Geography Reference
In-Depth Information
movement in total. The rate at which rainsplash transports material is
similar to the rate of transport by soil creep. However, while soil
creep occurs over a depth of soil and transports a large mass of mater-
ial, rainsplash only functions on the surface, moving individual parti-
cles. Ensuring there is a good vegetation cover to protect the soil
surface from rainsplash forces is a good way of reducing soil move-
ment by rainsplash.
If raindrops land on flowing water moving over the land surface
then their direct impact on the soil surface is reduced. However,
the flowing water itself can carry material. Where the flow over
the surface is shallow, the combined effects of raindrop impact,
which detaches sediment, and transport by flowing water over the
surface, are very effective and this combined process is called rain-
wash. When the water depth over the surface is deeper than 6 mil-
limetres, raindrop detachment is weak and so the initial movement
of a particle is more related to flowing water in a process called rill-
wash. This erosion process is common in major storms. Many
poorly vegetated areas develop temporary rills which are channels
formed during storms. Wetting and drying, or freeze-thaw accu-
mulates material that infills the rills between storms. However, in a
large storm, channels may form which are too large to be refilled
before the next event and these are known as
gullies
. These chan-
nels collect water in subsequent events, rapidly enlarging the gullies
further.
Where there is lots of weathered material but the erosion
process cannot carry it far (e.g. rain splash hitting the surface and
moving particles) then the erosion process is transport limited. In
situations where there is not much sediment available to be trans-
ported but the transport processes could, if there was more sedi-
ment available, carry a lot more, then this is known as a
supply-limited condition. Places where such transport-limited con-
ditions operate have a tendency to have a good cover of vegetation
and soil, and over time the steepness of slopes often declines. Land-
scapes where removal of material is mainly supply-limited tend to
have little vegetation and soil and have steep slopes which remain
steep as they erode. The landscape can therefore contain many
clues to the processes forming it. Convex hillslopes are associated
with creep or rainsplash. Concave profiles are generally associated
with rillwash. Mass movements generally lead to slopes with quite
