Geoscience Reference
In-Depth Information
•
Resistance: the greater the height of the fence, the lower its resistance. In building
the fence, a trench at least 30 cm deep is needed to firmly sink the supporting
elements of the fence. Similarly, these fence elements should be firmly tied
together between at two or more levels along the length of the fence to increase
its resistance.
This fencing technique has been shown to be very effective in Kanem, especially
as the wind is unidirectional (NE-SW). If the opposite is the case, a network of
different meshes would be required. As the fence will eventually be engulfed by
sand, its height will need to be increased once the sand has reached within 10-15 cm
of the top of the fence.
12.2.2
Hedges
After stabilizing sand by mechanical means, it is essential to fix the dunes defini-
tively by promoting vegetation growth. The final aim is to recreate the ecosystem as
it existed in the past, which means to conquer land degradation and shifting sands
by covering them with as dense a vegetation as possible.
The choice of forest species and local and exotic grasses for biological fixation
depends on the capacities of the species to adapt to this environment (its aridity
and soil structure). The best adapted local species are
Acacia spp, Balanites
aegyptiaca, Leptadenia pyrotechnica, Ziziphus mauritiana
and
Panicum turgidum
.
Exotic species that are also well adapted include
Prosopis chilensis, Prosopis
juliflora, Parkinsonia aculeata
and
Cajanus cajan
.
Taking into consideration the intensity of sand encroachment in Kanem, biologi-
cal fixation of shifting dunes is technically only possible with planted species grown
in nurseries. The production of plants relies on, among other things, harvesting
seeds, choosing an appropriate site and training nursery growers. Work should begin
in January each year and terminate in August. Plants grown in the nurseries should
be healthy and vigorous, with a height of 50-80 cm. They should be planted out in
mid-July, or as soon as soil humidity reaches a depth of 30-40 cm, at a density of 400
plants per hectare — to minimize the cost of the operation and to limit competition
between plants, thus favoring natural regeneration.
12.3
Involving the Local Population
The results over the course of the six-year experiment were very encouraging.
The method of intervention (using a participatory approach) was based on a
contractual agreement between local people and project organizers. The nature of
the relationship, the reciprocal engagement and the working methods were written
up into a standard contract defining the terms of agreement between the two parties.
Agreement to the terms of the contract reflected the two parties shared responsibility.
Any community wishing to subscribe to a contract to combat sand encroachment
must first organize itself into a united group, which becomes the principal actor in
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