Is your emergency spillway in virgin soil and have grass cover? If so, and if not too steep, it should handle occasional use. I have a 15" drain on my 1+ acre pond with over 20 acres of watershed, but I have at least 5' of freeboard on the dam, but less than 2' to the emergency spillway. One advantage for me is that the 15' main pipe drops a great deal so the pipe carries a lot of water once full. I lose fish during flooding, but that does not really worry me. Never seen any dead fish after flooding, and my emergent plants and water lilies seem to be able to keep the algae at bay except for some short periods of time.
