ON THE outskirts of Nouakchott, Mauritania, monsters lie in wait: hordes of sand dunes are poised to overwhelm this desert capital. They stand tens of metres high and are advancing towards the city like predatory snakes. While dunes move in slow motion compared to other natural disasters like earthquakes or tidal waves, they are every bit as damaging and just as impossible to stop.
But one man claims to have a solution to tame the dunes. His name is Jean Meunier, a Frenchman who spent his life teaching agriculture in more than 30 countries. After his retirement in 1993, he formulated a simple and elegant method to control dunes wherever they encroach on human habitation. Instead of futilely trying to stop the movement of sand, he harnesses the dunes’ very creator – the wind itself – to destroy them.
The strategy is called BOFIX and the idea behind it is to create local wind eddies to blow away dunes and keep them from reforming. Despite some early successes, Meunier’s ideas were met with scepticism from international aid agencies, so he used his own money to finance a BOFIX pilot project in Mauritania.
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For years he worked in relative obscurity, living in a modest neighbourhood of Nouakchott and driving an old car. But recently a geologist friend encouraged him to publish his ideas in a series of scientific papers. His work is now attracting the attention of a group of computer modellers who want to incorporate BOFIX into their predictive model of dune movement. If they are successful, they will be able to use Meunier’s ideas to reshape the desert at will, creating highways of wind that coax sand safely along pre-determined routes.
Finding a way to fight advancing dunes has become critical. In the past 50 years, desertification has increased worldwide and nomads have migrated to cities. Nouakchott, for example, has grown from 75,000 inhabitants in 1978 to nearly a million today. Desert city authorities have tried a number of dune-stopping techniques, but most are expensive and can be ecological nightmares.
Oil-rich governments in the Middle East bulldoze the dunes and then spray them with hydrocarbons to fix the sand in place. In the Sahara, villagers erect fences upwind of their settlement to collect sand into large, artificial counter-dunes. This approach works well until a large sandstorm topples the counter-dune and spreads the sand everywhere. Other strategies, including planting rows of small trees or shrubs in a checkerboard pattern that traps sand in an extensive horizontal sand storage system have been successful in China. But this often requires irrigation, and in most deserts water is too precious for that.
“In most desert countries, all you’ve got is sand and wind,” says Jim Jenkins, a physicist who studies granular media at Cornell University in Ithaca, New York. Yet sand and wind are all Meunier needs. His system consists of two components. First, fences placed in strategic locations are used to create turbulent winds that carve into the dunes, leading to their collapse. Second, drought-resistant trees are planted on the collapsed dunes to stabilise them.
One of Meunier’s first attempts to destroy a dune was in Nouakchott in 1995, where sand that had piled up over a few years had nearly reached the roof of a primary school. To remove the sand, he placed a fence made of semi-permeable plastic fabric several metres upwind of the school. The fabric filtered out the sand, allowing the cleansed wind to pick up speed, increase turbulence, and scour away the piled-up sand. It took about a month and the school stayed free of sand for years until the area was cleared for a new housing project.
Meunier did not stop there. He went on to devise a system to destroy the long, snake-like dunes that are the greatest threat to the city (see Map). These longitudinal dunes are at least 5 metres high, can be hundreds of kilometres long, and travel at tens of metres a year. They form when two seasonal winds come from diagonally opposed directions. In the case of Nouakchott, a wind known as the harmattan comes from the north-east or east during the dry season. During the rest of the year, the maritime trade wind comes from the north-west. The harmattan lifts the sand from the south-east flank of the dune and deposits it further ahead on the west-facing side. Then the maritime wind lifts sand from the dune flank that faces north-west and deposits it on the flank facing south-east. Over time, the bulk of the dune slinks towards the south-west.FIG-mg24015401.jpg
To destroy the longitudinal dunes, which Meunier calls “snakes”, he devised a fence he calls “the guillotine”. This is a V-shaped fence placed perpendicular to both the maritime wind and the harmattan. Instead of a semi-permeable fence like the one he used for the school, the guillotine blocks all the wind and forces it to go around the edges and over the top of the fence, where it forms eddies that blast away at the sand. In periods of strong wind, the guillotine can slice through a 5-metre-high dune within two months.
Several of these guillotines placed along the spine of the dune will do the trick. The displaced sand slides down into the valleys between the dunes. In Meunier’s vision, the valleys will eventually fill in and the whole region will become a sandy plateau that can be stabilised with rows of desert-loving trees.
But a sandy plateau isn’t always an ideal end point. Meunier has also tried to use BOFIX to save roads, such as the Route de l’Espoir, or road of hope, an east-west road that stretches from Nouakchott all the way to Nema in the east of the country and serves as an important commercial and psychological connection between these Mauritanian desert towns. Since its construction in the 1970s, the road has been plagued by dunes that are pushing further and further south. Meunier tried placing guillotine screens at the head of each dune, near where it threatens the road. The screens force the sand to spill to either side of the dune, and these mounds of sand in effect make a new dune that stretches east to west just north of the road. “We take a north-south dune and turn it into an east-west dune,” he says.
The project has yet to convince critics. The sheer volume of sand coming from the desert means the east-west dunes in turn become too high and spill onto the road anyway, says Monique Mainguet of the University of Reims in France, who worked on dune encroachment in Mauritania for 30 years. Meunier admits there are problems with the project, but he is convinced that with enough effort, the strategy will eventually pay off.
Meunier has applied the same tenacity to developing a method for destroying the crescent-shaped barchan dunes, or “turtles” in BOFIX parlance, that form on firm soils where the wind always blows from the same direction. He places two screens on the windward slope to make a V-shape, placing the point of the V near the bottom of the dune and each end at a point along the crest of the dune. The screens prevent wind from depositing sand within the V, creating an incision in the dune. Three of these V-shaped fences are sufficient to split the dune open. “If you destroy the shape of the dune, it becomes simply a pile of sand and it can no longer migrate,” says Meunier.
Once he destroys the turtles, Meunier must keep the wind from building new ones. The most logical solution is to plant vegetation to keep the sand in place, but that’s not easy in the desert. Nouakchott gets only 150 millimetres of rainfall per year, but about 5 to 6 metres below the surface of the dunes lies a damp zone that certain plants, such as a shrubby tree called Callotropis procera, can make use of. Meunier designed a rainwater collection system using bundles of discarded 1-litre soda bottles to funnel water down a column in the sand. The plant’s roots grow down this column and, after 8 to 10 months, reach the damp zone and can grow independently without irrigation. Meunier also placed a plastic bag over each tree to create a mini greenhouse to improve its growth even further.
Meunier formulated his ideas through trial and error, working with local labourers and consulting with geologist and friend Pierre Rognon at the Pierre and Marie Curie University in Paris. Since Rognon encouraged Meunier to present and publish details of his system (Secheresse, vol 11, p 309), a group of physicists headed by Hans Herrmann of the University of Stuttgart have become interested in enlarging their computer model of dune migration to include BOFIX principles. Until now, few researchers have taken the problem of stopping dune encroachment seriously. Geologists are more interested in events with very long timescales, while physicists study the flow of grains over a few seconds or minutes. Few have looked at what happens to a dune on a human timescale of, for example, 20 years.
One reason for the lack of interest is that dunes are so difficult to study in a laboratory. Their large size makes them difficult to fit into a wind tunnel, but scaling them down involves shrinking each grain, and as the grains get smaller they stop acting like grains and start acting more like dust. No one has yet figured out how to turn a pile of sand into a dune. Place a small mound of sand in a wind tunnel, blow some sand-laden wind over it and all you’ll get is a bigger pile of sand that will not assume the rolling shape of a dune, and won’t migrate.
Yet a predictive model of dune migration would be invaluable to desert communities, because the effect of interventions such as BOFIX screens can be hard to predict. Herrmann gives the example of a conveyor belt at the phosphate mine of Boukra, 100 kilometres east of La’youn in Western Sahara. The conveyor belt was built 15 metres off the ground so it would be well above the height of nearby dunes. But after a few decades, the sand filled in the entire 15 metres and claimed the conveyor belt anyway. “If the engineers who built the conveyor had been able to model dune movement over time, this type of thing might have been avoided,” says Herrmann.
In 2000, Herrmann together with Greg Sauermann, also at the University of Stuttgart, and Klaus Kroy at the University of Edinburgh developed a computational model of dune migration. Although previous models had described how dunes could form in the first place, the Herrmann model is the first that successfully simulates their migration.
The model follows principles first described by Ralph Alger Bagnold, a member of the British Army’s Corps of Royal Engineers, who trundled around the Libyan desert in a specially equipped model-T Ford during the 1920s and 1930s. Bagnold described how, as wind moves over the dune, it picks up sand grains and carries them forward in short hops, a process called saltation. To accelerate the grains, the wind gives up some of its momentum, causing it to slow. As this happens, the heavier grains fall back onto the dune, where they hit other grains that either splash back up in saltation or roll forward along the surface in a process called reptation, because the grains creep like a reptile. During saltation, the grains do not lift very far off the dune – nearly all the hopping happens within 60 centimetres of the surface. This, says Herrmann, explains why Meunier’s relatively low BOFIX fences are able to make such a huge difference to dunes much larger than they are.
The key to modelling these ideas on a computer is to realise that the amount of sand the wind carries varies enormously, says Herrmann. The wind that comes off the desert might sting the eyes and look sandy, but it is still not fully saturated with grains. It picks up new sand from the dunes and moves it along. Without this assumption, the dunes simulated in previous computer models did not migrate. “The main difference with our model is that the dunes can move,” he says.
Herrmann and his colleagues are now working to adapt their model to include the effect of BOFIX screens. When they are done, they should be able to predict not only where a dune will go next, but also how to tear it down or divert it. Herrmann is so taken with Meunier’s ideas that he is using money he received from a physics prize to pay for fieldwork and supplies for researchers at Nouakchott University. Eventually, they hope to have a tool that can be used to analyse a dune, test out the placement of BOFIX screens, and decide on the best configuration to eradicate the threat to settlements.
The hope is that, eventually, engineers will be able to use screens to channel sand into a “sand highway” that snakes through a town. This would not be a fast-flowing highway but a human-controlled dune. Screens set up along its crest would direct the wind to push sand grains in the required direction. For example, in Nouakchott, the sand grains would creep westward to the Atlantic Ocean. “There is a 3000-kilometre-[wide] sand sea to the east of Nouakchott,” says Herrmann. “There is always new sand coming. The sand has to go through.”
Last year, after a decade of fighting the dunes, Meunier’s failing health at the age of 76 forced him to abandon his BOFIX site, pay his workers their final salary, and return to the sand-free environment of Paris. But in Nouakchott, a non-profit organisation plans to use BOFIX screens to protect a 200-kilometre pipeline that they hope will bring water from the Senegal River to the city. Meunier’s ideas are already giving people the confidence to take on the sands.
