Geoscience Reference
In-Depth Information
Figure 23.1
Dunes migrate across a newly built road in Ras Al-Khaimah, United Arab Emirates. The most frequent dune moving
hazards result from small transverse dunes that migrate quickly in a unidirectional wind (photo: author).
the movement of dunes caused by such aeolian activ-
ity where small (1-2 m high) transverse dunes may mi-
grate >15 m/yr, therefore quickly burying roads and crops
(Misak and Draz, 1997; Al-Harthi, 2002).
Such aeolian hazards tend to be associated with active
dunefields and sand transport corridors in dryland regions
where topographic depressions accumulate sand-sized
material (see Chapter 17). Human activity within or on the
edges of such regions (e.g. the Siwa Oasis in Egypt on the
edge of the Great Sand Sea; Misak and Draz, 1997) involv-
ing the establishment of roads, railways, irrigation canals
and farmland then become susceptible to blowing sand
and dune movement (Watson, 1985, 1990) (Figure 23.1).
Such hazards are therefore often predictable in both time
and space and, where planning schemes to avoid such
hazards cannot be utilised, there are several engineering
techniques involving fences or surface treatments that can
be employed to reduce the impacts of blowing sand and
dune movement by enhancing upwind deposition of sedi-
ment, immobilising susceptible surfaces, enhancing sand
transport through the hazardous area or deflecting sand
movement away from the area to be protected (Watson,
1985, 1990). However, the maintenance of such schemes
is often sporadic and so comprehensive control of sand
drift can frequently be lacking (Misak and Draz, 1997).
The recent construction of roads and railways through
active dunefields in China provides an excellent exam-
dunes and mitigating measures that can be employed. The
Taklimakan Desert sits in the Tarim Basin and has ac-
cumulated shifting sand across 80 % of its 338 000 km
2
area. Between 1991 and 1995 a 560 km long highway was
built north-south through the desert in order to enable the
exploitation of petroleum reserves (Dong
et al.
, 2004).
As 447 km of the road was built through actively mo-
bile dunefields, it has proven to be an excellent laboratory
within which to test engineering measures to control blow-
ing sand and dunes. Specific problems were found where
the road dissected small (
2 m high) barchan dunes mi-
grating between 5.0 and 7.5 m/yr (Dong
et al.
, 2000) and
also where the road cut through the elongating fronts of
small linear dunes and sand sheets within intermegadune
flats (Dong
et al.
, 2004).
Sand control was effectively accomplished by combin-
ing four different measures, depending on the local situ-
ation.
Porous fences
about 1.1 m high and made of reed
or nylon were established along a 10-20 m wide belt on
the upwind edges of the highway. The fences reduce the
wind speeds across a fetch of 20-25 times the fence height
and enhanced sand deposition is evident both upwind and
downwind of the fence. There has been much research
into the design of porous fences with porosities of be-
tween 0.3 and 0.4 providing airflow characteristics most
suitable for reducing wind erosion (Lee and Kim, 1999;
Dong
et al.
, 2006). As fence porosity increases from 0
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