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The good news is that the main spillway seems to be holding up as they send 100,000 cubic feet per second (a little more than an Olympic swimming pool every second) down the damaged concrete chute. This is indeed lowering the lake level about one foot every three hours, as we crowdsource calculated yesterday.
From the L.A. Times:
Officials say they’re still releasing 100,000 cubic feet per second from the paved spillway. No water is going over the emergency spillway at this point.
“It’s hard to look at a crystal ball and predict how it’s going to evolve,” said Kevin Lawson of Cal Fire.
The flow into the lake is roughly 37,000 cubic feet per second, so they’re shedding a net 60,000 or so cubic feet per second.
They’re hoping to drop 8 feet per day.
It’s unclear if they’ll hit the target of lowering the lake by 50 feet before the next rain hits. But they’re expecting a smaller level of precipitation at a cooler temperature, so it may not run into the lake as quickly, giving them more time.
“We’re going to deal with that as it comes in,” said acting state Department of Water Resources Director Bill Croyle.
You can follow the dam data here:
The lake level is down 7 feet from yesterday afternoon when water was flowing over the brim of the so-called emergency spillway.
Here’s the LA Times’ diagram of what’s dangerous about the so-called “emergency spillway:”
That’s really just a concrete wall set on a mountain ridge. Spilling water directly down the wall raised fears that the dirt and rock under the wall was eroding and the whole wall might give way, allowing the top 30(?) feet of the reservoir to head toward the San Francisco Bay. But now the lake is maybe a half dozen feet lower than the brim.
The next set of rainstorms might not arrive until Thursday, whereas yesterday the forecast was for rain to begin again on Wednesday.
The last set of storms raised the lake level about 52 feet.
As the Duke of Wellington said about the Battle of Waterloo, it’s going to be a damned close-run thing.
Evacuees haven’t been told yet when they could go home. It could be a couple of weeks. Or maybe they’ll be let home and told they have to leave again by Wednesday. I don’t know.
And, of course, there could be well be more crises as future winter and spring storms roll through and the high altitude snow pack melts.
A reader asks: “What’s the worst-case scenario?”
First Case Scenario: due to water erosion, the concrete brim of the Emergency Spillway gives way in a rush and the top 30 feet of the 15,000 acre reservoir, 450,000 acre-feet, pours into the Feather River pretty much at once.
Worst Case Scenario: the hole in the concrete Primary Spillway has somehow or other been undermining the 770 foot tall dam next door in some unforeseen fashion, so the dam collapses and 3.5 million acre feet of water head down the river all at once.
They were worrying about the First Case Scenario yesterday, which is why they evacuated over 100,000 people. But it didn’t happen (yet).
The Worst Case Scenario is not supposed to happen.
Update: Here’s a picture of the eroding canyon in the so-called “emergency spillway” that threatened to undermine the concrete wall that is the brim of the reservoir. The water is on the left in the picture. Six workers are visible in yellow, which shows how huge the erosion ravine is.
It looks to me like the big danger is water spilling directly over the concrete lip onto the mountainside that it rests on and eroding the foundations. Why not quickly build some big chutes to carry water a few hundred feet down the mountainside so it doesn’t start eroding until it’s well below the foundations of the lip?
A cubic foot of water weighs something like 64 pounds, so weight would be an issue. So a ramp 100 feet long and 100 feet wide would have to carry 6.4 million pounds of water when the lake is overflowing the lip by 12 inches. That’s a lot. But it also doesn’t seem impossible.
Keep in mind that the next rain storm likely won’t be the last to hit the reservoir before the dry season starts in, roughly, May. Maybe it’s impossible to get anything in place before the rains come again later this week, but what about later this month or in March?