Page 102 - Fundamentals of Geomorphology

GEOMORPHIC MATERIALS AND PROCESSES 85

lesser roles. Ground-surface factors include vegetation
Wind velocity profile cover, roughness, obstacles, and topographic form.
Soil factors include moisture content, structure, and
density.

z Wind erosion
height, Wind erosion engages two processes – deflation and
Log 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
z 0
and to keep them airborne. Below that diameter, and
especially for clay particles, greater wind velocities are
Wind velocity, u z
needed to surmount the cohesional forces binding indi-
Figure 3.16 Wind velocity profile. 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
Ralph Alger Bagnold (1941). The wind-velocity profile readily lifted by turbulence and carried in suspension
(Figure 3.16) may be written as: in the atmosphere, the finest material being transported
z great distances. The world’s hot deserts are a leading
u ∗
u z = ln source of atmospheric dust. Even temperate areas may
κ z 0
produce dust. In south-eastern Australia, a wind-blown
where u z is the wind speed at height z, z is height above dust, locally called parna, covers wide areas. Soil erosion
the ground, κ (kappa) is the Kármán constant (which is by wind is well documented and well known (p. 313).
usually taken as ≈0.4), z 0 is roughness length (which Wind without grains is an impotent geomorphic
depends on grain size), and u ∗ is the shear or friction, agent; wind armed with grains may be a powerful ero-
defined as: siveagent.Abrasionisthecannonadingofrockandother
surfaces by particles carried in the wind – a sort of nat-

τ 0 ural ‘sandblasting’. Rocks and boulders exposed at the
u ∗ =
ρ a ground surface may be abraded by sand and silt particles.
Abrasion rates appear to be highest where strong winds
where τ 0 (tau-zero) is the shear force per unit area and carry hard sand grains from soft and friable rocks upwind.
ρ a (rho-a) is the air density. Sand particles are carried within a metre or two of the
In moving, air behaves much like water. As air is about ground surface, and abrasion is not important above that
a thousand times less dense than water, it cannot trans- height.
port such large particles. Nonetheless, the wind is an
agent of erosion and transport. The ability of wind to Wind transport
erode, entrain, and convey rock and soil particles depends
upon the nature of the wind, the nature of the ground Before the wind can transport particles, it must lift
surface, and the nature of the soil or rock. Crucial wind them from the ground surface. Particles are raised by
factors are the wind velocity and the degree of tur- ‘lift’, which is produced by the Bernoulli effect and the
bulence, with air density and viscosity playing local acceleration of wind, and bombardment by particles