There isn't always a spillpipe or culvert. Most ponds that I am familiar with that utilize a pipe as the primary means of discharging excess water also utilize an emergency spillway, which around here is usually a spot, or section of the dam (almost always at the end, or corner of the dam), that is lower than the rest. Its purpose is to provide an escape for water should the primary exit (pipe) become clogged, or be unable to keep up with the flow. Rather than allowing the water to cascade over the top of the dam, this built in "low spot" makes sure it goes where you want it to.
Two of our ponds utilize a spillway as the only means to discharge water. They've worked alright for 40 years, so whoever built the things must've had a pretty good idea of what's what. They are however, in the corners of the dams, not the middle.
