360 HYDRAULICS 



point, and the float is timed between the corresponding division 

 points. The partial areas of the two cross-sections are deter- 

 mined, and the mean of the areas of the corresponding division 

 multiplied by the corresponding velocity will give the partial 

 discharge of that division. The sum of the partial discharges 

 is the total discharge of the river. 



The mean velocity as observed by a rod float is to be taken 

 as the actual mean velocity only when the float is made to pass 

 close to the bottom. When the float is immersed only to a 

 depth *, the actual mean velocity is, 



in which V m is the measured mean velocity and d the depth of 

 water at which the measurement was taken. 



When a surface float is used, the actual mean velocity may be 

 obtained approximately by multiplying the measured mean 

 velocity by .8. 



FLOW OF WATER IN PIPES 



In determining the flow of water in pipes, the discharge in 

 cubic feet per second is Q = .7854d*v, in which d is the diameter 

 of the pipe in feet and v the actual velocity, in feet per second. 

 The theoretical velocity is v t = *J2gh, h being the static head. 

 This head h, which is available before the flow begins, sustains 

 losses during the flow due to skin friction between the water 

 and the pipe, to resistances at entrance, to bends and elbows, 

 and to other causes, resulting in a reduction of the theoretical 

 velocity. The actual velocity is, 



in which I is the length and d the diameter of the pipe, both in 

 feet; /, tne coefficient of resistance for friction; and c, the sum 

 of all coefficients for losses due to entrance, bends, valves, etc. 

 For a pipe whose length is more than 1,000 times its diameter, 

 called a long pipe, the value of 1 + c is very small in comparison 



