Geoscience Reference
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human activities, other natural factors, such as drought, compound its effects. The
considerable scale of the phenomenon is such that the means mobilized by the pop-
ulation have not proven sufficient to protect the region's important infrastructures
(inhabited villages, farmed and grazing areas, wells). It has largely been due to
government mobilization and the assistance of NGOs that a number of villages (such
as Tarfey, Rig-Rig and Barra) and certain wadis (Miou, Barkadroussou, Moto) have
been saved from being engulfed by sand. The successful actions implemented by the
pilot Kanem Agro-Forestry Pastoral Development Project can be used as a model
for future interventions against sand encroachment in arid and semi-arid zones. The
principal causes are the following: persistent drought, overgrazing, cattle tramping,
deforestation and relentless tree logging (evaluated at 250,000 tonnes in 1988),
and farming on sand dunes (rendering the dune surface vulnerable to the erosive
effects of the wind). In fact, 219 villages, 324 wadis and numerous boreholes and
grazing areas are increasingly becoming so threatened by sand encroachment as to
be potentially life threatening for the local population in Kanem.
The strategy used to fight desertification in Kanem hinges on the following four
imperatives:
The protection of threatened sites and the regeneration of ecological resources
The improvement of production systems
The reinforcement of institutional capacities
The development of a national scheme of land planning
It is within the framework of this combat strategy that the agro-forestry pastoral
development project in operation in Kanem from 1993 to 1998 contributed to
saving certain villages, wadis , schools and clinics from the catastrophe of sand
encroachment. Both structural and biological methods were used to halt sand
encroachment.
12.2
Structural Methods of Inhibiting Sand Encroachment
12.2.1
Brush Fences
Fencing consists of setting up a barrier (using date palms, Leptadenia branches,
thorny twigs and millet thatch) between the source of the sand and the threatened
area. As sand accumulates along this barrier, it forms an artificial dune, which
itself acts as a further obstacle. The fence is placed 200-300 m from the site to
be protected, orientated perpendicular to the dominant wind direction.
The following characteristics are required for the success of this approach:
The fence must have a certain permeability, to prevent the wind from destroying
it. The only criteria used is the quantity of material used: an average of 20-25 palms
or 4-6 branches of Leptadenia is used per meter;
Taking into account sand distribution along the fence according to wind flux, the
ideal height of the fence is between 1.5 and 1.8 m;
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