Sand dunes

What can be star or moon-shaped, hundreds of metres
high and can swallow villages?

A few years ago, one village on the edge of north China’s , Gobi Desert was anxiously awaiting a silent invasion of their houses and farmland. Sand dunes were marching towards them at 20 metres per year. Within two years, the fi rst houses vanished beneath the sand.
 More than 99 per cent of the world’s active sand dunes are found in deserts,but they can form anywhere there is little vegetation, a wind or breeze to move loose sand, and obstacles – rocks, bushes or even dead animals – that cause a patch of sand to settle. This includes beaches,
dried-up lakes and river beds. Once a sand patch forms, it traps sand grains as they bounce along in the wind. Around 95 per cent of sand grains move by jumping a few centimetres into the air and landing a few metres away in a process called saltation. When grains hit the ground, they collide with other grains and make them saltate. Sand grains build up on the patch until it forms a pile – a sand dune. The dune reaches its maximum height when sand is eroded from the crest at the speed it’s deposited, ensuring a constant height.
Wind erosion sculpts the upwind side of the pile into a gentle slope. The sheltered lee side of the dune – the slip face – is steepened by turbulent, backcurling eddies that form when the wind overshoots the dune crest. Dunes advance because sand is constantly removed from the windward side of the dune, carried over the crest, and dropped on the lee side.
 When the prevailing wind is coming from a single direction, dunes have a slip face and a windward slope at right angles to wind direction. More complex dunes are formed where the wind changes
direction. The biggest are some 300 kilometres (186 miles) long and up to 500 metres (1,640 feet) high, while the tiniest are under 5 metres (16 feet) long.
Dunes become inactive when the climate gets wetter. Plant roots bind the sand together, preventing dunes from growing and moving. Vegetated dunes in once-dry areas have slopes facing into
long-gone winds.