Kerala - Thiruvananthapuram   

Dams lack spillway capacity

THIRUVANANTHAPURAM: Preliminary reports about failure of the Moolathara dam (regulator) points to lack of spillway capacity and preparedness to meet emergencies.

The vulnerability of a dam depends on several factors, and one of the main factors responsible for dam failures is inadequacy of spillways. Several of the dams in the State do not have adequate spillway capacity, and maintenance of even the available capacity is lax. This is the case even with major dams such as Idukki.

The Mullaperiyar dam is a classic case of designing dams without an adequate spillway capacity. That was why the Central Water Commission recommended construction of additional spillways when it inspected the dam in 1979. Though Tamil Nadu constructed the additional spillways, even those were found to be inadequate.

Spillways are to be designed to handle probable maximum flood in the catchment — the biggest flood that is likely to occur over 100 years. There is even an opinion that one should plan for the biggest flood in 1,000 years as the probability of such a flood occurring during the life of a dam cannot be ruled out. The spillways should be capable of safely discharging waters.

With the additional spillways, the Mullaperiyar dam is capable of discharging flood waters at the rate of 6,003 cumecs (cubic metres per second). However, according to the Kerala government, a flood that resulted in a peak flow of 8,453 cumecs had occurred in the Mullaperiyar basin in the 1940s. Considering that, the spillways of Mullaperiyar should have been designed for a much higher value. This means that the dam should have more than double the existing spillway capacity.

A more surprising factor is that the spillways of the Idukki project at Cheruthoni are even lower than those of Mullaperiyar, though Idukki is a much bigger dam with larger catchments. The spillways of the Cheruthoni dam are capable of discharging only 5,100 cumecs. This means that the Idukki system at full reservoir level will not be able to handle the probable maximum discharge from Mullaperiyar, the same way as the Moolathara dam failed to handle the discharge from the upstream Aliyar dam.

The Aliyar dam was at its full reservoir level and Tamil Nadu could not have avoided the opening of the spillway shutters. Otherwise, the floodwaters would have posed a threat to that dam. It is yet to be explained why Kerala failed to open the shutters soon after receiving information from Tamil Nadu about the water release.

Water Resources Minister N.K. Premachandran told The Hindu that Tamil Nadu had released an excessive quantity of water — at the rate of about 23,000 cusecs, while the Moolathara dam could handle only 10,000 cusecs. The situation on the dam safety front in Kerala has been showing no improvement. This makes people ask if Moolathara is a dress rehearsal for a major failure.

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