Sunday, November 7, 2010

Seismic Threat to Mullaperiyar Dam

Within a span of 2 days, two tremors of mild intensity on 5/11/10 (M2.0) and 6/11/2010 (M2.9) occurred in the 50KM range of Mullaperiyar Dam suddenly caused great anxiety in the minds of people, earthquake scientists and dam engineers. Even though, these earthquakes are minor in nature, the above earthquakes are seen as indication of the increased seismicity of the region surrounding Mullaperiyar Dam, which was once thought completely dormant. These warning signals by nature must taken very seriously, even though the present tremors are not able to create any significant damage to the dam due to its low intensity and large distance from the Mullaperiyar Dam.

Upstream face of Mullaperiyar Dam before the recent repairs

The above tremors are coincident with the cosmetic beautification done by Tamil Nadu authorities to the grandma Mullaperiyar Dam to conceal its chronic ailments from the naked eyes. But the ailments of Mullaperiyar Dam are inherent and deep rooted. This dam was constructed during 1886-1895, when the dam engineering was at infancy. The dam is a composite structure with rubble masonry in lime surkhi mortar as facing on upstream and downstream and a lime surkhi concrete inner core. Later in 1980s, a concrete backing of 10 M width was added to the downstream of the old dam.

Arguments by Tamil Nadu and Counter Arguments

1. ARGUMENT: 37 dams exits in India, which are more than 100 years old and still in service and hence it is not prudent to conclude that Mullaperiyar is unsafe based only citing the age of the structure.

COUNTER ARGUMENT: It is true that there are more than 37 dams exists in India which are more than 100 years old (Source: National Register of Large Dams, 2002). But a close look into the above data shows that out of the above 37 dams, 30 dams are of earthen dams having around 20 m height and more over having only an average gross storage capacity of less than 1 TMC! Hence these earthen dams are only can be qualified to be as ‘earthen bunds’ and does not pose much threat. 

If we consider the remaining seven masonry gravity dams, the Mullaperiyar dam surpasses all others with its height of 53.64m and its huge storage capacity of 15.66 TMC. The second one on the above list, Khadakwasla dam, which had already overtopped in 1961 due to the failure of the Panchet dam in the upstream (Maharastra - Pune District).  The third one on the list is having a height of only 16.77m. Hence such comparisons have no meaning.

Mullaperiyar dam is a composite gravity dam which lost of much of its strength due to continuous leaching of lime from its core. This dam is situated in an active fault zone, which makes the dam vulnerable to failure in an earthquake of moderate intensity with an epicenter close to the dam. Also the high hazard this dam poses a direct threat to the lives and property of 35 lakhs people living downstream and also a threat to the mega storage Idukki reservoir. Hence Mullaperiyar dam has no parallels and considering its age, deterioration and high hazard nature, must be decommissioned and a new dam must be built.

Mullaperiyar Dam - A Permanent Threat to Kerala

View of the Mullaperiyar dam and the famous Thekkady lake

Lease Deed of 1886 and Supplemental Agreement

A lease deed was executed in 1886 between the Maharaja of Travancore and the Secretary of the State for India, for irrigation works by the Madras Presidency for diverting all waters of Periyar River of Kerala flowing into the land bounded by a contour line of 155 ft above the deepest point of the bed of the Periyar river to Vaigai basin for a period of 999 years. Two supplemental agreements were executed in 1970 by the successor Governments of Kerala and Tamil Nadu. As per the first supplement agreement, the lease rent was enhanced from Rs.5 per acre to Rs.30 per acre with a provision to revise the same in every 30 years. The second one permitted Tamil Nadu to generate electricity using the waters of Mullaperiyar.