X-ray computed tomography reveals that grain protrusion controls critical shear stress for entrainment in fluvial gravels

Hodge, Rebecca; Voepel, Hal; Leyland, Julian; Sear, David; Ahmed, Sharif

Abstract

The critical shear stress (τc) for grain entrainment is a poorly constrained control on bedload transport rates in rivers. Direct calculations of τc have been hindered by the inability to measure the geometry of in situ grains; i.e., the shape and location of each grain relative to surrounding grains and the bed surface. We present the first complete suite of three-dimensional (3-D) grain geometry parameters for 1055 water-worked grains, and use these to parameterize a new 3-D grain entrainment model and hence estimate τc. The 3-D data were collected using X-ray computed tomography scanning of sediment samples extracted from a prototype scale flume experiment. We find that (1) parameters including pivot angle and proportional grain exposure do not vary systematically with relative grain size; (2) τc is primarily controlled by grain protrusion, not pivot angle; and (3) larger grains experience larger forces as a result of projecting higher into the flow profile, producing equal mobility. We suggest that grain protrusion is a suitable proxy for assessing gravel-bed stability.

Citation

Hodge, R., Voepel, H., Leyland, J., Sear, D., & Ahmed, S. (2020). X-ray computed tomography reveals that grain protrusion controls critical shear stress for entrainment in fluvial gravels. Geology, 48(2), 149-153. https://doi.org/10.1130/g46883.1

• Journal Article Type: Article
• Acceptance Date: Oct 17, 2019
• Online Publication Date: Nov 22, 2019
• Publication Date: Feb 1, 2020
• Deposit Date: Sep 18, 2018
• Publicly Available Date: Nov 29, 2019
• Journal: Geology
• Print ISSN: 0091-7613
• Electronic ISSN: 1943-2682
• Publisher: Geological Society of America
• Peer Reviewed: Peer Reviewed
• Volume: 48
• Issue: 2
• Pages: 149-153
• DOI: https://doi.org/10.1130/g46883.1
• Public URL: https://durham-repository.worktribe.com/output/1314423