Geoscience Reference
In-Depth Information
Deflation and entrainment
KEY PROCESSES
Deflation
is the composite process by which fine-grained materials are sorted, lifted and removed from the land
surface by wind, prior to their subsequent deposition downwind. Wind first winnows or sifts clay, silt and sand grains
from coarser particles, too large to be deflated and therefore left behind as
lag deposits
. As effective wind velocity
increases, it imposes a surface shear stress or drag on exposed particles which may at first roll or creep without
becoming airborne. Shear stress increases exponentially with wind velocity from 2 N m
-2
at 1·3 m s
-1
to 100 N m
-2
at 13 m s
-1
. This establishes general motion, which interacts with air flow in the development of bed forms, discussed
later.
Ballistic impact or collisions between particles, as they begin to move, sets a broader field of particles sliding or
rolling in a process known as reptation. Larger sand grains may continue to move by such means alone. Fine particles
become entrained in the air stream when turbulent lift and shear stress exceed normal stress and friction. Wind
velocity increases rapidly above the land surface as friction falls, reducing pressure above the particles (
Figure 16.1
).
This creates a Bernouilli effect which draws fine particles upwards into the flow. Entrainment may also occur by
saltation as ballistic impacts from descending particles kick others into the air. Larger sand grains have low, short
trajectories and move mostly by saltation, whereas, once airborne, turbulence may support finer grains up to 200
μm in suspension indefinitely.
The sequence from creep, reptation, saltation to suspension occurs with increasing wind velocity or decreasing particle
size. Average grain size in sand seas lies between 100 microns and 1 mm, with a modal size approximately 300
microns. Entrainment and ballistic thresholds for medium and coarse sand (over 200 microns and 600 microns to 2
mm respectively) are 5·0 and 4·2 m s
-1
and 8·0 and 6·0 m s
-1
(with 5·5 and 4·5 m s
-1
for the modal size).
Deflation is the prerequisite for all other processes in the aeolian environment, including abrasion and sandblast, the
excavation of deflation hollows and creation of mobile bed forms. A special case can also be made for particulate
injectionby volcanic eruption and sea spray, adding ash, tephra and fine coastal materials to the atmosphere. Although
not part of the aeolian mainstream, they may occur in quantities capable of geomorphic and stratigraphic significance.
immune to deflation at low
matric forces
, approximately
1·5
10
3
Pa (permanent wilting point).
Long airborne residence times and incorporation at
higher altitudes above 1-2 km can maintain particles in
transit for 10
3-4
km and accounts for dust plumes
downwind and offshore of major sources. It is calculated
that 200-500 Mt yr
-1
is deflated from the Sahara to the
Atlantic by the Harmattan wind, over 75 per cent of which
is deposited within 2,000 km of the West African coast
though some reaches the Caribbean. The loess deposits of
China involve similar transport distances and it is not
uncommon for Saharan dust to reach northern Europe,
circling the western edge of blocking anticyclones. Rain
forces wet deposition earlier than dry fall-out. At lower
elevations, under 1 km, dust storms are common events
in arid environments and are indicative of soil desiccation
and the degradation of farmland there, and also in more
humid mid-latitude areas. Deposition of deflated material
occurs when wind velocity falls below the thresholds
Effective wind and deflation
The prime ingredients of aeolian landsystems are effective
wind velocities and turbulence, with a large supply of
incohesive particles of sand, silt and clay size. In practice
this requires that air flow is unimpeded by vegetation,
which displaces threshold velocities away from the surface
- and therefore the target materials - at its boundary layer
(see
Figure 16.1
).
Vegetation also retains soil moisture,
adds humus and binds grains - all of which increase
cohesion. Dry sand is incohesive and possesses only
friction strength under shear stress, whereas silt-clay
particles in the presence of moisture develop low cohesive
strength. Wind, like flowing water, operates over a wide
range of velocities but in practice aeolian processes occur
only in arid conditions where vegetation and soil are
sparse or absent. Air flow also contributes to aridity
through its removal of evapotranspired water and
steepening of hydrological gradients. Soils are virtually
