372 



NEW YORK STATE MUSEUM 



Experience in flows over dams of this length and with depths 

 as great as from 7 to S feet is as vet rather limited in this coun- 

 try, and the question was raised as to the best method of comput- 

 ing the discharge for a case like the one under discussion. The 

 engineers of the British government in India have had. in con- 

 nection with their large irrigation works, perhaps more experi- 

 ence in this class of measurement than all others combined, and 

 The formulas used by Them appear more rational in form than 

 those commonly used in the United States for such computa- 

 tions, and after some study it was decided to use these. As 

 many American engineers may not be familiar with these 

 formulas they are here reproduced. They take the following 

 form — 



Q = f LC (37) 



in which — 



Q — the discharge over a thin-edged clear overfall, in cubic 



feet per second, 

 L = the length of the dam in linear feet, 

 C = coefficient depending for its value on d. 

 g = acceleration of gravity == 32.2, 

 d = depth on crest, in linear feet. 

 Equation (37) may also take the form — 



Q = 5.35 L C V 3 1 . (38) 

 To find C for different values of d. we have — 



= 1- (° M (34 ; 6 + "J (39) 



This gives a series of values of C corresponding to (7. For 

 instance, for (7 = 0.25 foot, C = 0.651; for (7 = 0.50 foot. 

 C = 0.649, and so on. 



For a wide-crested dam the coefficient is further modified to 

 suit the actual width of the crest. For this we have given the 

 expression — 



'Equation (30) may bo written in a simpler form, 0=1 — 0.01 (34.G+d) 



