Prayers for members Harvey and Al Weidman., The Oroville Dam E-Spillway is failing. Options

[ConeDodger]

The dam itself is apparently holding, but the Emergency Spillway which is being used for the first time in the 50 year history of the dam is eroding away quickly and expected to fail. Oroville is below the dam.

Residents have been told to evacuate. (IMG:style_emoticons/default/sad.gif)

[Rand]

The dam is absolutely at risk if the erosion gets bad enough! That's the whole reason for the concern. Thankfully, it looks like that's not going to be an issue now that they've been able to get the water level down and stop the erosion. BUT, there's more rain in the forecast, so we can't assume everything is just going to be ok.

[GregAmy]

The dam is absolutely at risk if the erosion gets bad enough!

No, that's entirely wrong. The main 770' dam is not at risk in any way. In fact, if the erosion below the weir were to have continued up and completely taken it out , that would have, in fact, REDUCED the risk to the dam (that's what spillways are for). And even if the almost-entirely-impossible were to happen and both the weir and the primary spillway were completely taken out, there's no reasonable possibility that the erosion damage would migrate around that corner to the point where flows would erode the earthen dam. The most likely scenario in that case would be those spillway structures would collapse and take out that entire hillside below them down to bedrock all the way to the river (they were not even close to that scenario).

Now, if you're using the word "dam" to refer to the entire SYSTEM, then yes, each of those components is part of that "dam system". But, I'm using the word "dam" to refer to the 770' high primary structure that blocks off the main channel of the Feather River. And that structure was not, and is not, in any danger of foreseeable compromise in any way.

That's a fact, and not an alternative one. Yes, I am an engineer.

I think we'll find that this whole thing was a result of lack of imagination. When they designed this system in the 60s, they pretty much over-designed the primary spillway's capability to handle the water flows into that valley. As such they did not put as much emphasis on designing a stronger weir (which was intended only to take care of flows that the primary spillway could not.)

What I think they failed to imagine was a compromise to the primary spillway. "But for" that compromise, this would have been a non-event (combined flows have been less than what that system handled easily in the 1997 floods). That thought is supported by the last-minute scrambling to clear the hillside of potentially-clogging vegetation, grouting and rip rap below the toe of the weir and along the roadside, and surprise at the levels of erosion below the weir.

Mother Nature has little empathy for the hubris oversight of Man.