A real life problem, just for fun

[catkin]

Homework Statement

Around one of our buildings we have an "ant channel" of roughly square cross section (say 20 cm x 20 cm) with fish in it to control the mosquito larvae. The surface of the water carries particles giving a good indication of any surface movement.

Late one afternoon we observed the surface moving relatively quickly (say 3 cm/s). Suspecting a leak we followed the moving surface water and were amazed to find that the moving surface gave way abruptly to a still surface. Watching a little longer we observed that the interface between moving and still areas of the surface was itself moving -- at about half the velocity of the moving surface.

It appeared as if there was a discontinuity in the flow of water.

Homework Equations

None.

The Attempt at a Solution

I think I can explain the phenomenon. If no one gets it, I'll give clues.

 

[nova-ex]

Hm. I'm not sure. But can I hazard a guess? There's a change in the depth of the water?