Archive for November, 2009

Meandering across the digital divide V: Finis-The people

November 12, 2009
MissExp

Meandering in Vicksburg, Mississippi, 1945

This is the image I had in mind when I read the post at Highly Allocthonous early last month, that started me off on my own meanderings about the digital divide, that I am now closing the loop on.

From my side of the divide there is a certain amount of sediment that is being transported by the river of flowing time. I was 13 in 1945, living in Los Angeles, having been moved there with my family from a little town not far from Charlotte, North Carolina. My mind was certainly not on meandering streams.

Vicksburg, Mississippi is indelibly associated in my mind with images of the  civil rights struggle subsequent to 1945 that my wife and I were involved with, just as Dow corporation is associated in my mind with images of napalm and agent orange from later years.

All of which should have nothing to do with doing geology or teaching geology or being a scientist or blogging about any of these. Even so, it prompts me to wonder what was in the minds of those people in the photo. Men, apparently. Perhaps even Friedkin himself.

Individual motives cannot be read from photos, but institutional rationales are a little safer to surmise. That is, the people who payed for this research I think we can be confident about. They were out to control the river. First understand, then control.

We understand from this side of the divide that achieving that goal is not only undesirable, but is quite likely impossible. Perhaps a little too easily we attribute to the people in the photo aspects of vanity, arrogance,  even hubris. Perhaps a little too easily, I reacted the same way to the Post and its comment stream.

But a question lingers. Does restoring the river come down to controlling it, to making it move backward through historic time to some earlier ideal state?

Ideal state? Whose ideal? Who has the authority to define the ideal? Whence comes that authority? Questions as old as the human community itself.

Advertisements

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

November 10, 2009

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?

Prandtl

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.

Meandering acrosss the digital divide III: Pedagogy of a meandering stream

November 9, 2009

My teaching was clearly rooted on the other side of the digital divide from here and now. In it I developed a fairly consistent program for using meandering streams as a means of teaching how geology goes about answering its basic question (how did the Earth get to be the way it is?) by bringing to bear the non-historical sciences, in this case physics.

I started with an in-class problem. The students got a piece of paper with a meander loop drawn on it, and a data set with depth and water velocity. As I recall now I stole the data from something of Strahler’s. Actual stream gauge measurements. That required some explanation of measurement and operationalism. The students plotted the depth profile and the velocity profile across the meander loop. Needless to say, there was a close correlation: deeper water on the outside of the curve, running faster where deeper. Generalizing from this, I showed them how to draw the thalweg down the length of the stream shown on the map, completing the three-dimensional view of the  inter-relationship of process and form.
The next step was to, hypothetically, dump a load of mixed size sediment into the river at the thalweg on the cross-section and follow it downstream, using the stream competency diagram (particle size vs stream velocity). I had to resort to drawing my own version, even if there was one in the text. With it by interpolating velocity at max at the thalweg to zero at the bank, the students could figure out where to look for the sediment: on the next point bar downstream on the same side of the thalweg it started from and sorted out by size.
In the interests of saving time during the term, I dropped the in-class problem aspect and just lectured my way through these ideas. That doesn’t work nearly as well, but I needed to save the time for my other problems that I thought were more fundamental to geology- eight in all. The mysteries of log-scaled graphs, interpolation, and diagrams that show process but are too easily conflated with form just took too long to unravel.

After I felt that I had the students mind thoroughly into the stream, I went back to the cross-section and started talking about the actual point to point velocity and how that could be measured. I re-introduced the turbulence, showing where it maxed out on the outer part of the curve, below the surface, not at the bottom, sometimes with some helical flow. Then, with the bald assertion that the velocity plotted in the competency diagram was a surrogate measurement for the degree of turbulence, which we did not know how to measure, I further asserted-teachers in a classroom can get away with crap like that!-that if the stream was eroding, the point at which the erosion occurred was where the turbulence was maximum where, in turn, velocity changed the most in the shortest distance. That is, below the water surface, on the outside of the  curve, not at the bottom. Sediment removed from that point would leave the unconsolidated material above it un-supported, and it would collapse into the stream. All this stuff would be carried downstream to the next loop and depositied on the point bar, sorted out according to size. Frequently there were students who had swum in rivers, knew where the beach was and where the swimming hole was, and could testify about walking into the water over increasingly coarse sand, pebbles, etc.

The net result of these processes operating was the migration of the asymmetric profile sideways, leading to the side-cutting action of the stream and the accentuation of the meander loop, etc. By introducing some speed-up and slow-down of velocity around the loop, down-valley migration can be accounted for. Oxbow lakes, clay plugs resisting erosion(back to the competency diagram): it fits together beautifully. Occasionally you can see the click on the face of the student when all the various pieces of the puzzle fall into place.

All of this leads easily to saying that once the form of a meander is established, the process that occupies the form will re-create the form continually. The only remaining question was what caused the stream to wiggle in the first place.
I then went through the familiar examples from Leopold’s work of melt water streams on glaciers, the Gulf Stream, the jet stream in the atmosphere, even adding my own favorite examples, streams flowing across beaches, like at Alamere Falls. I could say at that point that meandering seemed to be an inherent property of fluids in turbulent flow, so we have to await a further understanding of turbulent flow from the people who are responsible for that sort of thing, the physicists.

Uncomfortably, the above development implies that all streams should meander. So that required some work on the conditions that rule in non-meandering streams: braided streams (variability of sediment load and discharge) and riffle and pool streams in canyons (flowing water can’t cut sideways into bedrock- the cutting tools are all in the bottom. They are doing the best they can.).

Meanwhile, geologically speaking we could say that the observational evidence indicates that we need refer to nothing beyond the current on-going process to explain the existing form. The independent variables in a stream system are the discharge and sediment load, which are both determined basically by climate. The internal dynamics of the process adjust the form to meet the needs of uniform dissipation of energy. While the process can approximate balance or equilibrium or steady state at the local time scale, so long as there is land above water, the planet Earth is not gravitationally in a stable configuration, and will continue to change at the global time scale. The internal energy residual and generated within the Earth is now, and has for nearly 5 billion years, acted to counter the force of gravity, fueling the continuing and shifting interactions at the surface of the Earth that we call geologic history.

Having spent the last four decades in front of a classroom, dealing with the structured ignorance of the students, I am a little intimidated by the staggering amount of new knowledge the scientific community has compiled while my back was turned, facing away from learning and toward teaching. To be a little more precise, I have been learning, but the thing I have been learning about is hidden behind that phrase “the structured ignorance of the students” and how that clashes with the fundamentals of science. In this process, I have been forced to re-learn those fundamentals again, but trying now to look at them from the point of view of someone who is, consciously or unconsciously, committed to a radically different frame of reference. But that’s not this story!

Alamere Falls 2006

The last Alamere: no beach, no stream