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Deserts and Winds
Definition of Desert
A desert is an area with less than
25 cm (10 inches) of annual precipitation
aridity index = potential evaporation/precipitation
greater than 4.0
Deserts may be cold, temperate or hot.
All major continents have one type of
desert or the other.
Distribution and causes of dry lands
Dry regions cover 30 percent of Earth’s
land surface
Two climatic types are commonly
recognized
• Desert or arid
• Steppe or semiarid
World's Deserts
about 1/5th of land
Atmospheric Convection and Subtropical Deserts
Tropopause Barrier
RAIN
Wind in the desert
Transportation of sediment by wind
• Differs from that of running water in two
ways
– Wind is less capable of picking up and
transporting coarse materials
– Wind is not confined to channels and can
spread sediment over large areas
Wind in the desert
Wind erosion
• Wind is a relatively insignificant erosional
agent with most erosion in a desert
performed by intermittent running water
• Mechanisms of wind erosion
– Deflation
– Lifting of loose material
– Deflation produces blowouts (shallow
depressions) and desert pavement (a
surface of coarse pebbles and cobbles)
Desert pavement is formed from larger
rocks and fragments left after deflation.
Desert
Pavement
Wind in the desert
Transportation of sediment by wind
• Mechanisms of transport
– Bedload
– Saltation – skipping and bouncing along
the surface
– About 20 to 25 percent of the sand
transported in a sandstorm is moved this
way
– Suspended load
Wind in the desert
Wind erosion
• Mechanisms of wind erosion
– Abrasion
– Produces ventifacts (stones with flat faces)
and yardangs (wind sculpted ridges)
– Limited in vertical extent
Yardang: A small wind-sculpted rock formation caused
by abrasion .
Figure 15.4
Wind in the desert
Wind deposits
• Significant depositional landforms are
created by wind in some regions
• Two types of wind deposits
– Dunes
– Mounds or ridges of sand
– Often asymmetrically shaped
– Windward slope is gently inclined and the
leeward slope is called the slip face
Formation of a desert blowout
Formation of sand dunes
Sand dunes in the
western United States
Wind in the desert
Wind deposits
• Two types of wind deposits
– Dunes
– Slow migration of dunes in the direction
of wind movement
– Several types of sand dunes including
transverse, longitudinal, parabolic
9. What are the three classes of dune forms?
We can simplify dune forms into three classes
1–crescentic2- linear3- star dunes, and others.
Crescentic dunes are divided into four types:
Barchan, Transverse, Parabolic, and Barchanoid
Ridge. Barchan dunes are crescent shaped dunes
with horns pointed downwind. Winds are constant
with little directional variability. Limited sand
availability. Transverse dunes are asymmetrical ridges
which are transverse (perpendicular) to the wind
direction. Surface has abundant sand supply.
Parabolic dunes are generated by vegetation, open
end faces upwind with U-shaped and arms shaped by
the vegetation. Barchanoid ridge dunes, are wavy,
symmetrical dune ridges aligned in right angels to the
winds. Formed from coalesced barchans. (See next
Barchan
a) barchan dune
WIND
b) parabolic dune
depression
c) longnitudinal dune
d) transverse dune
Parabolic
Longnitudinal
Longitudinal Dunes
Transverse
9. What are the three classes of dune forms?
(continued)
The next class of dunes is called linear dunes. They
are divided into two types: Longitudinal and Seif.
Longitudinal dunes are long, ridge-shaped dunes that
are aligned parallel to the wind direction and have two
slipface. Average 100 meters high and 100 kilometers
long and can reach to 400 meters high. Seif dunes
(means sword in Arabic) a sharp-crested sand dune
with curved edges, often several miles long. Runs in a
series of parallel ridges; most common in the Sahara
desert.
Longitudinal Dunes (left) and a Satellite photo
of Seif Dunes in Saudi Arabia (Right).
9. What are the three classes of dune forms?
(continued)
The third class of dunes is called a Star dune
(One type only). Star dunes are giant dunes;
Pyramidal or star shaped. Slipsurfaces in
multiple directions. Resulting from winds
shifting in all directions. (See next slide).
Star Dunes
Star
9. What are the three classes of dune forms?
(continued)
The last class is named “other” for other types.
There are two of them: Dome dunes and
Reversing dunes. Dome dunes are circular
or elliptical mounds with no slipface. Reversing
dunes are asymmetrical ridges formed
intermediately between star dunes and
transverse dunes formations. Wind direction
can alter their shapes between forms. (See
next slide).
Dome and Reversing dunes.
Wind in the desert
Wind deposits
• Two types of wind deposits
– Loess
– Blankets of windblown silt
– Two primary sources are deserts and
glacial outwash deposits
– Extensive deposits occur in China and the
central United States
10. Another form of material deposits are loess
deposits. How are loess materials generated? What
form do they assume when deposited?
Pleistocene glaciers advanced and retreated in
many parts of the world, leaving behind large
glacial outwash deposits of fine-grained clays
and silts (<0.06 mm). These materials were
blown great distances by the wind and
redeposited in unstratified, homogeneous
deposits named loess. Loess deposits form
some complex weathered badlands and some
good agricultural land.
2. Loess = silt and clay
Blown from glacial outwash plains, deserts,
and floodplains to areas where they have
been stabilized by vegetation
From rich top soils of the Ukraine to Germany,
China and the northern plains of U.S.
Figure 15.24: Desertification
Figure 15.24
Wind defects and Engineering solution
Wind Deffects:
Covering rail ways, roads,drainage canals,
erosion of wooden pools,cover building by
sand
Solutions:
Planting
Fencess
Bitumin coat