Environmental Engineering Reference
In-Depth Information
velocity over a bare surface and 10 cm tall grass layer and (b)
particle size-velocity relationship for entrainment (fluid) and
ballistic impact. Source: After Warren (1979).
EFFECTIVE WIND AND DEFLATION
The prime ingredients of aeolian landsystems are effective wind velocities and
turbulence, with a large supply of uncohesive particles of sand, silt and clay size. In
practice this requires that air flow is unimpeded by vegetation, which displaces threshold
velocities away from the surface - and therefore the target materials - at its boundary
layer (see box, p. 344, Figure 1). Vegetation also retains soil moisture, adds humus and
binds grains - all of which increase cohesion. Dry sand is uncohesive and possesses only
friction strength under shear stress, whereas silt-clay particles in the presence of moisture
develop low cohesive strength. Wind, like flowing water, operates over a wide range of
velocities but in practice aeolian processes occur only in arid conditions where vegetation
and soil are sparse or absent. Air flow also contributes to aridity through its removal of
evapotranspired water and steepening of hydrological gradients. Soils are virtually
immune to deflation at low
matric forces
, approximately 1·5 × 10
3
Pa (permanent wilting
point).
Long airborne residence times and incorporation at higher altitudes above 1-2 km can
maintain particles in transit for 10
3-4
km and accounts for dust plumes downwind and
offshore of major sources. It is calculated that 200-500 Mt a
−1
is deflated from the Sahara
to the Atlantic by the Harmattan wind, over 75 per cent of which is deposited within
2,000 km of the West African coast though some reaches the Caribbean. The loess
deposits of China involve similar transport distances and it is not uncommon for Saharan
dust to reach northern Europe, circling the western edge of blocking anticyclones. Rain
forces wet deposition earlier than dry fall-out. At lower elevations, under 1 km, dust
storms are common events in arid environments and are indicative of soil desiccation and
the degradation of farmland there, and also in more humid mid-latitude areas. Deposition
of deflated material occurs when wind velocity falls below the thresholds needed to
maintain motion but is often transient and aeolian sediments are characterized by their
relentless progress downwind. Sand and silt are totally segregated
en route
.
ABRASION
Ballistic impacts inevitably bring quartz grains into sharp contact with softer materials
which they abrade, at the same time removing their own angular edges, either when
matched for hardnesses or when grains are flawed. Fracture may also occur during
saltation, and 'collision splash' releases fines for further deflation. Abrasion occurs on
larger clasts incapable of deflation and exposed bedrock surfaces which may undergo
general surface lowering, at rates varying between 1-10 m ka
−1
and 10-50 mm ka
−1
in hot
and polar desert respectively. This difference is probably explained by the protective
effect of snow cover and the almost perennially frozen state of soil moisture. However,
deflated ice fragments become hardened as temperature falls and may mimic the hardness
