Roy Mathew

Calls for relook at CWC estimates

Researchers say flood estimates

are too low

Want climate change to be taken into account

THIRUVANANTHAPURAM: The risk to the Mullaperiyar dam from an unusual flood in its catchments may be higher than anticipated.

The Kerala government has been arguing that the probable maximum flood (PMF) of 1.12 lakh cusecs (6,003 cumecs) calculated for the Mullaperiyar basin was an underestimate. (Spillways of dams are built to safely handle peak floods. Underestimation of PMF and consequent inadequacy of spillways has been one of the reasons for dam failures worldwide.) The calculations fail to take care of a peak flood recorded in 1943 (8,553 cumecs). Tamil Nadu maintains that the 1943 data are unreliable.

CWC documents

The Kerala government has now obtained documents from the Central Water Commission (CWC) which show that the maximum water level in the reservoir under peak flood conditions can exceed 156 feet even if the PMF is assumed to be only 1.22 lakh cusecs. This will happen even when the flood impingement level is taken as 136 feet. Increasing the impingement level to 142 feet increases the risk as well as the maximum water level. Besides, the water level will then be above the legal limit set by the Mullaperiyar lease agreement with Tamil Nadu. Tamil Nadu has rights over water up to the contour level of 155 feet only under the terms of the agreement. Under maximum flood conditions, the water level could even hit the parapet level, which goes against all safety considerations. This risk exists even if it is assumed that the dam is strong enough to hold water up to 142 feet. Kerala has already placed this argument before the Supreme Court in the case filed by Tamil Nadu challenging provisions of Kerala’s dam safety legislation — the Kerala Irrigation and Water Conservation (Amendment) Act. The Act specifies that the water level should not be increased above 136 feet on safety considerations. In one of the documents (Design Flood Studies for Periyar Dam) obtained by the Kerala Irrigation Department under the Right to Information Act, the CWC says there had been severe limitations of data in calculating the PMF.

Revised estimates

However, researchers at the Charldon Hill Research Station, Somerset, United Kingdom, have come out with revised estimates of probable maximum precipitation (PMP) for peninsular India using historic data from India. Studies by P.R. Rakhecha and C. Clark, published in 2000, showed that design floods estimated for many existing dams in India were considerably lower than those calculated by them for the area using the revised PMP. They concluded that some dams in India might be unsafe in the event of probable maximum flood.

According to T. Shivaji Rao, Director of the Centre for Environmental Studies, Visakhapatnam, this called for a relook at the CWC estimates of spillway design floods for various dams including Mullaperiyar. Besides, there is opinion among experts that weather changes on account of greenhouse effect call for revision of estimates. Some officials suggest that an international study is called for on the safety of the Mullaperiyar dam. Concern is growing across the world that many dams designed in the past might be unsafe. The International Commission on Large Dams recommends that dams should be insured on the basis of the loss that could result from their failure. Dam break analysis and inundation studies (which are also required for providing insurance cover) are becoming mandatory in many countries.

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