Erosion has created a 300-foot-deep hole in the concrete spillway of Oroville Dam and state officials say it will continue grow.
State engineers on Wednesday cautiously released water from Lake Oroville's damaged spillway as the reservoir level climbed amid a soaking of rain.
Situated in the western foothills of the Sierra, Lake Oroville is the second-largest manmade reservoir in California after Shasta....
According to the article posted by mintjulip above, the spillway is not paved and has not been used since 1968. It is an overflow spillway away from the dam proper. Hopefully the abutment of the dam whereby his spillway is located is solid rock.
After watching a long movie of a helli cruising the spillway I see now that the water really does run horizontally for a couple of hundred yards before sloping down. So indeed it doesn't look like the dam's left natural structure is ever going to be in jeopardy. They can let it eat the entire mountain from the Hole down with out affecting the structure. And that's likely exactly what they're going to do rather than let the emergency spillway, which is merely a concrete lip go into operation.
You can see the water has pretty much stopped being muddy and only bedrock is remaining.
As an observation on the photo above, there's a lot of water seeping through the drains into the tailrace from the surrounding land. Ground conditions must be very wet there.
As of Monday (02/06/2017) the year-to-date rainfall totals for Los Angeles (which is still technically considered to be suffering drought conditions) was nearly 15 1/2 inches, already more than the normal yearly total, and it has been raining most of yesterday and more is predicted today as well as another storm possible by the end of next week. I don't think we'll break the yearly rainfall record for SoCal, which is close to 37 inches, but whatever we end-up with, it should take the edge off out local drought conditions. I realize that Northern California may break some records, and that's great because much of our municipal water comes from there via the state aqueduct system. This should also help the farmers in the Central Valley.
Perhaps one of our civil engineering hydraulics experts can comment on the drainage systems there. The side walls have some form of drainage, with the many outlets for that drain system spewing water. Could they be connected some how to a drain system under the slab? No way could one expect that slab system not to leak, so a drainage system under it would be mandatory or downstream the uplift would cause the slab to fail.. But where would it discharge? To to those side walls? Looking at the close up photo it appears that the slab is full of cracks, no reinforcing, and may wall have started an undermining process well up hill from the right side(in photo). I'd sure like to see a cross section of the slab details.
Today, Saturday 11 February, the primary (floodgate-controlled, 160 foot wide, concrete-lined) spillway that failed (see photo's above) has washed out another few hundred feet sideways and down the hillside.
Lake level is now up even higher at 900 foot (+) and has begun washing over the emergency spillway this morning.
The primary spillway is separated from the earth-fill dam by a raised hillside and some bedrock, but the erosion from the primary spillway concrete failure is washing into that hillside - Don't know how many hundred feet are left of "original" rock and hillside between the spillway and the earth dam itself.
The emergency spillway is of course not regulated at all, so it will continue flowing as long as lake level is above emergency spillway height.
The emergency spillway is beside the primary spillway, but looks like they are separated by a few hundred feet of of (what used to be) hillside and trees. SO water over the emergency spillway could rejoin the washout from theprimary spillway.
Don't know if they (California Water Resources Board ?) are going to close or throttle the primary spillway once the emergency spillway starts flooding. Seems like they would not want to throttle flow in the primary spillway, but, the primary is waashing hillside away.
So what is better?
Image shows the emergency spillway (with concrete lip now being overwashed and flowing over the dam road below), the controlled spillway entrance channel, then the raised hillside between the normal spillway and the dam itself, then the earthen dam,
Overspilling at 12,000cf/s now. I hope that little lip at the bottom is up to the task of stopping the
initial K.E. of the vertical water.
I was given a half-day tour of the Oroville power plant when I was in my 20s. Of the many technical
tours I've been lucky enough to have that was one of the most memorable. Fascinating. The whole plant
is underground.
BTW, I still remember seeing the construction of Oroville Dam (which ran from 1961-1968) when I was a kid. My grandparents lived in nearby Chico and had a cabin at Lake Madrone in the forested hills above Lake Oroville. According to my dad, my first trip to the cabin (and thus past the construction site) was in the summer of 1963. just before I turned 5. My family, coming up from Fresno, made 2 to 3 trips to the cabin each year until my grandparents sold it in about 1974. On every trip past the dam while it was under construction, we would stop for a while at a viewpoint and my dad would explain what was going on. I didn't understand much, but I was fascinated. My younger brother had no interest in it.
Fred
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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill
And, yes it looks like one frozen image frame as the water looks entirely static but if you
full-screen it you will see moving cars and the foreground tree dancing around.
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