Environmental Engineering Reference
In-Depth Information
Chapter 16
The work of the wind
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 denudation 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 27).
AEOLIAN PROCESSES
FLUID MOTION OF THE WIND
General dynamics of fluid motion relevant to the entrainment, transport and deposition of
earth materials, set out 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 (1000 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 controlling its effective
velocity and patterns of turbulence (see below). Air turbulence may extend through layers
 
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