BROAD-CRESTED WEIRS 167 



k . K . b H = 3-087 kbll = K'bl cub. ft. sec. 

 Thus corresponding to the values *82 and '87 of k, the values of K income 

 2-53 and 2'69. 



The validity of this formula receives remarkable confirmation from the 

 results of the tests on such weirs at Cornell (p. 163), where for all heads 

 between '5 foot and 4 feet, and for all crest widths greater than 5 feet 

 the values of K' lie between 2'59 and 2'69. 



A sloping crest increases the discharge, as does any rounding of the 

 up-stream corner. The effect of this rounding is not so pronounced as 

 with a thin-crested weir, diminishing as the crest width increases and 

 also as the head increases. Fteley and Stearns, experimenting on a 

 crest 4 inches wide, with radii of one-fourth, one-half, and one inch 

 respectively, found the effective head to be increased in the ratio 



I 1 H -- jj- ,-, this correction being applicable for heads of not less 



than '17 and '26 feet on weirs with radii of one-fourth and one-half 

 inch respectively. 



Bazin's experiments on crests having widths of 2'62 and 6'56 feet, 

 heads from '25 foot to T50feet, with an up-stream crest radius of 4 inches, 

 showed a mean increased discharge of 13'5 per cent, with the narrower 

 and 10 per cent, with the broader crest, while the United States Deep 

 Waterways experiments on a weir 22 feet wide, with 6 : 1 slope on each 

 face, showed an average increase of 2 per cent, with an up-stream edge 

 rounded to a radius of 4 inches. Experiments on weir B, Fig. 89A, 

 showed that a rounding of the up-stream edge to a radius of 4 inches 

 increased the discharge by about 4 per cent, at the higher heads. The 

 condition of the crest as regards roughness is found not to influence the 

 discharge by more than about 2 per cent. 



AET. 58. RISE IN SUEFACE LEVEL PEODUCED BY A WEIR. 



A dam or weir is usually placed across a stream with the idea of rais- 

 ing the surface level and increasing the depth of water up stream. The 

 distance to which this effect may be felt is sometimes considerable (see 

 Art. 89), and can only be determined when the rise in level in the 

 neighbourhood of the dam is known. This may be obtained if the dis- 

 charge of the stream before the introduction of the dam is known (since 

 this discharge will be unaffected by the presence of the dam) by equating 

 this, in cubic feet per second, to the flow over a dam of the required 



