No mechanism to monitor impact of tremors on dam

THIRUVANANTHAPURAM: As many as 35 tremors have been recorded in the vicinity of the Mullaperiyar Dam in the last decade. Yet, the century-old dam has no seismic instrumentation to monitor the changes in the lime-surkhi concrete or assess the vulnerability of the old structure in the event of an earthquake.The Dam Safety Review Panel formed by the World Bank and the Government of Tamil Nadu, as part of the Dam Safety Assurance and Rehabilitation Project (DSARP) headed by Dr G N Tandon, in its report dated July 1991 had recommended that ‘two accelerographs could be installed in Periyar Dam - one in old and one in new backing concrete to monitor the real response of the composite dam to seismic events in the area, the monitoring of which is said to be being regularly done at Idukki.’Accelerographs are used to monitor structures for earthquake response and are useful when the ground motion is very strong during earthquakes, when the more sensitive seismometers tend to go off-scale.The computed data is used to improve building design, or to help locate important structures in safer areas. The type of information gathered, such as rupture velocity, would not be possible with the standard seismometers.  While constructing Mullaperiyar dam, the dam engineers were not aware of  how to design the dams against the effect of seismic forces.In the 1960s and early 1970s a number of events including the major cracking of the 340-ft high Koyna Dam, the cracking of Hsingfengkiang Dam, a 345-ft-high concrete buttress dam in China and the significant sliding occurred in the Upper Van Norman following earthquakes made the engineers sit up and think about the inadequacies of the structure.In 1995, the Geological Survey of India (GSI) had again conducted a geological mapping of the Mullaperiyar Dam area as part of the Dam Safety Assurance and Rehabilitation Project aided by the World Bank.In its report, GSI had advocated to ‘study the seismogenic capability of the lineaments by instrumentation and monitoring of the Mullaperiyar area'."Even after a lapse of over 15 years, no action has been taken. If the seismic instrumentation was in place the exact response of the Mullaperiyar Composite Dam could have been gathered,’’ said James Wilson, member of Mullaperiyar special cell.It may be recalled that IIT Roorkee team had identified 22 major faults in a radius of 300 kms around Mullaperiyar dam site. Out of this, the Thekkady-Kodaivannalur Fault was identified as the one which will create most devastating effect on the Mullaperiyar dam site, which is capable of producing an earthquake of 6.5 Magnitude in Ritcher scale within a close distance of 16 km from the dam site.The intensities of maximum possible earthquakes due to the 22 faults and their effect on Mullaperiyar dam site were studied in detail. IIT Roorkee in their report on Seismic Hazard Assement recommended that the existing Mullaperiyar dam has to be checked for its safety since eventuality of its failure may result in huge economic and human loss.