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Irrigation And West Rapti Dam By AB Thapa There are three major
rivers that cross the Terai region of the
western half of Nepal before entering into
India. They are the Gandak, Karnali and Mahakali
rivers. These three rivers carry on an average
annual flow in the range of 3500 cumecs.
Such enormous flow of these rivers could
bring under year round irrigation a vast
area of lands to the north of the
Ganges river in the western Bihar and
eastern UP. Nobody has any doubt that
sooner or later the water drawn from
the giant hydraulic structures built in
Nepal would be transforming the vast tract
of lands in India to the north of
the Ganges river into one of the most
advanced regions in agriculture production.
Unfortunately such unbelievable opportunity to
greatly uplift the agriculture production in
India would mean nothing to our own
farmers in the western Terai particularly those
living in Kapilabastu, Rupandehi, Deokhuri and Banke.
Most of our farmers in these districts would
be forced to watch the miracle of
agriculture development on the other side of
the border in great amazement and perhaps in
disbelief to what they would have seen with
their own eyes while their parched
lands would continue to be denied
opportunity to quench in want of water. Most part of the
Deokhuri, Banke, Kapilbastu and Rupandehi districts are not
easily accessible to extend the canal system
to deliver irrigation water drawn from either
the Gandak or the Karnali. So we would be
finding it very difficult to use the
Gandak and Karnali waters in our own
country to irrigate sizeable area of lands
in above districts despite the fact that
the agriculture on a vary vast area of
lands extending as far as the Ganges in
the south would be thriving on waters drawn
from our major rivers. Fortunately what
we would have missed out in not
being able to use to a reasonable extent the
water drawn from our major rivers to irrigate
our own lands, the development of
the West Rapti dam project to some
extent could help to make up.
However, we should be extremely careful in
drawing the plan to implement the
West Rapti project. We could be easily
misled because it is a very complicated
multipurpose project. Apart from irrigation the
electric power generation component of this
project is also quite significant We
should completely refrain from doing anything
that would in anyway jeopardize the
prospect to use the full irrigation
potential of the West Rapti river.
It can hardly be denied that we
should abide by our commitment
not to kill the vast long term
development prospect of the West Rapti
project to score short term gains. West Rapti Development
Strategy The West Rapti high
dam project should be planned primarily to
focus on irrigation despite the fact
that it is a multipurpose scheme
because almost the 2/3 districts of the
Terai in the western half of Nepal would
be dependant on the Western Rapti river
for adequate supply of water needed to
provide round the year irrigation.
Unfortunately the West Rapti is a tiny
river by comparison with the Karnali or the
Gandak. The annual flow of the
West Rapti river is very limited. The
average annual flow of this river is only
in the range of 100 cumecs whereas such
flow of the Gandak or the Karnali is in
the range of 1500 cumecs. . Moreover, the
greater part of such total West
Rapti annual flow is carried in the
monsoon months when the demand for irrigation
water is very much limited. Thus there is
a need to explore the possibility
to provide a very large West Rapti
storage reservoir that would allow to hold
back if possible the entire flood
season water. In this way it would be
possible to extend the coverage of irrigable
area to the greatest extent possible. This
type of massive withdrawal of West Rapti
water from the storage reservoir for the
purpose of extending the irrigation coverage
would have very much positive impact on
power generation. It would be possible to
generate greater part of the electricity in
dry months when the demand for the
electricity in Nepal is the highest. Such
generation of electricity in dry season would
fetch higher price. The total area of
lands in 5 Terai districts to the West
of the Gandak river in need for water
to irrigate lands in dry season is very
large. It should be tried to extend
the total coverage of the irrigation
area to the maximum extent possible
by increasing the storage capacity of the
West Rapti reservoir. Nevertheless, fairly large
chunks of lands would still be not
having an access to supplies from the
West Rapti even after the entire project
would come under operation. Separate plans
should be carefully prepared to irrigate
such lands beyond the command area of
the West Rapti irrigation system by
drawing water from the Gandak and the
Babai although it could be costlier
to provide such irrigation system. During the monsoon season
the West Rapti flood damages are not
at all insignificant despite the fact that
this is relatively a small river. In
recent time almost every year widespread
submersion and the loss of life and
property in the Indian territory due to
the West Rapti floods are reported. The
rise of the West Rapti water level is
exceptionally high at the time of
big floods. In the last century until
1975 the recorded highest flood water level
of the West Rapti river at Birdsghet
near Gorakhpur area had exceeded the warning
stage level by a huge margin of over
9 feet. By comparison with the West
Rapti, the peak flood water level rises
in other rivers are relatively small. In
the same above period the recorded
highest flood water level of the Ganges at
Patna, Yamuna at Delhi, Gandak at
Bhainsalotan and the Bagmati at Hayaghat
around Samastipur had exceeded the warning stage
level by far smaller margin of just 3.4
feet, 5.4 feet, 2 feet, and 4 feet
respectively. No wonder the West Rapti high floods
devastate vast area of lands despite the
fact that it is relatively a small river.
Very large West Rapti storage reservoir
would help to keep at bay flood damages
across a vast area of lands in the
north-east UP of India. Despite the fact that
the main focus of the West Seti project
would be on irrigation, the power and
flood control components of the project
would not be insignificant. The same West
Rapti water on its way to irrigate
lands would be used at the beginning
to generate electricity without any negative
impact on irrigation. The storage dam
would provide protection against the West Rapti
floods to peoples in Nepal as well as
India. Such multiplicity of benefits accruable
from the West Rapti project makes this
scheme very attractive. Need For Bigger
Storage Volume The average annual flow
of the West Rapti river is in the
range of about 100 cubic meters per
second. It implies that the annual flow
of the West Rapti would be around
3,000 million cubic meters. Preliminary study
carried out to examine the possibility of
building a high dam at Bhalubang indicates
that it would be possible to provide a
storage reservoir over 3,000 cubic meters
in volume if the dam height is
raised to about 120 meters. If the dead
storage volume of the reservoir is presumed
to be about 400 million cubic meters as
in the past studies, the total regulating
capacity of the storage reservoir would be
over 85% of the total annual flow. In
the context of Nepal it might not
be wrong to roughly say that the flow
of West Rapti river might be fully
regulated to provide an uniform discharge
throughout the year if the regulating
capacity of the storage reservoir is over
60% of the total annual runoff. The
capacity of the proposed Bhalubang storage
reservoir would be far in excess of the
volume necessary to regulate the annual runoff
to provide uniform flow throughout the year.
In fact the capacity of the West Rapti
reservoir would be adequate to hold back
completely the entire monsoon flow of the
West Rapti. The storage of almost
entire monsoon flow would not have much
adverse impact since the demand for
irrigation water as well as electricity supply
would be very insignificant during the
monsoon period. The conserved monsoon flow
could be used to double the availability
of water in dry months lasting 3 to
4 months when the demand for water to
generate electricity and to irrigate lands
would be the highest. We should not shy
away from building the high dam to provide
storage reservoir even far in excess of
the annual flow if such decision suits
us. We can draw a lesson from the
High Aswan Dam built in Egypt. The annual
runoff of the Nile river is about 83,000
million cu. m. The High Aswan Dam constructed
in 1971 has a storage capacity of 164,000
million cu m nearly twice the annual flow of the
river Decision to provide such large
storage reservoir was made despite the fact
that the annual loss of water due to
evaporation is 12,335 million cu. m which is
very high. Bhalubang vs Naumure
Two sites have been
identified to build high dam across the
West Rapti river. The first one is at
Bhalubang. A pre-feasibility level study to
build a dam up to 120 meters in
height has already been completed by the
Department of Irrigation for irrigation
and also generation of power. Similarly a
preliminary study of the high dam further
upstream at Naumure has also been
carried out by the Electricity Department for
the generation of the electricity. The height
of the Naumure dam is in the range
of 200 meters. There is a need to
conduct through studies of both these
projects before taking decision to implement
any of them. Main Irrigation
Distributaries There would be two
main irrigation distributaries to draw water
from the West Rapti reservoir. The first
one would be an open channel that
would draw water from the tailrace of the
hydropower station at the foot of the
high dam. This canal running parallel to
the West Rapti river would be irrigating
lands in Deokhuri and Banke. It is also
equally possible that a certain stretch of
the West Rapti river itself be used
in place of canal to reduce the total
length of the distributary. The construction
works of the Sikta Irrigation project
which was abandoned after completing few
preliminary works should be restarted as an
integral part of the West Rapti project.
The next main distributary would be
a tunnel that would draw water directly
from the storage reservoir The tunnel
would deliver water to the hydropower station
located at a certain distance from
the storage reservoir. The total gross
water head at full supply level of
the storage reservoir is expected to be 220
meters. The water discharged from the power
house would be carried by open canal
running to the east. This canal could be
planned to irrigate lands in Kapilbastu, Rupandehi
and Nawalparasi districts as far as possible
to cover maximum area. Planning of the
sublateral canals should done giving due
consideration to existing irrigation systems such
as the Banganga project, existing ground water
projects etc. Total Irrigable Area The proposed West Rapti
project with greatly enhanced storage capacity
would be able to provide in total a
flow of more than 150 cumecs in dry
season of an average flow year. It might
be possible to irrigate about 120,000 ha in
dry season if it is presumed that the
coverage area under dry season paddy and
wheat would be almost equal. |
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