Mechanics of Sand Transport - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 35

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Mechanics of Sand Transport

The character of an aeolian landform must depend on how
the sand moves. This, of course, depends on the interaction
of the particles with the fluid: where can particles start
moving, how far do they move, how fast do they hit the
ground again. These all depend on the forces acting on the
grains, and thus in turn on the physical environment—
gravity and the properties of the fluid. The study of this
problem has traditionally been addressed somewhat sepa-
rately of underwater particles, and for particles in air, in
fluvial/marine and aeolian geomorphology respectively.
However, we can see these cases as merely two points in a
continuous space of environmental parameters.
Bagnold's (1941) account of the problem is lucid and
still worth reading. For the masochistic reader determined to
subject themselves to the full modern glory of this problem,
there are exhaustive, and sometimes exhausting, treatments
with countless pages of equations in several texts, especially
Greeley and Iversen (1985) and Zheng (2009), and the
review by Kok et al. (2012). Our aim here is only to give the
basic equations and numbers, and beyond that a familiarity
with the principal controlling effects, and how they depend
on the planetary setting.

onto other sand grains, it may rebound to a much greater
height than can be attained through aerodynamic lift forces
alone. Saltating sand grains that impact onto resting sand
grains cause a 'splash' of several of the impacted grains,
each of which follow paths much smaller than that of an
individual saltating sand grain. The lower energy of ejection
for the impacted grains is the primary reason for the reduced
scale of the splashed-out grain trajectories. Splashed sand
grains follow a kind of 'staggered random walk' pattern
across the surface of the sand, a process distinct from sal-
tation that is now referred to by the term reptation
(Anderson 1987). Reptation tends to occur at a scale that is
about one-sixth that of a typical saltation path length; both
theory and experiment have shown that reptation is a crucial
element to the growth of aerodynamic ripples on sand
(Anderson 1988). Saltating sand grains that impact onto
small gravels and pebbles can produce a gradual rolling
movement of the coarse particles in the downwind direc-
tion, in a form of particle motion termed creep or traction;
granule and pebble ripples are examples of aeolian bed-
forms that result from this form of wind-induced particle
motion where the large particles rarely lose contact with the
surface. Note that in fluvial transport, the term 'bedload' is
used for particles transported by creep and saltation.
If the velocity in turbulent eddies within the flow is
larger than the terminal velocity of the particle, the particle
is said to be in suspension, and such particles (usually silt
or dust) can be transported very great distances (even
hundreds to thousands of kilometers); large deposits of loess
found worldwide on Earth are prime examples of the
transportation of suspended small particles by the wind.
Note that fine silt and clay-sized particles can stick together
effectively and are generally too small to be lifted out of the
laminar boundary layer by the airflow alone. Often they
may be ejected by the impact of saltating sand grains, or
sucked out of the surface by the pressure drop in a vortex
such as a dust devil.

4.1 Styles of Particle Motion

Movement of particles by the wind can take several dif-
ferent forms (Fig. 4.1 ). The most common form of move-
ment of sand by the wind is called saltation, which refers to
the 'hopping' motion of the individual grains that results
when lifted grains gradually attain an increasing fraction of
the wind velocity before impacting the surface downwind at
a shallow angle (generally \10 with respect to horizontal).
Saltation path length can be affected by a variety of factors,
but generally medium-to-fine sand on Earth tends to saltate
along paths *10 cm in length (Bagnold 1941). If a saltating
sand grain happens to impact onto a large rock rather than