Geoscience Reference
In-Depth Information
16
The power of the wind in extreme meteorological events
is evident from hurricane, typhoon and tornado damage
to property and the resultant human misery. Wind
translates this power into geomorphic work indirectly
through its ability to drive waves in the coastal zone,
and move sand or silt to produce dunes or loess. Its
direct geomorphic impact, however, is much less closely
associated with planetary storm belts and is restricted
largely to redistributing and ornamenting products of
other processes. The predominance of quartz sand and
silicic silt in
aeolian
(wind-blown) sediments denotes
a final sorting of the residual fractionates of other denuda-
tion processes. Wind is linked romantically with ever
shifting sand seas of Earth's hot deserts and their nomadic
peoples. Common landform terms are often Arabic in
origin for this reason but the presence of coastal sand
dunes, extensive Pleistocene
loess
(aeolian dust) belts and
dustbowls on intensively farmed land in more humid,
mid-latitude areas is testimony to its opportunistic attack
on susceptible materials everywhere. Building sites, urban
landscapes in general and exposed mountain tops provide
additional sources of airborne particles. Wind agency is
also important in desertification, where it exacerbates
land degradation processes (see
Chapter 26).
in earlier chapters, show some variation in the wind
environment. Although aspects of laminar and turbulent
flow and the application of force are broadly similar,
density differences between aqueous and gaseous fluids
are very significant. Water is three orders of magnitude
more dense than air (1,000 kg m
-3
, compared with 1·22
kg m
-3
at sea level) and therefore applies greater force at
any given velocity. For example, stream flow maintains
particles over 100 mm in diameter (small pebbles) in
motion at some 6·0 m s
-1
. This is the wind velocity
required to
entrain
fine sand over 200 μm or maintain
coarse sand over 600 μm in motion. However, lower
density increases sorting efficiency, with particles falling
rapidly out of incompetent flow, and
grain ballistics
are
more effective in moving stationary particles on impact.
The transmission of force from moving to stationary
particles lowers the entrainment threshold for the
stationary particle.
Air flow is constrained in the boundary layer with
the ground like any other fluid but the nature of the
topographic surface is particularly important in control-
ling its
effective
velocity and patterns of turbulence
(see below). Air turbulence may extend through layers
10
1-4
m thick, unlike turbulent flow in the nearshore zone
and rivers, where it is restricted by much shallower water
depths, and air flow is not confined to narrow channels.
Aeolian bed forms can develop on a massive scale if the
sediment source is sufficient. Temperature also varies over
a wider range in air than in water, which increases its
influence on air viscosity. Threshold velocities for sand
particles decrease as temperature falls and density rises,
enhancing entrainment in cold climates.
AEOLIAN PROCESSES
Fluid motion of the wind
General dynamics of fluid motion relevant to the entrain-
ment, transport and deposition of earth materials, set out
