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 spillway 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 pressure 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 
