html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> From the archives: Not scour.

Thursday, January 11, 2007

Not scour.

On the eastern side of the Central Valley, the water is snowmelt, freshly run off granite outcroppings. It is so pure and sediment-starved that it sucks the cement out of concrete, leaving the aggregate in place. This is a terrible picture, but you are looking straight down a bridge pier, with smooth concrete at the top, some erosion at the high flow line, and inch deep erosion at the water line. It is only a couple feet down to the water. The structure is about sixty years old.

13 Comments:

Anonymous Anonymous said...

They need some Thompson's.

Justin

1:56 PM  
Anonymous ptm said...

That's super-cool.

Is that picture current? If so, why is there snowmelt midwinter - releases from reservoirs?

3:02 PM  
Blogger Megan said...

That picture was from my rice-mill trip last fall. Sometimes it is snowmelt that has waited in a reservoir for a while.

3:10 PM  
Anonymous Anonymous said...

Uh, why does water want sediment? Is this an osmosis sort of thing?

4:28 PM  
Blogger Megan said...

Yeah. I'm gonna gloss over that part, 'cause I loved physics and would have loved hydraulics with a better professor, but hated chemistry with a blinding hate. I think I took most college chemistry classes twice. Failed it the first time, passed it the second time, for maybe eight semesters in a row, including every summer. I got up to and through P-Chem that way. I switched majors when I realized I shouldn't be crying silently through my lectures. It got better after that, 'cause I had taken more chemistry than anyone else ever wanted of me again.

Anyway, yeah. Osmosis.

4:39 PM  
Blogger jens said...

Sediments, probably not. That gets pushed around regardless of the purity of the water.

But a very unsaturated solvent "wants" a solute...

6:43 PM  
Blogger poot said...

huh? i'm confused. i'd think water with more sediment already in it would erode things faster- the existing sediment would be abrasive, like sandpaper. i don't understand why clear water would erode concrete more.
my understanding of physics and chem are minimal at best; a layman's answer would be just fine.

6:43 PM  
Anonymous Mitch said...

I love Le Chatelier's principle because it explains (or describes, at least) such a wide range of phenomena. In fact, I'm sure the same thing goes by other names in other fields. If so I'd like to know what those are.

General chemistry I liked a lot; probably some of that was a good professor. But it was more than that--general chem had rules. I hated organic chemistry because it didn't have rules, in the main; it was mostly just a set of recipes to memorize. O-chem is why I switched to CS.

7:23 PM  
Anonymous Mitch said...

poot: maybe it would help to think of it as the water leaching out the cement rather than eroding it away.

Megan: was the downstream side much different from the upstream side?

7:25 PM  
Blogger Megan said...

Poot:
I didn't say it very clearly, but that isn't abrasive erosion. Mitch said it right when he said the water is leaching the cement away.

Mitch:
u/s and d/s were about the same. If you look at the Friant, you see cobbles instead of smooth concrete for the length of the canal. They aren't as deep as the ones in that picture, though.

9:34 PM  
Anonymous Anonymous said...

As an organic chemist, I think the whole 'memorization versus rules' thing is a byproduct of it being taught poorly. There are rules; they are clear and interesting.

I love organic chemistry; it's the best.

7:28 AM  
Anonymous Anonymous said...

Huh, so if it had sediment it wouldn't do that. Interesting. As a layperson, I wouldn't have figured the water's purity would make it leach more.

Spungen

2:23 PM  
Blogger jens said...

Even for a layperson it makes sense.

Put some sugar (or salt, or anything else that dissolves) into pure water, and it ....dissolves.

Pour some more in.

OK, keep going.

At some point or other, the solution gets "saturated" and nothing dissolves anymore. Since most large scale natural processes aren't all or nothing, it stands to reason that this is gradual (at first, things dissolve quickly...as saturation approaches, they dissolve more slowly).

So, at first, things would dissolve (leach) more quickly.

This makes sense to a layman like me. Whether it is actually true or not...go ask a chemist!

7:48 PM  

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