Meandering across the digital divide IV: what causes it to wiggle in the first place?

In my mind, streams follow a sinuous course, in single channels or multiple, even within fixed walls, because the flow is turbulent, perhaps following some demands of  chaos theory.

So I wondered what new stuff has developed over the years about sinuous fluid flow, turbulent flow versus laminar flow, chaos theory and so forth. I can’t remember how I got there, but I was looking at a map that showed the avulsion of the meandering channel of the Owens River. So I had to look up avulsion. Great word!
It led me to the book “Tectonic Geomorphology”: Burbank and Anderson (2001), which I went to Amazon and ordered, along with “California Rivers and Streams”: Mount(1995). I also have “Braided Rivers”: Best & Bristow ed (1998) coming from the Geological Society of London.
Going in the other direction, I dug out my Leopold “Fluvial Process….” (1964), and some other stuff:(Russell-“Rivers, Streams and Seacoasts”1967; Birot “The cycle of erosion in different climates” 1968). I guess there are two places I tend to go to for answers:old books in my basement and new stuff through my computer. Meandering I go, back and forth across the digital divide.

But what causes it to wiggle in the first place?


From Longwell, et alli; 1969 Physical Geology

I searched for Prandtl, not knowing anything about him. He was a pioneer in aerodynamics in the early 20th Century. Boundary layers and separations thereof, subsequent forces.

Wikipedia has a nice piece on turbulent flow and laminar flow. I clicked on a “See Also” topic titled “swing bowling”. It sounded too much like swing dancing to pass it by. Turns out it is a cricket phenomena where the bowler causes the ball to deviate sideways (wiggle?)  by heaving it at just the right speed to keep the airflow over half the ball turbulent and the other half  laminar. The ball, which has an equatorial seam, is kept half rough and half smooth by the players in the field. The sideways force comes about because the separations of the boundaries layers on the turbulent side and the laminar side don’t line up. I think I have that right.

In a stream, what corresponds to the two sides of a cricket ball? I taught that in a stream one can distinguish a laminar flow layer at the contact with the un-moving bed of the stream. As velocity increases away from this surface, at a specific velocity, the flow abruptly changes to turbulent. Then, as flow velocity continues to increase, the flow again changes abruptly to jet flow.  Typically, this occurs at the surface along the thalweg.

Rethinking this now, I wondered what was jet flow, the flow that produces those smooth standing waves on the surface of even small streams, like the one at Alamere Falls? Of course it looks like laminar flow, but it is going too fast, faster even than the turbulent flow.

But fast relative to what frame of reference? For a packet of water moving down the thalweg, the source of friction that would produce turbulence is not the bed of the stream, but the water packet immediately beside it, which is traveling at nearly the same velocity! The jet flow portion of the stream is an elongate lake being carried downstream on the rollers of turbulence. Any flow within it is laminar!

Somewhere in there in the separation of boundary layers on the two sides of the turbulent flow zone sandwiched between the laminar flow at the bed and the laminar flow at the thalweg, those swing bowling side-wise forces arise to start the stream to wiggle! I believe it!

But then I’m just an old igneous petrologist from the geosynclinal era gone to seed in the classroom, now out to pasture.


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