Vamaz;
I have a pond just inder one acre with a fairly good size watershed. Pond was constructed by daming a stream, so it sits in a kind of "valley " between 2 pastures. I used 8" drain pipe and can say my spillway gets used a few times a year with good rains. Because of that, I'm having difficulty getting any grass on it and am dealing with some erosion issue's. My soil is predominantly clay.
Anyway, if hindsight was forsight I would have installed at least a 12" pipe. Not very technical, just an observation from my experience, as our ponds sounds simular.
Joe
