You need to consider some sort of controlled drain system. IF my 1/4 acre pond, with excessive watershed, did not have a simple drain pipe...it would not have a dam today. You want to size the drain system to handle as much water as your pond is expecting to see in the larger rain events and then have the Emergency Overflow off to the side, preferably in virgin soils (like Bob said). My example, 1/4 acre pond with 20+ acres of water shed (about 10 times more than it should for my area), has a 15" diameter pipe through the dam, and a low path (emergency spillway) where the dam met the original landscape. We could not place the emergency spillway in completely virgin soils so we added freeboard to the dam that allows water to build up 3 foot above full pool before going over the dam. To date (going on 4 years), the emergency spillway has not seen water, but the water has risen to 2 foot above full pool. Without the through pipe...I am sure the spillway would have needed some maintenance at a minimum.

It's purely my opinion that a pond that can exist without a drain pipe is one that would have a hard time filling and staying at a reasonable filled level. This would be one with inadequate watershed for it's size.

Something else to consider. Try to put the spillway where it will have the least amount of slope on the back side. The less the water path drops per foot, the lower the water speed and the less it will erode.

Fish on!,
Noel