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Tipaimukh
Dam: A geo-tectonic blunder of international dimensions
Dr
Soibam Ibotombi
THE
proposed Tipaimukh dam is to be located 500 metres downstream
from the confluence of Barak and Tuivai rivers, and lies
on the south-western corner of Manipur state. It is a huge
earth dam (rock-fill with central impervious core) having
an altitude of about 180m above the sea level with a maximum
reservoir level of 178m and 136m as the minimum draw down
level. The dam was originally conceived to only contain
the floodwater in the Cachar plains of Assam but later on,
emphasis has been placed on hydroelectric power generation,
having an installation capacity of 1,500MW with only a firm
generation of 412MW (less than 30 per cent of installed
capacity). In order to appease the people of Manipur state,
the project proponent, NEEPCO, has been building up a list
of benefits that include high-class tourism, free power
sharing, resettlement and rehabilitation package and an
all round rosy picture of development.?
Over
the past decade and half, the issue of Tipaimukh dam has
created a lot of disenchantment in regard to scientific,
technical, economic and environmental feasibility of the
dam, especially concerning with the state of Manipur. An
attempt is, therefore, made here to provide a brief geological,
structural and tectonic account of Tipaimukh and its adjoining
region in terms of tectonic framework of Indo-Myanmar [Burma]
Ranges in general and that of Manipur in particular and
possible socio-economic impacts of the dam. Such a consideration
would reveal the nature and extent of the geo-tectonic risk
being taken by constructing a mega-dam at Tipaimukh.? ?
Some
basic geological information ?
TIPAIMUKH and its adjoining areas are basically made up
of Surma Group of rocks. The rocks of Surma Group are mainly
light grey to brownish grey generally medium to coarse grained
sandstones having occasional shale and silt/sand intervening
bands between massive to thickly bedded sandstones. Conglomeratic
(loosely cemented pebbles and gravel)) horizon at the base
of Bhuban Formation, though, can be observed in the field
easily due to its wide areal extent; other conglomeratic
horizons are generally often missing which is probably due
to their localised nature.? In general, this group of rocks
are predominantly arenaceous with subordinate shales. Usually
shales are less sandy and sandstones are less argillaceous.
Some typical natures of bedding similar to turbidite character
are also found at places. Like Barails, Surma Group of rocks
is also marked by primary structures such as cross bedding,
ripple marks, etc.?
All
these geologic features, lithocharacters as well as primary
structures suggest a different depositional environment
from that of the Disangs and Barails. So, these groups of
rocks as well as the younger Tipams are treated as molasse
sediments.?
The
rocks of Surma Group are well characterised by folds and
faults having regional strike similar to that of the Barails,
i.e. NNE-SSW. Fractures are also well developed which have
close relationship with the topographic features and drainage
patterns. The geometry of folds found in the region is quite
typical as in other parts of the Surma Basin and Western
Manipur. Antiforms are generally sharp and angular forming
ridges while synforms are broad and rounded representing
valleys and river beds. Such geometry of the folds might
have been controlled by hidden faults called blind thrusts.
And these thrusts could be potential earthquake foci any
time in future.
Geomorphic
and topographic features around Tipaimukh and its adjoining
region is also quite interesting not only because of thickly
vegetated low-lying hill ranges but also due to the intimate
relationship between the topography, especially the drainage
system, and the structural and tectonic lineaments of the
region. The drainage pattern of the Barak river and its
tributary system around Tipaimukh displays how delicately
Barak river takes a turn of about 360 degrees at Tipaimukh
giving rise to what is called, barbed pattern. Such a drainage
pattern is always resulted from the structural control of
the river. And practically the main Barak River opposite
to Tuivai River itself is also controlled by the Barak-Makru
thrust fault. Further it is also observed that main Barak
river course and its tributary system are all controlled
by faults and fractures as they all show rectangular to
sub-rectangular drainage patterns.? All these faults and
fractures cause localised shifting or deflection of the
main river course, and even at the confluence of Barak River
and Tuivai River. Such faults are potentially active and
may be focal and/or epicentres of any future earthquake.?
Northeast
region among six major seismically active zones of the world
tectonic setting of Northeast India is one of the most interesting
aspects in the tectonic framework of Southeast Asia. In
this region, two typical tectonic settings are found resulting
from the convergence between Indian and Eurasian plates.
The Eastern Himalayas represent a continent to continent
collision mechanism while the Indo-Myanmar Range is an island
arc type of subduction mechanism. The Indo-Myanmar Range,
therefore, evolved as an accretionary prism where major
structural and tectonic features spread out in the form
of an imbricate thrust system. The Tipaimukh area, about
which the dam is proposed to construct, lies in the Barak-Makru
Thrust zone of the imbricate thrust system.?
The
structural and tectonic pattern of Manipur is transitional
between the NE-SW trending pattern of Naga-Patkai Hills
and N-S trend of Mizoram and Chin Hills. The general structural
and lithological trend of the rock formations of the state
is NNE-SSW. It frequently varies between N-S and NE-SW although
sometimes NNW-SSE trends are locally common. Almost all
the major structural elements such as folds, thrust and
reverse faults follow this regional strike. Majority of
the extensional structures, e.g. normal faults, have WNW-ESE
trend. While the structures having neither compress ional
nor extensional affinities strike in the NW-SE and NE-SW
quadrants. Dip of the lithounits varies between moderate
to steep angles towards east or west. The geological and
structural settings suggest a very interesting tectonic
evolutionary history of the state.?
The
state, forming an integral part of the Indo-Myanmar Range,
lies in the boundary region of the Indian, Eurasian and
Myanmar plates having typical interaction nature. As a result,
the region is also one of the most seismically active zones
in the world (Zone V, earthquake zones of India).?
The
northeast region of India is one of the six major seismically
active zones of the world that includes California, North-East
India, Japan, Mexico, Taiwan and Turkey. So, it is essential
to have a brief discussion on these aspects also.?
Plate
kinematics ?
THE root cause of earthquakes in a particular region is
more or less exclusively a function of the tectonic setting
of that region and its proximity to plate boundary. Therefore,
the tectonic setting, plate movements and palaeo- and neo-stress
analyses of the region are very important aspects in order
to know about the seismic activity of that region. It not
only will reveal the deformation mechanism of the region
but also will provide knowledge about the structures that
may be easily reactivated as a function of the plate kinematics
in that region.?
Analysis conducted by the author about the plate kinematics
in and around Manipur reveals that the structural and tectonic
features of the IMR in general and that of Manipur in particular
evolved through the interaction between the Indian and Myanmar
plates rather than Indian and Eurasian (China) plates under
a simple shear deformation mechanism. From the analysis
it is found that the region has compression in the WNW-ESE
direction while extension lies in the NNE-SSW direction.
As a result, structures such as folds, reverse and thrust
faults oriented parallel to NNE-SSW direction will suffer
maximum compression and shortening while structures such
as normal faults, tension fractures and joints running parallel
to the WNW-ESE direction will undergo maximum extension.?
And
structures lying in the NW-SE and NE-SW quadrants will have
strike-slip movement. The faults and fractures around Tipaimukh
dam axis belong to the category that may undergo strike-slip
and extensional movements. So, these structures can be easily
reactivated causing small to considerable displacement along
them by any tectonic phenomena e.g. moderate and large earthquakes.
By such a process, if the dam axis is displaced by a few
centimetres a serious damage may occur causing a dam disaster
leading to huge loss of lives and property.?
Seismicity
?
Northeast India is one of the highest earthquake-potential
area in the world due to its tectonic setting, i.e. subduction,
as well as collision plate convergence. Analysis of earthquake
epicentres and magnitudes of 5M and above within 100-200km
radii of Tipaimukh dam site reveals hundreds of earthquakes
in the last 100-200 years. It is found that within 100km
radius of Tipaimukh, 2 earthquakes of +7M magnitude have
taken place in the last 150 years and the last one being
in 1957 at an aerial distance of about 75km from the dam
site in the ENE direction.?
Beside
the frequency of such large earthquakes within 100km radius,
it is also further observed that a number of epicentral
points align in the form of a linear array parallel to regional
strike NNE-SSW or N-S revealing how this Barak-Makru thrust
zone is seismically active. Another important aspect of
seismic activity is that shallow earthquakes are far more
disastrous than the deeper ones even if magnitude is relatively
low since destructive surface waves can be quickly and easily
propagated from the focus/epicentre. And majority of the
earthquakes that takes place on the western side of Manipur
are shallow (50km focal depth or less) which is due to the
tectonic setting of the Indo-Myanmar Range.?
Under
these circumstances whether it will be a wise policy to
construct a huge dam or not need to be thoroughly discussed
and investigated. The trend of earthquakes shows that the
regions which have experienced earthquakes in the past are
more prone to it; the magnitude of future earthquakes may
be uniform to the past ones; and the earthquake occurrence,
geological data and tectonic history all have close correlation
(Mollick). The Tipaimukh Dam site has been chosen at the
highest risk seismically hazardous zone.?
The
dam proponent, NEEPCO, claims that seismic hazards are being
taken care of through consultations with Rourkee University
(However, the government of India has requested NEEPCO to
also consult with the Geological Survey of India). Here
it is pertinent to state that extreme seismic hazards cannot
be addressed adequately or satisfactorily through consultations
with seismologists, as the risk inducing and impact factors
are mechanical, geophysical, tectonic and socio-economic
in nature.?
Dr
Soibam Ibotombi teaches earth sciences at Manipur University
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