TRAPEZOIDAL WEIRS 155 



While suffering from the disadvantage that only small quantities of 

 water are passed with a comparatively high head, the triangular has the 

 advantage over the rectangular notch where the flow is very variable, that 

 with either large or small heads it is equally easy to take accurate 

 measurements of the head, while with the rectangular notch with very 

 low heads this is practically impossible. 



On allowing for the velocity of approach, equation (4) becomes 



Q = ^ C tan |V 2 \(H + hf- fc*i cub. ft. per sec. 

 lo z l J 



v 2 

 Here h may be taken as 1*4 ^- , where v = velocity of approach. 



v 



In using the notch, care should be taken that the sides are equally 

 inclined to the vertical. 



Variation of Discharge with Head. 

 Since Q = K H^ 



_- 



Q ' TT 



It follows that a small change in, or error in estimating H, produces 

 2*5 times the percentage change or error in Q. 



ART. 54c. TRAPEZOIDAL NOTCH (Fig. 85). 



Let b be the bottom breadth of the notch. Let be the inclination to 

 the vertical, of the sides. 



The notch then becomes equivalent to a rectangular notch of breadth b, 



FIG. 85. 

 together with a triangular notch having an angle 2 B between its inclined 



