If you excavate your pond to an elevation deeper than the fall behind your dam, then you will not be able to "drain" your pond below that level.
However, you could use a trash pump in that situation. Therefore, you might design your system to partially pull down your water (as your elevations allow) and also use materials that allow you to attach a pump as needed if you desire to completely drain your pond.
Another option for rare draining with no nearby low spot - is to use 4" or 6" corrugated plastic drain tile pipe. This is by far the cheapest type of pipe in those dimensions and can be used to set up a temporary siphon that reaches several hundred feet downslope of the pond to reach a suitably low elevation.
