116
A. M. Sharaky
Annual
Ranges of speed (knots)
Scale of direction
0 5 10 15
%
%
Fig. 9 Wind roses of Dakhla Oasis, 1971–1980
q = V
2
(V − V t ),
(1)
where q = rate of sand drift potential, V = wind velocity at 10 m height and V t =
impact threshold velocity.
The energy of surface winds was classified in the desert regions, depending on the
average annual drift potential, into the following groups (Fryberger 1979): (1) lowenergy environment, with the drift potential (DP) less than 200 vector units (VU);
(2) intermediate-energy wind environment with DP ranging between 200 and 399
VU; and (3) high-energy wind environment in which DP is greater than 400 VU. The
threshold velocity of 0.2 to 0.3 mm quartz sand grains was estimated as 5.97 m/s
for many desert dune sands (Fryberger 1979). This value was rounded to 6.17 m/s
and applied in this study. The average value of the annual drift potential at Farafra
Depression (1882 VU) is much higher than that at Dakhla Oasis (787 VU). Dakhla
A. M. Sharaky
Annual
Ranges of speed (knots)
Scale of direction
0 5 10 15
%
%
Fig. 9 Wind roses of Dakhla Oasis, 1971–1980
q = V
2
(V − V t ),
(1)
where q = rate of sand drift potential, V = wind velocity at 10 m height and V t =
impact threshold velocity.
The energy of surface winds was classified in the desert regions, depending on the
average annual drift potential, into the following groups (Fryberger 1979): (1) lowenergy environment, with the drift potential (DP) less than 200 vector units (VU);
(2) intermediate-energy wind environment with DP ranging between 200 and 399
VU; and (3) high-energy wind environment in which DP is greater than 400 VU. The
threshold velocity of 0.2 to 0.3 mm quartz sand grains was estimated as 5.97 m/s
for many desert dune sands (Fryberger 1979). This value was rounded to 6.17 m/s
and applied in this study. The average value of the annual drift potential at Farafra
Depression (1882 VU) is much higher than that at Dakhla Oasis (787 VU). Dakhla