When the evacuation was announced I sort of hastily pasted into a chat channel that the dam was going to fail.
The main dam of course did not fail, but looking at the recent photos, the overall system sure did fail. I hadn't realized the extent to which the spillway had continued to erode.
There's basically no chance of the main dam failing. A failure of the emergency spillway is basically the worst case scenario. For the main dam to fail, water would have to get over the top, and the emergency spillway ensures that the lake stays below that level.
A failure of the emergency spillway would still be quite destructive, of course.
The base of the emergency spillway is bedrock, so it can only erode down that far. That's 30ft below the top of that spillway, which is why everyone talks about potentially having a 30ft wall of water hit downstream communities.
I'm not a geologist, but... Didn't the main spillway failure erode some of what was considered "bedrock"? Isn't the bedrock somewhat more erodable than the term "bedrock" would lead you to expect?
Some…yeah, where some is the 1.7 million cubic yards of rubble now clogging up the Thermalito Diversion Pool. [1]
When you look at photos or drone videos of the erosion near the main spillway, it's clear that an immense amount of rock has been eroded away (the color changes from brown to gray). [2]
Juan Browne discusses the type and strength of the rock in one of his excellent videos about the Oroville crisis. [3] (Note that he demonstrates the weakness of the rock where he is standing, which is across the river from the spillway and might not be the same rock.)
I'm not either, and I don't know! Total speculation: water partway down the spillway is considerably more violent than water just spilling over the top, so while bedrock may not resist the former, it can resist the latter.
Another wild guess: the main spillway hasn't eroded any bedrock, just concrete, dirt, loose rocks, etc.
If anyone actually knows versus my wild guesses, I'd be most interested.
Don't know the details of the projects but from the pictures posted (here and elsewhere) it seems that only topsoil was eroded.
Yes, bedrock is not indestructible, it could get eroded from water, but it takes times. Plus it get eroded slowly over time. The main concerns was the use amount of water released by the failure of the spillway. For sure the water keep flowing over the collapse spillway would erode the bedrock lowering it, but at a slower pace...
One thing I'm really impressed with is how many redundancies are built in to dams, and how their failures are intended to be gradual and not catastrophic and sudden with a full release of the reservoir contents.
ie, use the water to generate electricity. If that's not enough outflow, use the spillway. If the spillway isn't enough, the emergency spillway will drain more.
There's even another contingency (with more details), described here, that I haven't heard anywhere else:
> The emergency spillway is not auxiliary, and i don’t believe any competent geologist present during it’s construction would have considered its use as anything but a last ditch effort. It was designed as a fail-safe to prevent the loss of the main dam during the event of a half a million cfs inflow to the lake during a 200 to 1000 year event. The ogee weir extending from the main spillway gates was likely built to protect the main spillway gates. The bedrock was excavated deeper below the ogee weir to find more competent bedrock than the concrete wall extending northwards from the ogee weir, which was built to protect the ogee weir. The lack of concrete in the far NW corner of the parking lot where we saw considerable erosion and helicopters lowering bags of rocks into is not lack of foresight, but designed weakness built into a weak structure. The parking lot is built on highly weathered bedrock and is designed to function as a sacrificial plug located as far from the dam itself as possible, similar to the Auburn coffer dam failure of 1986. As headcutting progresses into the parking lot, water at elevation 900 and above is skimmed off. Once headcutting reaches the lake, then downcutting commences, “safely” lowering the lake till competent bedrock is found, maybe a hundred feet down, leaving the vast majority of the lake still in the lake and Oroville dam still standing, no matter the magnitude of the storm.
The normal water exit from the dam is through the power plant. If either spillway is moving too much water, the level of the pool below rises, and the power plant turbines can't run because there's backpressure at their exit. That happened this time, which increased the load on the spillway.
Right now, the lake level has been dropped to 50 feet below the dam top, to provide some safety margin for later storms. The crisis is probably over, and there will be heavy construction this summer.
The main dam of course did not fail, but looking at the recent photos, the overall system sure did fail. I hadn't realized the extent to which the spillway had continued to erode.