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Does the permeability of gravel river beds affect near-bed hydrodynamics?

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posted on 28.09.2017, 11:01 authored by James R. Cooper, Annie Ockelford, Stephen RiceStephen Rice, D. Mark Powell
The permeability of river beds is an important control on hyporheic flow and the movement of fine sediment and solutes into and out of the bed. However, relatively little is known about the effect of bed permeability on overlying near-bed flow dynamics, and thus on fluid advection at the sediment-water interface. This study provides the first quantification of this effect for water-worked gravel-beds. Laboratory experiments in a recirculating flume revealed that flows over permeable beds exhibit fundamental differences compared with flows over impermeable beds of the same topography. The turbulence over permeable beds is less intense, more organised and more efficient at momentum transfer because eddies are more coherent. Furthermore, turbulent kinetic energy is lower, meaning that less energy is extracted from the mean flow by this turbulence. Consequently, the double-averaged velocity is higher and the bulk flow resistance is lower over permeable beds, and there is a difference in how momentum is conveyed from the overlying flow to the bed surface. The main implications of these results are three-fold. First, local pressure gradients, and therefore rates of material transport, across the sediment-water interface are likely to differ between impermeable and permeable beds. Second, near-bed and hyporheic flows are unlikely to be adequately predicted by numerical models that represent the bed as an impermeable boundary. Third, more sophisticated flow resistance models are required for coarse-grained rivers that consider not only the bed surface but also the underlying permeable structure. Overall, our results suggest that the effects of bed permeability have critical implications for hyporheic exchange, fluvial sediment dynamics and benthic habitat availability.

Funding

This project was financially supported by the Natural Environment Research Council of the UK through NE/H020772/1 (awarded to Powell) and NE/H020993/1 (awarded to Rice).

History

School

  • Social Sciences

Department

  • Geography and Environment

Published in

Earth Surface Processes and Landforms

Volume

43

Issue

5

Pages

943-955

Citation

COOPER, J.R. ... et al, 2017. Does the permeability of gravel river beds affect near-bed hydrodynamics? Earth Surface Processes and Landforms, 43 (5), pp.943–955.

Publisher

Wiley © The Authors

Version

VoR (Version of Record)

Publisher statement

This work is made available according to the conditions of the Creative Commons Attribution 4.0 International (CC BY 4.0) licence. Full details of this licence are available at: http://creativecommons.org/licenses/ by/4.0/

Acceptance date

11/09/2017

Publication date

2017-09-18

Notes

This is an Open Access Article. It is published by Wiley under the Creative Commons Attribution 4.0 International Licence (CC BY). Full details of this licence are available at: http://creativecommons.org/licenses/by/4.0/

ISSN

0197-9337

eISSN

1096-9837

Language

en

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