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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

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


Authors

Hal Voepel

Julian Leyland

David Sear

Sharif Ahmed



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 Mar 29, 2024
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

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