I am no expert, but a carefully designed and large emergency spillway may take care of some of your concerns (at an elevation based on the size of your watershed and the height of the raod). Another thing to consider is the size and drop on your tube through the dam. More drop equals more capacity once the tube fills and a siphon starts (gravity pulling on the water through the pipe). Plenty of free board on the dam is also a great benefit. My 1.25 acre pond filled in 12 hours just 2 weeks after being built (in a deluge), and all of these features prevented any damage. However, there is no substitute for having a good contractor. I also had all of my excess soil placed behind the dam and sprung for a long pipe to maximize drop. The extra money was well worth it for me.