The proper approach with uncertainty is an expected value calculation. That way the uncertainty regarding rain within the watershed, the temperature, etc. can all be factored in.
What is missing in this nice analysis is the economic impact. If 200k were evacuated, and roughly estimating they have $100k in real estate each for housing, and say $50k each for business real estate in the flood risk area, that comes to $30B. Assuming some will not be a complete loss, say $15B in real estate loss if the emergency spillway fails (neglecting job and economic activity impacts, etc).
Now if you have a 5% chance of failure in this next storm, the expected value of the loss is $750M.
This value now can be used to decide how much effort should be put into emergency repairs, something less than $750M.
Say these numbers are in the ballpark. Is the emergency repair effort taking place sensible? I suspect not. To avoid a loss in the billions, you'd expect a bigger effort. I'd expect every shotcrete contractor in the western US on hand, rebar being placed by helicopter, steel beams, 50 pieces of heavy equipment, temporary tanks of diesel fuel, and a real sense of emergency. Not seeing it.
Seems like they are dropping super sacks of rock. They might be dropping super sacks of dry mix, but I highly doubt it - the money just wouldn't be there. If they're just looking to armor then the rock would more sense.
Shotcrete and concrete are difficult to place via helicopter. Both need to be pumped (or hand placed) to get a good product, which can't be done via helicopter.
Couldn't you put some kind of plastic membrane over the top of it once it's cured enough to not deform? The reason it needs so long to cure is to make sure the surface is hard enough to withstand erosion from turbulent water flow. If it's not directly exposed to the flow, you should be able to use it much sooner.
What is missing in this nice analysis is the economic impact. If 200k were evacuated, and roughly estimating they have $100k in real estate each for housing, and say $50k each for business real estate in the flood risk area, that comes to $30B. Assuming some will not be a complete loss, say $15B in real estate loss if the emergency spillway fails (neglecting job and economic activity impacts, etc).
Now if you have a 5% chance of failure in this next storm, the expected value of the loss is $750M.
This value now can be used to decide how much effort should be put into emergency repairs, something less than $750M.
Say these numbers are in the ballpark. Is the emergency repair effort taking place sensible? I suspect not. To avoid a loss in the billions, you'd expect a bigger effort. I'd expect every shotcrete contractor in the western US on hand, rebar being placed by helicopter, steel beams, 50 pieces of heavy equipment, temporary tanks of diesel fuel, and a real sense of emergency. Not seeing it.