Geology Reference
In-Depth Information
lesser roles. Ground-surface factors include vegetation
cover, roughness, obstacles, and topographic form.
Soil factors include moisture content, structure, and
density.
Wind velocity profile
Wind erosion
Wind erosion engages two processes - deflation and
abrasion. Deflation is the removal of loose particles by
the wind. Smaller sedimentary particles are more suscep-
tible to wind erosion than larger particles. Particles of
about 100 micrometres diameter are the most vulnera-
ble to wind erosion. Above that size, increasingly higher
velocities are needed to entrain increasingly large grains
and to keep them airborne. Below that diameter, and
especially for clay particles, greater wind velocities are
needed to surmount the cohesional forces binding indi-
vidual grains together. Deflation of sand-sized particles
is localized, and it takes a long time to move sand great
distances. Silt and clay, on the other hand, are far more
readily lifted by turbulence and carried in suspension
in the atmosphere, the finest material being transported
great distances. The world's hot deserts are a leading
source of atmospheric dust. Even temperate areas may
produce dust. In south-eastern Australia, a wind-blown
dust, locally called parna , covers wide areas. Soil erosion
by wind is well documented and well known (p. 313).
Wind without grains is an impotent geomorphic
agent; wind armed with grains may be a powerful ero-
sive agent. Abrasion is the cannonading of rock and other
surfaces by particles carried in the wind - a sort of nat-
ural 'sandblasting'. Rocks and boulders exposed at the
ground surface may be abraded by sand and silt particles.
Abrasion rates appear to be highest where strong winds
carry hard sand grains from soft and friable rocks upwind.
Sand particles are carried within a metre or two of the
ground surface, and abrasion is not important above that
height.
z 0
Wind velocity, u z
Figure 3.16 Wind velocity profile.
Ralph Alger Bagnold (1941). The wind-velocity profile
(Figure 3.16) may be written as:
u
z
z 0
κ
=
u z
ln
where u z is the wind speed at height z , z is height above
the ground,
(kappa) is the Kármán constant (which is
usually taken as
κ
0.4), z 0 is roughness length (which
depends on grain size), and u
is the shear or friction,
defined as:
τ
0
ρ
u
=
a
where
τ
0 (tau-zero) is the shear force per unit area and
ρ
a (rho-a) is the air density.
In moving, air behaves much like water. As air is about
a thousand times less dense than water, it cannot trans-
port such large particles. Nonetheless, the wind is an
agent of erosion and transport. The ability of wind to
erode, entrain, and convey rock and soil particles depends
upon the nature of the wind, the nature of the ground
surface, and the nature of the soil or rock. Crucial wind
factors are the wind velocity and the degree of tur-
bulence,
Wind transport
Before the wind can transport particles, it must lift
them from the ground surface. Particles are raised by
'lift', which is produced by the Bernoulli effect and the
local acceleration of wind, and bombardment by particles
with
air
density
and
viscosity
playing
 
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