There could be some issues if those storm drains enter the retention pond down low, at the base or bottom of the pond itself.
One of our retention ponds has two 3' culverts coming into it, and they are both at grade with the pond bottom.
Agree.
That could be the reason for the initial description of "create marsh or dredge". If the spillway has to be lowered to get the water down to a safe level for the drains to work properly, then it might create a marsh out of the pond. If they didn't want the marsh, they would need to dig it deeper.
A lot of us here would agree that it is more work(= more money) to rework an old pond, than it does to build a new one.
