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Turbulent and macro-turbulent structures developed in the benthic boundary layer downstream of topographic features

Turbulent and macro-turbulent structures developed in the benthic boundary layer downstream of topographic features
Turbulent and macro-turbulent structures developed in the benthic boundary layer downstream of topographic features
The characteristics and effects of large-scale flow structures developed in the benthic boundary layer downstream from large topographic features were analysed throughout a tidal cycle. The observed signature of the macro-turbulent features consisted of streamwise modules of low horizontal velocity and high suspended sediment concentration (SSC), alternating with modules of high horizontal velocity and low SSC. These modules extended 10 to 20 m streamwise and exceeded 1 m vertically, and are believed to be related to flow separation effects over large bedforms upstream of the deployment site. The macroscale flow modules intensified the ‘ burst-like ’ turbulent events and favoured sediment transport. ‘ Ejection-like ’ events were magnified during modules of decreasing horizontal velocity and increasing turbidity, whereas ‘ sweep-like ’ events were magnified during modules of increasing horizontal velocity and decreasing SSC. The enhanced turbidity of the macroscale modules may be the result of enhanced upward diffusion of sediment by ejection events, whereas the low-turbidity modules may be induced by increased downward transport of suspended sediment by sweep events. These hypotheses were supported by cross-spectral analysis performed on velocity and suspended sediment concentration time-series recorded at the site. An enhanced (negative) contribution of outward and inward interaction events to the Reynolds stress, compared to those reported in uniform BBLs, resulted in ‘ abnormally ’ low stress values.
BENTHIC BOUNDARY LAYER, SEDIMENT TRANSPORT, TURBULENCE, BOTTOM TOPOGRAPHY EFFECTS, BED FORMS, TOSCA, FLEMISH BANK
817-833
Lecouturier, M.N.
293c2933-2685-48a7-89b7-a3dc5984fe8e
Grochowski, N.T.
526f80ad-7673-425a-97b5-a754035d9bb8
Heathershaw, A.
6aaa194d-91eb-4006-8349-0a4f62118395
Oikonomou, E.
2cc51ab2-d9f0-4b9d-86e2-f4e95ad92b5c
Collins, M.B.
3b70278b-0004-45e0-b3c9-0debdf0a9351
Lecouturier, M.N.
293c2933-2685-48a7-89b7-a3dc5984fe8e
Grochowski, N.T.
526f80ad-7673-425a-97b5-a754035d9bb8
Heathershaw, A.
6aaa194d-91eb-4006-8349-0a4f62118395
Oikonomou, E.
2cc51ab2-d9f0-4b9d-86e2-f4e95ad92b5c
Collins, M.B.
3b70278b-0004-45e0-b3c9-0debdf0a9351

Lecouturier, M.N., Grochowski, N.T., Heathershaw, A., Oikonomou, E. and Collins, M.B. (2000) Turbulent and macro-turbulent structures developed in the benthic boundary layer downstream of topographic features. Estuarine, Coastal and Shelf Science, 50 (6), 817-833. (doi:10.1006/ecss.1999.0602).

Record type: Article

Abstract

The characteristics and effects of large-scale flow structures developed in the benthic boundary layer downstream from large topographic features were analysed throughout a tidal cycle. The observed signature of the macro-turbulent features consisted of streamwise modules of low horizontal velocity and high suspended sediment concentration (SSC), alternating with modules of high horizontal velocity and low SSC. These modules extended 10 to 20 m streamwise and exceeded 1 m vertically, and are believed to be related to flow separation effects over large bedforms upstream of the deployment site. The macroscale flow modules intensified the ‘ burst-like ’ turbulent events and favoured sediment transport. ‘ Ejection-like ’ events were magnified during modules of decreasing horizontal velocity and increasing turbidity, whereas ‘ sweep-like ’ events were magnified during modules of increasing horizontal velocity and decreasing SSC. The enhanced turbidity of the macroscale modules may be the result of enhanced upward diffusion of sediment by ejection events, whereas the low-turbidity modules may be induced by increased downward transport of suspended sediment by sweep events. These hypotheses were supported by cross-spectral analysis performed on velocity and suspended sediment concentration time-series recorded at the site. An enhanced (negative) contribution of outward and inward interaction events to the Reynolds stress, compared to those reported in uniform BBLs, resulted in ‘ abnormally ’ low stress values.

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More information

Published date: 2000
Keywords: BENTHIC BOUNDARY LAYER, SEDIMENT TRANSPORT, TURBULENCE, BOTTOM TOPOGRAPHY EFFECTS, BED FORMS, TOSCA, FLEMISH BANK

Identifiers

Local EPrints ID: 8955
URI: https://eprints.soton.ac.uk/id/eprint/8955
PURE UUID: fec44774-0f27-45d4-9c8e-4e0464261844

Catalogue record

Date deposited: 10 Sep 2004
Last modified: 17 Jul 2017 17:12

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