1. Title slide:
Soil erosion – causes, effects and prevention.
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2. Logging roads cut into a forested hillside; three landslips
can clearly be seen in the foreground. The results of uncontrolled
logging near the road are also very apparent (Philippines).
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3. Ease of access resulting from the incursion of a logging
road has resulted in the complete loss of forest cover. Run-off
from the road has formed large rills which will, if unchecked,
soon create gullies (Philippines). |
4. Slips do not necessarily occur where road cuttings are
more stable, but the banks will still be open to gully formation
(Venezuela). |
5. The increasing demand for food crops has forced farmers
to cultivate steeper slopes. In this area of secondary forest,
“slash-and-burn” techniques are being employed over a wide
area (Philippines). |
6. This farmer has planted cash crops on a steep slope; the
virtually non-existent ground cover will offer no protection
when the rains come and a serious loss of soil and nutrients
will occur (Philippines). |
7. Even with fairly dense planting, crops such as maize give
very little protection to the soil surface from either intense
rainfall or overland flow. In this case, the situation has
been made worse by cross-contour furrows (Malawi). |
8. This site is obviously a well established farming area,
but even though only the less-steep slopes are being used
to grow wheat, the need for terracing is still as great (Venezuela).
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9. In fact, a series of major gullies has started to form
and unless remedial action is taken soon, both the fields
and the buildings will be in danger (Venezuela). |
10. In a case such as this where the soil depth is very shallow,
land slips are always likely to occur soon after the indigenous
crop cover has been burnt off (Philippines). |
11. Even many years after incorrect cultivation, the steeper
slopes will be likely to suffer from solifluction – the slow
creeping of wet soil down a slope (Philippines). |
12. The problems of denudation can also be exacerbated by
over grazing and the need to collect ever increasing quantities
of fuel wood from the area surrounding a settlement (Botswana).
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13. Under certain conditions, the rainfall may infiltrate
the ground and be concentrated in a soft stratum, which will
gradually be eroded away. This effect, known as “piping”,
will only become obvious when the ground surface collapses
to form the start of a gully (Venezuela). |
14. This gully was formed through a maize field along the
line of a frequently used footpath and was understood to have
been formed during only one wet season. The farmer was planning
to try and stop further erosion by blocking the gully with
banana stalks to trap the sediment (Malawi). |
15. When run-off occurs across land with a shallow gradient,
gullies may form a braided pattern with channels that continually
split and recombine. The intervening soil ‘islands’ will gradually
be eroded away until the gully width may be measured in tens
of metres (Malawi). |
16. The results of soil erosion are seen here in the colour
of water passing over the spillway of a large dam – the water
is carrying a high concentration of sediment. This colouring
will continue to be present a number of months after the rains
have finished (Kenya). |
17. As the gradient of a river reduces, its ability to transport
sediment diminishes and so the material is deposited to form
a thick layer on the river bed. After the flood, the river
will re-work the sediment by cutting a smaller channel leaving
a high bank of alluvial deposits (Kenya). |
18. The meandering form of a river crossing a sediment filled
flood plain can clearly be seen in this aerial photograph.
The location of such meanders within the permanent banks cannot
be predicted and so the design of water off-takes presents
the designer with a major problem (Philippines). |
19. The fine silts and clays may never be deposited along
the course of a river and they can continue to form a coloured
plume where the river discharges into the sea. This can cause
problems for fish life and the survival of coral reefs, offshore
(Indonesia). |
20. A good example of correct land husbandry techniques being
used is seen here. The furrows run along the contours, and
the fallow areas in the background are under a good ground
cover. The contour bunds have also been covered with mulch
to protect them from the heavy rainfalls (Malawi). |
21. Very steep slopes are still capable of cultivation if
terracing is constructed. The first walls here are reputed
to have been built over 2000 years ago and are still being
used. The rice harvest is entirely rainfed with the water
cascading from one terrace to the next (Philippines). |
22. Good land husbandry practice requires that tilting and
bunding on sloping land should be carried out along the contours
as seen at this site in the foreground. The hills in the background
have not been protected (Venezuela). |
23. Part of a major reservoir catchment that has been stabilised
by a programme of reforestation. The young trees are located
in the regular network of circular depressions (Philippines).
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24. The erosive force of a naturally occurring, or manmade,
stream can be controlled by introducing a series of drop structures
to effectively reduce the flow velocity (Venezuela). |
