Hard to see how that slab stays in place under the two months or more of constant pounding its likely to see this spring.
When/if it goes the slabs to its side are likely to go shortly thereafter.
If it goes, let’s hope the next slab up from there stays put. It’s easy to see a domino effect if the slabs in general are not anchored well enough for that slope and volume of water.
Wildcard: If the rock underneath the slabs has been exposed to “weathering” over the last few decades it may no longer be as strong as it was when the spillway was first built. Based on the great pictures you have so kindly provided, portions of that rock look pretty fractured.
Another wildcard: Significant new erosion is certain if that slab goes. Lots more debris in the channel. At some point the generators would have to be shut down again, leading to the need for even higher flow through the spillway. A bit like an unstable feedback loop I heard about recently...
Someone suggested the possibility of just starting over on a new spillway next to this one which would be used until the new one is finished. At some point, maybe the cost and possible further risk of properly fixing the damage/design/construction errors on this spillway may justify spending resources on a new one altogether.
Ha! "..an unstable feedback loop.."
Heard from a Boeing electrical engineer (worked on designing amplifiers in Radar systems): "You design an amplifier you get an oscillator. You design an oscillator you get an amplifier."