q =- C Z^h^/Z 



(7) 



where 



= discharge of water per unit of weir-crest length 



C = weir discharge coefficient (which for the required accuracy can 

 be assumed constant = 0.6) 



g = acceleration of gravity 



h = head above the weir crest 



Equation (7) applies when the nappe of the overflow is aerated, i.e., when the 

 water level on the downstream side of the weir is below the weir-crest eleva- 

 tion. This situation occurs for only a short time during a tidal cycle since 

 the phase difference between the tides on each side of the weir is usually 

 small (Fig. 13). The more frequent situation occurs when the weir crest is 

 submerged as it is during most of a tidal cycle. For this case, the discharge 

 is given by 



= c /2 g ( hl - h 2 ) (1 hl +ih 2 j 



(8) 



where h is the downstream head over the weir crest and the other variables 

 are as defined for equation (7) (Fig. 14). The calculation of weir discharge 

 is illustrated by example problem 1. 



3.0 



2.5 



2.0 



1.5 



1.0 



0.5 







-0.5 



-1.0 



■1.5 



-2.0 



-2 5 



(water level on channel side below weir crest) 

 I I I I I 



0.3 0.4 0.5 0.6 0.7 



Time (fraction of tidal cycle ),t/T 



Figure 13. Conditions of weir flow at various times during a tidal cycle, 



28 



