Big Dams Are Not Sustainable
Big dams like Tehri are not sustainable. The main function of these dams is to store water in the monsoons and release the same in winters and summers for irrigation. This water is stored in a reservoir behind the dam.
The river brings sediment with water and the sediment gets deposited in the reservoir. This is especially so in the Ganga which carries much higher volume of sediments compared to other large rivers of the world. The reservoir behind the dam is filled with sediments in a short time. It can then no longer store monsoon waters. It becomes like a bucket that is filled with sand and cannot hold water
Engineers are aware of this problem. They try to solve this by making gates at the foot of the dam. These gates are opened occasionally. The water released from the gates carries the sedimentsdeposited behind the dam. The sediments are flushed out of the reservoir. However, the entire sediment of the reservoir is not flushed out. For example, the length of the reservoir is about 45 kilometers at Tehri whereas, the sediment which is flushed out for only about 1.5 kilometer behind the dam as shown in the picture below.
About 43 Kilometers of the sediment remain in the reservoir. So gradually the sediment fills up the bulk of the reservoir and the water storage capacity of the reservoir becomes proport5ionately less.
Study On Tehri Dam
Tehri Hydro Power Corporation has conducted two studies of this problem. The first study has been done by the Tojo Vikas International Company. This study says:
“Progressive loss of storage due to sedimentation in different storage zones are assessed as under:
Dead storage 0.6% in 3 yrs
Live Storage 0.59% in 3 yrs
Gross Storage 0.59% in 3 yrs”
The bottom portion of the reservoir is permanently filled up with water and is called “Dead Storage.” The upper part is called “Live Storage.” Here water is released in the summer and filled up again in the rains. According to the Tojo company, about 0.6percent of live storage and dead storage has decreased in three years. That is, 0.2 percent of storage has been reduced in a year. This reduction will accelerate as the Dead Storage gets filled and the incoming sediment gets deposited in the upper layer of Live Storage. If there all the sediments were flushed out then there would be no such reduction in the Live Storage. Thus the Tojo Company has stated that the useful life of the Tehri Dam will be approximately 130 years:
“Keeping in the view the above calculation by methods defined in the CBIP manual on the reservoir, the useful life of the reservoir can be taken safely as 130 years.” (Tojo Company’s report can see here).
The second study is done by Hydrologic Designer of IRI Roorkee Shri A.C. Pandey. Its results are almost the same as above. According to Mr. Pandey, the dead storage in Tehri Dam will be fully filled in about 170 years. The capacity of the reservoir will become zero in 465 years. (See detailed survey on Tehri dam done by Shri Pandey)
After the reservoir is filled up with sediments completely, the water will flow through the upper surface above the sediments as shown in the picture below:
Therefore, according to both studies, the capacity of Their Reservoir has already started decreasing. The Dead Storage will be filled in 130 to 170 years. After that, the reduction in the capacity of water storage of Tehri dam will accelerate. In nearly 200-300 years, the Tehri Dam will be completely filled with sediment and it will not be able to sustain a drop of water.
The question is should we build these dams for the short run gains of 200-300 years? The construction of these big dams should be stopped if we want sustainable development.
please write to Prime Minister Shree Narendra Modi for this issue.
