Aeolian Sand Transport Potential and Its …
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the linear and crescentic dune sands, respectively. Opaque minerals are the heaviest
mineral group recorded in the dune sands. They represent 80% of the heavy fraction.
The opaque minerals are goethite, hematite, magnetite, and limonite. The non-opaque
minerals are represented by a suite of minerals enriched in ultra-stable zircon and
tourmaline, and meta-stable minerals such as epidote, garnet, and staruolite. The
ultra-stable minerals are concentrated on the dune crests and the eastern flanks of
crescentic and linear dunes. Zircon is the most abundant non-opaque mineral in the
crescentic dune sands, averaging 35.5%, but it is next to epidote in the linear dune
sands (Sharaky 1990). The abundance of zircon indicates that these minerals are
being reworked from older sediments, most probably the Nubian sandstones.
3 Movement of Sand Dunes in Dakhla Oasis
The sand dunes in Dakhla Depression are in continuous movement southwards. The
sand dune encroachment threatens roads, houses, water wells, and cultivated lands.
Two groups of aerial photographs for the western area of Dakhla Depression (21 years
apart) were used to determine the rate of sand dune movement. The first group is a
set of 82 crescentic dunes, and the second set includes 25 linear dunes. They were
chosen to measure the sand dune movement of different dune belts, types, forms, and
sizes. The dimensions of the crescentic dunes including width and length, distance
from the toe of slipface to a distinct land mark were measured.
The Dakhla crescentic sand dunes range in movement between 0.5 and 14 per
year, in average 5.8 m per year. The dune movement differ from one place to another
based on local conditions. The most important factors of dune movements are dune
size and local wind velocity. In general, small crescentic dunes move faster than the
larger ones. The sand dunes in the northern part are smaller in size and move faster
than the massive southern sand dunes. The crescentic sand dune grow in width and
length in average 25 and 37 cm per year, respectively.
4 Sand Drift Potential (SDP) in Dakhla Oasis
The prevailing wind in Dakhla Depression is NNW (see Fig. 9). Winds of gale force
and above are rare in the depression. The annual average surface wind speed more
than 5.14 m/s is about 13% (Sharaky 1990). The Fryberger formula of sand drift
potential was applied to the average effective wind speed calculated from the wind
data of Dakhla and Farafra meteorological stations (1970–1980) (Fryberger 1979).
The wind values representing the relative rates at which winds of different average
velocities can move sands are called weighting factors (Fryberger 1979). They are
derived by substituting values of wind velocities (average wind speed of a velocity
category) for the weighting formula of Lettau (McKee 1979) as follows:
115
the linear and crescentic dune sands, respectively. Opaque minerals are the heaviest
mineral group recorded in the dune sands. They represent 80% of the heavy fraction.
The opaque minerals are goethite, hematite, magnetite, and limonite. The non-opaque
minerals are represented by a suite of minerals enriched in ultra-stable zircon and
tourmaline, and meta-stable minerals such as epidote, garnet, and staruolite. The
ultra-stable minerals are concentrated on the dune crests and the eastern flanks of
crescentic and linear dunes. Zircon is the most abundant non-opaque mineral in the
crescentic dune sands, averaging 35.5%, but it is next to epidote in the linear dune
sands (Sharaky 1990). The abundance of zircon indicates that these minerals are
being reworked from older sediments, most probably the Nubian sandstones.
3 Movement of Sand Dunes in Dakhla Oasis
The sand dunes in Dakhla Depression are in continuous movement southwards. The
sand dune encroachment threatens roads, houses, water wells, and cultivated lands.
Two groups of aerial photographs for the western area of Dakhla Depression (21 years
apart) were used to determine the rate of sand dune movement. The first group is a
set of 82 crescentic dunes, and the second set includes 25 linear dunes. They were
chosen to measure the sand dune movement of different dune belts, types, forms, and
sizes. The dimensions of the crescentic dunes including width and length, distance
from the toe of slipface to a distinct land mark were measured.
The Dakhla crescentic sand dunes range in movement between 0.5 and 14 per
year, in average 5.8 m per year. The dune movement differ from one place to another
based on local conditions. The most important factors of dune movements are dune
size and local wind velocity. In general, small crescentic dunes move faster than the
larger ones. The sand dunes in the northern part are smaller in size and move faster
than the massive southern sand dunes. The crescentic sand dune grow in width and
length in average 25 and 37 cm per year, respectively.
4 Sand Drift Potential (SDP) in Dakhla Oasis
The prevailing wind in Dakhla Depression is NNW (see Fig. 9). Winds of gale force
and above are rare in the depression. The annual average surface wind speed more
than 5.14 m/s is about 13% (Sharaky 1990). The Fryberger formula of sand drift
potential was applied to the average effective wind speed calculated from the wind
data of Dakhla and Farafra meteorological stations (1970–1980) (Fryberger 1979).
The wind values representing the relative rates at which winds of different average
velocities can move sands are called weighting factors (Fryberger 1979). They are
derived by substituting values of wind velocities (average wind speed of a velocity
category) for the weighting formula of Lettau (McKee 1979) as follows: