I kind of have the same scenario with my dam build. My dam will serve as road also across a deep ravine and I have zero place for an emergency spill way. My dam is going to be 25ft tall. I'm going to set my main overflow pipe to drain at 22ft. And set the second over flow pipe at 23 maybe 24ft. In case the first pipe can't handle the water volume in a big storm or the main gets clogged the second will drain.