By Prof. U.K. Choudhary
The 260.5 m high earth and rock-fill dam across the river Bhagirathi is located about 1.5 km down stream its confluence with river Bhilangana near Tehri town. The reservoir has a gross and live storage capacity of 3,540 million cubic m and 2,615 million cubic m, respectively. The head for live storage is 90 m and that of dead storage 170.5 m. The reservoir being created will extend 44 km along the Bhagirathi and 25 km. along river Bhilangana. The area of the land to be submerged in the reservoir is 5,200 hectares of which 1,600 hectares is cultivated land, 2,000 hectares is un-cultivated land and 1,600 hectares is forest land in which rare species of medicinal plants exist.
The entire Tehri town and over 95 villages have almost been submerged and more than 50, 000 people have been displaced. The normal reservoir level of Tehri lake will be 830 m and Tehri town is located between 630 and 730 m. The catchment area of the Tehri dam is 7,511 sq. km out of which 2,328 sq km lies above the snow line. The Tehri valley has phyllite, slate and scientose rocks. The slope of the Tehri reservoir is 6m/km. The rocks in the area show extensive techtonic activity. The geological mapping shows that a number of faults are located near the dam site. The annual yield of Bhagirathi at Tehri is 8,135 million cubic m. The average discharge from October to June is 107 cubic m/second, where as from July to September, it is 705 cubic m /second. Therefore the average discharge of the year is 256.5 cubic m/second. Thus, the time required to fill the reservoir is 160 days.
The water in the lower portion of the reservoir, which cannot be removed, known as dead storage, will be a store house of inorganic and organic matter. Their disintegration will be at a faster rate in the static water of the dead storage. The ions of different elements/substance will increase their densities and will cause pollution /contamination of the reservoir water. This will happen in the same way as a pot of copper/iron etc. ionizes the water kept in it. Therefore, the big dam of Tehri will pollute the water causing tremendous increase in the pressure. The flow and the quality of the water of the river are thus bound to change in totality. The density gradient will generate inside the reservoir in millions of cubic meter of reservoir water mixed with different materials, the Ganga jal is mixed under density gradient. The reservoir water, thus, circulates with the current known as density current. Under this action the few hundred cubic meters of the Ganga jal loses its complete identity.
The water in the lower portion of the reservoir, which cannot be removed, known as dead storage, will be a store house of inorganic and organic matter. Their disintegration will be at a faster rate in the static water of the dead storage.
The mixing of the reservoir water above the outlets (known as live storage) takes place with that of the water with the dead storage (reservoir water below the outlets). Thus, the Ganga jal is converted like the water of the reservoir. Further when the Ganga is inside the Tehri dam it interacts with the stored flood-water containing different types of chemicals. The purity of the Ganga water continuously diminishes with time due to the chemical reactions/interaction with the reservoir water. As the level of water in the reservoir increases, the velocity decreases, the turbulent energy slows down and sedimentation enhances. The organic matters are deposited inside the reservoir. The dense forest of 1,600 acres is submerged in Tehri reservoir. Their decomposition causes the formation of different acids. This spoils the soil of the reservoir. This finally causes water pollution. The sedimentation in the reservoir will be much more enhanced due to the sliding of the hills under variable saturated and unsaturated conditions on large sloping ground under pore-pressure. The situation will be similar to the human whose body is inside water for a longer period of time due to which his body vibrates and finally he falls down.
The Tehri reservoir as per its rock characteristics further will discharge the polluted water through seepage and cause landslides in the down stream. This result is clearly seen as the reservoir filling time is continuously increasing. This will further result in decrease in oxygen content and increase in B.O.D. load and the microbes inside the dam. The more time the river takes to fill the reservoir, the more is the Ganga-jal to be pure in terms of organic and microbiological pollutants. But this water does not have the quality of the river water. Inorganic pollutants are never removed. The ionization of inorganic load causes an increase in their concentration. The density of water, therefore, rises. Thus, the pollutants of Tehri reservoir will increase everyday. This is deposited in the lower level of the dead storage.
The dam, therefore, will work at the cost the Ganga jal. The essence of the Ganga jal is bound to be lost in the large volume of reservoir water. This will happen in similar way as the sweetness of the Ganga jal is lost and it entirely becomes saline water when it enters the ocean. Further, the reservoir water coming out from the Tehri dam cannot have the capability of absorbing more oxygen, this will be similar to the ocean water that inspite of hurricane does not absorb more oxygen from the atmosphere. In this way the self-cleaning potential of the Ganga jal is bound to diminish continuously. Further, the Tehri reservoir with large slope bearing weight of high column of water will start shaking, known as self-induced seismicity.
The Tehri reservoir has a slope that is six-times larger compared to Bhakra Dam reservoir and 18-times that of the T.G.P. dam of China, the biggest hydro power plant of the world. Therefore the Tehri dam may be affected by self-induced seismicity. Based on the above facts it seems as if it were the great Himalayas saying, ?I am not capable of keeping the Ganga inside the big dam. I am at present young, I have fractured and crushed rock. I am often subject to intense earthquakes. I have sloppy land, covered with valuable medicinal plants. I shall terribly subside and my soil will spread along the whole length of the river, the great plain of India will suffer floods and soil erosion and the eternal essence of the Ganga-jal will be lost. Therefore, do not put the Ganga inside the Tehri dam, this is the most valuable property gifted by nature to this earth, let her qualitative virtues be used by the whole of mankind.?
(The author is a Professor, Department of Civil Engineering, Benaras Hindu University. He is also founder of the Ganga Research Centre in the University.)