Geoscience Reference
In-Depth Information
1
Saltation proper
Wind
Sand Sheets
Bed load
(rolling and saltation)
Dune Sands
Reptation
Approach
angle
0.1
3m
Figure 18.18
The process of reptation where the impact of a
high-velocity saltating grain ejects other grains into the airflow
(after Anderson, 1987).
Loess
tion (Haff and Anderson, 1993). Much more research is
required to understand this process fully, but Anderson,
Sørensen and Willetts (1991) suggest that it could be ex-
tremely important in near-surface aeolian transport and
more recent modelling work confirms that transport flux
models produce more realistic simulations with the inclu-
sion of a reptation component (Andreotti, 2004; Namikas,
2003).
0.01
18.6.4
Saltation
Dust storms
0.00
10
30
50
70
100
200
The most intensively researched mode of aeolian transport
is that of
saltation
. This is the characteristic ballistic tra-
jectory of grains (
U
*
(cm s
-1
)
0.06-0.5 mm in diameter) as they are
ejected from the grain bed, are given horizontal momen-
tum by the airflow, descend to impact the grain bed and
then continue 'leaping' downwind (see Figure 18.19). The
impact of saltating grains on the surface often leads to the
splash of other grains into the airflow, which may then un-
dergo reptation or, if sufficient momentum is transferred,
saltation. Hence, saltation is a very efficient transporting
mechanism whereby a few saltating grains can rapidly
≈
Figure 18.17
The relationships between grain diameter,
shear velocity and mode of sediment transport showing the
distinction between the suspension of dust-sized sediment and
the bedload transport of sand-sized sediment;
U
f
is the particle
fall velocity (after Tsoar and Pye, 1987).
18.6.3
Reptation
The low hopping of several grains consequent upon the
high-velocity impact of a single saltating grain has been
termed
reptation
(Anderson and Haff, 1988) and is an im-
portant transitional state between the modes of creep and
saltation. On impact and subsequent rebound, a saltat-
ing grain may lose 40 % of its velocity (Willetts and
Rice, 1989; Anderson and Haff, 1991; Haff and Ander-
son, 1993). This energy imparted to the grain bed re-
sults in the ejection (or 'splashing') of perhaps 10 other
grains (Werner and Haff, 1988) with velocities at ap-
proximately 10 % of the impact velocity, often too low
to enter into saltation (Willetts and Rice, 1989; Ander-
Wind
w
1
Initial
u
1
u
2
Final
w
2
h
α
l
