Hello

I have a problem that hopefully someone can help me with. I need to work out the detention time of a pond as it drains from a snorkel. The problem is I have multiple draining holes in the snorkel at different heights which will obviously have different flow rates as water level changes due to differing pressure on different holes. Just wondering if there is some sort of equation out there for this problem as we have wracked our brains over it and cant figure it out.

Thanks, any help will be much appreciated

Sam