16 BULLETIN 831, U. S. DEPARTMENT OF AGRICULTURE. 



These were of comparatively small dimensions, however, and of a 

 rather low degree of efficiency — estimated at from 40 to 50 per cent. 

 A nigher state of development was later obtained in France, where 

 reinforced concrete was used and more efficient functioning effected by 

 proper proportioning of the essential parts of the structure. Fear 

 of the possibility of freezing weather interfering with the operation 

 of the siphon spillways is said to be the principal reason why Ameri- 

 can engineers failed to adopt this type of structure, but a modifica- 

 tion of the design led to the overcoming of this difficulty and to their 

 rapid adoption at a number of points throughout the United States. 

 The lowering of the intake leg well below the normal water surface, 

 and the draining of the outlet sealing basin to prevent the possibility 

 of clogging at that point, did away with the ice menace and also 

 prevented the possibility of debris collecting or lodging in the throat. 



There should be little argument against the use of the siphon spill- 

 way on canals or in connection with reservoirs where absolutely auto- 

 matic control without the maintenance of mechanically operated de- 

 vices is desired, and where there must be close regulation of the rise 

 and fall of the water surface. Economy of cost, space, and rapidity 

 of control greatly argue in favor of such structures, especially where 

 laws require close regulation of the pond level, as in Italy where 8 

 inches is the limiting range. It is also most efficient where immedi- 

 ate response to sudden rises in water level is essential, because of the 

 fact that it is brought into action to ite full capacity with the rise of 

 only a few inches, whereas the overfall spillway is not fully effective 

 until the danger point is reached and then is dependent upon the 

 stored head for acceleration of velocity. 



The use in this country of siphons as spillways is much more re- 

 cent than it is in Europe, some of the earliest examples being con- 

 structed in 1910 on' the New York State barge canal. 1 Since then 

 they have been used on the fore bay of a hydroelectric plant of the 

 Tennessee Power Co. and on the power canal of the Mount Whitney 

 Power & Electric Co. in California. Thejr have been used still more 

 recently on several projects of the United States Reclamation Service 

 and on a number of irrigation and power projects in California, 

 the notable cases being the Orland, Salt River, Yuma, and Sun 

 River projects of the Reclamation Service; the South San Joaquin 

 Irrigation District; the Sweetwater Dam near San Diego; and the 

 Southern California Edison Co. at Huntington Lake, Calif. 



As has been shown, the head utilized to produce flow in an over- 

 floAY spillway is figured from the surface of the water above the spill- 

 way to its crest, although the actual head available, but which is not 

 utilized, is much larger, being the total head from water surface 



1 Engineering Record, July 30, 1910, and Oct. 8, 1910. Eng. News, Nov. 17, 1910. 



