Geoscience Reference
In-Depth Information
some of the more common agents of erosion, which we now consider in more
detail.
Raindrop impact is the initial agent of soil particle detachment. The kinetic energy
(
E
k
) incorporated in falling rain is the product of total raindrop mass (
m
)andthe
square of the velocity (
v
) of the falling raindrops.
E
k
=
mv
2
(10.1)
The momentum (
M
) of a falling drop at impact is the product of
m
and
v.
M
=
mv
(10.2)
As might be expected, there is a linear relationship between the rainfall intensity
of any given rainstorm and the total raindrop momentum embodied in that storm.
The rate of detachment of soil particles is a function of total raindrop momentum
(Williams,
1969a
). At the moment of impact, some particles rebound upslope and
some downslope. The steeper the slope, the higher the number of particles projected
downslope. In any event, once run-off or overland flow occurs, detached soil particles
will be washed downslope.
Desert storms are often very local and very intense. We might therefore expect high
rates of soil particle detachment. This will only apply if the surface is not protected
by a more or less continuous layer of stones or 'desert pavement' and, of course, if the
surface is not bare rock. Surface rock creep is the slowmovement downslope of surface
rock fragments under the influence of gravity. The rate of movement is proportional
to the sine of the angle of slope, regardless of whether the initial disturbance of the
surface stones is triggered by frost or by raindrop impact.
A key factor limiting the impact of falling raindrops is the degree of plant cover or
surface mulch, because these will absorb much or all of the momentum and kinetic
energy of the falling drops. By way of example, on granite slopes of only 2 per
cent gradient in the seasonally wet tropics of northern Australia, for the same unit
momentum of falling rain, soil particle detachment was twenty to forty times more at
the start of the wet season when the surface was bare than it was during the height
of the rainy season when surface plant cover amounted to 30-40 per cent by area
(Williams,
1969a
).
One common effect of raindrop impact in arid areas is the creation of a thin surface
crust formed of silt and clay particles. Such crusts are often only a few millimetres
thick but can be many hundreds of times less permeable than the soil beneath them.
The immediate consequence of such crusts is to reduce infiltration of water into the
soil and produce a corresponding increase in surface run-off.
Where much of the surface consists of bare rock, run-off rates will be high. If,
as is common in many semi-arid areas underlain by granite or similar rocks, the
local relief consists of bare rocky hills, or
inselbergs
, rising above gently sloping
plains, the foot-slopes around the inselbergs are often quite densely vegetated. This is
