor flow depth and provide data to calculate average boundary shear stress. Monumented cross-sections were surveyed repetitively to identify the timing and magnitude of migration. An erodibility coefficient (k), expressed in cm ³/N-s was determined (average=.071) as an inverse function of the critical shear stress (τ_c) by in situ testing using a submersible jet-test apparatus. τ_c - values ranged over 4 orders of magnitude with an average of 135 Pa, well above the 78 Pa average boundary shear stress provided by the bankfull flow in the main knickpoint zone. However, local shear stresses of about 225 Pa have been measured, capable of eroding knickpoint materials. Maximum τ_c - values are as high as 400 Pa. Erosion of these more resistant materials, therefore, occurs by additional forces and mechanisms. During the period February - September 1999 knickpoint migration has ranged from negligible to more than 5 m. Field observations of failed cohesive blocks at the toe of knickpoints indicate that particle-by-particle erosion may play a minor role in certain materials and that more rapid erosion and migration occurs by mass failure. The average cohesive strength of these materials as determined by in situ testing is about 7 kPa and is enhanced by matric suction because they are unsaturated. Three principle mechanisms have been identified for erosion and migration of the cohesive knickpoints: (1) particle-by-particle erosion by shear forces and enhanced by upward-directed seepage forces within the streambed, (2) mass-failure of the knickpoint face, enhanced by turbulent scour which undercuts the base, (3) and by tension cracking of the headwall related to pressure release and stress-induced deformation. The relative dominance of these mechanisms is partly a function of the hydraulic and geotechnical resistance of the cohesive materials as well as the relative depths of headwater and tailwater. Copyright ASCE 2004.