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Sensitivity of interfacial hydraulics to the microtopographic roughness of water-lain gravels

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posted on 29.08.2013, 10:07 by Stephen RiceStephen Rice, Thomas Buffin-Belanger, Ian Reid
Flow within the interfacial layer of gravel-bed rivers is poorly understood, but this zone is important because the hydraulics here transport sediment, generate flow structures and interact with benthic organisms. We hypothesized that different gravel-bed microtopographies generate measurable differences in hydraulic characteristics within the interfacial layer. This was tested using a high density of spatially and vertically distributed, velocity time series measured in the interfacial layers above three surfaces of contrasting microtopography. These surfaces had natural water-worked textures, captured in the field using a casting procedure. Analysis was repeated for three discharges, with Reynolds numbers between 165000 and 287000, to evaluate whether discharge affected the impact of microtopography on interfacial flows. Relative submergence varied over a small range (3.5 to 8.1) characteristic of upland gravel-bed rivers. Between-surface differences in the median and variance of several time-averaged and turbulent flow parameters were tested using non-parametric statistics. Across all discharges, microtopographic differences did not affect spatially averaged (median) values of streamwise velocity, but were associated with significant differences in its spatial variance, and did affect spatially averaged (median) turbulent kinetic energy. Sweep and ejection events dominated the interfacial region above all surfaces at all flows, but there was a microtopographic effect, with Q2 and Q4 events less dominant and structures less persistent above the surface with the widest relief distribution, especially at the highest Reynolds number flow. Results are broadly consistent with earlier work, although this analysis is unique because of the focus on interfacial hydraulics, spatially averaged 'patch scale' metrics and a statistical approach to data analysis. An important implication is that observable differences in microtopography do not necessarily produce differences in interfacial hydraulics. An important observation is that appropriate roughness parameterizations for gravel-bed rivers remain elusive, partly because the relative contributions to flow resistance of different aspects of bed microtopography are poorly constrained.

Funding

This work was supported by the Natural Environment Research Council (NERC) [grant number NER/B/S/2000/00697]

History

School

  • Social Sciences

Department

  • Geography and Environment

Citation

RICE, S.P., BUFFIN-BÉLANGER, T. and REID, I., 2014. Sensitivity of interfacial hydraulics to the microtopographic roughness of water-lain gravels. Earth Surface Processes and Landforms, 39 (2), pp. 184-199.

Publisher

© The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd.

Version

VoR (Version of Record)

Publication date

2014

Notes

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. This article was published in the journal, Earth Surface Processes and Landforms [© 2014 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd.] and is also available at: http://dx.doi.org/10.1002/esp.3438

ISSN

0197-9337

eISSN

1096-9837

Language

en