to clarify: it's not the dam itself that's at risk of failure, but the emergency spillway. while this scenario would be catastrophic, it's far less catastrophic than the entire dam failing

Think about that statement, what does is mean to you that the "emergency spillway failed" ? The ES is the portion of the dam that is slightly lower than the overall height of the dam so that in the event the main spillway was unable to release water fast enough, the water would start spilling here rather than across the dam. If it "fails" then a portion of the dam ceases to exist. That releases water through that hole uncontrollably. At which point losing containment is generally a matter of when not if.

If it fails, it will be a double disaster, many people will lose property, and California will lose some or all of the water in its largest reservoir. The challenge is going to be keeping ahead of it through to the next storm.

The emergency spillway is around 30 feet high, the dam itself is 770 feet high. The amount of water released from a failure of the emergency spillway will be enormous, but still much smaller than the amount of water released by a complete dam failure.

To clarify, the emergency spillway is 30 ft below the top of the dam, not 30 ft high

http://www.sacbee.com/news/local/article132356269.html provides an excellent description of the challenge. The hillside where the emergency spillway is would be at risk.

Excellent link, thanks. I found the following comment sobering:

"Countryman: It’s not going to be the (main) embankment failure, but it’s a failure. If it does happen, there’s nothing saying that the ground is going to stay where it is. That force of water will start tearing that hill apart​,​ and it could eat back into the reservoir and drain the reservoir."

> At which point losing containment is generally a matter of when not if.

No, it's not. The local geology is fairly solid rock. The overflowing water will wear down the overlying soil and loose Earth, but should be sustained by the rock formation. It is a risk, certainly, but such a site is chosen because of the suitable foundation.

The issue right now is to see how the spillway fares over the night. If it handles the increased flow, then it should be able to continue to relieve the situation.

To be fair, I think it is a little ethically troubling to refer to the emergency spillway as anything other than that (e.g. auxiliary spillway), but it is designed to function in an emergency, not just fail catastrophically.

I'd love to get a source on this info. I've been looking for info on the geology of the exact area and the dam construction to assess this exact point.

Where did you find the info on the area being solid rock?

That would go a long way to giving an idea as to how bad any increased failure might be.

> California will lose some or all of the water in its largest reservoir

Lake Oroville is actually the second largest reservoir and one of six reservoirs with a storage capacity over 1M acre feet of water: http://cdec.water.ca.gov/cgi-progs/reservoirs/RES

The largest is Lake Shasta: http://cdec.water.ca.gov/cgi-progs/stationInfo?station_id=SH... with a storage capacity of 4.3M acre feet of water, approximately 800,000 more acre feet than Lake Oroville.

So far, the only failure foreseen is to the auxiliary spillway which would definitely lead to an uncontrolled release of water from the lake, but is a separate structure from the main dam.

There's definitely worries about the erosion on the main spillway moving back upstream towards the dam, but that is something folks are going to have to look at and consider over the remaining duration of the water season, unlikely to be a danger tonight.

No, the emergency spillway is part of the dam. Failure of the spillway could lead to failure of the entire dam. It's not likely, but it's possible. Once large quantities of water start going where they weren't designed to go, all bets are off.

You should look at the dam from maps or satellite. The spillway is off to the side, away from the dam (clearly, by design), and the emergency spillway is on the far side of the main spillway from the dam, so while the spillway failing would be terrible it should not damage the dam itself.

Looks like you're right. I read somewhere that the spillway was part of the dam structure, but now I can't find that reference any more, so I guess I must have misread it.

However, my original comment was still substantially correct. See http://www.sacbee.com/news/local/article132356269.html

"It’s not going to be the (main) embankment failure, but it’s a failure. If it does happen, there’s nothing saying that the ground is going to stay where it is. That force of water will start tearing that hill apart​,​ and it could eat back into the reservoir and drain the reservoir.

It certainly could, but they are literally separate structures, as repeated stated by the actual engineers who maintain the dam.

The hillside underneath the overflow spillway is literally washing away. You can see downhill portions of the spillway where the waters ate away 10, 20 feet of vertical elevation. If that erosion backs all the way up to the reservoir itself, then the lining could fail (much as the main spillway did). At that point, it doesn't matter if the dam is still standing, if there is a gash right next to it lowering the effective maximum water level by several feet.

I don't know enough about the geology of the area to know how likely it is to happen. But history does show that dams can go very quickly once erosion reaches the dam/reservoir, so alarm isn't exactly unwarranted.

Why is this being down voted? The geology of the area is such that the emergency spill off can't be eroded down to the valley floor. That's why the location was selected.