Landslides
are geologic hazards that occur on spatial and temporal scales in mountainous
landscapes and are frequently responsible for considerable loss of life and
property (McKean and Roering 2004). Landslides
are rapid mass-wasting processes in which gravity causes a variety of
slope-forming material, from soils to rock to artificial fill, to move downwards
in the slopes. The term "landslide"
encompasses five modes of slope movement: falls, topples, slides, spreads, and
flows. These are further subdivided by the type of geologic material (bedrock,
debris, or earth). Debris flows (commonly referred to as mudflows or mudslides)
and rockfalls are examples of common landslide types.
Methodology:
Data are collected through secondary sources such as books, articles,
project reports,
and websites. Also, data are collected from the
sites of the ministry of home affairs too.
Landslides
distribution areas in Nepal:
Nepal is a mountainous country with weak and fragile topography leads
to landslide frequently even though the area is well forested. Seasonal
landslide with starting of monsoon is not only the landslides that cause
fatalities and loss of huge property but also off seasonal landslide like
falling and rolling of large stone and dry landslide also cause the equal
damage.
In the context of distribution, the immature Siwalik region and densely
populated mountain areas are highly suffered from landslide problems. In the northern region of the country, being altitude greater than 4000m results in
alpine hazards including topples, high wind resulting in the rolling of large rocks,
ice leading to avalanche, debris flow, and glacier lake outburst flow.
In the lower side of the Himalayan region, due to the unavailability of plain and
favorable land for shelter and cropping, forest regions are being cleared. Rapid clearing of forest due to overpopulation
leads to desertification causing loose soil structure which is easily swept
away by ice flow avalanche and glacier lake outburst. Unscientific and
unplanned development of road networks in the Siwalik region for linking district
headquarters is also a cause of the frequent mass flow of debris and land in this
region. Monsoon reach late in the eastern part in comparison to a western part
therefore the chances of landslide is less in eastern part than in the western part
of the country. Almost all mountainous road networks suffer landslide due to
unscientific planning, construction and weak soil topography. Also, areas with
large and random deforestation, overgrazing, areas with construction work going
on, area near to mines, construction with explosives areas are more prone to
landslides.
Causes of landslides
in Nepal:
Almost every landslide has multiple causes. Slope movement occurs when forces acting
down-slope (mainly due to gravity) exceed the strength of the earth materials
that compose the slope. Causes include factors that increase the effects of
down-slope forces and factors that contribute to low or reduced
strength. Landslides can be initiated in slopes already on the verge of
movement by rainfall, snowmelt, and changes in water level, stream erosion, and
changes in groundwater, earthquakes, disturbance by human activities, or any
combination of these factors. The major cause and prime factor of landslide in
Nepal in context after the earthquake of 2015 A.D. is weak geology by the
shaking too.
Although if we
categorize the causes of landslides in Nepal, there may be broadly two causing
factor namely, natural and anthropogenic causes.
A.
Natural causes:
1.
Steep topography: Being a country with loads of
hills and mountain, sliding of lands, ice, rocks, and debris are very frequent
and prominent. The slope of the hills is almost more than 60 degrees. Almost many
high hills are barren and on heavy precipitation due to high relief slides
downwards.
2.
Unsteady rugged geology: Due to intense folding and faulting
induced by seismic and tectonic plates, soil and rocks became highly weathered
and fractured resulting in the sliding down of landmass with beginning of
heavy precipitation or offseason in the form of dry landslides. Sometimes, large
rocks roll downwards leading a huge mass of land to slide with it too. Nepal being highly brisk of the earthquake, lots of small to medium earthquakes occurring frequently leading to weak geography and fragile structures of the slope.
3.
Concentrated heavy precipitation: Due to undistributed
monsoon and heavy rainfall concentrating in certain locations leads to mudflow,
earth flow, and sliding of landmass causing huge loss of life and property. In
Nepal most of the monsoon rain occurs within a week which leads to flood and water
flow in landmass, which along with its force sweep away the landmass. At this
condition trees also cannot reduce the risk due to the force of water currents
and its pressure.
There are other
natural causes of landslides like forest fires which destroy the natural
vegetation and increase the risk of a landslide along with soil erosion.
B.
Anthropogenic cause:
1.
Hill cutting and deep excavation: In the
name of various infrastructure developments, hill excavation, or cutting has
been continuing in Nepal for a long time. Such excavations are extremely
unplanned and unscientific which ultimately cause various toppling, rock
rolling, dry landslide, and sliding of landmass with the onset of monsoon.
2.
Unplanned mining, infrastructure
development and use of explosives: Nepal being a developing country doesn’t
have so many mines and mining activity, but the process and methods of mining
by private companies is so unplanned and only profit-oriented causing massive
sinkholes. Use of explosives during cracking of large rocks for road
construction and mining cause shaking of land which causes landslides and debris
flow.
3.
Improper land use and farming practices: Due to the lack of proper plain land for cultivation and
shelter, people started to clear the forest and started the cultivation and
construction of houses in the high Himalayan region. Improper planning for the cultivation causes huge soil
erosion due to steep slopes. The pounding of water and clearing of vegetation for
cultivation in steep slopes also induce very serious effects making the slope
fragile and prone to landslides.
4.
Deforestation: Rapid increment in population results in the
encroachment in the marginal land for cultivation, shelter, timber, firewood, etc which leads to intensive deforestation. These factors have increased the surface runoff and
head and toe erosion, giving rise to landslides. The young Siwalik hill is so
fragile that there is a saying “once the forest covers of Siwalik region destroyed, it is
extremely difficult to re-grow it".
There is a huge loss of life and property annually in Nepal due to
landslides. The highest fatalities ever recorded since 2010 AD is in 2014 where
247 people lost their lives which are a very breathtaking incident for whole
Nepal.
Poorly engineered
construction and shelter on high steep slope cause massive casualties yearly
make people homeless.
Landslide mitigation
and its control methods:
Landslides pose a recurrent hazard to
human life and livelihood in most parts of the world especially the region that
have experienced the rapid population growth and construction taking place.
Hazards can be mitigated only through precautionary means for instance. By
restricting or even removing populations from an area with a history of
landslides, restricting certain types of land use where soil stability is a question, and by installing early warning systems based on monitoring of ground
conditions such as strain in rocks and soils, slope displacement, and
groundwater levels. Various improvements can be done to mitigate and control
frequent landslides of Nepal.
1. Earthwork for
stabilization of slide: Removal of unstable soil or rock mass at
the upper part of the slide is the beginning/basic procedure for the stabilization
of the slide. All the loose soils, detached blocks located at the upper part of
the slide were removed to reduce the load and shear force. During the removal
process fractured, cracked rock mass was chiseled and the gradient of the slope was
maintained to standard slope the gradient.
2. Water management: Drainage management plays a vital role in controlling the landslide. Management of drainage networks alone
significantly improves the stability of the land. Water management in the slope
consists of surface and sub-surface drainage that are capable to take away the
water to the natural drainage system.
3. Structural supports: Structural supports like
check dams, retaining walls, and embankments, gabion walls provide
external supports and prevent frequent sliding of the landmass.
4. Effective formulation of policy and its
promulgation: Proper policy should be formulated for carrying out any
construction works like dams, rural road network, hill excavation, etc, and
forbid them to use more explosives and encourage them towards sustainable
development. Demarcation of landslide-prone areas by regular soil study, aerial
photographs, consulting with local people, and plan the future development
accordingly.
5. Afforestation and soil conservation methods:
Afforestation barren hills and landslide-prone steep slope with good anchoring
vegetation, ground covering grasses, trees, etc that reduce the direct splash
effect improves infiltrations and increase the strength of soil with soil
compaction. Planting grasses with a deep fibrous root system, plants with broadleaf will help also to reduce gully formation and increase the infiltration
capacity of the soil.
6. Hazard mapping and public awareness: Hazard
maps indicate the possibility of landslides throughout the given area. An idle
hazard map shows not only the chances that a landslide might form at a
particular place but also the chance that it might travel downslope a given
distance. People living near the landslide-prone area should be made aware
of the regular weather reports and the possibility of the hazard using means
of communication and knowledge should be provided to them for better forest
management and to plant barren hill cover.
7.
Soil Bioengineering: soil bioengineering is an
applied science that combines structural, biological, and ecological concepts
to construct living structures for soil erosion, sediments, and flood control.
Technically it is defined as” the site-specific use of plant materials to construct
structures that perform an engineering function and are biologically active.”
The technology is simple, low cost, replicable and effective, requires little maintenance, and is environmentally friendly and sustainable. Bio-engineering does not replace the standard civil engineering process nor
does it offer magic solutions. It does, however, increase the number of
available ways to address roadside slope stability problems. It is very reliable and provides a permanent solution to the frequent
sliding of lands in developing countries like Nepal. The function of
vegetation on this technique has two major functions which are listed in the table.
|
Hydrological function
|
Engineering function
|
|
1. Interception: reduces the
raindrop splash by vegetation canopy
|
1. Catching: stem and roots can
catch loose materials
|
|
2
Restraint: a dense network of root binds the soil
|
2. Armouring: vegetation reduces
water sensitive soil
|
|
3
Absorption: roots absorb surface and underground
water reducing saturation level.
|
3. Anchoring: Taproots of vegetation and binds
interlayers of soil.
|
|
4. Infiltration: plant residue
increase infiltrations reducing the chance of slides
|
4. Drainage: dense rooted
vegetation makes it easy in both surface and underground drainage.
|
Source:
Resource manual on flash flood management
Generally, local species of vegetation is best for bioengineering
because they will easily thrive on that condition; easily establish them as we
need fast adapting plant species in the given condition. Local species of plants
are also resistant to many diseases as compare to newly introduced species.
Major species that are used for bioengineering purpose are broom grass (Thysanolaena maxima), vetiver grass (Vetiver zinzaniodes), Babio (Eulaliopsis binata), kans (Saccharum pontaneum), Khar (Cymbopogon microtheca), Bhujetro (Butea minor), chilaune ( Schima wallichi), Khayer ( Acacia catechu), Dhanyero (Woodfordia fruiticosa), Sisau (Dalbergia sisso).
A case study of Krishnabhir is a perfect example of the success of
bio-engineering. Krishna Bhir situated 82.5 kilometers
from Kathmandu along the Prithvi highway until 2004 was notorious for
disrupting the highway to the capital of the nation. Every year until 2004, the
place was a nightmare for thousands of commuters traveling along the highway
with the landslide with being people stranded on either side of the highways
for hours or even days. The transport of the essential supplies was halted. In
2000, an entire side of the mountain slithered down the Trisuli River during
the monsoon blocking the highway for 11 days. Even after the debris was
cleaned, it would be blocked again following a slight spell of rain. But then
in 2004, Krishnabhir got a total facelift. According to Naresh Man
Shakya, an engineer at the Department of Roads and the in charge of the project,
they used a combination of water and debris management along with
bioengineering, the problem was resolved. The team used special grass to trap
debris, armored the surface, and anchored the topsoil, thus stopping the
slide. Culverts, inclined at 15 degrees, were set up underneath the
reconstructed highway over the drained and leftover debris. A retaining wall
was constructed at the toe of the slide adjacent to the river. Different other
structures such as wire bolsters, wire mesh, seeding, rofa boards (German
technology) and seedlings were also used. The total project was completed under
Rs 40 million unlike as proposed by a Japanese constructor who studied the
landslide and proposed to fix it for over Rs 2 billion. The successful plan was
homemade and it is one of the 100 major landslide sites that have been stable
since2003-2004.
Developmental work should be carried out in more
sustainable way without disrupting the natural condition. Proper and intensive
installation of rain gauge and weather station should be carried out to obtain
the information so that pattern of landslides which helps to alert the people
of landslide-prone area.
CONCLUSION:
Being a mountainous country, every year, Nepal is suffering from the problem
of landslides causing a huge loss of economy, human lives affecting the
mountain environment adversely. Therefore, an improved understanding of the
natural processes, their impacts on natural hazards, and their relationships
with human activities on the mountain slopes should be explored to reduce the
effects of the landslides. The main hazard attributing factors in Nepal
Himalaya are steep slope, improper land use pattern, rock types and associated
discontinuity, soil type and, soil depth. The conversion of forestland to
arable land and shrubland has been an accelerating hazard and also played a central
role in the development of slope instability and surface erosion. The
superposition method of landslide hazard mapping is found to be a useful tool
for delimiting the areas susceptible to sliding. With proper government
planning and the use of technology along with some innovative ideas, conservation of
such disaster permanently is possible and loss can be reduced forever.
Using both nature and science infrastructure development can be carried out in an eco-friendly way and mitigating measures can be applied which sustain and
preserve the degrading land structures with the halt of landslide.
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