Geoscience Reference
In-Depth Information
(b) Rill erosion (Figure 28 on p. 110)
Rill erosion generally occurs on areas of cultivated ground after a heavy rain event.
It takes the form of numerous relatively small, narrow rills all running down the
slope eroded to the depth of cultivation or a little more. Significant volumes of
topsoil can be removed and deposited further down the slope where it may be
intercepted and held by vegetation, or where the slope decreases and the water loses
its energy and the soil is deposited.
(c) Gully erosion (Figure 29 on p. 111)
This occurs where water runs off slopes and is concentrated in drainage lines.
Gullies start at the lower end of the drainage line and work their way up the slope
over time. A number of side gullies can form and join into the main gully causing
extensive damage to the landscape. Gullies may be from only a few metres deep to
a huge size such as the Grand Canyon in America. Soil may be deposited in fan
formation at the end of the gully, or be washed into streams or rivers causing
turbidity in the water.
(d) Tunnel erosion (Figure 30 on p. 112)
When the topsoil is relatively pervious and the subsoil is dispersive (sugary) as in a
sodic soil, water may soak into the soil profile and wash channels of subsoil from
under the topsoil, forming tunnels. The topsoil may later collapse, forming a gully.
The displaced subsoil is typically deposited as a fan where the water comes to the
surface.
(e) Stream bank erosion (Figure 31 on p. 113)
When stream and river banks become denuded of vegetative cover, they offer little
resistance to the erosive force of water caused by f lood f lows or even the wave
action caused by speed-boats. Floods can significantly change the course of a
stream or river by bypassing bends or washing new channels. High vertical banks
on the outside of a bend are particularly prone to erosion, with the soil collapsing
into the river and being washed downstream. This soil may be deposited as
sandbanks on the inside of bends, or deposited many kilometres away where the
slope decreases and the water in the river slows down.
(f) Landslips (Figure 32 on p. 114)
These occur on steep slopes with little vegetative cover to stabilise the soil. After
prolonged periods of rain when the soil is fully saturated, gravity can cause areas
of soil to slump and slide downhill where it is deposited as a mound.
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