>>>The main spill way is not failing regressively.<<<
The main spillway is failing regressively. The extreme turbulence of the failed mid section of the spillway has been moving upward in breaking off pieces of the concrete slabs. This is from the undercutting of the substructure. The worst effects are from “cavitation erosion”. The secondary undercut erosion is from the tremendous hydraulic turbulence. These are know civil design factors to proper spillway construction. They use large block “spillway aeration devices” to disperse the flow (just as this design has at the bottom of the spillway - large angled concrete pillars/blocks).
This spillway was not constructed on solid bedrock. It was constructed on “weathered” rock. This type of rock is very fractured but may remain stable if no powerful erosive forces are applied.
To prevent any sub erosion of the cement slabs of the spillway, a capture drainage system will redirect small leakages from any faults in the expansion joints et al. This is because the design must be protected from any erosion under the slabs.
There are a numerous images that have shown the progression of this erosion on the main spillway. How quickly it progresses is what the engineers are carefully watching. They don’t have much choice right now. Dump now, fix later & hope nothing progresses too quickly.
While there has been some regressive action, that action has largely been stable. that is evident from views of the spillway over the past few days. I am very well aware of the effects of ‘cavatation’
yes, the spillway wasn’t built to 2017 standards of engineering. yet your assumption of a lack of solid rock is not accurately founded. the failure site is a zone of highly weathered rock immediately stream of competent bedrock - day 1 photos make that apparent. Believe me, I’m a P.G. living downstream of the dam - I’ve been involved very closely at examining the geology and spillway construction as is
Again, I’ve been looking at the same images and the retrograde action is not as severe as you are insinuating.