What is the Rule Curve?
Rule Curve is a tabulation which specifies quantum of storage of water or empty space to be maintained in a reservoir during different times of a year, based on the rainfall data for 35 years.
It is a ready-reckoner in decision-making for officials in charge of the dam, for smooth operation of shutters, especially for moderation of flood, during monsoon times without having to seek permission from the top hierarchical ladder.
First dam to have Rule Curve
According to Tamil Nadu Water Resources Organisation, Mullaperiyar is the first reservoir to have Rule Curve implemented, in the country.
Water of the Mullaperiyar dam, has remained a bone of contention between Tamil Nadu and Kerala for over four decades. After a long legal battle, the Tamil Nadu Government was allowed by Supreme Court to raise the water level from 136 feet to 142 feet in 2014. However, Kerala insisted with the Supreme Court to insist that Tamil Nadu, which maintains and operates the dam, prepare a Rule Curve for flood control and flood management.
It had earlier alleged that sudden and unannounced release of flood water through Mullaperiyar dam caused severe damage to Kerala. Though Tamil Nadu Government delayed it with various reasons, it prepared the draft Rule Curve for the reservoir in consultation with Hydrology Wing of the Central Water Commission. And it was first implemented in October 2021. Water was released from the dam much before it could touch 142 feet, raising a huge hue and cry among the farmers and politicians of the State.
Restriction of water level
The Rule Curve comes into effect between June 10 and November 30, during which the dam gets maximum inflows from the South West Monsoon.
The upper limit of the water level has been fixed for every block period of 10 days of each month based on 35 years of data on water inflow and irrigation pattern. It has been fixed between 136 feet starting June 10 to 142 feet in September 20. Then again, the upper limit is reduced from September 30 and then gradually raised to the maximum permissible level of 142 feet by November 30.
The fear of Tamil Nadu is that Kerala was indirectly trying to restrict it from maintaining the water below the maximum level of the 142 feet permitted by the Apex Court, in the reservoir which has 152 feet as the full reservoir level and to dissolve the sub-committee formed under the Supreme Court’s direction to monitor the dam safety.
As a result of implementation of the Rule Curve, Tamil Nadu will have to keep the water level below the permitted maximum level of 142 feet for 150 days in a year, the period when the dam gets heavy inflows.
Mullaperiyar dam has the unique limitation of lesser head sluice discharge (withdrawal of water from the dam through tunnel towards Tamil Nadu side) with a maximum discharge capacity of 2,300 cusecs.
This is minimum in comparison to the dam’s maximum flood design of 1.24 lakh cusecs. In 2017,the dam received 30,000 cusecs of inflow.
Surplus water is released towards Idukki district of Kerala through shutters.
Eventually, the Rule Curve implementation over the years would be used by Kerala Government, citing the age of the reservoir, in disallowing Tamil Nadu’s efforts to further increase the water level to 152 feet upon taking up strengthening works of the Baby dam.
Stake-holders not consulted
While the FRL of the reservoir is 152 feet, calculating the Rule Curve with 142 feet as the base has reduced the maximum water storage in the dam during monsoon time.
Normally, water yield from the rainfall would fill up Mullaperiyar dam three times in a normal year and five times in a good year. Consequent to implementation of Rule Curve, the limitation in head sluice would force Tamil Nadu to part away with a huge quantity of water to Kerala during floods.
The preparation of Rule Curve was not done in consultation with the stakeholders, claimed the farmers of Tamil Nadu who benefit from the waters for irrigation.
Deficit Periyar-Vaigai basin
Periyar-Vaigai basin being a rainfall deficit basin is able to produce foodgrains making use of Mullaperiyar dam water. Any attempt to reducing the quantum of water flowing to Periyar-Vaigai basin through implementation of Rule Curve will be a great disservice to the nation, the farmers of the region say.
In contrast to general fact that the width of rivers widen as they progress towards the sea, the width of Vaigai river shrinks in Ramanathapuram district as it goes closer to the Bay of Bengal. This only indicates the hard truth of Periyar-Vaigai basin being a rain deficit basin.
In other words, under the Rule Curve method, water is not allowed to be stored to the permissible maximum level at the time when the reservoir receives huge inflows. And permitting Tamil Nadu to maintain it at 142 feet after the monsoon, when there will be little or no inflow, would defeat the very direction of Supreme Court allowing Tamil Nadu to increase the water level to 142 feet, farmers argue.