146 



HYDRAULICS AND ITS APPLICATIONS 



the final agreement is, in general, fairly close, as is shown by the follow- 

 ing table, which shows the comparative values of Q per foot of crest length 

 for a suppressed weir : 



It may be shown that neglect to allow for the velocity of approach will 

 lead to an error which may amount to 6 per cent, where A (the area oJ 

 approach channel) = 2 b H, but is reduced to "7 per cent, if A = 6 b H, 

 and to '25 per cent, if A = 10 b H. Thus with a suppressed weir it is 

 in general important that this should be taken into account, while with 

 two end contractions it is usually unnecessary. 



Braschmann's formula : 



Q = 1-3838 + '0386 |- + 'WW_\ b V~~2~g H* c.f.s. 

 I B H I 



the dimensions being in feet, is much used in Germany. Here B is the 

 width of the approach channel. The formula only applies to rectangular 

 weirs with two end contractions and free bottom contraction, and agrees 



well with that of Francis, where B ~ 4 b. 



The following values of C were determined by Professor Dwelshauvers- 

 Devy, of Liege, from experiments on a small rectangular notch 2*593 

 inches wide, and with heads varying from *4 to 4*3 inches, 1 with no 

 velocity correction : 



Eecent experiments on a sharp-crested weir 2'505 feet long, height of 

 crest 8*05 feet, and contractions suppressed 2 showed that over a range of 



; Proceedings Inst. Civil Engineers," vol. 94, p. 333. 



; BuUetin of the University of Wisconsin," No. 216, p. 283. 



