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 care 
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 Yerona 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, PI. XV) , is 
additional to the old overflow spillway on the south end, which in it- 
self has been remodeled and extended as 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. 
