In recent years, the controversy over the Mullaperiyar dam has acquired new dimensions. A purely technical matter has turned into an emotional and political issue between Kerala and Tamil Nadu. The entire fight is centred on whether the water level in the Mullaperiyar reservoir should be raised by two meters as demanded by Tamil Nadu, to cater to the needs of additional water for irrigation and power. Kerala opposes this on the ground that the additional water would increase the load on the 116-year-old dam, possibly resulting in a break in the dam, and a disastrous flooding of thickly populated downstream areas in Kerala. Further, the Kerala government fears damage to the dam from seismic tremors. The issue is now being examined by an “empowered committee” of the Supreme Court.
For the best decision, the Mullaperiyar issue needs to be considered from all technical and engineering aspects. It is surprising, therefore, that the silting of the reservoir has not been taken into consideration by any party. Rather, it has been totally neglected. In any dam, silting is a natural hydrological and sedimentological process by which sediments flowing from the upstream catchment area in the river water get deposited in the reservoir.
The process of sedimentation reduces the life of a reservoir. The estimated life of any reservoir is of the order of 120 to 180 years (broadly speaking, not more than couple of centuries). This is the reality. At present most of the Himalayan dams are heavily silted. According to available silt figures, the famous Bhakra dam has lost about 50 per cent of its storing capacity. The Jaldhaka Dam near Siliguri, West Bengal, in the Himalayan range is almost completely silted.
As per available records, the storing capacity of Mullaperiyar Dam has been lost by about 50 per cent during the last 116 years. The rate of silting in peninsular India is less compared to the alluvial rivers of Himalayan region. It may take another 150 to 200 years for the Mullaperiyar dam to get fully silted up — certainly the dam will not last 999 years. Some people argue that some old historical “dams” have stood for the last 1800 years and if these could survive for such a long time, why not the Mullaperiyar dam. But the observation is not based on a scientific understanding of water storage systems.
These ancient “dams” are in reality anicut, or weirs, with a height of about not more than three metres or so. In the engineering lexicon, these are not known as dams. When they overflow during heavy rains, weirs do not get damaged. Further, there is not much silt accumulation in weirs. In an engineered dam, when water flows over the top of the dam, it collapses. With the passage of time, due to siltation, the storing capacity of Mullaperiyar reservoir is bound to decrease steadily, and the availability of water will consequentially continue to decrease. Even if the request of one party to increase the water level is partially or fully granted, it cannot be a final or permanent solution. With the addition of silt every rainy season, the volume of water in the dam would continue to reduce and the demand to raise the water level would crop up again.
Dynamic process
It needs to be realised that the silting of a reservoir is a dynamic process that cannot be stopped or terminated. The silting puts a finite time limit on the life of a reservoir, a fact that, at present, the parties involved may find difficult to accept. The best way to overcome the present impasse is to keep a watch on the silting of the reservoir. When it is silted up to 75 per cent of storing capacity, it would be time to put emotions and political differences aside and start preparing to create an alternate source for water. This, of course, is not an engineering rule, but just a suggestion in the present case. The cumulative accumulation of silt in the reservoir is not dangerous to the stability of the dam as it does not exert any dynamic pressure on the body of the dam. The only danger of silt deposition is effective reduction in the storage capacity of the reservoir.
De-silting of dams has been projected as a solution to sedimentation, and has been tried in some recently constructed dams. Aswan Dam in Egypt was President Nasser's development dream. The Nile is famous for carrying heavy sediments and the dam is heavily silted up. Some experiments were tried to stir the accumulated silt and flush it through an outlet at the bottom of the dam, provided in the design. But the experiment did not produce the desired or expected results. In the case of Mullaperiyar dam such an experiment is not even possible due to its design, type of construction, age of the dam and the building material.
It is hoped that the point of silting in the Mullaperiyar reservoir would be considered by all the parties involved. This would involve studying and making a proper estimation of the accumulated silt, the rate of silting in the foreseeable future, perhaps over a time span of 50 years, and a realistic estimation of the future life of reservoir on account of the siltation. If any decision is taken ignoring the silting of the reservoir, that decision is certainly not going to solve the Mullaperiyar problem.
(The author is a research seismologist: arun_bapat@vsnl.com)
