Vertical stand pipe using either SDR-35 or SCH40 PVC, sized accordingly to acreage that drains into it, not necessarily pond size. 4" minimum diameter if its in open country, 6-8" minimum if its likely to get debris, could take even large if it has a lot of water flowing through it. You'll need to drain the pond partially to install it, we do quite a few of these on old ponds that are leaking. If doing it with the pond full, pump or siphon the pond down about 3-4' (you can get by with less, but your over flow ends up real close to the bank. Start out as far as you can without letting water get in to your excavation, trench straight down 3-4' then carry a 1/4" per foot grade on your trench all the way through the dam. If you want to drain the pond completely just trench through the dam, same height and location as the original pipe. Along with a vertical stand pipe every pond should also have an earthen emergency overflow (flat broad based ditch, seeded down in KY31 Fescue or the like). Grade of emergency overflow should be 8-12" above water level on a small pond. Below are some pics are of a pond with an old metal overflow that was leaking that we replaced.
Dug through old pipe, poured concrete plug and filled old stand pipe:
New vertical stand pipe installed:
Anti-siphon collars:
