Pond Boss Forums POND CONSTRUCTION Building a dam Adequate bottom to surface drain system size?

What Esshup says. If you have a good NRCS guy for your area he can figure it for you based on rainfall patterns and watershed. Mine did for me.

That said, I have a 3 acre pond with approximately 15-20 acre watershed (I need to actually map it out some day to say for sure). We are in the wettest part of Kansas with 42" annual average rainfall but with significant sized rain events (12" above "normal" so far for 2015). I have an 8" overflow pipe angled down at I forget what angle but has increased flow once it is covered and goes into siphon mode.

I would not want any smaller. If it were me, even though your pond is smaller and less watershed, I would go with the 8" unless cost is very important to you. 6" sounds like it would be adequate but if you can swallow the initial cost, go with the 8". It will be less prone to plugging with debris or FA like the 6" would be. There is going to be a significant cost difference going from 6" to 8" but it still should not cost an arm and a leg.

I was sent a picture of our pond by my daughter and it appears with all the recent rain and flooding the pond was to the bottom of my dock support which is 8" channel iron. If that is truly the case the water would be 3" deep over the emergency overflow and flowing full bore out the 8" pipe. I'm not worried as the emergency overflow area is very large and have a lot of dam height above full pool. But water will be flowing for a day or two out the pipe.

That is one other thing about pipe size. During a heavy rain event, a smaller pipe will let the pond rise higher during the event and will flow longer before the pond is back down to full pool. A bigger pipe will remove the excess water quicker and bring the pond back down to full pool quicker. Don't know if this matters to you, but it is the way it will work.