The plan we are considering (at substantial additional $$$$) is to skip the overflow pipe, use the planned spillway as the overflow and construct / excavate a 500 foot ditch that will engage at the 100 year flood event, line this ditch with rip rap etc.... This ditch will shed into my back 40 (if ever used).
Just so you know a 100-year event has nothing to do with 100 calender years. It is a term to describe the 1 in 100 chance of getting a set amount for a set area in a given year. I was in a meeting about storm runoff this year where it was said in my area of Alabama a 100-year event occurs about every 3 calender years now. I always get nervous when people use the term 100-year event and (if ever) near each other. The 100-year term is very misleading since in you life time you are more likely experience one.
