Darly,
You really need more information and accurate information to make determination on spillways. Based on what you provided I get, but check the math:
@5mph- 219.999 cfs
@7mph- 308.001 cfs
which is a CRAP ton of water. No pipe, unless it is 20' (slightly exaggerated or maybe not) in diameter is going to handle that much water. Other factors that need to be considered, area of watershed, rainfall numbers of 25, 50, and 100 year storms, type of spill way (hooded, drop inlet, siphon, etc.), length of pipe, and free-board between pipe and emergency spillway.
Note: The more freeboard you have, the more temporary water storage capacity you have which in turn lowers your required outlet pipe flow requirement. Many factors to consider.
My advice: I am not an expert but that is a LOT of water, so to prevent future complications I would contact your county extension office or someone who has experience with these calculations and your local conditions.
