14 



BULLETIN 155, U. S. DEPARTMENT OF AGRICULTURE. 



On most irrigation systems the head of water carried fluctuates 

 more or less and, as a rule, is far below normal for a considerable 

 period in the spring and again for a time in the fall. At such times 

 siphons and pipe lines may not run full. This condition may be 

 unfavorable to their life and, as a precaution against it, gates have in 

 a few instances been placed at the outlets as a means of throttling 

 the discharge so as to keep the pipe full at all times. Such provision 

 was made at the outlet of the 84-inch pipe line of the Pueblo Eocky 

 Ford Irrigation Co., and the same practice might be followed to ad- 

 vantage in many other places. 



HALF FRONT ELEVATION SECTION A-B 



Fig. 3. — Intake of " Poison Basin " siphon, King Hill project, Idaho. 



To prevent weeds or coarse debris of any kind from entering 

 pipe lines, gratings are usually provided at intakes. However, 

 unless carefully watched, the accumulation of weeds at the grating is 

 liable to obstruct the entrance so as to cause the water to overflow 

 canal banks. The danger of this, and of the serious damage which 

 might result in man} 7 instances, have led to the removal of gratings 

 which could not be inspected frequently. For irrigation service, 

 where water is not carried during the winter, iron gratings are very 

 satisfactory, but in places where ice is troublesome wooden gratings 

 are considered better, particularly if they project above the water, for 

 the reason that ice does not form on the wood so readily. 



