Arid environments - Environmental Sedimentology - page 184

in a fossilized landscape, but their impact upon
sediment systems is considered to be minimal.
In contrast the landscape response to aband-
oned mining is more dynamic in semi-arid envir-
onments. The Rio Tinto fluvial system in the
Province of Huelva, south-west Spain has a mean
annual rainfall at Huelva of around 500 mm yr −1
(Instituto Nacional de Meterologia). The annual
discharge ranges from close to zero in the summer
to 11 m 3 s −1 in the winter. The lowest 20 km
of the catchment is tidal, with a mean tidal
range of 2.2 m. The waters of the river are acidic
(typically pH 2.0 -2.5), but become less acidic
with the addition of Atlantic waters on flood tides.
The Rio Tinto lies within the Iberian Pyrite Belt,
one of the largest deposits of its type in the world.
The main rich ore body is 5 km long, 750 m wide
and 40 m deep. It contains abundant Fe, Cu, Zn,
Pb, Ag and Au. Mining of the sulphide deposits
has been occurring with increasing efficiency
for some 5000 years, ceasing in the late 1990s.
The impact of improved mining technologies and
efficiencies can be demonstrated by comparing
the amount mined in the Rio Tinto catchment in
million metric tons (values taken from Davis et al.
2000). The Tartessans removed 3 million tonnes
over 4200 years (5000 -2800 yr BP); the Romans
removed 24.5 in 200 years (2000 -1800 yr BP);
and the British (Rio Tinto Zinc) re-moved 1600
in the last 200 years. These mining operations
have liberated abundant suspended sediment
and solutes into the river system. Cores of 1 m
depth taken throughout the catchment from the
channel margins and dated with 210 Pb and 14 C
show overall rates of sediment accumulation
within the catchment of 0.3 cm yr −1 . High levels
of pollutants (e.g. Cu at 970 ppm for 840 yr BP
and 2466 ppm at 3640 yr BP) were found to pre-
date the RTZ opencast mining (Davis et al. 2000).

5.6.1 Aeolian hazards

The physical detachment, entrainment and trans-
port of solid particles has been enhanced in many
regions where the erodibility of the soils has been
exceeded by the erosive agents, which include
humans. Soil erosion exacerbated by humans
has been recorded from the time of Plato (fourth
century BC) to more recent history (e.g. O'Hara
et al. 1993). Aeolian removal of sediments is
a major problem in arid environments. Wind
erosion affects 39% of arid areas susceptible to
human degradation (UNEP 1992). Agricultural
mismanagement can lead to severe erosion and
sources for available dust can also be exacerbated
by human activity, especially where major water
diversions take place. Mitigation of these pro-
cesses ranges from physically removing or limiting
the source area of the sands, building protective
barriers and building to reduce the impact of
structures (Table 5.6).

5.6.1.1 Sand dune encroachment

Arid environments are characterized by sparse
or no vegetation. Thus free, mobile sand dunes
(section 5.3.4) are common in these areas. Dune
soils have proved popular for agriculture in
some regions, as they are less prone to salin-
ization than other areas. These resources can be
easily overexploited so that vegetation is lost.
Once the vegetation is removed, moisture loss is
increased and restabilization unlikely. In some
areas this is further exacerbated by quarry-
ing, for example in Kuwait, for urban building
projects. Thus, partially stabilized dunes may
become reactivated. The drifting sand becomes
a problem for road transport and can bury
settlements.
Urban areas themselves pose a problem in arid
environments as they offer an obstacle to sand
movement. In addition, associated urban activ-
ities may exacerbate the magnitude of aeolian
problems through the loss of vegetation, etc.
Sand and dust problems are thus prolific in the
literature on desertification. Areas dominated by
sand dune encroachment experience the burial
of structures and transport links, and features

LIVING WITH DESERTS : AN ENGINEERING PERSPECTIVE

5.6

Deserts provide engineering challenges where the
arid environment may have to be traversed or
inhabited. This section will consider the engineer-
ing management and remediation of sediment-
based hazards in desert environments today.