Opinion (Spotlight Weekly)

Langtang Water To Kulekhani Via Kathmandu

By AB Thapa

The Melamchi Water Supply Office had brought out in their web site comments on the articles on Melamchi project published in this journal some time back. Such prompt reaction of the Melamchi Office in defense of their plan deserves to be taken up positively. It has provided an opportunity to bring forward to the notice of the general public various technical, economic and policy issues related to this project. This article is the continuation of the previous article published very recently in this journal on April 25 on Melamchi Office comments.

Is It Only A Dream ?

It is not a surprise that many people would have apprehension that the proposed Langtang water diversion into the Kulekhani reservoir through Kathmandu could be nothing more than a dream. The grandiose of the title of the total plan itself can confuse anybody. They can easily be misled.. The whole concept can be much more easily understood if each element of this diversion plan is explained separately. Before going any further to present the plan to divert the Langtang river into the Kulekhani we need to be very clear that according to the existing Melamchi project plan about 40 kilometers long tunnel would have to be built anyhow to cater the demand for water supply in Kathmandu valley from 2011 onward. Out of such total length of the tunnel, the length of the Melamchi tunnel would be 28 kilometers and the length of the Yangri and Larke diversion tunnel about 12 kilometers. The Melamchi tunnel hydropower is also a component of the original Melamchi water supply project plan prepared under the UNDP assistance though. it is now considered to drop this component despite the fact that the UNDP study had recommended to provide the hydropower because it could contribute to reducing the cost of drinking water to Kathmandu residents.

Let us now look into the NEAís plan to divert the Langtang river at Ghore Tabela for power generation. Even according to the Melamchi Water Supply Officeís reporting the Langtang diversion is an excellent power project. It is proposed to discharge the Langtang water after the power generation directly into the Trisuli river. Now we come to the crux of the problem. What is going to happen if the plan of the water resources ministry to divert the Langtang river at Ghore Tabela for the generation of electricity is combined with the plan of the water supply ministry to deliver additional water to Kathmandu valley on top of the Melamchi water? Will such plan to generate electricity nicely dovetail with that of the water supply ministry ? What would be the comparative advantage of such multipurpose project vis-‡-vis two separate projects delinked with one another ? It can hardly be denied that it is the duty of the NATIONAL PLANNING COMMISSION to examine such possibilities. Nonetheless it also is the obligation of each individual ministry to insure that every aspect of the project within its jurisdiction is honestly examined in the broader interest of the country.

If the alignment of the tunnel to draw Langtang water at Ghore Tabela for power generation is changed and the water after the power generation is led into the Melamchi river as against the original plan to discharge into the Trisuli river, there might be even slight decrease in the total combined cost of the Melamchi water supply project and the Langtang power project. However, such rerouting of the Langtang power tunnel could open up completely new vistas of possibilities for the generation of cheap electricity.

In this brief article three separate components of the Langtang diversion into the Kulekhani have been examined. To a certain extent each of these three components can also be seen as a separate project independent of one another. The first would be the Langtang diversion at Ghore Tabela. What would happen if the Langtang river is diverted into the Melamchi instead of the Trisuli river ? The next would be the Langtang storage dam at Jaithang and its contribution towards the augmentation of the dry season flow of the Melamchi and Kulekhani rivers. The third is the diversion of the combined flow of the Langtang and Melamchi rivers into the Kulekhani reservoir.

Langtang Diversion into Melamchi

According to the NEA study the minimum flow of the Langtang river at Ghode Tabela is about 4 cumecs. Such flow could be more than the combined flow of the Yangri and Larke rivers. There is a level difference of about 1600 meters between Ghore Tabela and the Trisuli river. It is planned to utilize this big head to generate power and for this purpose a 12 km long power tunnel would be necessary.

It would be possible to obtain a total gross head of about 1600 meters or even more if the Langtang water is conveyed to the Melamchi river instead of diverting it into the water surplus Trisuli river. Such diversion could be a substitute for the diversion of the Yangri and Larke rivers to augment the flow of the Melamchi. It could, thus, spare us the trouble to cut off the Larke and Yangri water supply to Indrawati hydropower stations for power generation. A 20 km long power tunnel would be needed to deliver the Langtang water from Ghore Tabala to the Melamchi project intake and such tunnel would have been 8 km longer than the power tunnel needed to divert the Langtang water into the Trisuli river. We should not forget that it would be necessary by 2011 according to the UNDP feasibility study of the Melamchi project to build the 12 km long Larke and Yangri diversion tunnels to augment the dry season flow of the Melamchi river. If the Langtang water is diverted into the Melamchi river, it would not be necessary neither to build the 12 km long diversion tunnel to deliver the Yangri and Larke waters into the Melamchi nor to build the expensive headworks across those two rivers to divert the water. Thus the utilization of the Melamchi route for the Langtang river power project would have involved the construction of power tunnel 4 km shorter in length and would also have allowed to dispense with the construction of expensive headworks across those two rivers( Yangri and Larke).

There would not be any reduction in total gross head for the power generation despite the changes of the Langtang diversion route from Ghore Tabela- Trisuli to Ghore Tabela ñMelamchi. Out of the total gross head of 1620 m between the Ghore Tabela and the Sundarijal tailrace, about 1300 m head would be used by the Langtang hydropower that discharges water into the Melamchi intake and the rest by the hydropower to be provided near the entry point of the Melamchi tunnel into the Kathmandu valley.

Langtang Storage Dam at Jaithang

The Langtang storage project is one of the schemes identified in the UNDP supported Gandak Basin Master Plan study. According to the UNDP study report a dam about 120-m in height is proposed on the Langtang river near Jaithang. The total drainage area of the Langtang at Jaithang is 150 sq. km. as against a drainage area of 250 sq. km. at Ghore Tabela. The volume of the reservoir would be about 180 million cubic meters. The elevation of the FSL would be 3995 meters which is well below the permanent snow line. The Melamchi Water Supply office has provided information in their web page that the height of the proposed Langtang storage dam at Jaithang should be raised to 300 meters to arrive at a total storage volume of 180 million cubic meters. It would be very much appreciated if they could provide further information in more detailed form about the recalculated height of the Langtang storage dam that completely disproves the findings of the UNDP supported Gandak basin study. According to the UNDP study the regulated flow of the Langtang river is proposed to be tapped for power generation only at Ghore Tabela about 15 km downstream at an elevation of 3028 meters. However, it appears to be worthwhile to build another hydropower near the Ghore Tabela to utilize the potential of the Langtang river between the diversion dam at Ghore Tabela and the storage dam at Jaithang. The hydropower would be directly drawing water from the storage reservoir.

There can be some concern about the thickness of the river bed materials wherever a storage dam is proposed to be built. Fortunately in case of the Langtang high dam, the bed rock might be found at lesser depth because the gradient of the river is too steep. Another equally important issue could be the glaciological problems which are briefly described hereinafter.

Threat From Langtang Glaciers

Glaciers occur exclusively in mountainous regions above the snow line. Mountains below the snow line carry only seasonal snow. The snow line of the southern Himalayan region is approximately at an elevation of 5000 meters above the sea level. Snow accumulates and glaciers are formed at heights above the snow line. Constant avalanches of snow from the slopes feed the upper reaches of the glaciers. The surface layers of ice and snow grow thicker. The glacier begins to slide under its own weight and enters the valley like tongue of moving ice. The part of the glacier at elevation below the snow line melts into water and such melted water is drained into the river. The position of the termini of the glaciers remains almost unchanged if the phenomena such as the retreating ( or advancing if it is opposite) of the glaciers recently observed in the Himalayan region are discounted. In case of the Langtang storage dam at Jaithang and its environs the full supply water level in the storage reservoir would be well below the level of the snow line and thus at a distance from the tongue of the Langtang glacier. It is difficult to find any direct relationship between the reservoir water level and the existence of the Langtang glacier. However, it would have been a completely different matter had there been a glacier lake in between the storage dam and the termini of the glacier as in case of the Dig Tsho glacier. In 1985 the Dig Tsho glacier lake outburst flood (GLOF) completely destroyed the Namche Hydropower station. Nonetheless it can not be denied that a study of the Langtang glaciers in full detail would have to be an important part of the future feasibility study of the Langtang storage dam project.

Diversion from Kathmandu to Kulekhani

The diversion of the surplus Melamchi and Langtang waters( initially without a storage dam at Jaithang) from Kathmandu valley to the Kulekhani reservoir could be a very simple and at the same time the most cost effective proposition despite the fact that at first glance it might appear to be a highly complicated engineering task. Let us take a hypothetical case that we are going to draw only 4 cumecs water somewhere near Chobar out of the combined flow of the Bagmati. Melamchi and Langtang rivers. We might have to build a 15 MW pump station to lift the water to a height of about 300 meters above the river water level which might be in height equivalent to the full supply level (FSL) of the Kulekhani storage reservoir. It is equally possible that instead of one big pump station we might need several small pump stations with a total capacity of about 15 MW that would be completely dependant on topography. A 15 km long waterway might be needed to carry the water into the Kulekhani storage reservoir out of it the length of the tunnel could be about 9 kilometers. The total cost of such diversion could be about US $ 20 million

After the completion of the above described Langtang diversion into the Kulekhani reservoir, there would be a net increase of about 250 GWh firm power in annual generation of the Kulekhani Nos 1,2&3 hydropower stations even after subtracting the energy spent on pumping. Thus the additional generation of the Kulekhani hydropower stations after the Langtang diversion would be two times greater than the present annual generation of Kulekhani No 1 Station. It would be possible to increase to such a great extent the electricity generation absolutely without any additional investment in civil structures and electrical equipments of the Kulekhani dam and hydropower stations.

At present the NEA is paying to the private developers at an average rate of about US Cents 6 per KWh. Out of such purchase, the firm energy is only about 50% and the rest is secondary energy. The purchase rate of the exclusively firm energy could be about US Cents 9 per KWh if it is presumed that 3 KWh secondary energy would be equivalent 1 KWh firm energy though in the World Bank supported Upper Karnali Project feasibility study the seasonal energy has been valued only at 10% of the firm energy based on the expected net value of such energy for the thermal fuel displacement. Thus even a small scale Langtang diversion into the Kulekhani could help to increase the annual income of the Kulekhani hydropower stations by a wide margin of US $ 22.5 million whereas the total investment in the pumping system to deliver the water into the Kulekhani reservoir could be only about US $ 20 million.