The Iowa Institute of Hydraulic Research has posted an excerpt of one of their older videos, Fluid Motion in a Gravitation Field:  Flow Over Weirs.  The video, developed with the support of the National Science Foundation, covers, among other things, the need for aeration of a weir and demonstrates the effect of Froude number as it relates to weir flow.

If a weir is used for flow measurement, both the upper and lower surface of the nappe (the body of water discharging over the crest of the weir) must be kept at atmospheric pressure. 

For open channel flow measurement the upper surface of the nappe is exposed to the atmosphere.  The lower surface of the nappe, however must be aerated (in the case of rectangular weirs) or the weir channel must be of sufficient with (v-notch weirs) to ensure aeration.

Aerated Flow over a Thin-Plate Weir

If aeration of the underside of the nappe is removed, the air under the nappe will gradually be carried away.  As this occurs, the difference in pressure will cause the nappe to be depressed – collapsing against the downstream face of the weir.

Unaerated Flow over a Thin-Plate Weir

 

 

Source:  YouTube