I did some rough calculating, and it looks like you would need an 18” pipe for your overflow. My degree is in civil engineering but I am not an engineer and am a land surveyor, so you might want to get that number checked. That’s assuming 25% runoff, 15 acres, and a 2inch an hour rain. That results in 7.5 cfs of flow. You would have to look up rainfall data for your area but a true design to a 25 year flood would probably have a lot higher rainfall intensity, maybe as high as 5 inches per hour. Your pond isn’t that big and it doesn’t look like anybody is going to die if it washes out.
Thanks for the info Dave. I think you are probably pretty close. There is no damage that could be done in a washout but sure don't want to have that happen after doing the work to build it. I would like to get back there during a hard rain to see in person how much water is flowing through there.