30(3 



HYDRAULICS AND ITS APPLICATIONS 



at C and C'. The proportions often adopted in practice are indicated in 

 Fig. 132. 



Figure 133 a and b shows sections which are sometimes adopted for 

 large sewers to the same end, the hydraulic mean depth being fairly high 

 even with a small discharge. 



Effect of varying m or i. 



Assuming v = C V m i 



we have 



d m 



or 



, ^ \/ * i v , 



d r V - . d m = 77 . d m 

 2 m 2 in 



d v d m 



v 2 m' 



Thus a small increase in m produces one-half its percentage increase 

 in v. 



Similarly it may be shown that - - = <y^, so that in calculating v, and 

 therefore the discharge, from this formula, any error in the value 



FT a. 133. 



assumed for the slope will lead to one-half the proportional error in the 

 estimated discharge. 



