January 29, 2010

Converting an ogee spillway into a siphon spillway:Some considerations

Filed under: spillway,Uncategorized — Rajnikant Khatsuria @ 11:53 am




Formerly Addl. Director

Central Water and Power research Station, Pune, India

E Mail: rmkhatsuria@rediffmail.com


The discharge over an overflow spillway is a function of the head measured above its crest. Enclosing the crest and making the resulting conduit flow full, thus converting an ogee spillway into a siphon spillway, can substantially increase the effective head. The head on the siphon spillway is then the difference in elevations between the reservoir level and the water level at the spillway outlet. This property can be utilized to increase the discharge capacity of a spillway without raising the reservoir level or lowering the crest level. This can be an effective way for the rehabilitation of a spillway that has undergone hydrologic re-assessment with an increased inflow.

There are, however, several issues, which require serious considerations:

• An ungated spillway, preferably with crest piers supporting bridge would be an ideal choice from consideration of construction facility.
• The profile of the upper membrane forming the siphon would have to follow the profile of the spillway surface for obvious reasons. This would rather be an inefficient configuration as far as priming of the siphon spillway is concerned. A flow deflector suitably located downstream of the crest may be a solution.
• The height of the siphon barrel at the crest would have to be greater than the maximum depth of overflow for achieving a sizable increase in the discharge.
• The conversion to the siphon spillway would involve large scale construction modifications like provision of siphon inlet at the entrance, embedding the barrel in the existing crest piers, construction of the barrel portion downstream of the piers and provision of air vent for depriming.
• If the span width is significantly larger as compared to the depth of barrel proposed, the barrel may require to be divided into two or more compartments from structural considerations. This feature is known to result in reduced coefficient of discharge.
• Finally, model studies would be indispensable for arriving at a suitable design, keeping in view the scale effects involved in such a study.

There is no information about any of the existing ogee spillways converted into siphon spillway. However, if the above mentioned difficulties could be overcome, such a conversion has a potential for increasing the discharge capacity considerably without lowering the crest or increasing depth of overflow, as shown by the following example.

The spillway in question has a span width of 20m and the maximum depth of overflow of 10m, as shown in Figure 1. The discharge per span is calculated as 1382 m3/s, assuming a coefficient of discharge of 2.185.The difference between the FSL El 110m and water level El 65m at the spillway outlet is 45m.

Figure 1: The original spillway

This spillway is proposed to be converted into a siphon spillway as shown in Figure 2. The overall cross section of the barrel is 20m wide x 12m high, with a1m thick dividing wall forming two barrels of 9.5m x 12m.

Figure 2: Proposed conversion to siphon spillway

 This configuration is likely to reduce the coefficient of discharge C in the formula

substantially. A conservative value of C=0.37 has been assumed for the present. Because of the increased discharge, the water surface at the outlet of the spillway would now be around El 70m, giving an effective head Ha of 40m. With the values of C=0.37, A=228 sq.m, the discharge through the siphon is calculated as 2363 cum/s.

This shows that a considerable increase in discharge capacity can be achieved with the proposed modification. The required size of the siphon barrel can be calculated for a given increase in the discharge. It should be ensured that the velocity at any section of the barrel is not more than about 12m/s to avoid cavitation.

Copyright: Neither this article nor any part of it may be copied in any form without the written permission of the author.



  1. 1. This is a good idea to increase discharge of an existing spillway without lowering the crest or raising the head. However, the construction modifications required to bring up a siphonic structure would be so much as to be very difficult, if not impossible. Particularly, dismantling existing concrete would be so large as to damage the rest of the spillway.

    2.The coefficint of discharge for the siphon of 0.37 seeems to be too small as compared to a conventional value of 0.67. How do you explain this?

    Comment by K. Sharma — February 5, 2010 @ 10:36 am | Reply

    • 1. It is true that large scale modifications would be required. This would mainly comprise digging grooves in the base concrete for anchoring reinforcement and constructing barrel. Carrying out this job by conventional hydraulic breakers would certainly damage the structure by microcracks. However, the advanced technique involving use of wire-sawing and concrete bursting would enable safer ways of modifications. With the advancemnets in concrete technology such modifications would be easily handled.

      2. In the case of siphon barrel divided by intermediate walls, the coefficient is found to depend on aspect ratio of each element. If the ratio of width to depth approaches unity, Cd drops down to about 0.37.

      Comment by Rajnikant Khatsuria — February 7, 2010 @ 6:09 am | Reply

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