h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Some time ago I happened to read an article on the Coriolis effect which was titled something like "on the meanders of streams" Some of the conclusions drawn by the author seemed contradictory to the statements in this posting, which is most likely because the author was wrong (it was published in the 40's) In brief the article says that (in the northern hemisphere) if you face downstream you should notice that the surface water moves right to left, while the water at the streambed moves left to right, due to the coriolis effect. That along with the effect of friction and erosion will cause a stream to more effectively dig out the left bank, causing it to meander to its left until it no longer can, and the lay of the land swings it back to the right.
I've spent several summers as a rafting guide in the states, and have told this to many clients, and have always been able to convince them that it is the truth. It probably helped that the author of the article was Albert Einstein, so nobody ever questioned it's validity...
Anyway, I was just curious if anyone has read Einstein's paper, and knows whether or not it is scientifically sound. Either way, I'll probably keep telling it to the tourists - it tends to keep them quiet.

Can we assume that I know less about physics than the good people who wrote this entry and Albert Einstein, and I realise that I may be being a bit dense here, but if that stream effect really happens, doesn't it make a difference what direction downsteam is?

And if not surely this would cause the stream to circle - why does the 'lay of the land' force it back?

This is all very difficult - I'm going to put my brain on ice, see if that helps.

Results of very little thought...

Surface right to left, streambed left to right = anticlockwise movement. So the left bank would be driven down into the bed, the *right* bank would be scraped up into the flow, so by this description, the right bank should wear away first.

Of course, if Coriolis effect was large enough to generate this effect, the left/right bank debate would depend on whether the river was flowing north or south (Northward flow = effect as described above, southward flow = vice versa). If it was flowing east or west, there wouldn't be any effect.

I do tend to think that turbulence within the flow would more than cancel out any tiny effects of Coriolis force (oops, "effect"). But thanks for the response! I love stuff that makes me think, and this pretty much falls into that category...


H

As Einstein explained it, the counterclockwise (as we say on this side of the puddle) motion of the stream, along with the lack of friction at the surface causes the water which hits the left bank to be moving at a relatively (this is Einstein, remember) high speed, and this increased speed is what causes left bank dirt to be erroded at a correspondingly higher rate. Left bank is then carried along with the (now) right-ward moving bottom water, which is slowed by friction with the streambed, and is then deposited on the right bank where the water is moving across the stream most slowly. This was how he reasoned that the stream should meander to the left, at least until the effect of gravity became strong enough to counteract the momentum of the water (I guess he's saying that the water can actually run uphill a little bit, after taking a 180 degree left turn).
Of course he also didn't believe in quantum mechanics, which seems to work, so maybe the little guy wasn't all he was cracked up to be. Still, its an interesting bit to ponder, if not interesting enough to woo a girl at the pub...

Over geologic time, a small effect might add up, especially if it was consistent, and the randomness of the turbulence was not.

Hmm. True - *if* the effect wasn't completely drowned out. Analogy: if I stand at the front of an Iron Maiden concert, quietly whispering the words to a Kylie Minogue song, it doesn't really matter how long I stand there, nobody will hear me. And a good thing too. The "Coriolis induced" currents could only have an effect (I think) if the turbulence somehow stopped drowning them out (assuming the flow was turbulent at all, of course).

In actual fact the above is nonsense because I know better than most that the majority of river flow is well below the turbulent region (Reynolds number <2100 in case you need to know...) so in fact the Coriolis effect could conceivably have this effect. On the other hand, things like the geology of the bank would also have much larger effects as well. Isn't the universe complicated?

H.

On the off chance that anyone is still reading this thread - the essay that I origionally refered to is one of several in a book titled "Einstein: Ideas and Opinions" - having recently re-read it I think that I can safely say that I did a very poor job of expressing the authors theory. If you've read this far down the thread, you might just as well go down to the bookstore and flip thru the book for yourself...

I haven't actually read this book, but I've always been taught that coriolis doesn't have any real effect on streams etc. There's usually stronger forces involved (such as turbulence) that over-ride the relatively small effect of coriolis.