Landslides and soil erosion are naturally occurring geological processes that can be exacerbated by human activities like deforestation, construction, and agriculture. Landslides involve the downslope movement of earth and rock due to external forces like rainfall, earthquakes, or melting snow. Soil erosion is the removal of topsoil by wind and water. Common types of erosion include sheet erosion and gully erosion. Mitigation measures include terracing slopes, contour ploughing, and maintaining vegetative cover to control erosion and stabilize slopes. Uncontrolled human activities that disturb land and vegetation can increase the risks of landslides and erosion.
2. Landslide
The term ‘landslide’ is generally used to
denote a downslope movement of mass of
earth, debris or rock down a slope due to the
action of external forces such as rainfall,
snowmelt, volcanic eruption, earthquakes,
anthropogenic activities etc.
4. In the context of Nepal…
Landslides are more often in our country
because of it’s geographical landscape.
The main reason is rainfall, beside that
snowmelt and earthquake may be the
important factors for landslide in Nepal.
Human activities like construction,
deforestation, agricultural practices in steep
areas, overgrazing also creates a path for
landslide to occur.
5. Landslides occur when the stability of the slope
changes from a stable to an unstable condition. A
change in the stability of a slope can be caused by a
number of factors, acting together or alone.
Landslides cause loss of lives and properties from the
area where landslides occur or from the downslope
area due to the transport of the landslide materials
(sediment).
6. • Natural Factors
• Concentrated precipitation
• Seismic activity: The Himalayan Range lies
in a high seismicity belt. Several active faults
have mapped.
• Rise in groundwater table in the monsoon.
• Perched water table
• Inadequate size and choking of roadside
drains
• Liquefaction
• Swelling and Shrinking of Clay
7. Anthropogenic Factors
• Deforestation: intensive deforestation has taken
place in most parts of the Himalaya excepting the
higher Himalaya in the last decades
• Improper land-use:
(a) Agricultural practices on steep slopes
(b) Irrigation on steep and vulnerable slopes
(c) Over-grazing
(d) Quarrying for construction materials without
considering the condition of the terrain
• Construction activities
8. • Deep-seated Landslides
• Landslides in which the sliding surface is mostly
deeply located below the maximum rooting
depth of trees (typically a depth greater than 10
metres) are called deep-seated landslides.
• It is usually believed that the deep-seated
landslides are triggered by moderate rainfall
intensity distributed over long periods. Deep-
seated landslides are generally slow moving in
nature and rarely claim lives, but may cause
high property damage. The failure modes in
such cases are generally rotational or complex
types.
9.
10. Shallow Landslides
• Landslides in which the sliding surface is located
within the soil mantle or weathered bedrock
(typically to a depth from a few decimetres to
several metres) are categorized as shallow
landslides. The surface of the slope in steep hilly
and mountainous regions is quite often underlain
by a plane of weakness lying parallel to it and
therefore, shallow landslides are predominant.
• In many cases, the shallow landslides are fast-
moving and are extremely destructive, causing
wide-spread damage and casualties. Shallow
landslides can pose grave threat to life and property.
11.
12. Fall: A fall starts with
detachment of soil or rock
from a steep slope along a
surface on which little or
no shear displacement
takes place. The material
will then descend largely
through air by falling or
rolling
13. Topple: A toppling occurs
as a result of overturning
of blocks rather than
sliding or falling. It is a
forward rotation, out of
the slope, of a mass of soil
or rock about a point axis
below the gravity of the
displaced mass.
14. Slide: A slide is the downslope movement of a soil or
rock mass occurring dominantly on the surface of
rupture or relatively thin zones of intense shear
strain.
Transitional Rotational
15. Flows: In flows, materials move as a coherent but
constantly changing mass, involving internal shear or
mixing of the mass and even sorting based on
particle size and position in the flow. The distribution
of velocities in displacing mass resembles that in a
viscous fluid.
Earth flow Debris flow
16. • Spread: The term ‘spread’ refers to an extension of
cohesive soil or rock mass combined with a general
subsidence of the fractured mass of a cohesive
material into softer underlying material. The spread
may result in from liquefaction or flow of softer
materials.
17. Creep: Creep is the imperceptibly slow, steady,
downward movement of slope forming soil or rock.
The movement is caused by shear stress sufficient
to produce permanent deformation, but too small to
produce shear failure.
18. Physical damage- Debris may block roads,
supply lines (telecommunication, electricity,
water etc.)
Loss of property.
Casualties – death and injuries to people and
animal.
Changes in the surface landscape.
19. Land use practices such as:
Areas covered by degraded natural vegetation
in upper slopes are to be afforested with
suitable species .Existing patches of natural
vegetation should be preserved.
Any developmental activity initiated in the area
should be taken up only after a detailed study
and slope protection should be carried out if
necessary.
In construction of roads, irrigation canals etc.
proper care is to be taken to avoid blockage of
natural drainage.
20. Total avoidance of settlement in the risk zone
should be made mandatory.
Relocate settlements and infrastructure that fall
in possible path of the landslide.
No construction of buildings in areas beyond a
certain degree of slope.
Avoid building on or near steep, high risk slopes
and avoid modifications that increases the risk
of landslide.
21. Retaining walls can be built to stop land from
slipping (these walls are commonly seen along roads
in hill). It’s constructed to prevent smaller sized and
secondary landslides that often occur along the toe
portion of larger landslides.
Surface drainage control works: The surface
drainage works should be implemented to control the
movement of landslides accompanied by infiltration
of rain water and spring flows.
22. Increasing vegetation cover is the cheapest and
most effective way of arresting landslides. This
helps to bind the top layer of soil with layers
below, while preventing excessive runoff and soil
erosion.
Gabion net, gabion wall and wire bolsters
should be constructed in landslide occuring
Places to prevent damages.
23.
24. Soil erosion is a naturally occurring process on all
land.
Erosion occurs when soil is transported from one
location by wind and water and moved to a new
location, such as lakes and rivers.
The agents of soil erosion are water and wind, each
contributing a significant amount of soil loss each
year.
Soil erosion may be a slow process that continues
relatively unnoticed, or it may occur at an alarming
rate causing serious loss of topsoil.
25. Types of Soil Erosion
Rain drop or splash erosion: Erosion preceded by
the destruction of the crumb structure due to the
impact of falling raindrop on the surface of soil is
termed as splash erosion.
Sheet erosion: It is the fairly uniform removal of
soil in thin layers from the land surface, often
scarcely perceptible, especially when caused by
wind. Areas where loose, shallow topsoil overlies
compact soil are most susceptible to sheet erosion.
Rill erosion: A form of water erosion in which
numerous very small and more or less straight
channels are produced; the channels get obliterated
by ordinary use. It can be removed by normal tillage
operations.
26. Gully erosion: A form of water erosion in which
gullies are produced by combination of unattended
rills.
Stream bank erosion: Stream banks are eroded by
water either flowing over the sides of a stream or
scouring at the base. It is aggravated by removal of
vegetation, over grazing or cultivation near the
stream banks.
28. Causes of soil erosion
Ploughing
Ploughing with machinery
compacts soil. This creates
channels for rapid water
flow especially on slopes.
Deforestation
When trees are deforested
the root holding soil are
removed which then
leaves bare top soil
Vulnerable to extreme
wind and water damage.
Fertilizers
Soils are not given time to
recover naturally when
soil is used intensively
Overgrazing
When large number of
animals are grazed in a
small area the vegetation
is removed faster than it
can regrow leaving the
topsoil exposed.
29. Effects of soil erosion
- Loss of soil fertility.
- Loss of ability to hold water.
- Pollutes water, kills fish.
- Damages crops through sandblasting of young
seedlings
- Desertification
30. Mitigation measures
Terrace farming-slopes are made
into flat step areas or ‘terraces’
which means that water and
soil are held by the walls thus
minimizing the erosion.
Contour ploughing-this is when
ploughing takes place across
hillside. This reduces the flow of
water(runoff) across the land
surface thus preventing erosion.
31. Strip farming- Different crops
are planted and harvested at
different time that means the
amount of bare soil is minimized.
Stone lines- These are kept in
place following contour lines
meaning runoff is reduced
and soil can be collected and
redistributed. This is a cheap
And easy method.
