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Hydraulic control in open channels: upstream and downstream controls

In an open channel, a high-velocity supercritical flow can only be controlled from upstream, e.g. by a spillway gate or by an uncontrolled weir crest. This is termed upstream control. A fluvial, subcritical flow is best controlled from downstream, e.g. by a gate at the downstream end of a canal. That is, a downstream control.
Hydraulic structures that cause critical flow (e.g. gates and weirs) are used as control sections for open channel flows. At the control section the discharge can be calculated once the depth is known. Some structures are built specifically to provide means to record the flow rates simply by measuring the critical flow depth: e.g. broad-crested and sharp-crested weirs.
A control section ‘controls’ the upstream flow if it is subcritical and controls also the downstream flow if it is supercritical. Two classical examples are the sluice gate and the broad-crested weir.
The three video movies illustrate seminal examples of hydraulic controls. The first movie presents a vertical sluice gate, with the flow direction from left to right. The second movie shows a broad-crested weir upstream of a stepped spillway. The third movie presents a prototype operation north of Brisbane, Australia on 22 May 2009. The gated spillway crest controls the smooth steep chute operation. An increase in gate opening would results in an increase of discharge outflow.
Hubert Chanson.

Additional notes
YouTube channel >>
Hubert Chanson
Hubert Chanson
HENDERSON, F.M. (1966). "Open Channel Flow." MacMillan Company, New York, USA.
MONTES, J.S. (1998). "Hydraulics of Open Channel Flow." ASCE Press, New-York, USA, 697 pages.
CHANSON, H. (2004). "The Hydraulics of Open Channel Flow: An Introduction." Butterworth-Heinemann, 2nd edition, Oxford, UK, 630 pages (ISBN 978 0 7506 5978 9).

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