WATER CONDUCTORS 113 



As the use of flumes becomes less and less as hydro-electric 

 work becomes more permanent in character, it is suggested that 

 for preliminary estimating purposes the cost of low-pressure pipe 

 lines be used instead of using the presumably lower cost of flume 

 construction. 



Tunnels. Where the proposed route of the waterway encoun- 

 ters mountain ridges it is often advantageous if not absolutely 

 necessary to go through these by means of tunnels rather than to 



Area of waterway 151 sq. ft. 



Wetted perimeter 45 ft. 



Hydraulic radius 3.3 



Friction coefficient .014 



Grade .002 



C. 130 



FIG. 59. Typical Tunnel Section. 



excavate deep cuts or go around. The question as to which method 

 should be chosen is one of first cost as well as of maintenance. 

 Tunnels are, of course, safer and their upkeep is usually low as 

 compared with open canals, especially if these are built on the 

 hillsides where they are exposed to dangers from boulders striking 

 them, undermining, etc. 



Tunnels may be either of the pressure or non-pressure type. 

 When of considerable length they are usually of the former type 

 so that the drop may be utilized as useful head. They are almost 

 always lined with concrete, the thickness of the lining varying from 



