Gumboot,
WOW! 24,700 cu yds. of fill.
This seems like a lot of earth fill for a 6 acre pond. How tall is this proposed structure? Are you sure you need a 36" tube for 120 acres of pasture drainage? You may want to contact the local NRCS folks and see if you can get a preliminary design.
3:1 front and back slopes will probably more than sufice. (Again check with your State Regs.)
I've been involved in several dam building projects (my own included) and 12,000 cu. yds. for a farm pond would be considered huge!
Granted my experience is west, and we don't have near the runoff and/or storm events you are likely to have, but I think I would seek another opinion.
good luck,
ED
