Publication detailsHodge, Rebecca, Voepel, Hal, Leyland, Julian Sear, David & Ahmed, Sharif (2020). X-ray computed tomography reveals that grain protrusion controls critical shear stress for entrainment in fluvial gravels. Geology 48(2): 149-153.
- Publication type: Journal Article
- ISSN/ISBN: 0091-7613, 1943-2682
- DOI: 10.1130/G46883.1
- Further publication details on publisher web site
- Durham Research Online (DRO) - may include full text
Author(s) from Durham
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.