Idukki dams under serious risk of failure

By Roy Mathew

THIRUVANANTHAPURAM, DEC. 19. The dams of Idukki project are facing serious risks of failure on account of a variety of factors.

These include the precarious condition of the Mullaperiyar dam upstream, the inadequacy of the spillways, slope instability in the catchment areas, siltation, earthquakes and lack of proper instrumentation to monitor the Mullaperiyar, Idukki, Cheruthoni and Kulamavu dams.

Tamil Nadu has long been arguing that the Mullaperiyar dam could withstand a water level of 145 feet or higher. However, experts from Kerala dispute this. Last week, experts from both Kerala and the Centre came across fissures and leaks in the dam body. A report of the Central Soil and Materials Research Station is expected to be submitted in a week's time.

The dam was constructed at a time when hardly any official regulations or codes of practice in construction existed. Effects of uplift force and seismicity were not considered in the design. No drainage gallery had been provided, as in modern dams, to release pressure. The dam was constructed as one long continuous block using rubble masonry and lime surkhi mortar and without providing construction joints.

According to engineer, Mr. M. K. Parameswaran Nair, who is representing Kerala in the Committee set up by the Central Water Commission to study the issue, the strengthening works undertaken by the Tamil Nadu Government to ensure the safety of the Mullaperiyar dam were not adequate. Mullaperiyar had a history of sweating and leakage from the time of its commissioning. In the 1930s and the 1960s, the Government had resorted to large-scale grouting and guniting.

The total cement used for this was 542 tonnes. According to Tamil Nadu's own figures, the annual leaching noticed was of 30.488 tonnes a year.

So, it was clear that more cement than that used for grouting has leached out, leaving holes in the dam body. Moreover, there was no guarantee that the entire hollow space inside the dam had been filled up during the grouting.

Mr. Parameswaran says that though Tamil Nadu had given a concrete backing on the rear face as part of strengthening measures, this was not fully integrated with the old structure. There was a gap between the old and new structures and hence the whole would not be acting as a monolith.

Tamil Nadu's claim that the dam was strong was based on the assumption that the hybrid structure is a monolith. Though drainage galleries were provided through the new structure, these had become ineffective through clogging.

So, there was no mechanism to release pressure developing in the foundation. Considering all this, the failure of the Mullaperiyar dam in a heavy flood was a distinct possibility.

Dams are designed to handle the biggest probable flood that can occur any time in 1,000 years. Each dam has a probable maximum flood (PMF) estimated for its design. In the case of Idukki project, the probable maximum flood is 8,000 cumecs. The dams and spillways are designed to handle an inflow of 5,100 cubic metres per second.

The balance flood was to be absorbed by the capacity available between Full Reservoir Level (FRL) and the permissible Maximum Water Level.

The PMF incorporated into the design of Mullaperiyar was 2,495 cumecs. Idukki was designed on the assumption that the maximum flood discharge from Mullaperiyar would be only 2,495. However, the PMF of Mullaperiyar basin was revised as 6,003 cumecs as per the recommendations of the Central Water Commission. The Idukki project, according to Mr. Nair, would not be able to handle such an inflow if the Idukki reservoir is at FRL or near to it.

The chances of both the projects getting caught in a probable maximum flood situation simultaneously are high considering the fact that their catchments are similar and adjacent.

This means that even the normal operations of the Mullaperiyar dam at full reservoir level of 152 feet (during monsoons) would be at a heavy risk to Idukki, not to speak of a failure of the Mullaperiyar dam.

The situation calls for urgent construction of additional spillways at Idukki reservoir. Otherwise, the reservoir levels would have to be kept down. The situation is compounded by the fact that the Idukki's own estimated PMF needs revision.

After Idukki dams were built, several smaller rivers had been diverted into the reservoir to augment the availability of water. This would have raised the PMF.

At anytime, a failure of the Mullaperiyar dam could be catastrophic for Idukki. The project, built to hold 2,000 million cubic metres of water, will have to cope with a sudden inflow of up to 443 million cubic metres of water from 50 km away. Hundreds of towns and villages in three districts of the State would be wiped out if the dams of Idukki fail.

Even in its normal operation, earthquakes, siltation and instability of slopes in the catchment area pose some risks. Idukki dam is designed to withstand a horizontal acceleration of 0.05g. and vertical acceleration of 0.025g. This means that it can withstand earthquakes bigger than those reported from the area now.

However, it is to be noted that even smaller dams, like the Lower Periyar, were now designed to withstand horizontal acceleration of 0.1g. However, landslides in the catchment area can cause dam failure as the reservoir could overflow if large masses of earth falls into it.

Studies by the Department of Geology of the University of Kerala and the Centre for Earth Science Studies have shown that many slopes in Idukki district, including those in the catchment are unstable. Recent studies have also indicated that the reservoir is silting up heavily, reducing its storage capacity.

The technical committee constituted by the State Government has found that the Mullaperiyar dam had some instrumentation only in the backing concrete portion. These were four uplift pressure cells, four strain metre groups with no stress strain metre, four joint metres and eight thermometres. Set up sometime in the eighties, these instruments were either not functional or not being systematically monitored. The Committee observed that the instrumentation was neither effective nor the monitoring systematic.

The monitored data are seldom interpreted.

The situation with respect to the dams of Idukki project are also not much different. Most of the embedded instruments are not functioning and data from other instruments are not being interpreted properly.

In a presentation at the Institute of Engineers here a few years back, retired engineer of the Board, Mr. M. Sasidharan, had pointed out the Board was unable to calculate the real stress pattern existing in the dam.

Malfunctioning of instruments and abnormal seepage had been noticed. Abnormal press development in foundation drains had also been detected. In many areas, total neglect, incompetence and inaction were occurring. Mr. Sasidharan, when contacted on Tuesday, said that the situation was no different now.

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