SPILLWAYS FOR RESERVOIRS AND CANALS. 33 



high, the latter being in Big Creek itself. In 1917 these dams were 

 all raised 35 feet to an elevation of 6,955.5 feet. The dam known as 

 No. 1 was the only one provided with a spillway, the other two being 

 constructed above any possibility of overflow. An overflow crest 645 

 feet long was built in dam No. 1 over which flow begins after the 

 water surface in the reservoir exceeds an elevation of 6,950 feet. The 

 downstream face was built in a series of steps, each 4 feet high and 2 

 feet wide, and to prevent any great amount of water going over the 

 dam, a battery of 7 siphons was installed to operate when the water 

 on the spillway crest had reached a depth of 6 inches or more. 



The cross-sections of these siphons are shown in figure 12, and it 

 will be noted that they are not of uniform design in that five of 

 them have the outlet legs inclined toward the intake and two of 

 them slope away from the intake. They are all of reinforced 

 concrete with throats 3.5 feet high and 12 feet wide, containing an 

 area of 42 square feet. Because of the sloping topography of the 

 canyon side where the siphons are built, they also vary as to operat- 

 ing head, three of them being designed to utilize 12 feet, two of them 

 15 feet, one 18 feet, and one 22 feet fall from head to tail water. The 

 intakes extend 4 feet below normal water surface and are tapered 

 from an area 9.25 feet high by 15.5 feet wide to that at the throat, 

 42 square feet, and the draft leg is uniformly of the same area as 

 that at the throat. Except for extreme care in making the concrete 

 dense enough to exclude air leaks in the siphons, no other precautions 

 were taken to effect a special lining other than a coat of " gunite," 

 which was applied after the forms were removed. Air control was 

 provided by an 8 by 27 inch air duct connecting the upstream face 

 of the dam with the crown of each siphon chamber, the top of the 

 air inlet being at the same elevation as the overflow siaillway crest. 

 A sliding gate is fixed over these air ducts, the vertical movement of 

 which is 16 inches, which makes possible the regulation of the siphons 

 to a point that far below the point where siphonic action would 

 ordinarily cease. 



A peculiar condition was developed as a result of the varying heads 

 under which the siphons were to operate in that the sealing basin of 

 the upper, or lower-head, units would tend to overflow into the ad- 

 jacent basins, causing a rise of water in the siphon chambers at the 

 outlet end and transmitting adverse air piessure to the intake arm 

 and resulting in the lowering of the water there and retarded priming 

 of that particular siphon. To eliminate this condition there was in- 

 stalled a system of relief pipes connected with the crowns of the 

 siphons so that any air pressure in one siphon produced by the action 

 of another could be expelled by the siphon in action. This was suffi- 

 cient for those whose outlets were of the same level, but the pipes for 

 149907°— 20- 3 



