Geoscience Reference
In-Depth Information
23
Dunes and People
Dunes are beautiful structures that deserve study in their own
right. But lest the reader be confronted by demands to justify
their study, we offer a few examples here of how dunes
directly affect human livelihoods. Of course dunes have
influenced human history: whether in the age of Alexander
the Great, or the Gulf War or the many conflicts in between,
conflict in the desert places special demands on men,
animals and machines. We touch upon these in
Sect. 1.3
;
here we discuss the ongoing war of people against sand. The
fact that dunes move on timescales perceptible to humans is
a remarkable one, and is much of the fascination that
motivates this topic. For civil engineers trying to keep roads
clear, it can be a considerable inconvenience.
Settlers in the northwestern USA found some challenges
with dunes that move, such as those along the Columbia
river gorge (see Figs.
23.1
and.
23.2
). These turned out to
be poetically ephemeral, no longer existing, presumably
because the sand source has been choked. Yet even in the
1960s, Oregon dunes threatened agricultural lands, and
efforts to mitigate the problem inspired Frank Herbert (see
Occasionally, dune movement can have a positive side:
Andreotti et al. (2002) quoting a Ph.D. thesis by Oulehri,
note that the difference in migration rate between small and
large barchans dunes was known to Saharan people in
ancient times. Barchans were used to protect goods from
pillage: a small or a large dune would be chosen in which to
bury a bundle of goods, depending on when it was desired
to recover it. Further south in Africa, it is said that local
Namib peoples will lay dead bodies in the path of a dune in
order to bury them—and it will be sometime before it
reappears. It is perhaps for this reason that the area is known
as the Skeleton Coast.
Many oasis towns in the Sahara have long confronted
sand encroachment, a prominent example being InSalah in
Algeria, which has a large dune more or less in the center of
town. But the Saharan sands most famously threaten Nou-
akchott, the capital and largest city (population today
*900,000, compared with only 75,000 in 1978) of Mau-
ritania. The city's name is from the Berber 'Place of the
Winds', which hints at its predicament. The combination of
north-westerly trade winds and the harmattan, a seasonal
easterly wind, causes snake-like linear dunes to march
south-west towards the coast (Fig.
23.3
).
Nouakchott has nowhere to go—it is pinned between the
advancing dunes and the sea. In some cases, however, the
advance can be accommodated as just part of the cost of
living somewhere. For example, the desert oasis of Liwa in
the United Arab Emirates is a major center of date culti-
vation. It is striking in satellite imaging as defining an arc
within which large (2 km wide, 100 m tall) megabarchans
are found. These megabarchans are slowly advancing
southeast, at perhaps 0.1 m/year; this is apparent in that the
northern boundaries of the date plantations are sharply
defined by the slipfaces of the dunes (Fig.
23.4
) and in some
places trees can be seen in the process of being buried
(Fig.
23.5
). A tree may yield fruit for several decades, so
the dunes advance only about one tree-length (and thus, in a
typical plantation, one tree-spacing) over the useful life of
the tree. So you plant the tree anyway and you get what you
get.
French scientist Jean Meunier has pioneered a range of
methods ('BOFIX'—of course, many of these methods
have been long known, although less systematically, to
desert people worldwide) to break up dunes using fences
that divert the wind, spawning eddies that slice through the
dune in a matter of months; other (permeable) fences can be
used upstream to interrupt the sand transport to dunes
threatening individual buildings (Zandonella 2003). The
Food and Agriculture Organization (FAO) of the United
Nations has undertaken major planting efforts, growing
seedlings that might stabilize the dunes in a sustainable
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