SPILLWAYS FOR RESERVOIRS AND CANALS. 37 



least one case because of the lack of area in which to build it. At 

 this point it was estimated that the maximum inflow from the drain- 

 age area was about 700 second-feet, which the siphons must take cafe 

 of during heavy rainfall and in addition limit the fluctuation of 

 water surface to 1 foot. A weir to discharge the inflow stated, with 

 the limiting head on its crest, required a length of 200 feet which 

 was not permissible under the conditions and a spillway consisting 

 of four siphons and a 20-foot waste weir for floating trash, required 

 but 57 feet between abutments. Each siphon has a throat area 7.75 

 feet wide by 1 foot high and acts under a head of 10.5 feet calculated 

 to discharge 160 feet. The 20-foot weir with the 1 foot head will 

 discharge 70 second-feet, making a total of 710 second-feet. The in- 

 let of the siphons is well submerged to prevent the entrance of float- 

 ing bodies and is further protected by screens. The inlets were 

 flared to twice the normal area to reduce to a minimum the loss of 

 head due to entry, and the throat of the siphons were each made 1 

 foot high, because it was desired to retard full siphonic action until 

 the tubes were completely filled at that point. They were flared 

 again to a section of 2 feet by 4 feet at the outlet end. Air vents 6 

 inches high by 12 inches long were provided for each siphon, pierc- 

 ing the wall at low-water level, thus regulating the action at that 

 point. 1 



Contrasting examples of requirements of overflow and siphon spill- 

 ways are furnished in Italy, where the limiting lengths of spillway 

 sites are in a ratio of about 6 to 1. The two plants in question are 

 a hydraulic power plant near Milan, which is equipped with or- 

 dinary overflow spillway, and another at Verona with a siphon spill- 

 way. The siphon is confined within a space 59 feet in length and 

 limits the surface fluctuations to 3 inches, while the overflow spill- 

 way at Turbigo, near Milan, is 300 feet in length and does not dis- 

 charge the required volume until 2 feet of water pours over its crest. 

 In addition the overflow spillway is supplemented by three gates au- 

 tomatically worked by electric motors. 2 The conclusion as to control 

 devices is self-evident. 



The battery of 6 siphons recently completed for the spillway at 

 the north end of Sweetwater Dam, California (fig. 2, PL XV), is 

 additional to the old overflow spillway on the south end, which in it- 

 self has been remodeled and extended a;: a result of the damage by 

 the flood of 1915. There is also an emergency overflow spillway 500 

 feet long in the center of the dam. The siphon spillway is the largest 

 of the type constructed to date, having an intake area for each unit 

 of 144 square feet, a throat area 6 by 12 feet, or of 72 square feet, an 

 outlet opening 8.5 by 12 feet, or of 102 square feet, and operating 



1 Engineering News, vol. 64, No. 15. 2 Engineering News, Apr. 20, 1911. 



