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The broad-crested weir

A broad-crested weir is a flat-crested structure with a crest length large compared to the flow thickness. The ratio of crest length to upstream head over crest must be typically greater than 1.5 to 3.
In most practical situations, the approach flow velocity is small and the upstream flow conditions are subcritical. Critical flow conditions occur at the weir crest. When the crest is "broad", the flow streamlines are parallel to the crest, and the pressure distribution above the crest is hydrostatic. Downstream of the weir, the flow is supercritical in absence of downstream control.
Graphically the relationship between specific energy and flow depth at a broad-crested weir shows the transition between the upstream and downstream flow conditions. And critical conditions are observed at the weir crest.
Hubert Chanson.

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Hubert Chanson
Hubert Chanson
HENDERSON, F.M. (1966). "Open Channel Flow." MacMillan Company, New York, USA.
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).
FELDER, S, and CHANSON, H. (2012). "Free-surface Profiles, Velocity and Pressure Distributions on a Broad-Crested Weir: a Physical study." Journal of Irrigation and Drainage Engineering, ASCE, Vol. 138, No. 12, pp. 1068–1074 (DOI: 10.1061/(ASCE)IR.1943-4774.0000515)
EDLIN, S., LU, Z., and CHANSON, H. (2020). "The Broad-Crested Weir." Generic Document, The University of Queensland, School of Civil Engineering, Brisbane, Australia (ISBN 978-1-74272-311-2). {https://espace.library.uq.edu.au/view...}

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