Extreme saltation - the movie

 
A couple of years ago, one of the great pleasures of my red Ford Mustang California dreamin’ road trip was a guided tour of the spectacular Oceano dunes by an enthusiast for, and expert on, this sea of sand. I have kept in touch with Kevin and recently received links to some videos that he has been working on. I started watching and my jaw dropped – they are extraordinary. I plan that this will be the first of a series of posts enjoying these movies, but to start, I have selected one that I find quite mesmerising. This is five minutes of closely observed moving sand, grains in exquisite detail, lit by the sun, the soundtrack the wind and the hurtling grains. I do appreciate that not everyone would find five minutes of moving sand “mesmerising” but I would venture that folk who read this blog might well. So, go to this link, crank it up to full-screen and HD (if your connection will permit), maybe make some popcorn (because, as Kevin notes, it will remind you of the action in the movie), sit back and enjoy:

http://www.youtube.com/watch?v=BX7AZTUxwcA

Fast and furious, eh? Non-stop action, the thrill of the chase, the endless conflicts…

Now, if you feel so inclined, go back to my post on Ralph Bagnold, “The man who figured out how deserts work.” And then check out “Splashing around on Mars.” In the latter, I talk about saltation and Bagnold’s work on the movement of sand – the way in which the wind causes grains to skip and leap in graceful but frenetic and attenuated parabolic trajectories, crashing into their colleagues and sending them, in turn, on their way. This is the mechanism by which huge amounts of sand move long distances in the wind and induces the other mechanism: surface creep or reptation. Saltating grains plummet to the surface and knock others along. Watch the video again and note the wonderful way in which both of these processes are captured, the sunlight picking up the ballistics of the saltating grains and the the busy rushing crowd of their reptating friends. Bagnold illustrated his wind tunnel work and the process of saltation with the following figure (from his enduring classic, The Physics of Blown Sand and Desert Dunes, reproduced by permission of Stephen Bagnold).

This is precisely what you can watch, in glorious technicolour, in Kevin’s video.

But a word of warning: filming blowing sand destroys cameras. Here is Kevin’s dismantled (and carefully diagrammed) in the hopes of removing the penetrative grains. As he writes, “ Sand and particulates packed all the buttons to where you couldn't even press them and even worked inside the lens and between the sandwiched polarizer elements as well! Tiny little hooligans.”

I’ll end with a couple of quotes from Bagnold – as I watched the movie I found it difficult to avoid the sensation of sand as a living thing, a communal organism, on the move with a purpose. In spite of his fundamentally scientific and empirical approach, Bagnold also saw something lyrical, organic and alive in the moving dunes with “the atmosphere of instability and continual impending movement they suggested—waves that ought to heave but remained still day after day, breakers just going to break but waiting motionless till suddenly unloosed by a gale to overwhelm the intruder, sixty-mile-long worms whose notched segments one expected to see starting a concertina motion.” And “One had a feeling that something was still going on, that the old Gorgon of the myths, in destroying all forms of life as we know it, had but freed the sand to organise itself into these huge forms, and that a creeping movement, slow but inexorable, was taking place even as we were among them.”

“In 1929 and 1930, during my weeks of travel over the lifeless sand sea in North Africa, I became fascinated by the vast scale of organization of the dunes and how a strong wind could cause the whole dune surface to flow, scouring sand from under one’s feet. Here, where there existed no animals, vegetation or rain to interfere with sand movements, the dunes seemed to behave like living things.”

Thanks, Kevin.

Comments

[Richard Bready]

Amazing camera work. Astonishing quality. I especially like 4:05 through 4:22, where I can best see the extent of local changes. I went back and watched the rest looking at a square inch of screen, and it's like time-lapse pictures of biological changes: appearances, disappearances, displacements back and forth. The overall force also reminds me of biological processes, but huge ones: wildebeest migrations, sardine swarms, driver ant swarms. Thanks indeed.

[Danielle]

Fascinating; thanks for posting!