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

X-ray computed tomography reveals that grain protrusion controls critical shear stress for entrainment of fluvial gravels
X-ray computed tomography reveals that grain protrusion controls critical shear stress for entrainment of fluvial gravels
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.
0091-7613
149-153
Hodge, Rebecca
b1d32ae8-df98-4726-a2f7-83f34a01ddfd
Voepel, Harold
7330972a-c61c-4058-b52c-3669fadfcf70
Leyland, Julian
6b1bb9b9-f3d5-4f40-8dd3-232139510e15
Sear, David
ccd892ab-a93d-4073-a11c-b8bca42ecfd3
Ahmed, Sharif
37570e92-ba6b-4e03-9144-c70fa7722c51
Hodge, Rebecca
b1d32ae8-df98-4726-a2f7-83f34a01ddfd
Voepel, Harold
7330972a-c61c-4058-b52c-3669fadfcf70
Leyland, Julian
6b1bb9b9-f3d5-4f40-8dd3-232139510e15
Sear, David
ccd892ab-a93d-4073-a11c-b8bca42ecfd3
Ahmed, Sharif
37570e92-ba6b-4e03-9144-c70fa7722c51

Hodge, Rebecca, Voepel, Harold, Leyland, Julian, Sear, David and Ahmed, Sharif (2020) X-ray computed tomography reveals that grain protrusion controls critical shear stress for entrainment of fluvial gravels. Geology, 48 (2), 149-153. (doi:10.1130/G46883.1).

Record type: Article

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.

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More information

Accepted/In Press date: 17 October 2019
e-pub ahead of print date: 22 November 2019
Published date: 1 February 2020

Identifiers

Local EPrints ID: 436831
URI: http://eprints.soton.ac.uk/id/eprint/436831
ISSN: 0091-7613
PURE UUID: c77c3283-57c6-48e9-9578-e9cfdaec269e
ORCID for Julian Leyland: ORCID iD orcid.org/0000-0002-3419-9949
ORCID for David Sear: ORCID iD orcid.org/0000-0003-0191-6179
ORCID for Sharif Ahmed: ORCID iD orcid.org/0000-0002-3290-3592

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Date deposited: 10 Jan 2020 17:34
Last modified: 18 Feb 2021 17:08

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