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



with the flaring outlet. In each unit the inlet is well submerged, with 

 its upper end 5.5 feet below the water surface. The inlet is 3.5 feet 

 high and 6 feet wide, protected by a vertical screen with f-inch bars 

 spaced 4 inches center to center. The over-all efficiency was tested 

 using the formula Q = kAV2gH and gave a coefficient of discharge 

 of 0.65. These siphons are not provided with water seals on their 

 outlet ends, but have free discharge. The whole structure, gate in- 

 cluded, was placed in a length of 90 feet and the cost was one-third 

 that estimated for the overflow type of spillway. 



UNITED STATES RECLAMATION SERVICE SIPHONS. 



The United States Reclamation Service has located a siphon about 

 12 miles below the heading of the Yuma Project, where the main 

 canal leaves the foot of the mesa and turns southward toward Yuma. 

 At this point there is a drop of about 12 feet in water level, bringing 

 the canal to the level of the lower valley. It is intended to develop 

 power here to be delivered to a point below Yuma and used for 

 pumping water from the main canal through a lift of about 80 feet 

 to the top of the mesa, for the irrigation of some 30,000 to 40,000 

 acres of land. A battery of 5 siphons has been installed. They can 

 be adjusted to discharge at different levels and all employed when 

 the canal is running full, the combined theoretical discharge being 

 1,488 second-feet. They have been tested operating as a battery at 

 efficiencies ranging from 68 to 70 per cent and in combinations from 

 64 to 80 per cent. The area of the smallest or throat section, which 

 was used in the computations, is 11.35 square feet, and at the outlet 

 end 21 square feet. In the tests for efficiency the actual drop between 

 water surfaces was 11.87 feet, which was certainly all the head avail- 

 able for producing velocity, but the partial vacuum registered by 

 the mercury gage showed an equivalent of 15.60 feet of water. 



This was noted as indicating that the siphon was acting in a man- 

 ner similar to a compound diverging tube under pressure and having 

 a discharge coefficient greater than one and which may even have 

 been greater than two. It also indicated that the draft tube should 

 flare, which was the case in this instance. The observed depth over 

 the lip of throat necessary to start siphonic action was 0.35, 0.40, 

 0.35, 0.15> 0.35, and 0.40 foot respectively for the siphons as listed 

 above and with 5.35 feet of water over the lip of the outlet, and they 

 ran uniformly one-tenth foot higher in each case with the outlet sealed 

 with 6 feet of water. They are of reinforced concrete and cost about 

 $23,000 complete. The regulation of siphonic action is by means of a 

 specially designed sliding air valve shown in figure 9. This same 

 design is used on the installations at the Salt River project near 

 Phoenix, Ariz. The operation of these control valves was satisfac- 

 tory, except that there was not enough vertical movement to permit 



