Bronco,
I'm not sure about siphon systems, as we have either used straight pipes with hooded inlets, or drop structures going into straight pipes for all our dams. However our pipe inlets are obviously set right at the water level. Then the emergency spillway is 1-2 ft above the top of the pipe inlet (most of our pipes are 18+ inches in diameter). Then we have 2.5-3 ft of elevation between the emergency spillway to the top of the dam. Thus the top of the dam is always at least 5 ft above the normal water level.
These are designed to handle at least 25 year storms completely through the principal spillway (the trickle tubes). In our area that's a 3-4 inch rain in a 24 h period. Last year we had 2 events of 7+ inches overnight, and the water came over the emergency spillway about 1 ft deep, but was not close to over-topping the dam.
I'd always recommend over-building a dam in this way. It is a few more bucks, but well worth the peace of mind.
In your part of the country with more significant rainfall (including hurricane storms) I would think you would want much more than 2.5 ft of freeboard.
Bill
