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Text:
Marie Le Breton
UNESCO reports that one-third of the Earth’s
surface is threatened with desertification. Although
human activities are responsible for the process, little is known about how the sediment
is transported. One way of improving our understanding
of this subtle blend of fluid mechanics and granular
physics is to study the sand ripples or dunes that
form, spread and constantly shift in the desert. Scientists
at the Paris Laboratory of Physics and Mechanics of
Heterogeneous Media* have spent the last three years
studying the dynamics of barchan dune formation in
the coastal desert of Southern Morocco. These dunes
are formed by unidirectional winds and can travel up
to about a hundred metres per year. Previous computer
simulations have shown that isolated dunes will collide
and gradually merge
to form a single giant dune. However, this is not what
actually happens. Barchans arrange themselves into
corridors of crescent-shaped dunes stretching for hundreds
of kilometres with their horns pointing downwind. The
researchers have managed to break this phenomenon down
into equations and identify the dynamic mechanism that
dictates barchan size. The complexity
of the sediment transport involved in the process lies
in the interaction between the grains of sand and the
wind. Any disruption, such as a change in wind direction,
destabilises the surface of the dunes and generates
sand waves on their slopes. These waves tumble down
the barchans’ horns, creating a succession of
small dunes in the wake of the now-unstable dunes.
These small dunes may collide with larger, slower dunes and destabilise
them in turn. By generating new small dunes, this volatility
controls the size of barchans and prevents them from
growing indefinitely. For its next trick, the research
team will be studying what happens on Mars where the
atmosphere is eighty times less dense and the dunes eighty times
larger than on Earth. There, basic research can revel
in the study of formations spanning a distance of up to six hundred metres.
*CNRS, ESPCI, Universities of Paris-VI and VII
(France). |
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