Environmental Engineering Reference
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a sharp drop in the influx of montane and Sudano-Guinean pollen into the Chad area
between 5000 and 4000 year BP.
22
Saharan climates have also undergone significant changes in recent centuries, based
on an analysis of historical sources, meteorological records, and studies of lake and river
fluctuations.23
23
For example, from the early sixteenth century until the eighteenth century, the
Sahel experienced increased rainfall in comparison with the present century; in the eighteenth
century, there were frequent famines and droughts; there was increased precipitation in the
late nineteenth century; and in the early twentieth century, there was marked aridity along
the Saharan margins. The rivers and lakes showed low positions around 1912, again in the
early 1940s, and in the period since the late 1960s. This last fluctuation caused a marked
diminution in the area and volume of Lake Chad, and it led to a marked increase in the level
of dust-storm activity in a great belt from Mauritania to Sudan.
24
1.3 Southern Africa
There are three main desert areas in southern Africa. In the west, there is the coastal des-
ert of the Namib
25
that extends some 1750 miles from south of Luanda in Angola to St.
Helena bay in the Republic of South Africa (Figure 1.4). It is bounded on the east by the
Great Escarpment and forms a narrow strip, generally less than about 94 miles wide. In the
central and coastal parts of the Namib, hyperaridity prevails (the mean annual rainfall at
Walvis Bay is around 0.91 in.), although fog and dew are of frequent occurrence. The desert
15°
23°
Walvis
Bay
Sandwich
Harbor
Height (ft)
Spacing (ft)
Linear dunes
Complex
Compound
straight
Compound
anatomosing
Simple
Kuiseb R.
4500
Conception
Bay
Tsondab R.
24°
3000
300
300
Cresentic dunes
Barchans
Simple
75
4500
Meob Bay
300
>6000
Sossus
Vlei
>150
25°∞
Compound
150
75
150
6000
Star dunes
3000
3000
Zibars
75
Sand sheets
150
4500
3000
26°
75
0
20
km
Lüderitz
27°∞
FIGURE 1.4
Map of the Central Namib Desert. (From previously unpublished data. With permission.)
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