I think some may depend upon the type of pond also. Ours are dammed up ravines, and the earthen spillways are located at the junction of the dam and the sides of the ravine. Most importantly, in my eyes anyway, is the fact that whoever designed and built our ponds didn't simply leave the deep water at the dam.....in the areas of the overflow, there is a considerable "shelf" of shallow water that extends back into the pond.
This means that we don't have exceptional depth at the overflow section, which would tend to allow fast moving water to cut "deep" into the dam. As it stands, we have approx 30' of shallow shelf, 20' wide.
Since I've been here, I have personally experience two 500 year rain events, and what they are calling a 1000 year event. Never saw the water get deeper than 2" over the spillway. Pretty slick.
