Look at my #1666, The Addick flood reservoir is basically a big levee. There are outflow weirs that have a certain width. They start draining as various elevations are reached.
If the inflow is substantial, the 108 elevation outflow weir only lets out a certain cubic foot per second due to its width. The detention continues to rise until the next outflow point of 111 comes into play, etc. etc. The outflow points get bigger as it goes up to protect the main levee/dam from overtopping.
not sure where this is but looks like a whirlpool in/near barkers
https://twitter.com/TedLandK5/status/902610514810896384