It's where velocity head is exchanged for height head in open channel....typically caused by tailwater height. The height of the water surface can abruptly change by 1..5..10..20 ft in height.
Some holes are actually hydraulic jumps. Cats allow for some of the forces to be transmitted thru and... around the open space between the tubes.
Gary Strome firstname.lastname@example.org
Friends Don't Let Friends Drive PVC Boats