160 



HYDRAULICS AND ITS APPLICATIONS 



crest width w to head, at which the nappe, springs clear, depends on the 

 head and is usually less than '5, as shown in the following table : 



Where the crest is sufficiently wide to cause adherence of the nappe, 



FIG. 88. 



the weir is termed broad-crested. In such a weir the stream springs 

 clear of the crest at its up-stream edge A and again makes contact at B, 

 as in Fig. 88, so that the discharge is reduced by the friction offered by 

 the surface from B to C. An increase in the head diminishes the length 

 of this portion of the surface, but increases the velocity of flow, so that 

 some one particular head will give the minimum loss due to this cause, 

 and will give a maximum coefficient of discharge. As the width of crest 

 is increased the friction loss increases and the coefficient of discharge in 

 consequence decreases. 



Bazin experimenting on flat-crested weirs having crest widths ranging 

 from "164 foot to 6'56 feet, and with heads from '25 foot to 1/5 feet, found 



