It sounds like every child’s dream – your very own 16-hectare sandbox to play in, complete with a real bulldozer to shovel the stuff about.
The prospect was no less exciting for Clement Narteau, a French geologist based in the Geological Fluid Dynamics Laboratory in Paris, who in 2007 set out to create an enormous, life-size “lab” in the wastes of an Inner Mongolian desert.
But the sand castles Dr Narteau set out to create in 2007 would be built not by bucket and spade, but by the combined forces of wind and time.
If his theories were correct, after flattening the desert with bulldozers he and his Chinese colleagues would be able to stand back and watch, fascinated, as nature reasserted her dominion and sand dunes were created before their eyes.
The three-year experiment was no mere whimsy. On it hung the validity of the core theory of how sedimentary particles are moved about and deposited – in other words, the study of how entire landscapes are formed and altered, from the deserts and polar regions of Earth to the wastelands of distant planets .
Until now, only computer modelling has been able to predict such things and “reverse engineer” landscapes so scientists can figure out the history of a climate. What was missing was real-world confirmation that the basic theory on which the models were built was correct.
“Such work has been either theoretical or carried out in wind tunnels,” says Dr Narteau, who lectures at the Institut de Physique du Globe de Paris.
“But really it is impossible to do in a wind tunnel, that’s the main point. Recent developments in dune physics reveal that there is a minimum dune size and in Earth’s atmosphere it is approximately 20 metres long. So if you want to study the development of dunes in a wind tunnel, you need a tunnel hundreds of metres long. Impossible.”
And dunes cannot simply be scaled down to be studied in the lab, he says.
“Playing with the size of the particles is not really possible, because the behaviour of the particles becomes erratic.”
To create his giant sandbox, Dr Narteau chose to collaborate with Chinese scientists. This was partly because in the Tengger region, 1,000 kilometres west of Beijing, they had access to vast amounts of the right sort of windblown desert, but also because in developing large solar projects in the region they had mastered techniques for flattening the desert.
“Ours is a big experiment, but compared to what is being done there for the construction of solar panels it is just nothing,” he says. “In the neighbourhood they are building solar farms everywhere and flattening dunes over tens of square kilometres.”
Dr Narteau and his colleagues from the Laboratory of Desert and Desertification at the Chinese Academy of Sciences set out the grand scale of their ambitious experiment in typically understated scientific terms in a paper that has just been published in an international journal, Nature Geoscience.
To the untrained eye, the emergence of oblique dunes in a landscape-scale experiment begins with little fanfare: “Aeolian dunes in many arid environments on Earth are shaped by seasonally varying bimodal wind regimes.”
Sand dunes, in other words, are created by winds from two directions, which vary according to the time of year – a truth consistent in deserts from the Tengger in Inner Mongolia to the majestic sand sculptures found around the Liwa oasis in the south of the UAE.
“However, the dynamics of dune evolution under such wind regimes are difficult to investigate at the time and length-scales of laboratory experiments. Here we report results from a landscape-scale experiment.”
Scientists never use words such as “unique” or “ground-breaking” when writing about their own work, though in this case both would have been appropriate. This had never been done before and, quite literally, they were breaking ground, razing established dunes right back to the desert floor.
After flattening a dune field over 16 hectares of the Tengger Desert, between March 2008 and October 2011, they watched what happened, measuring the winds and the changing topography over three years and seven months.
The result? It does not sound much to a non-geologist. In fact, it sounds like little more than common sense. They were able to show that “individual dunes propagate in different directions according to the prevailing wind”.
But there was a bit more to it than that – and this hard-won nugget of data about the behaviour of desert sand in the face of more than one wind confirmed the value of the models upon which countless geological speculation, careers and papers have been built.
“The orientation of sand dunes,” they proved, “is controlled by the combination of the normal contributions of the two dominant winds, with respect to their relative strengths and directions, such that crests form an oblique angle of 50 degrees with the resultant sand flux.”
And then came the punchline: “Our landscape-scale experiment suggests that the alignment of aeolian dunes can be used to determine wind forcing patterns on the Earth and other planetary bodies.”
Peer praise has been pouring in for this vast piece of work.
“They’ve done something quite brilliant,” said Douglas Jerolmack, a geophysicist at the University of Pennsylvania. “It’s the kind of result that says our theoretical understanding is actually validated in the natural, messy world.”
The theory, “Maximum Gross Bedform-Normal Transport”, or MGBNT to its friends, has many applications, says Dr Narteau.
“You have small sediment everywhere on Earth and also on all planets and that’s the way the topography is moving,” he says. “The way the Earth is evolving is through the transport of sediment. Dunes reveal this transport of sediment, so this is very important.”
“Dunes” also occur, for example, in rivers, “and when you have transport in rivers you need to study dunes; when you have contaminants in rivers these are stored within the dunes. So every time you want to study the transport of mass, on Earth and other planets, you have to confront the dynamics of dunes.”
Dune-bashing will never be the same again. Especially when you learn that it takes about 1,000 years to “grow” a dune 100m tall.
Today, says Dr Narteau, the study of sand dunes has become a very popular discipline, because the science “can be applied to so many subjects”, not least global warming and the encroaching desertification that threatens vital food production in many parts of the world.
“Dunes attract scientists because they are nice objects with lots of variables, and all the variables – in shape, size and behaviour – are not understood yet,” he says.
“And, of course, dunes attract people because they are beautiful.”
Behind his scientist’s facade, Dr Narteau is also something of a romantic. He says he likes the thought that his work links him to the man widely credited with starting it all – and to the romance of an earlier age when the deserts of the world remained shrouded in a mystery that attracted western adventurers.
The study of sand dunes “begins in the Second World War, with the fantastic story of Ralph Bagnold”, he says with undisguised enthusiasm.
Bagnold, whose work is still regularly cited in scientific journals today, was a British army officer who followed in the footsteps of warrior-academics such as Lawrence of Arabia.
During the war he founded and led the legendary Long Range Desert Group, a highly mobile, self-reliant force that operated behind enemy lines in the vast wastes of the Libyan Desert, gathering intelligence and blowing up stuff.
Unsurprisingly, Bagnold came to learn a lot about desert travel – he pioneered the partial deflation of tyres now common practice among dune-bashing weekenders. But he also developed a love of the desert and a scientific fascination for the formation of sand dunes.
Somehow, despite the ferocious fighting in North Africa, he found time to study and to write, and his book, The Physics of Blown Sand and Desert Dunes, published in 1941, remains a relevant text in the field.
“I am sure Bagnold would be interested,” says Dr Narteau. “It’s obvious that any scientist would want to test his prediction against nature and that is basically what we have done, by creating an experiment on the scale of 1:1.”
Dr Narteau is clearly delighted to report that he will be returning to the Tengger desert in April, when the winds blow hard through the region.
“Unfortunately,” he says, they will have to flatten the desert again and redo the first part of the experiment. In a nutshell, they were taken by surprise by the speed at which dunes began to form in the first few months.
“We did the first measurement after four months, and in four months it had gone from flat to a dune field of an average height of one metre. We missed everything in between.”
What a chore. Though one senses that, for Dr Narteau, the beauty of the evolution of the dunes that will form before his eyes will compensate for the inconvenience.
