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Flow separation at the inner (convex) and outer (concave) banks of constant-width and widening open-channel bends

Murphy, Brendan J.; Blanckaert, Koen; Kleinhans, Maarten G.; Mclelland, Stuart J.; Uijttewaal, Wim S.J.; Murphy, Brendan; van de Kruijs, Anja; Parsons, Daniel R.; Uijttewaal, Wim S. J.; Chen, Qiuwen

Authors

Brendan J. Murphy

Koen Blanckaert

Maarten G. Kleinhans

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Dr Stuart McLelland S.J.McLelland@hull.ac.uk
Deputy Director of the Energy and Environment Institute

Wim S.J. Uijttewaal

Brendan Murphy

Anja van de Kruijs

Wim S. J. Uijttewaal

Qiuwen Chen



Abstract

There is a paucity of data and insight in the mechanisms of, and controls on flow separation and recirculation at natural sharply-curved river bends. Herein we report on successful laboratory experiments that elucidate flow structure in one constant-width bend and a second bend with an outer-bank widening. The experiments were performed with both a flat immobile gravel bed and mobile sand bed with dominant bedload sediment transport. In the constant-width bend with immobile bed, a zone of mainly horizontal flow separation (vertical rotational axis) formed at the inner bank that did not contain detectable flow recirculation, and an outer-bank cell of secondary flow with streamwise oriented rotational axis. Surprisingly, the bend with widening at the outer bank and immobile bed did not lead to a transverse expansion of the flow. Rather, flow in the outer-bank widening weakly recirculated around a vertical axis and hardly interacted with the inner part of the bend, which behaved as a constant-width bend. In the mobile bed experiment, downstream of the bend apex a pronounced depositional bar developed at the inside of the bend and pronounced scour occurred at the outside. Moreover the deformed bed promoted flow separation over the bar, including return currents. In the constant-width bend, the topographic steering impeded the generation of an outer-bank cell of secondary flow. In the bend with outer-bank widening, the topographic steering induced an outward expansion of the flow, whereby the major part of the discharge was conveyed in the central part of the widening section. Flow in the outer-bank widening was highly three dimensional and included return currents near the bottom. In conclusion, the experiments elucidated three distinct processes of flow separation common in sharp bends: flow separation at the inner bank, an outer-bank cell of secondary flow, and flow separation and recirculation in an outer-bank widening. © 2012 John Wiley & Sons, Ltd.

Journal Article Type Article
Publication Date Jun 6, 2013
Journal Earth Surface Processes And Landforms
Print ISSN 0197-9337
Electronic ISSN 1096-9837
Publisher Wiley
Peer Reviewed Peer Reviewed
Volume 38
Issue 7
Pages 696-716
APA6 Citation Blanckaert, K., Kleinhans, M. G., Mclelland, S. J., Uijttewaal, W. S., Murphy, B., van de Kruijs, A., …Chen, Q. (2013). Flow separation at the inner (convex) and outer (concave) banks of constant-width and widening open-channel bends. Earth surface processes and landforms : the journal of the British Geomorphological Research Group, 38(7), 696-716. https://doi.org/10.1002/esp.3324
DOI https://doi.org/10.1002/esp.3324
Keywords Flow separation; Open‐channel bend experiments; Morphology laboratory
Publisher URL http://onlinelibrary.wiley.com/doi/10.1002/esp.3324/abstract;jsessionid=2D4A94C33DE038123B47EA55477B4143.f02t03