SPILLWAYS FOR RESERVOIRS AND CANALS. 19 



The formula Q = kAV2gH is used as the basis for figuring dis- 

 charge. While this expresses the quantity discharged, the accurate 

 design of one of these structures will necessarily require more thor- 

 ough search into the theory of the hydraulic features such as would 

 enter into any tendency to retard flow and thereby reduce the 

 efficiency of the whole structure. Coefficients, as has been stated 

 above, have merely been assumed, and it is to be regretted that more 

 data have not been developed to prove just how correct this practice 

 has been. Forms and dimensions of the different functioning parts 

 must necessarily be deduced from theoretical considerations, and on 

 the results of data obtained from practical installations of such 

 structures as have furnished any such information. 



Theoretically, fixed rules have been adopted and practice has shown 

 the advisability of having the overpour crest of the throat at the same 

 elevation as the normal water level in the reservoir. For starting 

 siphonic action the inlet must be sealed by the water as soon as the 

 water in the reservoir rises above this normal height and climatic or 

 other conditions may make this a matter of choice as to which of two 

 methods will be used. The inlet lip can be made to extend down to 

 the normal water surface or a very slight distance below it, so that 

 when the water drops below this level the air is admitted and siphonic 

 action ceases; or the inlet lip is extended to a considerable depth 

 below the water surface, and the highest point in the crown of the 

 siphon is connected with air inlets either fixed or regulated to admit 

 air to the crown at the desired stage of water surface and break 

 siphonic action. This latter case is used and is desirable when float- 

 ing material may interfere with the proper action of the siphon. 

 The contrast in the two designs is shown by text figures 6 and 7 as 

 installed in Europe. 



The discussions the writer has had with various engineers who have 

 designed and installed siphon spillways have led to the belief that 

 the assumed coefficients have been uniformly low ; that refinements to 

 a reasonable degree would lead to higher efficiencies ; and that data to 

 determine the reliability of coefficients of discharge and possibilities 

 of future design must be obtained from existing installations. Any 

 attempt to discuss the ideal proportions of the parts of a siphon spill- 

 way in order to approach the maximum efficiency would only revert 

 to the general assumptions taken from foimulas whose applicability 

 has not been proven, but merely taken in all cases as being adaptable 

 to this comparatively undeveloped structure. 



In a series of experiments carried on at Throop College of Tech- 

 nology in California and at Fort Collins, Colo., several small models 

 of siphon spillways were tested and some very interesting informa- 

 tion obtained, proving that the conclusions from standard formulas 

 to develop the proportionate dimensions of the various parts were 



