WATER CONDUCTORS 107 



,-, , , ,. .. Area of Cross-section , 



The hydraulic radius, r = -^ rf . _ . , the wetted per- 



Wetted Perimeter 



imeter of the cross-section of a channel being that part which is 

 in contact with the water. 



For an open canal, the grade or slope, s, is the ratio of the fall to 

 the length in which the fall occurs. For a closed penstock under 

 pressure, it is the ratio between the loss in head due to friction 

 to the length. (See also page 117.) 



The velocity of the water in a canal should be kept below that 

 which would cause erosion of the bed. It should, however, be 

 large enough to prevent vegetable growth from forming or silt 

 from being deposited. Assuming the bottom velocity to be 

 about 75 per cent of the mean velocity, the figures in Table XXIX 

 represent the safe values which are widely used in determining the 

 permissible velocities of water in open canals. 



TABLE XXIX 

 SAFE MEAN VELOCITIES * 



Very fine sandy soil or loose silt . 50 



Pure sand 1 . 00 



Light sandy soil, 15 per cent clay 1 .20 



Light sandy loam, 40 per cent clay 1 . 80-2 . 00 



Coarse sand 1 .50-2.00 



Loose gravelly soil 2 . 50 



Ordinary loam 2 . 50 



Ordinary firm soil or loam, 65 per cent clay. ... 3 . 00 



Stiff clay loam 4.00 



Firm gravelly clay soil 5 . 00-7 . 00 



Stiff clay 6.00 



Conglomerates, soft slate 6 . 50 



Stratified rocks 8.00 



Small boulders 8.00-15.00 



Hard rock 13.33 



Concrete 15.00-20.00 



B. A. Etcheverry, " Journal of Electricity, Power and Gas." 



The most advantageous cross-section to use, from the hydraulic 

 point of view, would be that which gives the smallest wetted per- 

 imeter or the largest value of the hydraulic radius. This would 

 mear a semicircular section, but it is seldom used on account of 

 the difficulties in building. A trapezoidal section is, however, 

 generally used, and by letting the bottom and sides be tangents 

 to an inscribed semicircle, as in Fig. 53, the best hydraulic results 

 will be obtained; the slope, i.e., the angle 6, being 60. 



