Opinion (Spotlight Weekly)

Water Energy Commission And Sun-Kosi Project

By AB Thapa

During the official visit of our prime minister to India in December, 1991 it was agreed to take up for detailed study the Kosi High Dam Project. However, there was not any mention in the agreement between the two governments neither about the Sun-Kosi Dam Project nor about the Kosi navigation. Nobody disagreed that the Kosi High Dam project is very important and a detailed study of this project is in the best interest of our country. The matter that worried us the most in the Water Energy Commission (W EC) was the fact that the Kosi project was going to be taken up for detailed study in isolation disregarding the Sun-Kosi project and the Kosi navigation. We feared that both these projects could be precluded forever. The Sun-Kosi project and the Kosi project would be mutually exclusive of one another if the Kosi project is implemented first. We would have thus forfeited a golden opportunity forever to irrigate vast area of lands in the Eastern and Central Terai. Similarly it would have been very difficult, if not impossible, to convince India to agree on our proposal to develop Kosi waterway linking Nepal with the seaport once all matters related with the Kosi project are decided. WEC vehemently raised this issue. It was extremely vital that the proposed Kosi study would not be allowed to bypass the study of the Sun-Kosi project and the Kosi navigation. Finally the WEC was able to convince the highly skeptical authorities in our government that the study of the Kosi project should be carried out side be side with the study of the Sun-Kosi project and the Kosi navigation. WEC agreed to prepare the basic strategy reports on Kosi dam project, Sun-Kosi dam project and Kosi navigation. These studies were made by ourselves without the slightest involvement of the foreign expertise. Those reports can be seen in the WEC library. It had been tried to cover in the above reports few important issues related to above projects to justify our stand that the Sun-Kosi high dam should be built first It has been explained in the report that the building right away of the Kosi high dam according to Indian proposal could have grave consequences for the people particularly those living in the lower reaches as a result of wide spread flooding. The experience gained in course of the operation of the Boulder dam built across the Colorado river in the USA as early as 1935 under physical conditions similar to that of the Kosi river helps to arrive at a such conclusion. Boulder dam problems including few other cases are described below.

Boulder Dam Induced Floodings

The Colorado river course in the USA was subjected to adverse geomorphological changes even during the construction of this 221 meters high gravitational arc concrete dam. Within six months of the closure of the river diversion gates in 1935 the river bed was lowered by from 2 to 6 feet over a distance of 13 miles. By 1947 the river bed in the 77 mile canyon reach had been lowered between 6 and 14 feet. Owing to the exposure of rock ledges the river became stable. However, at Needles, about 130 kilometers away, the river bed rose by 6 meters necessitating the construction of very expensive flood control structures.

Chinese and Egyptian Experiences

Danjiangkou storage dam project on the Hanjiang river, a tributary of the Yangze, was also subjected to serious river bed degradation after the completion of the construction. The volume of the storage reservoir is 16 billion cubic meters and the total drainage area at the dam site is about 95,000 square km. This river used to carry every year 115 million tons of silt load. After the construction of the high dam, the degradation of the Hanjiang river bed extended over 650 km up to its confluence with the Yangtze river and the annual sediment erosion had ranged upto 27 million tons, which was about 25% of the annual silt load carried by the river. After the completion of the storage dam, the sediments carried were the eroded materials from the channel bed and were much coarser and thus with greater settling velocity despite the fact that the peaks of floods were much attenuated and which in turn reduced the transport capacity by as much as 41%.

After the completion of the High Aswan Dam on the Nile river in Egypt an untoward process of the downstream river bed degradation was observed. Such lowering of the river bed reduced the minimum guaranteed water depths which indeed was a great impediment to navigation on the river Nile. During the high floods the releases at Aswan used to exceed 350 million cubic meters per day despite the fact that the total volume of the Aswan storage reservoir is about 168 billion cubic meters which is almost twice the annual flow of the Nile river at Aswan. An emergency spill channel has now been built to overcome the downstream degradation problem. The 55 km long channel diverts the excess water from the reservoir to the Toshka depression located in the desert west of the Nile river. The channel starts to operate as soon as the water in the reservoir rises above certain level.

Prior to Kosi High Dam

At present the Kosi river debouching into the Terai plain near Chatra brings with it every year an enormous volume of sediments. At the beginning the coarse materials are deposited; only the finer material of silt and clay is carried forward which are then deposited in the lower reaches of the Kosi and further beyond in the Ganges, Hoogly and the Bay of Bengal. Needless to explain that the Indo-Gangetic alluvial plain has thus been built by alluvial material which has been brought by the erosion of the rising Himalayan mountain chain.

The Kosi river in upper and middle reaches can be termed braided river. A braided river is relatively wide with poorly defined unstable banks, and is characterized by a steep, shallow water course with multiple channel divisions around alluvial islands. In the vicinity of the Chatra the longitudinal slope of Kosi river bed is almost one meter in every kilometer. The river bed slope decreases to about one meter in every four kilometers near Hanumannagar. Near Nirmali region ( in India) the bed slope could be about one meter in every five kilometers.

The bed slope of the Kosi river is very small in its lower reaches. It is about one meter in every twenty kilometers. Based on the classification of the channel patterns, the Kosi ceases to be a breaded river in its lower reaches. It becomes a meandering river.

In Post High Dam Period

The Kosi river would continue to pose serious threat to the life and property of a great number of people even after the completion of the Kosi high dam if every aspect of this vast project is not thoroughly examined including the problems associated with the fluvial geomorphology. It hardly needs to be further explained that after the construction of the high dam the regime of the Kosi river would be completely changed. The storage reservoir would be acting as a desilting basin for the water flowing in the system. All the bed-material load coming into the reservoir would drop out within the reservoir. Water released from the reservoir would be quite clear. The Kosi dam would have significantly big impact on upstream river regime. As a result, there would be aggradation of the river bed, loss of waterway capacity, change in river geometry, increased flood stage, change in base level for tributaries, deposition in tributarties near confluences etc. Similarly there could be even greater downstream changes in the post dam period. These changes are channel degradation, possible change in river form, local scour, bank instability, reduced flood stage etc.

River Channel Dynamics and Kosi Dam

The study of Kosi geomorphology is indeed a big challenge. Such study is expected to play dominant role in taking proper decision to finalize the scope of the project. Past works of the scientists like Einstein H.A.( son of Albert Einstein), Khan H.R., Lane E.W., Leopold & Maddock, Schumm S.A., Simon D.B. etc. point out that the sediment size, channel gradient and river discharge play the decisive role in deciding the dynamics of any river channel, ie whether it is subjected to aggradation or degradation.

In case of the Kosi dam proposed by India the volume of the storage reservoir is only about 13 billion cubic meters. The volume of the Kosi reservoir is going to be relatively too small by comparison with other reservoirs proposed to be provided for flood control. The proposed Karnali reservoir has a total storage volume of about 39 billion cubic meters despite the fact that the Karnali is a smaller river. Thus the high dam built at Barahachetra would not be able to reduce significantly the high peaks of the Kosi floods. During such high floods very big quantity of clear water would be discharged from the reservoir through the dam into the river. Unfortunately such clear water spilling into the river in very big quantity can play havoc with the whole downstream reach of the Kosi river channel. Moreover the longitudinal slope of the Kosi river bed throughout the upper reach is very steep so even the coarse materials deposited at considerable depths inside the river bed could be easily scoured and carried way further downstream. The slope of the Kosi river bed is quite gentle in middle reach and way further downstream in the lowermost reach it is extremely flat. So it becomes increasingly difficult to transport the sediments much further. The eroded bed materials from the Kosi river channel would be mostly deposited either in middle reach or in the lower reach. In either case it would result to widespread flood damages. This problem would be solved if the Sun-Kosi dam is built at the first.

Sun-Kosi Dam at the First

After the completion of the Sun-Kosi dam project about 50% of the Kosi river silt would be held back. The dam would also be able to regulate partially the Sun-Kosi river floods which would help to lower the maximum peak floods of the Kosi river. In this way it would be possible to reduce the maximum flood discharge as well as the total quantity of sediments being brought down every year to Terai from the mountain region. It also could be possible that in the post Sun-Kosi dam period a kind of state of equilibrium would be maintained between the forces that resist the downward movement of the sediments and the hydrodynamical force of the Kosi flow. In this way the Kosi flood problems could be solved temporarily. If it is found that even after the implementation of the Sun-Kosi dam project the flood and silt problem has not been resolved satisfactorily, the Tamar No.1 storage project could be taken up. After the completion of both these projects the Kosi flood problem could be solved for the next 50 years. Thus we would have at our disposal ample time to carry out leisurely the full study of the Kosi high dam project without endangering the life and property of millions in our region.

"Whenever I appear to be angry, remember, it is only love in another form. For, I have not even an iota of anger in Me.."

- Satya Sai Baba