Tyler,
Essup's correct in stating what your first step should be contacting NRCS to see if they can do an engineering design for you. They can determine the watershed area, calculate the projected amounts of runoff for various amounts of precipitation, and design the structure so it would be able to handle that amount of water.
It is truely amazing to see how much water can flow through a watershed after a heavy rain. About a month ago we had 5 inches in 4 hours and all our NRCS designed dams functioned properly. All the dams except 1 were able to pass all the water through the principle spillway (tube). The other dam had water flowing through the secondary (earthen) spillway for a short period of time.
I'm assuming that your dam was constructed based on what "looked" good based on normal flows. One of the pics seems to show a fairly narrow emergency spillway. On our dams the principle spillway tube is designed to handle a 25 year storm by itself. Thus they range from 18 inches up to 4 ft in diameter. There are at least 2-4 ft of elevation from waterlevel to the emergency spillay elevation. This ensures that the tubes can flow completely full before the water reaches the emergency spillway. Then there is another 3-4 ft of elevation from the emergency spillway to the top of the dam. The emergency spillways are between 10-30 ft wide and have a slight slope down the backside to the existing creekbed. We get sod forming grass like brome grass growing on the emergency spillway. Then when water flows over it the grass lays over and protects the soil, much like a thatched roof.
If NRCS can't/won't help, try to get an engineering firm to look it over. It would be interesting to see what they say. Based on what you describe you might need several large principle spillway drop structure tubes (3-4 ft dia) with a couple feet of elevation up to the emergency spillway. Then a spillway 30-50 ft wide with another 4-5 ft up to the top of the dam. It is easier to overbuild a dam, rather than keep rebuilding the same one.
Bill
