E.M.Smith says:
Geoff:
Please stop embarrassing yourself.
First off, the spillway is on the other side of a small mountain from the Dam.
Yes, the dam shoulder rests on one side of that mountain (or really really big hill) but the spillway is on the other side of the peak from the dam. They are two unrelated structures with bedrock between them.
Second, as the picture posted here shows, the dam is incredibly thick at the base and that base includes a concrete footer bonded to bedrock. So don’t expect to get water “cutting under the dam” ever.
BTW, I watched it all be built and I can assure you that the picture is how it was built. Also BTW, that “clay core” that looks so skinny in the picture was wide enough for road graders, compactors, and very large dump trucks to work on it and pass each other. (Though it gets skinny enough at the very top to “only” be about two large dump trucks wide…)
There is NO leakage from the spillway anywhere NEAR the dam
Next up, the spillway is built on top of a natural ridge made of rock. It isn’t “dam like” other than having water on one side and a river on the other. The surface rock does weather, so in a major overtopping it will erode, but under that it is a very solid bedrock for 1500 feet. I’d guess we lose at most 100 feet off the peak of that natural ridge in the case of an overtopping.
@A.P.:
Nope. Seepage under the dam, if any would have to take a hard left turn, go about 1/4 mile through bedrock, and then make a hard right turn, to end up under the spillway. Not going to happen.
What might have happened is 50 years of slow erosion of the landfill under the center part of the spillway chute from precipitation onto the rock fill next to it (backfill after the pour). and potentially also significant water intrusion from a poorly maintained joint in the bed of the spillway allowing water to erode under it. There was a known issue in that location, repaired, and likely not sealed enough.
@R. Shearer:
Nope. The spillway is NOT on the dam. It is separate from the dam and built on a natural rock ridge that is slightly lower than the dam top. This is by design.
It is IMPOSSIBLE to overtop the dam and it is IMPOSSIBLE for any class of spillway failure to damage the dam. (It can flood the powerhouse at the base).
It is possible for the natural ridge to be over topped and erode down to bedrock. The original design docs state that “some damage is expected” in the case of the emergency spillway being used. That means the original Engineers looked at it and figure out it WILL erode (the natural ridge) but not so much as to be catastrophic. (Where that means flooding in Oroville and some of the central valley, but not out of line with historic floods).
Now there is a question of what 50 years of weathering have done to that particular rock type. It is hard and sturdy when deep, but at the surface oxidizes and becomes more friable. After 50 years of being exposed (from the bulldozing / raking / rock breaking process of the spillway build) just how deep is the weak oxidized zone? I would guess about 50 feet to 100 feet max. That leaves about 800 feet of solid sturdy bedrock (as you can see at the break in the concrete spillway…)..
@All:
Please folks, get it clear in your minds that the spillways (regular and emergency) are not on the dam and are separated from it by a natural mountain and built on a natural rock ridge.
Problems on the spillways are NOT problems with the dam. Different structures. Different sides of a 1000 foot mountain.
Storm: 10 trillion gallons over next 7 days for CA #LakeOroville watershed
Ping to article and comments, see graphics at # 9 .
Thanks, Ernest.
Saw that.
Just mazing....
They must think there could be a problem, because they have called up some of the National Guard and have the rest on emergency standby. The fact that they have done that plus created a no fly zone, tells me they are very worried.