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Saptari Can Be Devastated By Kosi By AB Thapa It is reported
based on the past studies that the Kosi
river is on the verge of shifting to the
east far away from its present course.
The people of Nepal living in the Sunsari
and Morang districts might be heading for
a natural disaster of an unprecedented
scale. Unfortunately only very few know
that there is equally a possibility
that the Kosi river might devour
a substantial proportion of the Saptari
district before this river changes its
course to the east. The people living on
right bank side of the river just
downstream Kosi barrage must have been noticing
for quite some time that the Kosi river
is persistently attacking the right bank
during the flood seasons. It is indeed a
warning of imminent great danger. It has
already been reported in the preliminary study
made in the WECS ( Water and Energy
Commission) that there is a possibility that
the Kosi could again shift to the
west. How Saptari People
Suffered All the elderly people
living in the Saptari district not
far away from the Kosi river
would not have much difficulty to
recall that some 50 years ago the
channel of the Kosi river flowing
across the Sunsari district from north
to south was gradually shifting laterally
from east to the west. The river
was already quite close to Hanumannagar.
It is reported that the Kosi. River debouching
into the Terai plain at Chatra and
joining the Ganges River near Kursella in India
by that time had already laid waste
about 500 square miles of land in
Nepal alone. People living in the eastern part
of the Saptari district were panic
stricken. They were leaving their
ancestral homes in thousands to find shelter
at safe places far away from their
villages carrying with them whatever few things
they could take with them. The
headquarter of the Saptari district at
Hanumannagar was swiftly moved to
Rajbirag which was at a safe distance
away from the Kosi River. Fortunately the
shifting of the Kosi River course
to the west was held in check by
the embankments built on either side of
the river some 50 years ago. Now
all those who are involved in the Kosi
River study are saying that the
embankments built to protect the life and
property of millions in India and Nepal
might have already outlived their usefulness.
What is going to happen the next?
Every common man in the Saptari
district would be expecting an answer.
Perhaps to get an answer to this question
it is important to know why the
Kosi River could again pose threat
to Saptari. Rise in Kosi River Bed
Level The Kosi River brings
every year an enormous quantity of sediments
from its catchments in the mountains. Sir
Claude Inglis an expert on Kosi had
attribu;ted the shift of the Kosi River
channel to excessive sand load carried by
the river. Leopold and Maddock considering
Kosi behaviour had stated that a braided stream
will tend to shift laterally at a rate
dependent on the rate of accumulation of
material being deposited. As one course becomes
higher than possible adjacent paths, the river
would shift. Data published in the
American Society of Civil Engineering in
March, 1966 indicate that in the period between
1938 and 1957 every year on an
average about 100 million cubic meters of
sediments used to be deposited on the
Kosi River bed. The maximum such deposition
was around Nirmali in India not far
away from the Hanumannagar. There was very big
change in sediment deposition pattern immediately
after the completion of the Kosi barrage
in 1963. The results of the Kosi River
channel study for post barrage period
have been published by V.C. Galgali, Central
Water and Power Research Station, Pune (India), and
Gohain & Prakash of Roorke University. All the
past studies confirm that the Kosi River
bed just upstream of the barrage
has significantly aggraded due to
sediment deposition. On account of ponding,
sediment deposition had occurred, flattening the
bed gradient. The bed slope of the river
in the pond area was abut 0.61 m per km
in the year 1956 prior to construction
of the barrage, which became flatter to
about 0.42 m per km in the year 1969,
ie in six years of the functioning
of the barrage. Studies were made to determine
the sediment deposition based on post
flood 1963 and 1970 surveys These studies
indicated that about 35 million cu.m. sediments
had deposited in the pond length
of about 10 km upstream of the barrage,
giving an average depth of about 0.4
m in about 8 years with a rate of
bed level rise at about 0.05 m per annum. Studies made in the
years 1963 and in 1970 indicated that the
Kosi River bed between the barrage
and 22.5 km downstream has lowered by 0.25
m. Subsequent studies for the years
1970 onward to 1978 have, however, shown that
the bed has in general started getting deposited
and such deposition is even now continuing. Shifting Tendency of
Kosi After the completion of
the Kosi barrage, the river with reduced
sediment charge was expected to develop more
or less stable central channel downstream
the barrage. Such a central channel would
have been at a distance from the
embankments built on either side of the
river. Annual survey had been conducted to
study the changes in the river regime. The
river, originally flowing along the right
flank, was diverted in 1963 onto the barrage,
which was built close to the left bank.
However, the Kosi River after flowing across
the barrage veered to the right to
occupy again the right bank channel and
caused a breach in the western embankment
at 13 km. The Kosi was attacking a
section of the western embankment between Dalwa
( 12 Km ) and Kunauli (17 Km). In 1966, the attack
suddenly shifted to the length of the
embankment in the reach between 8 and 9 km, and
19 and 21 km. Past observations have shown
that until 1970 the Kosi was attacking
the western embankment up to a distance
of about 23 km from the barrage. After 1971 the river
moved onto the eastern side close to the eastern
embankment. It was seen flowing close to
the eastern embankment upto a distance of
about 22.5 km from the barrage. Based on
the available information the Kosi was flowing
in this reach very close to the
eastern embankment until 1978. After this very
strong protection works were provided to
strengthen the eastern embankments Thereafter
the main Kosi channel has shifted again
towards the western bank over a long
stretch of the river immediately downstream
of the barrage. Some very interesting
observations have been reported about the
regime of the Kosi in post barrage
period. It was found that despite
considerable efforts the objective of stabilizing
a central course of the river below the barrage
could not be achieved. The river has been all
along showing the lateral instability. Now
when the Kosi river bed level is
expected to be much higher by
comparison with the levels of the initial
periods the lateral instability could
pose a serious threat. What Could Happen
The Next ? The energy content of the
water discharged across the barrage into the
Kosi in the post barrage period would
certainly be far greater by comparison
with such energy before the construction. Such
excessive content of the energy in
the discharged flow from the barrage would
have been spent to induce
scour of the coarse sediments deposited
deep inside the river bed and
also to transport them a short distance
further downstream. The freshly deposited coarse
sediments blocking the river course
might be to a considerable extent responsible
for lateral shifting of the Kosi
channel downstream of the barrage.
Moreover, the excessive energy would also have
been spent to erode the embankments
built on either side of the river At
present the retrogression of the Kosi River bed
might be still observed, though on
a very short stretch immediately after the
barrage, due to upstream accretion of
sediments. Further downstream the river
bed is gradually rising as a result
of continued deposition of sediments.. The
relationship between the sediment deposition
and shifting of the river course
explained by Leopold and Maddock in a
very simple way has already been presented
at the outset ì A braided stream
like the Kosi will tend to shift
laterally at a rate dependent on the rate
of accumulation of material being deposited. As
one course becomes higher than possible
adjacent paths, the river would shiftî. It
implies that the Kosi would swing across
the embankment as soon as the level
of the Kosi river bed exceeds certain
height. The river can be expected to
shift to the west because according to
renowned experts on Kosi like Rennel and
Shillingfeld it is the general direction
of the Kosi channel oscillation that had
started some 250 years ago from a
region some 130 km to the east from
the present position of the Kosi. The
Kosi shift to the west would have
continued until the point where it empties
into the Ganges meets the mouth of the
Gandak. The Ball is in
Our Court Needless to say
that people living in Morang, Sunsari and
Saptari districts could be heading for a
big disaster. For the time being
this problem could be solved by building
the Sun-Kosi storage dam project that
would provide all the year round irrigation
across the vast area of Nepal Terai
and also cheap electricity on a large
scale that would suffice for meeting the demand
for electricity of the whole country for
many years. By this time we should have
already made significant progress up the path
to implement this very important project.
Unfortunately we are not at all seen to
be interested to pay due attention to
resolve the problem of Kosi flood disaster
that is threatening the life and property
of millions in our region. As far as
the India is concerned we could expect
full support from their side. They have even
agreed to provide financial assistance to
conduct detailed study. Needless to mention
that Nepal should take the main initiative
to implement this project because it is
our project. Excerpts from the past agreements
between Nepal and India related with the
Sun-Kosi project are presented below which
clearly show that the ball is in our court. The Revised Kosi
Agrement, 1966 - HMG shall have every right
to withdraw for irrigation and for any other purpose
in Nepal water from the Kosi river and from the
Sun-Kosi river or within the Kosi basin from
any other tributaries of the Kosi river as may
be required from time to time. The Union (it
indicates India) shall have the right to regulate all
the balance of supplies in the Kosi river
at the barrage site thus available from time to time
and to generate power in the Eastern
Canal. Kosi Study Agrement, 1997
- Detailed study of the Sun-Kosi high dam
project and the Kosi high dam
project would be carried out. The Kosi dam
study would also include the detailed study of
a waterway to link Nepal with the Ganges.
The navigation study would cover both the
options - a separate navigation canal and
the use of natural channel. India has
agreed to bear the entire cost of all
these detailed studies. |
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