What isn't discussed much in the press is how close they came to the largest dam disaster in US history.
As you can see from the pictures, the "bedrock" under the emergency spillway erodes quickly, even with a modest amount of water flowing over it. How would it have handled a flow 100x larger, if the spillway structure had collapsed?
It could have easily eroded down 100 or 200 feet under the intense flow. That would have discharged 50% of the total reservoir volume, destroying tens of billions in real estate and possibly killing thousands.
Fortunately, the dam inflow slowed enough, combined with the main spillway outflow, to stop flow over the emergency spillway before it collapsed. So lucky.
I think this was intentional. A friend of mine who was a geologist working for the Park Service before he got the coding bug and I hired him in the dot com days told be while this was going on, "You know they are lying right?"
I asked about that and he pointed out that the dam was failing, and we were watching it fail. He pointed out the spillway shooting sideways and the amount of progress it had made in just the couple of days between the emergency spillway issue and then. He drew a line on the screen showing how the water cuts away the rocks and that the material above falls in, and becomes like sanding bits in the water flow to cut away still more rock. He estimated that two more weeks of rain at the levels we had experienced would result in total failure.
None of which the press was reporting. And we talked about why that might be. And other than the loss of life (which could be minimized by an evacuation order) there wasn't a whole lot they could do. They certainly couldn't repair the dam while water was flowing out of it. But at the same time they would have a several days at least of knowing it was going to fail before it actually did because of the amount of material that would need to be removed.
He suggested that it was in everyone's best interest to keep the panic and stress levels low until they needed to be higher. And I can't say I disagreed although felt it would be really annoying to have my stuff washed away when folks knew it was a possibility and didn't even offer me the chance to move it first.
So cross your fingers for a sunny March and a slow melt of the Sierra snow pack.
> it would be really annoying to have my stuff washed away when folks knew it was a possibility and didn't even offer me the chance to move it first.
They were more concerned about getting the evacuees out of the way before they were worried about getting the evacuees' stuff out of the way. Houses, furniture, and belongings can be replaced after a disaster. A bunch of people twiddling their thumbs in anxiety over a potential failure while not ordering evacuations to minimize or prevent loss of life is potentially criminal.
Also, people tend to panic. Telling them that there is a probable problem, and ordering a mandatory evacuation long before danger is imminent so that, if the worst case scenario occurs, the evacuees will live gives people hope that officials are just being overly cautious. Telling people that they have 5 hours to evacuate before it is guaranteed that all of their belongings, and their lives, will be lost forever will cause panic and potentially loss of life in the chaos alone.
I agree with you. The interesting question in the discussion was how you weigh panic, economic cost, and human cost. I hope that we'll get to see the decision tree they used at some point.
> But at the same time they would have a several days at least of knowing it was going to fail before it actually did because of the amount of material that would need to be removed.
That's the part where they risk being wrong and end up getting a thousand people killed. The notion of cutting that so close, such that just several days is the buffer between killing a thousand or more people and not, is a very obnoxious premise if that's what they were actually doing.
Yes I think they were utterly bonkers to assume that a failure of the emergency spillway would be limited to "only" the top 30 feet of water. It seems likely that the torrent would have quickly cut down deeper than just the bottom of the wall.
Looks like the side of the spillway or roadway was damaged, since the concrete appears to be broken, although the spillway wall itself might be ok below that vantage point. It's just really hard to tell from that angle, but all of the official drone footage/photos from DWR don't include that area of the spillway, which is curious.
Just moved here. I just realized after looking at a map, it's clear a dam failure would have had little to no impact on the sea level closest to San Jose.
As you can see from the pictures, the "bedrock" under the emergency spillway erodes quickly, even with a modest amount of water flowing over it. How would it have handled a flow 100x larger, if the spillway structure had collapsed?
It could have easily eroded down 100 or 200 feet under the intense flow. That would have discharged 50% of the total reservoir volume, destroying tens of billions in real estate and possibly killing thousands.
Fortunately, the dam inflow slowed enough, combined with the main spillway outflow, to stop flow over the emergency spillway before it collapsed. So lucky.