Explainer: How flood waters are being managed in Idukki’s Cheruthoni dam

There is a debate whether one or two shutters of the dam should have been opened before.
Explainer: How flood waters are being managed in Idukki’s Cheruthoni dam
Explainer: How flood waters are being managed in Idukki’s Cheruthoni dam
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Twenty-nine dead and more than 15,000 people evacuated in four days following heavy rains and flash-floods. Calling the rains that have battered the state of Kerala “unprecedented”, Chief Minister Pinarayi Vijayan on August 9 termed the situation “highly alarming”. With water levels rising, 22 dams have been opened in the state – something that has never happened before.

With the Periyar River in spate following continuous rains, Idukki, one of the worst-hit districts, saw the Cheruthoni dam opening its shutters after 26 years. This is only the third time since the dam was commissioned in 1976 that its shutters have been opened – the first being 1981 and again in 1992.

Comparisons have been made with the December 2015 floods in Chennai, with a few experts pointing out that a trial run of opening shutters should have been held before the water reached the Full Reservoir Level.

Former KSEB chief engineer, and a dam safety expert, N Sasidharan is of the opinion that opening all shutters of the Cheruthoni dam in a span of 18 hours was unwarranted and the government should have released water through trial run.

James Wilson, an engineer who works with the Kerala government as special officer of  inter state water advisory committee, says this is inaccurate. While the delay in opening the Chembarambakkam reservoir and the indiscriminate release of water resulted in floods, Wilson notes that the discharge from Cheruthoni dam has been gradual.

Flood storage and management in Idukki

The Idukki hydroelectric project comprises three dams – the Idukki archdam, Cheruthoni dam and the Kulamavu dam. Shutters or spillways to regulate and control the release of water discharged from the dam are located only at Cheruthoni dam.

He goes to explain that there are two levels in a dam – the Full Reservoir Level (FRL) and the Maximum Water Level (MWL). In the Cheruthoni dam, the Full Reservoir Level is at 2,403 feet and can be maintained for weeks or even months, while the Maximum Water Level is at 2,408.5 feet. When water levels are close to touching FRL, the dam is no longer in a position to hold the inflow of flood waters, he says, and puts the very safety of the dam at risk. This is where the shutters or spillways come into operation, with the timely release of water. 

“Spillways are like a pressure cooker valve. Dams can hold water to a certain level. What spillways do is allow the release of water in a controlled and safe manner. Just like a pressure cooker will burst if a valve is not working, similarly a dam will burst without spillways,” says Wilson.

The space between FRL and MWL is flood storage. Wilson further points out that water at flood storage can be maintained only for a short interval – no more than a week. Between 2400 feet and MWL, there is flood storage capacity of 5.748 TMC, or in other words, the dam can hold an inflow of 66,500 cusec a day, notes the expert.

The Kerala government on August 9 decided to open one shutter of the Cheruthoni dam on a trial run when the water levels reached 2398.98 feet. The next day, officials opened four other shutters over the course of a few hours. And while Wilson notes that the shutters were raised gradually giving state officials more flexibility in regulating the flow of water, sources say that a controversy had already been brewing by the time Cheruthoni dam was opened. Reports suggest that dam safety officials were at loggerheads with Kerala State Electricity Board (KSEB), which owns and operates Cheruthoni dam. While dam safety officials wanted the shutters to be opened earlier, KSEB was reluctant. It was after much pressure that KSEB officials relented opening four shutters on August 10.

Sasidharan says the government, Kerala State Electricity Board and Dam Safety Authority should have taken into consideration the rising water level, especially as there was prediction of heavy rain.

"We should always allow some space for the rising water. The reservoir should be managed in a way keeping in view the rising water so that the water doesn't affect people downstream. Gradually open one or two shutters, but opening five shutters at a time is not warranted," he says. 

Dismissing the idea that at least one shutter of Cheruthoni dam should have been opened earlier keeping in mind those downstream, Wilson says, “The maximum discharge through the dam with 5 shutters has been only 750 cumecs or 26500 cusecs, while the maximum capacity of 5 shutters at FRL is 3879 cumecs or 137000 cusecs. In reality, the 5 shutters only used it's 19.3% capacity. This is equal to raising one shutter at full capacity or 2 shutters at half capacity. So, the question is whether KSEB discharged its full capacity and answer is a big ‘no’. It's depressing that 'experts' equate shutter numbers opening without discussing quantity of discharge.”

Wilson adds, “There was no delay in decision making. Since IMD has forecast rain for two more days, officials were prepared. There is more room (to accomodate more inflow of water) in the reservoir.”

As of 11am on Sunday, the water level at Cheruthoni dam is at 2399.20 feet, a dip from 2401.76 feet at the same time a day earlier. With all five shutters still open, the outflow of water is at 750 cumecs or 26,486 cusecs, as opposed to an hourly inflow of 669 cumecs or 23,625 cusecs of water. Allaying fears, Wilson further points out, “Even if the inflow were to be 80,000 cusecs, the reservoir will be able to contain that.”

And although alarm bells are ringing in the district, the expert points out that the Periyar basin has witnessed greater floods in 1924 and 1961 – much before the Idukki project came up. The 1924 flood witnessed a discharge of 4 lakh cusecs at Aluva, while 1961 recorded 2.5 lakh cusecs.  

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