Formerly Addl. Director
Central Water and Power research Station, Pune, India
E Mail: email@example.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.
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