Researchers are studying the complex physics
of menacing sand dunes.
by Trudy E. Bell and Dr. Tony
Next time you're at
the beach or in the desert, climb a sand dune in bare feet on a
windy day. Stand still in various places on the gently sloping windward
side. Watch how wind-driven sand grains appear to jump an inch or
two above the dune, stinging your ankles and making the dune's surface
appear to be in constant motion ever upward toward the crest.
At the dune's crest,
kneel to examine closely what's happening. Watch how airborne sand
grains fall and cascade down the steep lee slope in tiny avalanches.
Start hiking down the lee side; notice how suddenly still the air
feels, especially just past the dune's crest.
You've just observed
how dunes grow.
More importantly, you've
also just seen how dunes can migrate - a grave concern in nations
where the relentless advance of desert dunes is a serious threat
to habitation and agriculture. In arid northern China, for example,
dunes are advancing on some villages at a rate of 20 meters per
year. Parts of Africa and the Middle East are likewise threatened.
How do you stop a moving
sand dune? In some places people simply drench the sand with oil
- it's effective, but not very good for the environment. Sand fences,
like snow fences, can also help,
Photograph courtesy Philip
is from Great Sand Dune National Monument, Colorado.
although in many cases
their design is little more than guesswork. Engineers are disadvantaged
because there's no complete physical theory for the behaviour of
"Moving sand dunes
are an example of granular flow - a poorly understood branch of physics,"
explains James Jenkins, a professor of theoretical mechanics at Cornell
Physicists have long
had neat mathematical equations that fully describe the behaviour
of solids like bricks, liquids like water, and gases like air. But
granular materials like sand dunes don't quite fit in any of those
sometimes act like solids and sometimes like fluids," says Jenkins.
"The transition from one behaviour to the other can be very rapid."
Gravel in the back of a dump truck, for example, sits virtually
unmoving in a solid pile, even as the truck bed begins to tilt - until
a certain angle is reached, and then suddenly it all tumbles downward
in a thundering river of rock. Modern physics cannot predict the
Grainy substances are
so hard to figure out because they're so complex. In a heap of unmoving
sand, for instance, each grain interacts with five to nine immediate
neighbours all at once. The transitional state, when the heap begins
to move, is scarcely easier: Although each grain is simultaneously
interacting with maybe only three to five neighbours, those are
not the same neighbours from one moment to the next. Even a supercomputer
can't keep track of all the interactions.
NASA is supporting
Jenkins' research to understand such flows. "Our work involves experiments,
field studies, modeling, and numerical simulation of wind-blown
sand," he says. "We're trying to understand the mechanisms of dune
migration and what makes heaps of sand turn into moving dunes."
It's all part of NASA's mission to understand and protect our
Landsat image reveals sand dunes advancing on Nouakchott,
the capital of Mauritania
Sand dunes fascinate
Jenkins (along with his collaborators in Gainesville, Florida, and
Rennes, France) because they manifest three aspects of granular
The first is saltation.
"The word comes from the French sauter, meaning to leap or
jump," Jenkins noted. Saltation happens above the gently sloping
windward sides of dunes when grains are suspended in mid-air by
turbulent puffs of wind, fall and strike the sand again, and then
rebound and eject other grains - which then can do the same. "Under
the right wind conditions, saltation can become a self-sustaining
system of jumping sand grains moving along a dune," clearly visible
as swaying patterns of sand about ankle height moving upward toward
the dune's crest.
Image credit: NASA. more
view of ordinary sand.
The second is sheet
flows, an extension of saltation when the wind becomes strong
enough that sand grains begin to collide with one another in mid-air.
"In sheet flows, the mass transferred is extremely large," Jenkins
says, in some sandstorms moving entire dunes impressive distances - up
to tens of meters in a major storm, enough to engulf individual
houses or roads.
The third is avalanches
of sand down the steep lee side of a dune. Together with sheet flows,
avalanches allow an entire dune to move in a sandstorm "a little
like a tank tread," Jenkins said, with sand particles continually
circulating from the top to the bottom of the dune.
Jenkins's goal is to
characterize sheet flows and avalanches using partial differential
equations that model the movement of sand grains as if they were
particles in a fluid. "These equations should contain within them
the way avalanches scale with viscosity, velocity of turbulent wind,
grain diameter, and gravity," he pointed out. With such equations
in hand, it might be possible to anticipate the onset of dune migration,
to predict where they'll go and how fast.
His goal is quite a
challenge. Among other things, the exact form of an individual dune
depends on the consistency of wind direction. If windblown sand
comes from one prevailing direction, for example, a dune will be
a crescent-shaped barchan. If winds switch direction seasonally - say,
coming from the southeast for half the year and from the southwest
for the other half - a dune will be linear. If wind direction is
erratic, a dune may be star-shaped.
causes saltation, or jumping grains, on the windward side
of sand dunes.
But the payoff may
be significant. Not only might such characterization be useful in
designing fences or other restraints effective at mitigating the
advance of threatening dunes; it could also be a boon to planetary
"If we can fully describe
dunes on Earth," Jenkins observed, "we should be able to do so on
other planets, too, like Mars." Of course there are no cities on
the red planet for sand dunes to swallow. Not yet. But perhaps,
like the equations of granular motion, it's just a matter of time...