Hydraulics of compound channels

Lambert, Martin F.

Title: Hydraulics of compound channels
Creator: Lambert, Martin F.
Relation: University of Newcastle Research Higher Degree Thesis
Resource Type: thesis
Date: 1993
Description: Research Doctorate - Doctor of Philosophy (PhD)
Description: An investigation into developing practical methods of predicting the stage-discharge relationship in compound channels is reported. Four methods have been investigated of which three have been shown to give improved estimates of the stage-discharge relationship in compound channels without the need for laboratory based empirical correction factors. The first approach investigated did not give reliable predictions of the stage-discharge relationship but showed that common assumptions used to determine the energy dissipation gradient in gradually varied flow may lead to misleading results. As a result, it has been recommended that the momentum equation be used in favour of the energy equation to analyse gradually varied non-uniform flow in open channels, particularly where compound channels are involved. The approaches have been validated using experimental results from the large-scale flood channel facility at HR Wallingford in the United Kingdom and by independent experiments carried out at the University of Newcastle, Australia. The majority of the comparisons have been made on straight compound channels but results from another large-scale experimental investigation of a doubly sinuous compound channel at HR Wallingford have also been presented. The turbulent interaction that occurs between the main channel and its flood plains causes traditional methods, based on the assumption that the two area act relatively independently from each other, to substantially over-predict the discharge. To account for this, a correction to the traditional approach can be calculated by internally dividing the cross-section into small vertical elements and applying a simple turbulent model. The vertical elements interact through lateral shear stresses on the sides of the elements and the magnitude of these lateral shear stresses are determined in the proposed mathematical model using the mixing length hypothesis. This approach provided a good estimate of the stage-discharge relationship over a substantial overbank depth range using a single coefficient that was held constant. A modification o this approach, to explicitly allow for the effects of secondary currents, is also presented. An alternative approach is proposed in which the stage-discharge relationship is considered as a mathematical relationship between two nominated physical limits. This approach incorporates the advantages of traditional methods and allows the stage-discharge relationship to be quickly established for the overbank flow region without the need for laboratory based correction factors. The approach is easy to apply and produced good agreement with nearly all experimental results examined. Experimental results from a large-scale model of a doubly sinuous compound channel constructed at HR Wallingford are also presented. THe flow structure in the model has been examined using flow velocity and direction measurements and flow visualization techniques. The flow velocity and direction measurements showed that common assumptions about channels of this nature were invalid. Flow visualization revealed complicated and as yet not well understood flow structures. A comparison of the observed stage-discharge relationships from this model with predictions using the alternative approach also showed good agreement.
Subject: compound channels; stage-discharge; momentum equation
Identifier: http://hdl.handle.net/1959.13/1310998
Identifier: uon:22133
Language: eng

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