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Comment by marze | Hacker News Reader

Very misleading regarding Oroville.

It wouldn't have been a "30 ft. wall of water", it would have been half the reservoir volume as the failed emergency spillway "bedrock" quickly eroded.

And the emergency spillway failure mode wasn't because of age, it was a design flaw from the day the dam was built.

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The bedrock that the actual dam is built on is "hard amphibole"

https://kb.osu.edu/dspace/bitstream/handle/1811/58502/SES-Se...

This material will erode, but not so quickly that the main body of the lake wouldn't be shut off.

The "30 foot" figure is the depth of the water above this bedrock. Had the emergency spillway failed, this is what would have been released without any controls.

The main spillway was built on alluvium. I've posted elsewhere in the comment thread that cavitation was the primary cause of the concrete erosion.

The emergency spillway operated for a day at 5% of design flow, and that minor flow caused enough erosion to threaten to undermine and collapse the spillway structure.

Higher flows, like 100x higher, would have cut a deep channel where the emergency spillway was.

TFA's picture of the failed spillway is hilarious. That concrete can't be more than 4 inches thick! That would work for a driveway, barely. It seems really out of place on a giant dam. As soon as there was a crack, the underlying gravel and earth would start eroding, and a complete failure like that pictured was inevitable.

The spillway is 15" thick.

http://www.mercurynews.com/2017/02/17/oroville-dam-what-made...

The article also explains that cavitation caused the concrete to fail.

"The spillway is 15" thick."

That is surprisingly thin. Plenty of concrete surface that you walk (not drive) on every single day is 6 or even 8 inches thick.

I'm sure it's within spec and built to fulfill their design, etc., but ... I would hardly call it "burly".

6-8 inches is terribly wasteful and a bit of a nightmare for anyone following who wants to fix a pipe. Obviously a thick path is needed sometimes, but 8 inches will take a 10 ton truck just fine.

So how thick does it need to be?

Cavitation at Glen Canyon Dam chewed through 3 feet of reinforced concrete and dug a 32' pit along with multiple cavitation gouges in a few minutes.

https://m.youtube.com/watch?v=v98omCq1kRA

http://www.popsci.com/scitech/article/2003-03/water-vapor-al...

Why is everybody talking abut thickness when the concrete did not break but "eroded?

Because they are arguing from being smart in a different field and experience with things that are several orders of magnitude off the scale of what they're dealing with.

Things that work on a laptop with the dev server don't work at scale, and you won't even understand the problems till you've had some theory/experience with the problems that scale adds.

At a guess because they have no experience or knowledge on dimensioning something made out of concrete.

You can land an A380 on a runway 17" thick...

This was definitely strong enough, just not ready to withstand cavitation and given that bronze ship screws suffer from it it is hard to see what kind of liner would have solved this problem entirely.

Solving it completely may ask for a totally different design, likely not a different material or thicker concrete.

Previous images of the spillway were deceiving because of the scale involved.

Here's a more recent photo with humans standing near the broken concrete. You can see how massive that structure actually is (was?).

http://www.mercurynews.com/wp-content/uploads/2017/03/damage...

Great link. Haha, I wasn't even off by an order of magnitude!

The scale of that thing is off the charts, it is easy to mis-guess the size of things on those pictures because of a lack of references.

Here is one wit people for scale:

http://i.imgur.com/5TFQ6lW.jpg

I'm not sure you saw the scale of that spillway. It is _massive_. Something like 300ft wide and 3000 feet long. An entire football field wide. It's quite obviously not 4 inches thick, but admittedly at the scale of it, 15" doesn't seem like a lot.

There's a bunch of great pictures on imgur with "humans for scale" https://imgur.com/gallery/mpUge

The scale is deceptive. Take a look at the report on this https://www.youtube.com/watch?v=5xa_Q1R7mGs&t=132s and you will see a thick concrete spillway with 1.5" rebar. All that was ripped off by the force of the water!

The scale is very deceptive. The estimates are a million cubic yards of erosion and repair bill of about $250 megabucks. Of course, the smart move isn't to fix it exactly back to how it was exactly but to engineer the most value/$ TCO fix with the least cost and effort.

There's probably 20 Hoffman-crew-sized rented vehicles, and subcontracted employees and value-add diesel fuel ain't cheap either. It's going to take probably 9 months, at a minimum.

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