Geoscience Reference
In-Depth Information
loss of housing and transport routes through
burial and erosion.
Ultimately, in many river systems, the sus-
pended sediment will end up in a playa or lake
system. Reservoirs in arid areas are thus subject
to high rates of sedimentation, so that their half-
lives are considerably reduced. For example, the
Tarbella Reservoir on the Indus lost 6% of its
capacity within 5 years of construction (Ackers
& Thompson 1987). Thus, siltation is a primary
concern for reservoirs in arid zones, with miti-
gation taking the form of slope stabilization by
vegetation, check dams to retain sediments in
gully systems and maintenance of the reservoir
though flushing.
Another potential problem associated with
water in arid environments is the development
of karst and pseudokarst. The lack of water
means that deposits with a low unconfined com-
pressive strength such as gypsum can become
scarp-forming caprocks. Arid environments also
commonly supply the necessary geological and
soil-chemistry conditions (e.g. unconsolidated
sediments, abundant dispersive salts such as
sodium) for the subsurface flow of water, lead-
ing to the development of subsurface piping or
'pseudokarst'. Where these features develop they
offer problems for construction and, as they
are liable to change though collapse and natural
expansion (Fig. 5.20), can lead to the destabil-
ization of existing structures.
Fig. 5.20
Megapipe developed in gypsiferous Quaternary
deposits, south-east Spain.
weathering in urban areas are through (i) chem-
ical alteration (e.g. corroding reinforcing bars in
concrete and chemically altering concrete) and
(ii) volumetric changes due to salt weathering
(e.g. leaching of salts from foundations causing
subsidence, or hydration leading to expansion
and ground heave).
5.6.3 Salt hazards
Salt weathering is a major feature of deserts
(Doornkamp & Ibrahim 1990). Salt weathering
accelerates the breakdown of source rocks to
silt-grade material (Goudie 1984) thus making
them more susceptible to other sediment erosion
agents such as wind and water, and exacerbat-
ing dust problems. It also attacks the fabric of
houses, roads (Cooke et al. 1982; Doornkamp
& Ibrahim 1990) and important archaeological
features such as the Sphinx in Egypt. In many
cases accelerated salt weathering may be directly
related to increased salt in the local environment
as a function of irrigation (Goudie 1977) or
building-site leakages. The main impacts of salt
5.7
FUTURE ISSUES
The population of Earth has increased from some
2.5 billion people in the 1950s to 6.1 billion in
the year 2000. This growth in population creates
greater pressure to use resources in arid environ-
ments and increases the risk of desertification.
In all it has been calculated that there are some
49 million square kilometres on Earth at some
risk of desertification (Eswaran et al. 2001).
Most of this degradation is associated with un-
sustainable land management practices, which
have an impact on the soil and vegetation and
