HYDRAULICS 



353 



WEIRS 



A weir is a dam or obstruction placed across a stream for the 

 purpose of diverting the water and causing it to flow through a 

 channel of known dimensions, which channel may be a notch 

 or opening in the obstruction itself. The notch is usually rect- 

 angular in form. 



There are two general types of weirs, namely, those with end 

 contractions, as in Fig. 1 (a), and those without end contrac- 

 tions, as in Pig. 1 (6) . 



Crest of the Weir. The edge of the notch over which the 

 water flows, as shown in cross-section at a, Fig. 1 (c) and (d), is 



FIG. 1 



called the crest of the weir. In all weirs, the inner edge of the 

 crest is made sharp, so that, in passing over it, the water touches 

 along a line. The same statement applies to the inner edge of 

 both the top and the ends of the notch in weirs with end con- 

 tractions. For very accurate work, the edges of the notch 

 should be made with a thin plate of metal having a sharp inner 

 edge, as shown in Fig. 1 (d); but for ordinary work the edges 

 of the board in which the notch is cut may be chamfered off to 

 an angle of about 30, as shown at (c). The top edge of the 

 notch must be straight and set perfectly level, and the sides 



