30 



BULLETIN 831, U. S. DEPARTMENT OF AGRICULTURE. 



and 1,400 second-feet at the intake and outlet ends respectively. The 

 siphons are to operate under a head of 12 feet, at an average velocity 

 of about 14 feet per second, assuming 0.50 as a coefficient of discharge. 



The structure is of re- 

 inforced concrete 

 throughout, with a 

 detached air chamber 

 for the regulation of 

 the siphons. The air 

 control has not given 

 satisfactory results 

 because of its isola- 

 tion ; otherwise the 

 structure is a good ex- 

 ample of the combi- 

 nation of facilities 

 for the control of su- 

 perdrainage on side- 

 hill canal or where 

 melting snows or ob- 

 struction below the 

 structure might cause 

 an abnormal rise of 

 the water level in the 

 canal at the site of the 

 siphon. 



A structure to regu- 

 late the Arizona 

 Canal of the Salt 

 River project, at the 

 Arizona Power Plant, 

 is shown in figure 8. 

 The area of the throat 

 section is 10 square 

 feet, flaring to a maxi- 

 mum section of 15 

 square feet at a point 

 10 feet above the lip 

 of the outlet, where it 

 tapers to the same 

 form and cross section 

 as that of the throat. This is the only case of the tapering draft tube 

 known to the writer and it is to be regretted that there is no inf orma- 

 ion as to the efficiency of the structure. Such a series of tests as 

 would give results enabling the engineering profession to ascertain 



