This whole emergency evacuation makes no sense to me. They cut the flow on the main spillway from 100,000 to 50,000 cubic feet per second to push water to the emergency spillway and it immediately started dangerously eroding at only 6,000 to 12,000 cfps. So they cranked the main spillway back up to 100,000 cfps and as I type this the water level has dropped back down well below the top and the erosion danger has passed. The damage on the main spillway looks severe but not at immediate emergency levels. But in the mean time they have unnecessarily caused panic among 180,000 people with no warning and generated national headlines. It just seems like a lot of overreaction.
> they have unnecessarily caused panic among 180,000 people
Hindsight is 20/20. If the dam had failed, people would be saying "they unnecessarily killed 180,000 people because they were too lazy and politically timid to order an evacuation."
It's like tuning sensitivity vs. specificity in medical tests. In this case the cost of a false negative (thousands of deaths) is a lot higher than the cost of a false positive (a few days of disruption). So the threshold is set low so as to minimize the total expected cost, summing probability of outcome * cost to society. Perhaps counterintuitively, the goal is not to minimize the odds of being wrong.
We should expect to see more false positives than false negatives. If it were any other way, the system would be working sub-optimally.
The damage to the main spillway is extreme. Look at the pictures, it's more of a semi controlled waterfall at this point. If erosion moves upstream on the spillway, which it eventually will, once it reaches the gates they will need to stop the spillway or risk the dam itself.
Their emergency spillway is already unusable, so this means just get everyone out and wait for the dam to blow once the main spillway erodes too far back
They didn't (and still don't) know the full extent of the damage to the concrete emergency spillway structure. If it failed, the areas evacuated would flood. It's still possible it might fail, they really won't know until morning.
The concrete spillway is the main spillway. The emergency spillway is a concrete lip draining onto an unpaved hillside. It was partly because of the damage to the main spillway that they allowed the lake level to rise to the level of the emergency spillway, but when it appeared to be eroding alarmingly quickly, they opened the main spillway 100% in spite of knowing the lower half was already severely damaged.
