Chapter
Apr 26, 2012
Knickpoint Erosion and Migration in Cohesive Streambeds
Authors: Andrew Simon, Sean Bennett, and Mark W. GriffithAuthor Affiliations
Publication: Building Partnerships
Abstract
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 cm3/N-s was determined (average = .071) as an inverse function of the critical shear stress (τc) by in situ testing using a submersible jet-test apparatus. τc — 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 τc - values are as high as 400 Pa. Erosion of these more resistant materials, therefore, occurs by additional forces and mechanisms.
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© 2000 American Society of Civil Engineers.
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Published online: Apr 26, 2012
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Andrew Simon
USDA-ARS, National Sedimentation Laboratory, Oxford, MS, 38655
Sean Bennett
USDA-ARS, National Sedimentation Laboratory, Oxford, MS, 38655
Mark W. Griffith
USDA-ARS, National Sedimentation Laboratory, Oxford, MS, 38655
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Terms of Use: ASCE Library Cards are for individual, personal use only. Reselling, republishing, or forwarding the materials to libraries or reading rooms is prohibited.
Terms of Use: ASCE Library Cards are for individual, personal use only. Reselling, republishing, or forwarding the materials to libraries or reading rooms is prohibited.