I'm no expert by any means, Mike, but have a couple of thoughts from the peanut gallery. First off, I echo the sentiments that have been expressed already - great looking project!
One question I have is whether or not all of that extra distance of pipe is really necessary. If you don't need it and can cut some of it down, the buoyancy issue will obviously be reduced. I'm sure you've thought of that.
Another thought is to drain the water low enough and then use some manner of anchor on the excess pipe. Perhaps weld some rebar to form an upsidedown "U" that goes over the piping and into the ground. You'd obviously need to anchor it in concrete or something equivalent so it would hold against the positive buoyancy.
Like I said, those may not be useful suggestions at all, but since I appear to be the first one to take a stab, I figured what the heck! Besides, if I don't throw my ideas out to be refuted, how will I ever learn how to deal with that situation if I ever find myself in it! Good luck with a viable solution.