Pond Boss Forums POND CONSTRUCTION Building a dam Question about overflow pipe size

Sorry in advance if I sound like the noob that I am. Still trying to figure all of this out before we look for contractor to help move dirt.....


Recently we found a piece of land that looks like it will nicely support a 1/2 - 3/4 acre pond in western Illinois. After meeting with our local nrcs guy I walked away understanding that our biggest challenge will be getting all the extra water that won't fit in the pond out of the area out and down the creek bed without screwing up the dam.

Couple of vital stats he gave me:
Watershed area: 234 acres (rule of thumb here is 20 acre watershed = 1 acre pond)
Hydraulic length = 5400 ft

10 year storm data
24 hrs precipitation (p) = 4.63"
Runoff = 2.15"
Peak discharge (cfs) = 288.9
Ia/p = 0.14
Volume of runoff (Ac-Ft) = 42.0

25 year storm data
24 hrs precipitation (p) = 5.60"
Runoff = 2.94"
Peak discharge (cfs) = 407.2
Ia/p = 0.12
Volume of runoff (Ac-Ft) = 57.4

So given all of this, how do I begin to calculate the size of overflow pipe(s) I will need to move this water. Additionally does everyone size these so that the overflow will take care of a 10 year storm and you rely on spillway to make up the difference between a 10 and 25 year Storm? Or do you just build a lager freeboard into the damn so that it can hold more to absorb the 25 year rain?