The air-regulated siphon

A siphon is a pipe connecting two reservoirs of different water levels, in which the pipe pressure would locally become less than atmospheric. The siphon is a simple and efficient form of drain using natural gravity as the key driver. However, the start-up of a siphon, called priming, requires to exhaust the air from the top of the siphon, and the process of priming/depriming a siphon can cause instabilities and un-wanted flow patterns.
The air-regulated siphon is a relatively recent development which allows amounts of air into the siphon during the priming, ensuring a smooth, gradual, controlled priming action, and more controlled siphon spillway operation and downstream flood release.
The experience in air-regulated siphon spillway design suggested that:
the inlet geometry and shape have a strong influence on the head-discharge relationship of the siphon spillway;
the air demand in Froude-scaled physical models is nearly always under-estimated;
the position of the nappe deflector is best optimised using physical modelling;
the tailwater level would best drown slightly the outlet;
A seminal contribution (Ervine and Oliver 1980) further discussed the compliance between model and prototype for siphon spillways, based upon 40 years operational record at Eyebrook Reservoir (UK), including 25 years of operation with an air-regulated siphon spillway system and a 1:5 scale physical model study of the air-regulated siphon spillway structure.
The operation of air-regulated siphon spillways constitutes a basic application of fluid mechanics to open channel hydraulics and hydraulic structures, including small dams and weirs. This is further discussed in a number of relevant RUclips video movies in the same channel at:
{ / @hubert_chanson }
Hydraulic Engineering and Applied Hydrodynamics in Hubert Chanson RUclips channel { / @hubert_chanson }
Fundamentals of open channel hydraulics [Playlist]
Advanced hydraulics of open channel flow [Playlist]
Hydraulic structures [Playlist]
The Equation of Conservation of Mass in Fluid Mechanics { • The Equation of Conser... }
Equation of conservation of energy in fluid mechanics { • Equation of conservati... }
The siphon { • The siphon }
The inverted siphon { • The inverted siphon }
Acknowledgements
Late Prof. Alan ERVINE
References
ERVINE, D.A. (1976). "The Design and Modelling of Air-Regulated Siphon Spillways." Proceedings of the Institution Civil Engineers, London, Part 2, June 1976, 61, pp. 383-400.
ERVINE, D.A., and OLIVER, G.C.S. (1980). "The full-scale behaviour of air-regulated siphon spillways." Proceedings of the Institution Civil Engineers, Part 2, Vol. 69, pp. 687-706.
STREETER, V.L., and WYLIE, E.B. (1981). "Fluid Mechanics." McGraw-Hill, 1st SI Metric edition, Singapore.
CHANSON, H. (1997). "Air Bubble Entrainment in Free-Surface Turbulent Shear Flows." Academic Press, London, UK, 401 pages (ISBN 0-12-168110-6).
HARDWICK, J.D., and GRANT, D.J. (1997). " An adjustable air-regulated siphon spillway." Proceedings of the Institution Civil Engineers, Water, Maritime & Energy, UK, Vol. 124, June, pp. 95-103.
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).