SUBMERGED WEIRS 



157 



greater than is given by the formula, the error increasing as the length 

 is diminished. For lengths less than 3 feet the side inclinations should 

 apparently vary as follows : 



ART. 55. SUBMERGED WEIRS. 



Where the water on the down-stream side of a weir rises above 

 of the sill we get a submerged weir. 



If Hi and H 2 (Fig. 86) be the 

 heights of the free surfaces above 

 the sill of a rectangular weir, the 

 flow over the upper part of the 

 section, of a depth HI Hz, may 

 be considered as a free discharge 

 into air, and that over the lower 

 part, of depth H 2 , as a discharge 

 through a submerged orifice. 



Thus for the upper portion 



the level 



FIG. 86. 



i = Ci b V 2 g (Hi 



lower Q 2 = C*b ~^g H* ( 

 If we assume Ci = C 2 = C, we have for the total flow 

 iW^-HS (Hi - H 2 ) - 



= Cb 



= C b 



Hi 



Taking into account the velocity of approach, we have 



v , _.. p, 



(Hi - 



