Experiments on Gravel‐Sand Transitions: Examination of Washload Deposition

Dingle, Elizabeth H.; Venditti, Jeremy G.

Abstract

An abrupt transition in bed grain size occurs in river systems. Over a short downstream distance, often only a few channel widths, the bed surface fines from gravel (∼10 mm) to sand (∼1 mm). This is the gravel-sand transition (GST), and it is the only abrupt downstream reduction in grain size within fluvial systems. There are several theories for the origin of the GST, including size-selective deposition of bimodal grain size distributions and the rapid onset of washload deposition due to changes in particle suspension properties at shear velocities of ∼0.1 m/s. Here, we present a laboratory experiment examining changes in fluid and sediment dynamics across a GST. We developed a stable gravel bed reach that was just below the threshold of motion, then fed sand. We observed sand carried as washload in the gravel reach fall out of suspension, forming a sand bed and a stable GST. Shear velocity was 0.09–0.10 m/s upstream of the GST and <0.10 m/s downstream, consistent with the washload deposition hypothesis. We were then able to perturb the position of the GST by systematically varying discharge and/or sand supply, shifting it downstream with an increase in discharge or a reduction in sediment supply. A decrease in discharge or increase in sand supply caused upstream migration. Our observations support an abrupt change in washload transport conditions across a narrow range of shear velocities, consistent with the washload deposition theory and measurements taken across GSTs in natural river systems.

Citation

Dingle, E. H., & Venditti, J. G. (2023). Experiments on Gravel‐Sand Transitions: Examination of Washload Deposition. Journal of Geophysical Research: Earth Surface, 128(7), Article e2023JF007116. https://doi.org/10.1029/2023jf007116

• Journal Article Type: Article
• Acceptance Date: Jul 5, 2023
• Online Publication Date: Jul 25, 2023
• Publication Date: 2023-07
• Deposit Date: Sep 25, 2023
• Publicly Available Date: Sep 25, 2023
• Journal: Journal of Geophysical Research: Earth Surface
• Print ISSN: 2169-9003
• Electronic ISSN: 2169-9011
• Publisher: American Geophysical Union
• Peer Reviewed: Peer Reviewed
• Volume: 128
• Issue: 7
• Article Number: e2023JF007116
• DOI: https://doi.org/10.1029/2023jf007116
• Public URL: https://durham-repository.worktribe.com/output/1747829
• Publisher URL: https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2023JF007116