HYDRAULICS 357 



Character of Channel Value of n 



Coarse rubble masonry and firm compact gravel . .020 



Well-made earth canals in good alinement 0225 



Rivers and canals in moderately good order and 



perfectly free from stones and weeds 025 



Rivers and canals in rather bad condition and 



somewhat obstructed by stones and weeds. . . .030 

 Rivers and canals in bad condition, overgrown 

 with vegetation and strewn with stones and 

 other detritus, according to condition. .035 to .050 

 EXAMPLE. Find the discharge of a rough-plank sluice 24 in. 

 wide, when the depth of the water in the sluice is 15 in. and the 

 fall 3 in. in 100 ft. 



SOLUTION. The slope s = .25-f-100 = .0025; the wetted peri- 

 meter p = 2 + (2X1.25) =4.5 ft.; and the area A of the water 

 cross-section = 2X1.25 = 2.5 sq. ft. The hydraulic radius is, 

 therefore, r = 2.5 -r- 4. 5 = .5556. The value of n for unplaned 

 timber is .012 ; therefore, 



' 



a 14.7 



/ .00155\ .012 

 .5521+ (23+) X-p= 



\ -0025 / ^ 5556 



Substituting the values found in Chezy's formula, 

 v = 114.7 -\T5556 X .0025 = 4.27 ft. per sec. 



Therefore, the discharge is 



Q = Av = 2.5X4.27 = 10.675 cu. ft. per sec. 



Discharge of Large Streams. The discharge of a large body 

 of water, when it is impracticable to construct a weir, is deter- 

 mined by measuring, on one hand, the mean velocity v at a 

 cross-section of flowing water by means of floats or by the use 

 of special instruments, and, on the other hand, by ascertaining 

 the area A of that cross-section. Then, the discharge 

 Q = Av 



The current meter affords the most convenient and accurate 

 method of measuring velocities of a stream. One form of this 

 instrument is shown in Fig. 1. The number of revolutions of 

 the buckets b depending on the velocity of the flow is recorded 

 electrically on the dials m and n. . The relation between this 



