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The Hindu on indiaserver.com : Fresh concerns over safety of Mullaperiyar dam

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Thursday, December 14, 2000


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Fresh concerns over safety of Mullaperiyar dam

By Our Special Correspondent

THIRUVANANTHAPURAM, DEC. 13. The occurrence of an earthquake of the magnitude of about 5 on the Richter scale has raised fresh concerns about the safety of the Mullaperiyar dam.

While some experts are of the view that the dam has long outlived its normal life, others maintain that it can continue in service because of the strengthening works undertaken by the Tamil Nadu Government. However, engineers of Kerala and Tamil Nadu do not see eye to eye on the subject. And the earthquake has changed the position further.

Tremors had been occurring in and around the Idukki reservoir ever since the dam was filled. There is not much data on whether tremors had occurred before the commissioning of the dam. Seismographs had been put up at Kulamavu and other places only with the construction of the Kulamavu dam.

Most of the tremors recorded there were of such low magnitude that the local people had not come to notice them. As a correlation had been noticed between the filling and emptying of the reservoir, scientists had suggested that these could be reservoir-induced. However, subsequent tremors put certain questions marks on this assumption. The magnitude of the tremors was expected to come down as the geological formations under the reservoir had `settled down'. However, this was not happening. The tremor at Nedumkandam in 1988 measured 4.5 on the logarithmic Richter scale. The after-shocks measured 4.1 and 3.8 on the scale. It has been more or less confirmed now that yesterdays's shock was of the magnitude of 5 on the scale. The after-shocks also came close to this reading. In 1994, a shock of the magnitude of 4.3 on the Richter scale occurred at Wadakkanchery followed by another of the magnitude 3.0 in 1996. In between, a number of tremors of lower magnitudes have occurred in different parts of Kerala and Tamil Nadu. This pointed to tectonic activity in the area.

Idukki and its surrounding areas now have about a dozen large reservoirs, the oldest being Mullaperiyar. Most of them are over geological fault lines. Moreover, many of the slopes in the catchment areas are unstable. This increases the risk of large chunks of earth falling into reservoirs leading to dam failure. (The failure of the arch dam at Viont in Italy had occurred this way). The Mullaperiyar, which was built before tectonic studies became a norm for dam design, is prone to damage from quakes. This is especially true of the Baby dam. In the recent non- destructive testing of Baby dam by the Central Soil and Material Research Station, fissures had been found inside the dam structure. Further studies would be required to see how far the main dam structure would withstand quakes.

The risk here is not the collapse of the Mullaperiyar dam alone. If the dam gives way when the reservoir level is high in both Idukki dam and Mullaperiyar, Idukki would not be able to hold the inflow. Idukki dams are designed to withstand earthquakes. However, the spillways at Cheruthoni do not have the capacity to release such a flow and keep the dam safe. Many dam failures across the world had occurred because of deficiency of spillways. In the case, earthquakes and landslides can compound the problem.

Meanwhile, the Kerala State Electricity Board authorities are examining the dams under it for any damage from the quakes. However, their observations would be severely limited from the fact that several of the instruments for monitoring the dams are not functional. Some have malfunctioned in the past and gave wrong readings. The stress strain meters in the Idukki arch dam, for example, gave values in the nineties that suggested that the dam would burst.

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