After serious thinking Steve Walker wrote :
If the outlet were placed at the bottom of the dam, there would be a decent
head until the reservoir was virtually empty.

That works with a small pipe where the surface tension helps, but not
in larger pipes of this size.

Theoretical maximum head is 32 feet, practical is much less than that
even trying to pump - you are not so much pumping it up to a pump as
sucking it up to a pump and letting gravity deliver it down the
discharge pipe.

Even if the flow were not as great as with the pumps, more syphons could be
used or they could be just used to supplement the pumps and drain the
reservoir faster.

Both ends of the pipe would need to be under water, or the syphon
effect would be lost immediately as air entered the discharge pipe. The
syphon effect doesn't work quite so well in a large pipe unless both
ends are well submerged.