City Swallowing Sand Dunes

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.

Wind causes saltation, or jumping grains, on the windward side of sand dunes.

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.

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 geologists.

"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...