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

Hodge, Rebecca and Voepel, Hal and Leyland, Julian and Sear, David and Ahmed, Sharif (2020) 'X-ray computed tomography reveals that grain protrusion controls critical shear stress for entrainment in fluvial gravels.', Geology., 48 (2). pp. 149-153.

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.

Item Type:Article
Full text:(VoR) Version of Record
Available under License - Creative Commons Attribution.
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Full text:(VoR) Version of Record
Available under License - Creative Commons Attribution.
Download PDF
(1134Kb)
Status:Peer-reviewed
Publisher Web site:https://doi.org/10.1130/G46883.1
Publisher statement:© 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license.
Date accepted:17 October 2019
Date deposited:29 November 2019
Date of first online publication:22 November 2019
Date first made open access:29 November 2019

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