Exactly. It's not like a dam breach where you have a constant and steep incline into deep water. You've got a taper up on one side and down on the other. So

Exactly. It's not like a dam breach where you have a constant and steep incline into deep water. You've got a taper up on one side and down on the other. So your initial breach is going to only have a small cross section. You get something like river rapids as a result. And then erosion quickly does its thing, but much less quickly than it does when water goes over the top of a dam because the much shallower grade means that the amount of material that needs to be moved to get a deeper breach increase more quickly than in the case of a dam. The video from Malaysia is water flowing into what was formerly a mine pit so (like a dam) the wall is going to be about as steep as the geology can support. Natural geology tends to create much more mild slopes.