Nappe: 
The nappe is the sheet of water that flows over the weir crest. When the nappe is free-falling, there is often a gap between the underside of the nappe and the top of the weir.

Note:
Pressure Beneath the Nappe: 
The pressure beneath the nappe can become negative (less than atmospheric pressure) due to the air entrainment and the free-fall of the water. This creates a suction effect in the area beneath the nappe. This negative pressure can sometimes cause the nappe to cling to the weir, or in other cases, air may enter under the nappe, depending on flow conditions.