The new dam proposed by Kerala on Mullaperiyar is slightly higher than the old dam and is capable of storing more water.
The altitude at the rock level of the new dam will be 820.53 metres above the sea level. The old dam is also built on rock of about the same level. The new dam will have height of 160 feet from the bed level whereas the height of the existing dam is 158 feet (excluding parapet wall).
The maximum water level fixed for the new dam is 155 feet from the bed level which is the same as that for the old dam. The full reservoir level (FRL) of the new dam is fixed at 136 feet. The full reservoir level of old dam was 152 feet before it was lowered to 136 feet over safety considerations.
The storage capacity of the new dam will be 11.353 thousand million cubic feet (TMCft) at full reservoir level which is 143 million cubic feet more than the storage of the old dam at water level of 136 feet. The yield from the reservoir during the course of the year is estimated to be 20.5 TMCft (with 75 per cent dependability). This is about 15 per cent more than the water being drawn by Tamil Nadu at present and five per cent of it is proposed to be used for summer flows down the river to Idukki. The balance is proposed to be allocated to Tamil Nadu.
The maximum reservoir level is set much higher than the FRL considering projections of the probable maximum flood by Delhi IIT and limitations in releasing water through the spillways. (The difference between FRL and MRL of reservoirs like Idukki is much lower.) The discharge through the spillways may have to be limited to 1.22 lakh cusecs because the banks of the river downstream are populated.
If Tamil Nadu agrees to construction of new dam, differences could still arise over the FRL level. As Tamil Nadu does not accept the projections of the probable maximum flood by Delhi IIT, it may claim that the FRL could be raised to store more water. Already, it has demanded that the State should get more water from Mullaperiyar.
A notable aspect of the design is that Kerala has fixed the maximum water level at such a level that considerable portion of the Periyar Tiger Reserve would be submerged in the event of a flood. Vegetation and ecosystems had developed in those areas following lowering of the reservoir level of the existing dam in 1979. Kerala had consistently taken the stand that these areas could not be submerged again in view of the enactment of Forest (Conservation) Act in 1980. Now, its own design permits flooding of the areas.
