Whenever I model a cross section passing through a pond/lake, I run into a dilemma:
How deep is the effective flow? At what elevation do I set my blocked obstruction? Is it the normal pool elevation? No, that does not make sense if there is a dam at the downstream end and the pool elevation is 10 feet below the top of dam due to a pipe or side spillway. Certainly in a 100-year flood situation, the dam is blocking much more flow than what can pass through a pipe. At the same time, I am sure there is some water flowing below the elevation of the dam.
Is there a “rule of thumb” that anyone uses to decide how much area to block off?