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PLANNING PROBLEMS: Kosi River Development
By DR. AB THAPA
The Kosi River is posing a serious threat to life and property of millions in Nepal and India. As a result, the resolution of the Kosi flood problem has become a matter of the utmost importance. The Kosi development to resolve the flood problem has emerged as the greatest engineering challenge because it involves the construction of a dam perhaps the highest in the world on one hand and on the other it could result to a serious adverse impact on environment if the whole project is not planned with great caution. Downstream river bed degradation consequent upon the construction of the high dam could be one of the problems that would have far reaching adverse environmental impact.
Why Kosi Dam Needed
Solution to the problem of keeping at bay the Kosi flood disaster had been found a long time ago. Dr. A.N. Khosla, a prominent scientist in the field of hydraulic engineering and then Director General of Irrigation, Power, Flood Control and Navigation in the Government of India, had given his opinion in 1940s that storage dams would be needed to control the Kosi river for averting the flood disaster. On his leadership field survey works started immediately after the Second World War in 1946 while India was still under the British rule. The study recommended that a high dam would be needed to control the Kosi river floods. Surprisingly, in 1950s the idea of building the Kosi high dam was abandoned and in its place embankments were built on both sides of the river. It is not difficult to perceive that the embankments were built to confine the Kosi river within a certain fixed perimeter. It was only a temporary solution. Ultimately a storage dam would be needed.
Proposed Kosi Dam Highest in the World
The height of the Kosi dam proposed by India in 1940s was 269 meters. At that time the world's highest dam was the 221 meters high Hoover dam on the river Colorado built in 1936 in the United State. Thus the proposed Kosi dam not only topped in height the Hoover dam which was at that time the highest in the world but very significant is the fact that the proposed Kosi dam exceeded the latter dam in height by a very big margin of almost 50 meters. It clearly allows to consider that even at that time India must have realized that the height of the Kosi dam should be raised to a maximum possible extent to be constrained only by technical limitations.
Gravity type concrete dam had been selected for the Kosi river. The dam height had been stretched to an extreme limit for this type of dam even based on the global experience of the dam planning as of now. Even up till now there are only two concrete dams in the world only slightly exceeding the proposed Kosi dam in height. They are the 285 meters high Grande Dixene gravity dam built in the Switzerland in 1961, and the 272 meters high Inguri arch dam built in the former Soviet Union in 1980.
Kosi Dam Must Be Still More High
Recent studies of the Himalayan region provide a whole bunch of information very useful for planning high dams. Such information were extensively used for detailed planning of the Karnali high dam. The Karnali studies carried out at an enormous cost by a consortium of renowned consulting firms of several countries under the aegis of the World Bank in close consultation with the governments of Nepal and India have greatly helped to realize that compared to the earlier perceptions the magnitude of the problems to be actually encountered in dam planning in the Himalayan belt would be far more serious and also complicated. This has been amply reflected in finalization of the design of the Karnali high dam.
By comparison with the recalculated height of the Kosi dam based on the recent Karnali project study, the originally proposed Kosi dam height would have been very much on the lower side. It is quite possible that India might have realized even at that time that the height of the proposed Kosi dam must be further raised for an effective flood control but at that time the high dam technology was not yet fully developed to allow to increase further the Kosi dam height. It was already a very bold decision go up to a height of 269 meters. The technology of embankment type dam better suited to fragile geological conditions was not yet well developed to be applied to dams of this height. So we should not be surprised if India had taken the decision not to build the Kosi dam project in immediate future due to technological limitations. Thus under such circumstances the building of embankments on either sides of the river would have been at that time the only option left for the Kosi flood relief in near future. .
Downstream Degradation Problems
Sediment flow across the dam would be virtually blocked after the implementation of the Kosi high dam. Thus only the clear water would be discharged from the reservoir during the flood seasons when the reservoir is full. It is certain that the Kosi river bed downstream of the dam would be eroded and the coarse river bed materials would be carried and deposited on the lower reaches of the river where the bed slope of the river is relatively mild. The shifting of the river bed materials further downstream could result to severe flooding Such adverse phenomena have been observed after the construction of several large storage dams in the past. The lower reaches of the Kosi are already prone to frequent flooding after the completion of the embankments. The flooding situation would be extremely grave if the Kosi river bed suddenly rises due to piling up of the coarse bed materials carried from the upper reaches.
There is a low-lying plain in the mid-fan area along the eastern side of the Eastern Embankment of the Kosi. It extends from Supaul to the west of Simri-Bakhtiarpur. This area already remains water-logged during a substantial part of the year. The surfacial sediments of the area are silty muds with high organic content. It is regarded that the seepage of water from active Kosi River, which is at higher level by comparison with the adjoining areas, is the cause of water-logging.
The southernmost part of the Kosi fan is drained by north-south flowing groundwater fed streams which join the meandering Kosi River flowing eastwards at the bottom of the fan. Even at low flood discharges these streams merge into the Kosi river and result to flooding in adjoining areas on the fan. At peak floods, the Kosi submerges large parts of this plain.
Colorado River and Hoover Dam
Colorado River is in the southwestern United States . It is the major source of water for the surrounding region, which receives very little rainfall. People depend on the Colorado River for irrigation, but they also use it to generate hydroelectric power and to supply water to urban areas The Colorado River system, including the 2,330 km long Colorado River and its tributaries, drains an area of 637,000 sq km. The Colorado River basin ’s total runoff is approximately 700 cubic meters per second.
Hoover Dam is situated in Black Canyon on the Colorado River near Las Vegas , Nevada on the border of the states of Nevada and Arizona . At the time of construction it was a major engineering achievement. The massive dam is 221 m high and 379 m long at the crest. The reservoir formed behind the dam, Lake Mead , is one of the largest artificially created bodies of water in the world. It covers an area of 603 sq km and its shoreline measures 885 km. The hydroelectric generators of Hoover Dam, which are capable of supplying nearly 1.5 million kilowatts of power, provide electricity to Arizona , Nevada , and southern California .
Construction of the Hoover Dam began in 1931 as part of the Boulder Canyon project and was completed in 1936. Originally named after President Herbert Hoover, the dam was later called Boulder Dam, but in 1947 its initial name was restored.
Serious degradation phenomena had been observed in Hoover Dam of the USA . Below the Hoover Dam clear water caused severe degradation of the river bed. Within six months of the closure of the river diversion gates in 1935 the 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.
Nile River and Aswan High Dam
Nile River , located in northeastern Africa , is the longest river in the world,. From its principal source, Lake Victoria, in east central Africa, the Nile flows north through Uganda , Sudan , and Egypt to the Mediterranean Sea , a distance of 5584 km. From its remotest headstream in Burundi , the river is 6,695 km long. The river basin covers an area of more than 3,349,000 sq km.
To raise water levels for irrigation in the 20th century, several dams were built across the Egyptian Nile. The first dam on the Nile , the Aswān Dam, was built in 1902 and heightened in 1936. The Aswān High Dam, completed in 1970, impounds one of the world’s largest reservoirs, Lake Nasser . Construction on the mammoth dam began in 1960. By mid-1968 construction was essentially completed. The last of the 12 Soviet-built turbines was installed in 1970, and the dam was formally inaugurated in 1971. The dam has a generating capacity of 2,100 megawatts of electricity.
Aswān High Dam is located near the city of Aswān . The dam is about 6 km upstream from the much smaller Aswān Dam. The High Dam has an embankment 111 m high that extends 3,600 m across the river. Named in honor of Egyptian president Gamal Abdel Nasser, Lake Nasser covers an area more than 480 km long and 16 km wide. The water is used to irrigate farmland. . During the rainy season the dam also controls the yearly flooding.
Although the Aswan High Dam has provided many benefits to the area, it also has caused problems. The formation of Lake Nasser inundated many villages along the Nile in both Egypt and Sudan , forcing thousands of people to relocate. Farmland along the Nile is becoming less fertile and productive because it no longer receives the fertilizing silt from the river’s floodwaters.
Annual summer flooding of the Nile once deposited rich sediment along its banks, creating fertile farmland. However, the Aswan High Dam now controls the flooding, drastically reducing sedimentation and fertility. The dam’s environmental impact has been profound, as stretches of the river above the dam have become clogged with silt, and decreased flooding has led to increased erosion and greater salt content in the soil and water of the delta. Local communities and ancient sites in Egypt and Sudan were either submerged or relocated because of the dam.
After the construction of the Aswan High Dam nearly all sediment deposited in the reservoir. Despite the fact that the capacity of the Aswan Dam reservoir is very large, the releases at Aswan during high floods exceed 350 million cu. m. per day. As a result, serious downstream degradation problems have been faced, which led to lowering of the water depths over the sills of the navigation locks provided to bypass the dams built across the Nile River. It greatly hampered river navigation. A special project was undertaken to resolved this problem. A 55 km long spill channel has been built to divert Nile water. The channel diverts excess flood water from the reservoir to Toshka Depression located in the desert west of the Nile when the water level in the reservoir reaches certain height.
(Dr. Thapa writes on water resources)
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