Keep in mind that it is a good idea to draw down at 12" / day or less; this is to allow the water content in the soil, and in the dam in particular, to decrease. A faster drain could result in sloughin of the dam face or pond sides, especially if they have a steep slope. BTW, I drained my 3/4 acre pond quite easily with 3" thinwall pvc pipe. I made a flapper-type check valve for the pond side, and used a cap on the discarge end. If the water is warm a cap on both ends would work just as well.
