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Environment
Landslide Spells Doom for
the Land of Seven Sister
by VK Joshi
Far
flung seven states of northeastern India , viz. Meghalaya, Assam ,
Arunachal Pradesh, Nagaland, Manipur, Mizoram and Tripura are fondly
called as seven sisters. These states are divided into 72
districts, 412 blocks and 41240 villages. Tea, hydrocarbons with a gas
potential of 30 billion cubic meters of natural gas and Timber are the
mainstay of the economics of the area. The turbulent rivers of the
region hold a promise of generating 50000 megawatts of electricity.
Despite such rosy projections these states have their own geo-hazards
like landslides which hamper the growth.
It is a coincidence that politically sensitive and insurgency prone J&K
and the northeastern states are geologically sensitive too. The present
problem of these states is the human attrition. In the geological past
these areas experienced a violent collision between two landmasses which
made them geologically sensitive.
Close your eyes and try to visualize the geographical map of India. The
Himalayan arc takes two distinct sharp turns. One is near Gilgit north
of Kashmir and the other on the northeast at Shillong plateau. This bend
of the Himalayas is due to the collision of the Indian Plate with the
Asian Plate some 20 million years ago. The collision still continues and
makes these two areas geologically sensitive. In other words these areas
are tectonically active.
It is common experience that the slope forming material gets more
weathered in tropical climate. Northeast India is a classic example of
tropical climate, high rainfall and dense foliage. The last of the
factors is however, under great stress due to indiscriminate felling of
trees to make room for the houses. Apart from human efforts in reducing
the forests, the nature too has uprooted trees in the past during
earthquakes.
Guwahati, the district headquarters of Kamrup in Assam , is the gateway
to the seven states. It has a crucial role in the economic, industrial,
commercial and educational development of entire northeast region. The
gargantuan population pressure has hit Guwahati too. The plain areas are
now jam-packed. People are trying to construct dwellings on the
surrounding hill slopes.
In a hilly terrain the slopes remain in a state of balance as long as
they remain undisturbed. It is also a fact that the network of roots of
the trees bind the soil and check downward movement of the lose soil.
Construction of a house on a hill slope not only demands a clearing but
also flattening a portion by cutting a wide terrace. The activity
immediately disturbs the angle of repose of all the material lying on
the slope. Gravitational pull aided by water makes rocks, lose soil etc
slide down.
The first major landslide of Guwahati city took place in Nabagraha area
in 1972 says Ranjana Borah of department of Geological Sciences, Gauhati
University , Guwahati. This was followed by other major slides in
Dhirenpara, Dispur, Kacharibasti, Rupnagar, Madgharia, Sarania,
Kalapahar Colony, Bazar Hillside, Kharguli etc. The list clearly shows
how things have deteriorated between 1972 and today.
 Landslides
are sudden and swift in Guwahati. Rock mass or soil cum rock mass
suddenly moves down, particularly during the monsoon. Other reason for
such movements is due to lack of drainage or blockade of drainage by
human agencies. It is common practice on hill slopes to provide adequate
drainage. If an outlet is denied water enters the weathered surfaces and
attempts to come out by generating pore pressure. The sudden movement of
slope forming materials is due to this pore pressure.
Open joints in rocks are lucrative for water to enter and saturate the
rocks. Dispur, Kacharibasti, Nabagraha, Dhirenpara and Rupnagar areas in
Guwahati have such rocks. Abetted by anthropogenic activity rocks in
these areas get disturbed and during rains become a nuisance by their
sliding.
A study carried out by Geological Survey Of India (GSI) is that Assam is
prone to landslides and Guwahati is one of most sensitive areas for the
hazard. Between 1985 and 2000 more than 70 persons have perished in
landslides in this area says the study conducted by S.Chopra and V.C.
Srivastava of GSI. The incessant rains of 2004 took away 11 lives and
more than 80,000 people were rendered homeless.
Roger Bilham a renowned seismologist from America , based on his
observations on the trends of earthquakes, he postulates a major
earthquake for Guwahati area. In the eventuality of a high magnitude
earthquake, landslides are inevitable. Dr. Arvind Sinha of GSI has
calculated that seismogenic landslides in the past dislodged
approximately 1.505 X 1010 m3 of material successively in 1897 and 1950.
Such mass wasting process is not only hazardous for the habitats but
also for the mighty Brahmputra River. In one of his essays, Dr. S.R.
Joshi of Centre for Science Education, NE Hill University, Shillong says
that the Brahmputra carries one of the highest sediments loads in the
world, about 332 million metric tonnes annually throughout its course.
Recent satellites photograph reveal that Brahamaputra is continuously
shifting southwards and in some places, may be migrating at rates as
high as 800 m/yr. The shifting of the river is distinctly evident in the
districts of Dibrugarh, Morigaon and Sonitpur, where the river has
already shifted 2 to 8 km. in the last ten years.
A shifting river erodes the toe of the slopes, which leads to
landslides.
A number of corrective measures have been suggested by the geologists
and the engineers involved in geo-technical studies of such slide prone
areas. Most significant of these is development of a surface and
subsurface drainage system for areas like Guwahati. The drainage has to
be so effective that rain water is prevented from entering the affected
areas. In the areas prone rock falls benches constructed on the slope
collect rocks or boulders and prevent their rolling down the slope.
Bitumen or asphalt material can be used to seal the gaping joints of
rocks to prevent entry of water. Breast Walls or Retaining Walls are
normal convenient methods of checking landslide along roads.
The Border Road Organization (BRO) has successfully used 'Geo-textiles',
a tough and resilient fabric indigenously made of jute. The fabric acts
as a binder for the soil and holds it. V.K. Yadav a Chief Engineer with
the organization developed this novel method of growing bamboo seeds
under the cover of jute geo-textile. Bamboo shoots sprouted through the
textile cover and the roots gripped the soil to prevent soil creep. The
BRO has also locally placed high resolution satellite cameras to record
movements if any on the hill slopes. Such measures have been
successfully used elsewhere in the World and the results have been
encouraging.
Technologically we are on sound footing as far as the landslides are
concerned. Landslide Hazard Zonation Maps are available, people can
chose safer areas for construction of habitats. However, social
awareness needs to be carried out on war footing. Engineering solutions
can not stop the Himalayas from rising and as such landslides will
remain inevitable. A strict enforcement of rules by the government and
an active participation of the society to avoid hazard prone areas will
save many a lives.
November 19, 2006
Image: A landslide blocks a road in
Assam.
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