108 



WATER CONDUCTORS AND ACCESSORIES 



The ideal cross-section from the hydraulic point of view is, 

 however, not always the best to adopt. There are other factors 

 which must be considered, such as the cost of construction, 



whether lined or unlined, 

 the character of the soil, 

 seepage, safety, grade, and 

 velocity. No specific rules 

 can be laid down to cover 

 all cases and each installa- 



FIG. 53.-Cross-section of Canal. tion must be treated indi ' 



vidually. 



A concrete-lined canal having the least wetted perimeter will 

 require the smallest amount of material, while the steeper sides 

 mean less excavation. Such a canal can furthermore be given a 

 steeper grade, if sufficient fall is available, and thus a higher 

 velocity, so that the cross-section can be small for a given quan- 



Nole:- 



Approx. & J^cu. ft. of .concrete 

 per foot length of ditch. 

 Surface of concrete on inside ojE 

 ditch to be made smooth 



^^ v Slope of ground 



Water 



Concrete 3 'thick _ 



FIG. 54. Open Concrete-lined Canal. 



tity of water. This rs advantageous especially on hillsides, and, if 

 the soil is hard and the excavation difficult, a concrete-lined canal 

 may be cheaper than an unlined one. In other instances the soil 

 may be of such a porous nature that lining is essential to prevent 

 excessive seepage. (See Figs. 54 and 55.) 



NOTE. For " Flow of Water in Channels," see Bulletin No. 194 U. S. 

 Dept. of Agriculture. 



