Geoscience Reference
In-Depth Information
Practically, the erosion threshold determined from experiments is generally a
“saltation threshold”, that is, the minimal wind velocity for which a sustained
particle movement is detected (Iversen and White 1982 ). This is also the case for
field measurements of erosion thresholds performed with sensors that detect the
impacts of grains in saltation. Such sensors have detection limits, which generally
prevent the detection of the very first particle movement but provide an estimation
of the saltation threshold defined as the wind velocity below which the saltation flux
is negligible.
5.3.1
Influence of Soil Particle Size
For loose and dry soil particles, Bagnold ( 1941 ) first proposed an expression of
the threshold of movement of soil particles initiated by the wind. It is derived
from the balance between and particle weight. It expresses the dependence of
the erosion threshold with particle diameter D p and the air and particle density a
and p , assuming soil particles have a spherical shape. Wind-tunnel measurements
have shown an increase of the erosion threshold with decreasing particle size
below
100 m(Chepil 1951 ). This increase is explained by an increase of the
interparticle cohesion due to electrostatic forces (Iversen et al. 1976 ). Due to inverse
size dependencies of the cohesion forces and particle weight force, there is an
optimum particle size (
60 m) for wind erosion, that is, a particle size for which
U t is at a minimum. Assuming the electrostatic cohesion forces as a power function
of D p , Iversen and White ( 1982 ) proposed a formulation to predict the saltation U t ,
fitted on a large set of wind-tunnel measurements for various particle densities and
diameters. Shao and Lu ( 2000 ) proposed an improved formulation of the erosion
threshold based on an explicit physical formulation of the Van der Waals and
electrostatic forces involved in the interparticle cohesion forces.
A N p gD p
a
0:5
U t D p D
a D p
C
(5.3)
p f.Re t /
with A N D
0:0123:
Re t is the Reynolds number at the erosion threshold, that is, Re t D
( U t . D p )/ v
where is the kinematic viscosity of the air. The term = a D p accounts for the
interparticle forces, being adjusted to wind-tunnel measurements (from 1.65
10 4 to 5.10 4 kg s 2 ). Figure 5.3 illustrates the dependence of U t on D p .The
general shape of the curve is similar for the expressions of Iversen and White
( 1982 ) and Shao and Lu ( 2000 ). Differences appear for small particles (<100 m)
and for the minimal threshold and the associated particle diameter. The minimum
erosion U t
20 cm s 1 , predicted by Iversen and White ( 1982 ), is in agreement
with wind-tunnel measurements for D p < 100 m and with field measurements
(Gillette and Stockton 1989 ). The minimum U t
of
predicted by the expression of
 
Search WWH ::




Custom Search