City Swallowing Sand Dunes

"Granular materials 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 avalanche.

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.

This Landsat image reveals sand dunes advancing on Nouakchott, the capital of Mauritania

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 home planet.

Sand dunes fascinate Jenkins (along with his collaborators in Gainesville, Florida, and Rennes, France) because they manifest three aspects of granular flow.

Image credit: NASA. more A close-up view of ordinary sand.

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.