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Flow reversal over a natural pool-riffle sequence: a computation study

Flow reversal over a natural pool-riffle sequence: a computation study
Flow reversal over a natural pool-riffle sequence: a computation study
A computational study is presented on the hydraulics of a natural pool-riffle sequence composed of mixed cobbles, pebbles and sand in the River Lune, northern England. A depth-averaged two-dimensional numerical model is employed, calibrated with observed data at the field site. From the computational outputs, the occurrence of longitudinally double peak zones of bed shear stress and velocity is found. In particular, at low discharge there exists a primary peak zone of bed shear stress and velocity at the riffle tail in line with the local maximum energy slope, in addition to a secondary peak at the pool head. As discharge increases, the primary peak at the riffle tail at low flow moves toward the upstream side of the riffle along with the maximum energy slope, showing progressive equalization to the surrounding hydraulic profiles. Concurrently, the secondary peak, due to channel constriction, appears to stand at the pool head, with its value increasing with discharge and approaching or exceeding the primary peak over the riffle. The existence of flow reversal is demonstrated for this specific case, which is attributable to channel constriction at the pool head. A dynamic equilibrium model is presented to reconstruct the pool-riffle morphology. A series of numerical modelling exercises demonstrates that the pool-riffle morphology is more likely produced by shallow flows concentrated with coarse sediments than deep flows laden with low concentrations of fine sediments. It is concluded that channel constriction can, but may not necessarily, lead to competence reversal, depending on channel geometry, flow discharge and sediment properties.
pool-riffle, flow reversal, bed shear stress, channel morphology, fluvial processes, mathematical modelling
0197-9337
689-705
Cao, Zhixian
7642b644-eaa0-4ace-9356-3f75bf8e9e41
Carling, Paul
8d252dd9-3c88-4803-81cc-c2ec4c6fa687
Oakey, Rob
096b1d91-ed03-473c-895f-1f164c5e77b2
Cao, Zhixian
7642b644-eaa0-4ace-9356-3f75bf8e9e41
Carling, Paul
8d252dd9-3c88-4803-81cc-c2ec4c6fa687
Oakey, Rob
096b1d91-ed03-473c-895f-1f164c5e77b2

Cao, Zhixian, Carling, Paul and Oakey, Rob (2003) Flow reversal over a natural pool-riffle sequence: a computation study. Earth Surface Processes and Landforms, 28 (7), 689-705. (doi:10.1002/esp.466).

Record type: Article

Abstract

A computational study is presented on the hydraulics of a natural pool-riffle sequence composed of mixed cobbles, pebbles and sand in the River Lune, northern England. A depth-averaged two-dimensional numerical model is employed, calibrated with observed data at the field site. From the computational outputs, the occurrence of longitudinally double peak zones of bed shear stress and velocity is found. In particular, at low discharge there exists a primary peak zone of bed shear stress and velocity at the riffle tail in line with the local maximum energy slope, in addition to a secondary peak at the pool head. As discharge increases, the primary peak at the riffle tail at low flow moves toward the upstream side of the riffle along with the maximum energy slope, showing progressive equalization to the surrounding hydraulic profiles. Concurrently, the secondary peak, due to channel constriction, appears to stand at the pool head, with its value increasing with discharge and approaching or exceeding the primary peak over the riffle. The existence of flow reversal is demonstrated for this specific case, which is attributable to channel constriction at the pool head. A dynamic equilibrium model is presented to reconstruct the pool-riffle morphology. A series of numerical modelling exercises demonstrates that the pool-riffle morphology is more likely produced by shallow flows concentrated with coarse sediments than deep flows laden with low concentrations of fine sediments. It is concluded that channel constriction can, but may not necessarily, lead to competence reversal, depending on channel geometry, flow discharge and sediment properties.

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More information

Published date: 2003
Keywords: pool-riffle, flow reversal, bed shear stress, channel morphology, fluvial processes, mathematical modelling

Identifiers

Local EPrints ID: 14708
URI: https://eprints.soton.ac.uk/id/eprint/14708
ISSN: 0197-9337
PURE UUID: 6ba20b50-c36f-4351-8420-1b4e64b61618

Catalogue record

Date deposited: 24 Feb 2005
Last modified: 15 Jul 2019 19:33

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