You have about double the length of what I have, lots of pipe. I was referring to the needed pipe diameter. I thought most counties could give you watershed acreage (which you have), runoff coefficients, and rainfall amounts from 10, 25, 50, and 100 year storms for your area. The county here gave me that and actually preformed the calculations for cubic feet of flow. Easier to do with a computer program than by hand as the it runs on a curve. I prefer the numbers as the county here tends to exaggerate on the sizes needed. They said I needed a 26 ft wide emergency spillway, while in reality I needed 16 feet based on the numbers. Our guys are pretty helpful though, shame you do not have the same service there as it is quite beneficial.
