Idukki dams

The expert committee has reported that the overflowing loose soil and soil dumped on the river sides down stream would silt the Idukki reservoir down the river. However, it is nobody’s case that such siltation would seriously threaten Idukki either from the safety point of view or reservoir capacity. The real threat will arise, if the Mullapperiyar dam gives way at the peak of a heavy monsoon. Though Idukki may have the capacity to hold the entire storage at Mullapperiyar, a dam failure during monsoon would present a different picture. The waters would bring down much soil and floating debris which can clog spillways and exert considerable stress on the dams downstream. And Idukki is not the safest of dams.

I discuss Idukki dams in more detail here because condition of the Mullapperiyar dam has a direct bearing on the safety of the dams of the Idukki Project. The level of risk arising from a dam is a function of probability of the dam failure and its consequences. Taken individually, the probability of a dam failure is very high in case of Mullapperiyar and low in case of Idukki. However, the consequences are relatively low in case of Mullapperiyar and very high in case of Idukki. Taken together, the risk level is on the high side both in terms of probability and consequences.

Many have come to think that an arch dam is safer than a concrete gravity dam. This is not true. A gravity dam, as the name implies, holds the water because of its sheer weight. On the other hand, the arch dam holds the water because of its design features. Hence, any damage to the structure of dam is more serious in the case of an arch dam. For the same reason an arch dam requires better monitoring.

Mr. Phong Nguyen, heading the Dam Safety and Maintenance wing of Hydro Quebec2, Canada, told me in an interview that in case of arch dams, the foundations come under great stress. This was because of the to and fro movement of the arch when water fills and empties the reservoir. The arch can withstand this movement and pressure eminently. But that is not the case with the foundation where the arch ends. The foundation develops problems because of the stress resulting from this movement.
He noted that he Manic-5 multiple arch dam3 in Quebec is facing serious problems such as bulging cracks in the dam. Hydro Quebec was spending two to three million dollars a year to monitor the dam. It had also spent millions of dollars for grouting and other repair work. Arch dams, he said, were very difficult to monitor and very expensive to repair.

Incidentally, about 87 per cent of dam failures involving concrete dams are reported to have occurred because of foundation failure. In the case of arch dams, the risk of a foundation failure is on the higher side as indicated by Mr. Nguyen.

The Idukki arch dam, at the time of construction, was equipped to monitor even the slightest movement of the dam body and changes in stress. But almost all of the 16 systems which were set up for this purpose are now either malfunctioning or inoperative. The remaining systems are being used sparingly to monitor the dam. Consequently, no data is available after 1990 on the condition of the dam from a safety point of view. This is a serious situation.

I am not detailing the possible consequences of failure of either Mullapperiyar or Idukki to avoid a scare. But it is telling commentary on the Kerala State Electricity Board (KSEB) that it is not properly monitoring the biggest dam in the State on which much of the State’s prosperity and lives of thousands of people living downstream depends. I am also refraining from talking about emergency action plans as it is pointless to speak about such costly plans when the KSEB is not caring about even monitoring of the dam’s condition. I conclude quoting Nguyen:

“Risks increase in the absence of proper maintenance and surveillance.”