Mean flow and turbulence structure over exposed roots on a forested floodplain: Insights from a controlled laboratory experiment
Mean flow and turbulence structure over exposed roots on a forested floodplain: Insights from a controlled laboratory experiment
The time-averaged and instantaneous flow velocity structures of flood waters are not well understood for irregular surfaces such as are created by the presence of roots and fallen branches on forested floodplains. Natural flow structures commonly depart systematically from those described for idealised roughness elements, and an important knowledge gap exists surrounding the effects of natural flow structures on vertical exchanges of fluid and momentum. An improved understanding of the flow structure is required to model flows over forested floodplains more accurately, and to distinguish their dynamics from non-vegetated floodplains or indeed floodplains with other vegetation types, such as reed or grass. Here we present a quantification of the three-dimensional structure of mean flow velocity and turbulence as measured under controlled conditions in an experimental flume using a physical reproduction of a patch of forested floodplain. The results conform in part to existing models of local flow structure over simple roughness elements in aspects such as flow separation downstream of protruding roots, flow reattachment, and the lowering of the velocity maximum further downstream. However, the irregular shape of the surface of the floodplain with exposed roots causes the three-dimensional flow structure to deviate from that anticipated based on previous studies of flows over idealised two-dimensional roughness elements. The results emphasise varied effects of inheritance of flow structures that are generated upstream—the local response of the flow to similar obstacles depends on their spatial organisation and larger-scale context. Key differences from idealised models include the absence of a fully-developed flow at any location in the test section, and various interactions of flow structures such as a reduction of flow separation due to cross-stream circulation and the diversion of the flow over and around the irregular shapes of the roots.
Reesink, Arnold
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Darby, Stephen
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Sear, David
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Leyland, Julian
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Morgan, Peter
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Richardson, Keith
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Brasington, James
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26 February 2020
Reesink, Arnold
e9a3724d-3430-4fad-b85e-b27ed7343e53
Darby, Stephen
4c3e1c76-d404-4ff3-86f8-84e42fbb7970
Sear, David
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Leyland, Julian
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Morgan, Peter
c8f71e7a-8eca-4036-b6b9-e7f5b1a58e70
Richardson, Keith
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Brasington, James
bed2c201-e84a-461d-bcc6-4c8d952da803
Reesink, Arnold, Darby, Stephen, Sear, David, Leyland, Julian, Morgan, Peter, Richardson, Keith and Brasington, James
(2020)
Mean flow and turbulence structure over exposed roots on a forested floodplain: Insights from a controlled laboratory experiment.
PLoS ONE, 15 (2), [e0229306].
(doi:10.1371/journal.pone.0229306).
Abstract
The time-averaged and instantaneous flow velocity structures of flood waters are not well understood for irregular surfaces such as are created by the presence of roots and fallen branches on forested floodplains. Natural flow structures commonly depart systematically from those described for idealised roughness elements, and an important knowledge gap exists surrounding the effects of natural flow structures on vertical exchanges of fluid and momentum. An improved understanding of the flow structure is required to model flows over forested floodplains more accurately, and to distinguish their dynamics from non-vegetated floodplains or indeed floodplains with other vegetation types, such as reed or grass. Here we present a quantification of the three-dimensional structure of mean flow velocity and turbulence as measured under controlled conditions in an experimental flume using a physical reproduction of a patch of forested floodplain. The results conform in part to existing models of local flow structure over simple roughness elements in aspects such as flow separation downstream of protruding roots, flow reattachment, and the lowering of the velocity maximum further downstream. However, the irregular shape of the surface of the floodplain with exposed roots causes the three-dimensional flow structure to deviate from that anticipated based on previous studies of flows over idealised two-dimensional roughness elements. The results emphasise varied effects of inheritance of flow structures that are generated upstream—the local response of the flow to similar obstacles depends on their spatial organisation and larger-scale context. Key differences from idealised models include the absence of a fully-developed flow at any location in the test section, and various interactions of flow structures such as a reduction of flow separation due to cross-stream circulation and the diversion of the flow over and around the irregular shapes of the roots.
Text
journal.pone.0229306
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Accepted/In Press date: 3 February 2020
Published date: 26 February 2020
Additional Information:
Funding Information:
This research was supported by award NE/E009832/1 (to S.E.D and D.A.S) from the UK Natural Environment Research Council (NERC). The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. The authors are grateful for the constructive commentary provided by the reviewers, editors, and Koen Blanckaert. The original data and basic scripts are available online.
Publisher Copyright:
© 2020 Reesink et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Local EPrints ID: 438411
URI: http://eprints.soton.ac.uk/id/eprint/438411
ISSN: 1932-6203
PURE UUID: 89b6a9a3-d5a9-4ab8-8de0-86b9595be40b
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Date deposited: 09 Mar 2020 17:32
Last modified: 17 Mar 2024 03:04
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Author:
Arnold Reesink
Author:
Peter Morgan
Author:
Keith Richardson
Author:
James Brasington
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