It's unnecessary. The current emergency spillway is already on the other side of a hill made out of bedrock from the dam itself.

All of the concern about the 'failure' is a matter of loss of control of water release, not of breach of the main dam structure.

I thought the main danger, and reason for evacuation, was a potential uncontrolled failure, due to headward erosion, of the emergency spillway dam structure. (Thats the structure between the regular spillway and that parking lot)

https://twitter.com/TaberM_PE/status/831149440262164480

Yes -- which would only result in the loss of control of the amount of water above the level of the bottom of the spillway, or the bedrock under the spillway. That's maybe the top 30 feet at the most.

While that's still a lot of acre feet, and it would create a 'wall of water' situation in the first surge (hence the evacuation), it's not as significant as the dam failing and the entire reservoir emptying through it.

I'm sure this is a stupid question, but would it be possible to build essentially a large "siphon", so that the discharge of the water didn't result in erosion directly below the spillway?

Seems like it would be pissing in the ocean (or I guess that analogy in reverse?), but when I saw the newsreels of the helicopters dropping bags of rocks into the spillway hole, that also seemed like it would take a couple thousand of those drops to make a difference.

They're letting 100,000 Cubic feet per second flow over the spillway. Niagra falls has an average flow rate of 85,000 feet per second.

I don't think you're going to be able to make a siphon quickly that is large and robust enough to handle a significant fraction of that.

You're basically talking about building another emergency spillway. We could build one, sure, but why would it be any better than the existing one? It would still have the same risks of e.g. rapid erosion (making a concrete one isn't going to be any quicker than repairing the main spillway, I would think), and whatever was at the bottom of it would probably be less prepared to receive a surge than the river is.