Page 103 - Fundamentals of Geomorphology

86 INTRODUCING LANDFORMS AND LANDSCAPES

already in the air.The Bernoulli effect arises from the fact great distances. Sand particles may be lifted into the
that wind speed increases swiftly away from the ground lower layers of the atmosphere, as in sandstorms, but
surface, so that a surface particle sits in a pressure gradi- will fall out near the point of takeoff. Dust particles
ent, the top of the particle experiencing a lower pressure may be carried around the globe. Dust storms may
than the bottom of the particle. The Bernoulli effect carry 100 million tonnes of material for thousands
is boosted where airflow accelerates around protrud- of kilometres. A dramatic dust storm, which car-
ing objects. However, the most effective mechanism ried an estimated 2 million tonnes of dust, engulfed
for getting particles airborne is bombardment by par- Melbourne, Australia, on 8 February 1983 (Raupach
ticles already in flight. So the movement of particles et al. 1994).
is slow when a wind starts, as only lift is operative, 4 Creep and related near-surface activity. Coarse sand
but it picks up by leaps and bounds once saltation and and small pebbles inch forward by rolling and slid-
associated bombardment come into play. ing with the momentum gained from the impact
Wind transport encompasses four processes – of jumping sand particles and down the tiny crater-
saltation, reptation, suspension, and creep (Figure 3.17): slopes produced by an impacting particle.

1 Saltation. Sand grains bound, land, and rebound, It should be stressed that saltation is the key process.
imparting renewed impetus to other sand grains. Once saltation cuts in, it powers all the other processes,
Such motion is confined to short distances and especially creep and reptation. Even the entrainment of
heights of about 2 m. fine particles destined to become suspended is mainly
2 Reptation. On hitting the surface, saltating grains induced by jumping grains.
release a small splash-like shower of particles that The dividing line between saltation and suspension
make small hops from the point of impact. This appears to lie at about particles of 100 micrometres
process is reptation. diameter. Particles smaller than 100 micrometres have
3 Suspension. Particles of silt and clay lifted into the fall velocities lower than the upward velocity of the tur-
atmosphere become suspended and may be carried bulent wind and so stay in the air until the wind abates,

Outgoing
saltating grain

Suspension

Incoming
saltating grain
Creep
Reptation Approach
angle
Creep

Figure 3.17 Modes of grain transport by wind.
Source: Adapted from Livingstone and Warren (1996, 13)