EW,
This is the first time that I have seen a stand-pipe BEHIND the dam as a principal overflow...probably reveals my relative rookie status at this. My principal spillway design will use a pair of 12" diam smooth bore corr. plastic drain pipes set at normal pool, horiz thru the dam and then angled downward (still within the dam) to discharge at point even with the grade behind the dam. The actual drain will be exactly as you depict in your dwg, minus any standpipes or the like...it will continue on a horiz (minimal slope) to a similar area just short of the principal discharge. I am going to leave the end buried at a point that is still below the frost line and mark it with a stake. I hope I never have to use, but if I do...a little digging and let 'er rip.
I would be concerned for a principal spillway that is deep underwater...what happens if it gets clogged? This is the reason I am using 2 principals...to increase the odds if one gets clogged. And, even then, it ain't much to clear 'em.
