150 



HYDKAULICS AND ITS APPLICATIONS 



nappe is said to be fully aerated. In a suppressed weir discharging 

 between the walls of a discharge channel of its own width, the nappe 

 will remain in contact with these walls, and tends to eject the air enclosed 

 in the underlying space. For free aeration it is necessary for air holes 

 to be provided in these wing walls. 

 If no provision is made for such an air supply, a partial vacuum is 



a. b c 



FlG. 80. Depressed, Drowned, and Clinging Nappes. 



produced beneath the nappe ; the latter is said to be depressed, and the 

 discharge is increased (Fig. 80 a). 



In an extreme case of the depressed nappe the whole of the air behind 

 the nappe may be ejected, when this space is occupied by a turbulent 

 eddying mass of water which does not itself join in the general motion 

 of the nappe. The latter is then said to be drowned or wetted under- 

 neath (Fig. 80 I). 



With very low heads the nappe may adhere to both crest and down- 

 stream face of weir (Fig. 80 c). 



Bazin gives the following as the relative discharges from a thin-edged 

 weir 2*46 feet high and with a head of '656 foot under different nappe 

 conditions : 



