Need help figuring out what size pipe I need for dam. I live in South Alabama. The average ran fall is 57 inches annually. Soil type is Clay. Been trying to figure out the watershed for pond site. Also there is a small stream that will feed the pond.

Hard to know without watershed numbers. Given the 57 inches, clay soil, and small stream, I'd say you should err on the side of large rather than small. Emergency spillway should be for emergencies, not common occurrences.

You get a lot of bang for your buck by upsizing. For instance, my pondbuilder wanted an 8 inch pipe, but fortunately had to install 9 3/8 inch. That means close to 40 percent greater maximum flow, obviating the need for the emergency spillway outside of the most extreme events!

Thanks,
Any idea how I can calculate my watershed numbers?

I'm no expert, but I believe there's an app for that. You need a topo map for it to work.

Bear in mind that slope is important. A gentle slope will reduce runoff, as the water is more likely to seep into the ground. A steep slope increases runoff. And grassy fields slow the runoff much more than trees.

There are people here who really are experts in this stuff, and I'm sure they'd be willing to help. Topo map is a must!

Look at Google Earth or similar, and look for an app that can pull in topo info.

NRCS ran all the watershed and max flow rate for storm events for me, at no cost. Call your local office and tell them what you are looking for. They have access to data and tools that are unavailable to the general public.