I have a similar project to do for a customer, its about a 4 acre pond, that has no overflow pipe, they have been letting it overflow off to the side of the dam, goes across to the next valley, not around behind the dam, according to the size of trees on this dam the pond has to be thirty plus yrs old.
Its still about 40 feet from the actual high water mark of the pond then is dropping straight down in a 12 ft deep by probably 15 wide gully, obviously undercutting back toward the pond a few ft every yr, I plan on regrading the gully into a slope down to the lower elevation then lining that slope with a riprap channel, using a fabric underlayment under the riprap, to let the overflow water down to the lower elevation without erosion, just about exactly like Tbar's picture with concrete but an a much bigger scale and using riprap. just waiting till it is dry enough to get trucks back semi close to it to deliver the rock, about a 1/4 of a mile thru a brushy field.
Am not as big a fan of the concrete version of that because I have seen, and had to fix way too many that didnt work, unless it is done perfectly, like Tbar's appears to be, you can get a small pencil thin stream to trickle under it and that pencil thin stream will undercut the whole slab in very little time, and concrete will do what it does and bridge that undercut so that you dont even know its doing it until a lot of damage has been done, rip rap will keep settling to the deepest hole and will not allow under-cutting. its definitely not as aesthetically pleasing as concrete but this is in the back of his hunting farm so he doesnt mow or brush hog anything anyways. Good Luck, just my 2 cents
