John Ditchfield very kindly photographed some airholes on the Kennet & Avon Canal and sent on the photos. The captions are his comments.
Good length of overflow. Water flowing.
No overflow.
(Rebuilt) small openings are above the level of the top of the adjacent lock gate. However, just upstream there is another paddle or sluice gate on the canal side (next photo), so perhaps that is used to control the level.
The upstream paddle or sluice gate.
Overflow in action, preventing water overflowing gates.
Note: this lock connects the canal with the River Avon, and the building in the background was a steam pumping station, presumably for topping up the canal from the river.
Many thanks to John for taking the trouble.
Irish waterways history 