EW,
Unless I am reading your intent wrong, I would maintain serious concern for providing suitable flow for a principal spillway. You have indicated that you may have many acres of run-off feeding your pond. This, combined with the acre-feet of containment at normal pool, are driving factors in providing correct out-flow. It appears that your project may approach 30 - 40 acre feet...just a guess. If this is combined with 50 - 100 ac of run-off, your principal spillway pipe diameter better be substantial....likely a pair of 12" dia or one 18" - 24" dia. This is where I may be mis-reading you. It appears that you plan on using a drain pipe at the base of the dam as a principal outlet. This being the case, you will need a similar cross section as noted above.
IMO, I still shy away from the idea of having no riser pipe on the water side (plus a trash rack and anti-vortex baffle). If my assumptions are correct about your water volumes and run-off, you need to consider the massive rain scenario and where that water is gonna go. The goal is to provide enough water passage without use of the emergency spillway, knowing full well that in an armageddon, the emergency can handle the short term burst without damaging erosion.
You have a GREAT project going. I would hate to see you make a decision without all the possible angles on the radar.
