NAUMURE OR BHALUBANG SITE FOR HIGH DAM
West Rapti Project
By Dr. AB Thapa
It is reported in local newspapers that the Government is intending to implement the West Rapti Project, which will have a dam at NAUMURE for power generation. Hydropower generation is a component of the West Rapti Project but, more important, this river is vital to irrigation development in five districts of Southern Nepal. We should take utmost care to ensure that the irrigation potential of this river would not be placed in jeopardy.
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 together with 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.
West Rapti a Lifeline for Four Districts
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 while their own 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. .
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 surplus 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.
Substantial Flood Control Benefits to India
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 million 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.
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 be done giving due consideration to existing irrigation systems such as the Banganga project, existing ground water projects etc.
(Dr. Thapa writes on water resources)
