Tamil Nadu had fixed the Probable Maximum Flood (PMF) for Mullaperiyar as 2.12 lakh cusecs, which was endorsed by the Central Water Commission in their Memorandum on Rehabilitation of Mullaperiyar Dam (1986). As the recommended PMF was much lower than the recorded maximum flood of 1943, Kerala requested CWC to supply the workouts of arriving at the above figure to verify its correctness. But the response to the above request was negative.
Subsequently, Kerala managed to get the above details in April 2008 under the RTI Act. The flood routing studies of the CWC revealed that when the PMF impinges the reservoir at 136 ft, the water level would go up to 156.40 ft with one vent inoperative and 156.02 ft with all vents open. Hence, the Mullaperiyar dam would overflow even with the PMF of 2.12 lakh cusecs and as such the dam is hydraulically unsafe. It was quite unfortunate that to circumvent this, it was recommended to increase the height of the parapet wall of this old dam with reinforced concrete construction. This recommendation would increase the water pressure on the dam and increase the threat of failure during flood. Also any rise of water level above 155ft will be contravention of the lease deed of 1886.
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