A town in Western Australia had to move because sand dunes swallowed it
In the 1890s the settlement of Eucla, in Western Australia, was relocated because drifting dunes were burying it. Dunes can travel startlingly fast: between 1954 and 1959 a group of crescent-shaped dunes in China's Ningxia Province advanced more than 100 metres a year, and similar speeds have been measured in Egypt's Western Desert.
A dune moves grain by grain. Wind rolls or bounces sand up its gentle upwind face, called the stoss side, and at the crest the sand tumbles down a steeper downwind slope, the slip face. Repeat that endlessly and the whole mound creeps forward. Dunes can be built from quartz, gypsum, calcium carbonate or even snow, and they form underwater too, on river beds and the sea floor.
Wind direction largely decides the shape. Where it blows steadily one way and sand is scarce, barchans form: crescents wider than they are long, their horns pointing downwind. With more sand they merge into long ridges lying across the wind. Winds from two directions build seif dunes, named after the Arabic word for sword, knife-edged ridges with two slip faces. In the Sahara they can reach 300 metres high and 300 kilometres long. Where winds come from many directions, star dunes rise instead, pyramids with three or more arms that grow upward rather than outward. In China's Badain Jaran Desert some reach 500 metres.
Parabolic dunes invert the crescent. They start where wind scoops a hollow out of vegetated sand, and plants pin down their trailing arms while the bare nose pushes ahead, so their tips point upwind; the longest known arm stretches 12 kilometres. Researchers once credited plants with holding them steady, though water retained in the sand now looks like the bigger factor. Where winds regularly reverse, dunes grow a small temporary slip face on the opposite side, erased when the dominant wind returns.
On coasts, rows of dunes behind the beach absorb storm waves, and people sometimes build artificial ones for protection. The word itself reached English from French around 1790, borrowed earlier from Middle Dutch.
Source: Dune
