In Kerala, there’s no reason to damn the dams

A ‘normal Monsoon’ is what IMD predicted for Kerala this year and it was way off the mark. And yet, there are reasons to believe that the state’s multipurpose dams saved it from greater devastation

In Kerala, there’s no reason to damn the dams

James Wilson

They call it the ‘great deluge of 99’ while referring to the natural calamity in 1924 when large parts of Kerala were inundated. The figure 99 refers to the year 1099 of the Malayalam calendar. Ninety four years later, the state finds itself devastated once again.

Unfortunately, an unseemly blame game has clouded the more germane issues. Otherwise responsible people are seen trying to apportion blame to the state government, the management of dams, the failure of the Meteorological Department to correctly predict the deluge or give an assessment of the rainfall and so on.

The truth is that nobody could have prevented the deluge. Man is indeed a pygmy before nature and a natural calamity of this magnitude is unpredictable.

As in 1924, the entire state was affected in 2018. Kerala has 1564 villages and just about half of them, 774 villages to be precise were inundated. Out of a population of about 3.48 Crores, more than 54 lakhs, one sixth of the population, were affected by the deluge.

A major controversy brewing in the State is whether the Kerala State Electricity Board Ltd (KSEBL) messed up its operation of the dams during the deluge, precipitating the crisis.

Kerala has 53 large dams, out of which KSEBL maintains 33 dams, Irrigation Department, 19, while the remaining one is managed by the Kerala Water Authority.

The combined live storage capacity under the reservoirs created by these dams are a meagre 5136 MCM. That is just 6.6 % of the average annual surface water inflow through its 44 rivers. There are 4 other dams too inside Kerala State territory, which are being managed by the Tamil Nadu Government viz. Mullaperiyar, Parambikulam, Peruvarippallom and Thunacadavu dams, together have a combined live storage capacity of 670 MCM.

So, the reservoir systems under all 57 dams inside the state can store only a meagre 5806 MCM (5.8 BCM or 205 TMC). Compare this with the Hirakud reservoir in Odisha, which has 5820 MCM live storage capacity.

What is more, only seven reservoirs, which have a moderate storage capacity of 200 MCM or more and they hold 74% of the total live storage capacity of this 5806 MCM while the rest of the reservoirs store very little water.

In Kerala, there’s no reason to damn the dams

Many of the major irrigation projects and drinking water schemes depend upon the tail waters from storage reservoirs created by KSEBL. Hence it is not fair to paint the operation of these multipurpose projects as pure economical consideration by the KSEBL. In our State, no dam is constructed exclusively for flood control. So the pertinent question is that where you can draw a line to keep these limited storages systems empty without affecting the lean season demands when we are at the mercy of vagaries of monsoon?

The reservoir operation adopted by KSEBL thus give due weightage to the summer needs of the irrigation and drinking water projects below them. The long-term operation of reservoirs are often based on the Long Range Forecast (LRF) of IMD (which predicted normal monsoon with 96% to 104% of the Long Period Average, LPA) as well as historical inflows.

Even the prediction for August-September, second half of S-W Monsoon, was 95% of LPA with +/-8% and not speaking about any heavy rainfall monsoon second half!

Even the bulletin on IMD’s website published on 9th August 2018 for the forecast for 9th to 22nd August 2018, does not predict heavy or very heavy rainfall for Kerala State. Recollect that IMD predicted a similar normal monsoon in their long term forecast for the year 2017, wherein Kerala faced an acute drought year and the stored water in these reservoirs was the only relief then. Also many times in the past, we have seen that North-East Monsoon used to fail when South-West Monsoon was intense.

In Kerala, there’s no reason to damn the dams

It is therefore pointless to argue that KSEBL should have kept releasing water from their reservoirs well in advance, anticipating a storm that nobody had predicted.

So, was it a man-made disaster or a natural calamity of immense proportions? Several observers have been comparing the long term rainfall during this water year, with the long term rainfall received in 1924, and concluding that rainfall this year has nowhere been even close to what was received in 1924.

This is a total misinterpretation of data and a misleading comparison without understanding and acknowledging the nuances of the extreme rainfall events.

For example, you can carry 50 kg weight on your head for some time each day without breaking your neck for 30 days; but if the same 1500 kg (30x50 kg) of weight falls on your head all of a sudden, your neck would break.

Likewise, extreme rainfall disturbs the equilibrium of the river flow regime and causes extreme flooding, even in areas which are not usually prone to flooding. Consequent to this, extreme swelling of streams, waterfalls or normally dry ditches, even in areas which are normally at low risk, happens.

In Kerala, there’s no reason to damn the dams

In Kerala, the flooding worsens in the plains as the backwater lakes get swollen due to huge backwater effect when the discharge due to a deluge is many times more the quantity of water that is discharged through the limited exits to the sea.

Indian Meteorological Department (IMD) describes rainfall greater than 244.5 mm as ‘extreme rainfall’ and rainfall between 124.5-244.4 mm as ‘very heavy rainfall’.

Hydrologists who design spillways and dams look for extreme rainfall events like 1-day, 2-day, 3-day storms. The question is what was the rainfall observed in the catchment areas? Whether they qualified to be extreme?

Hard data speak for themselves. Between August 15 and 17, Idukki reservoir in Periyar basin received a total of 825mm of rainfall in three days. It was 235mm on the 15th, 295mm on the next day and again 295 mm on the 17th August. Similarly, Kakki reservoir in Pamba basin received 812 mm of rainfall in three days and 1032mm in four days days! This 3-day storm of 15th-17th August 2018 with its eye at Peerumedu is more or less equal to the 3-day storm happened between 16th to 18th July 1924 with its eye at Devikulam, which is still being used as the Standard Project Storm. These type of extreme rainfall events, are capable of filling our reservoirs with small/medium storages within a few hours to a day or so and we have no choice but to spill the excess water beyond its permissible storage.

In Kerala, there’s no reason to damn the dams

Clearly, IMD was not able to provide any advance warning about this storm. It was not their fault because these storms can gain in intensity within a couple of hours.

In the district-wise forecast for Kerala by the IMD, on August 14 and August 15 Idukki was to receive very heavy rainfall (124.5mm to 244.4 mm), and heavy rainfall (64.5mm to 124.4 mm) on August 16 and light to moderate rainfall (2.5 mm to 3.5 mm) on August 17.

So, would it have helped if Idukki reservoir spillway had been opened early and would it have helped avert the calamity? Several ‘experts’ have suggested that KSEBL should have started a trial run/opening of spillway from 31st July itself when the water level touched 2395.28 ft (live storage of 1329.10 MCM). They also argue that a controlled release of 50 cumecs from that day to 9th August, when KSEBL finally opened the spillway, would have averted the tragedy.

But they conveniently forget the fact that the inflow into the Idukki reservoir was reduced from 21.75 MCM (31/07) to 7.97 MCM (6/08) and increased to 16.46 MCM (8/08). The average inflow during these nine days was 13.51 MCM/day while the KSEBL was operating the Moolamattom Power House and discharging on an average 10.09 MCM/day to the Muvattupuzha river. So, the net rise in the flow into the reservoir during the suggested period was a mere 3.42 MCM/day.

In Kerala, there’s no reason to damn the dams

So, let’s examine two scenarios of opening of the spillway of Idukki considering the fact that a controlled release of 50 cumecs, means a total discharge of 4.32 MCM/day:

• If the spillway had been opened between 31st July to 9th August and in 9 days, the spill would be 38.88 MCM.

• Assume another scenario. If KSEBL had started releasing water from Idukki, when its storage crossed 80%, from 19th July (when water level was at 2380.46f ft) to 9th August, in 21 days, the spill would have been 90.72 MCM.

The pertinent question is would such a reduction of 38.88 MCM or even 90.72 MCM would have made any significant difference when Idukki had received an inflow of 929.65 MCM and it had spilled 759.93 MCM in the heavy spell between August 10 and 20? Even if KSEBL had operated Idukki at its 80% storage capacity, it would have made no significant change in the spill water compared with the above huge inflow due to the extreme storm!

Did Idukki discharge more water than what it received from its catchment? While the inflow peaked to 2,532 cumecs at 10 PM on 15th August, Idukki actually released only 1500 cumecs, while its maximum capacity is 5000 cumecs. So, Idukki reservoir even during peak inflow absorbed more than 1000 cumecs. At the same time, Idamalayar reservoir discharged 1500 cumecs more.

People conveniently forget two important factors. First, the spill from the reservoirs was less than the inflow received and there is a huge contribution of water from the catchment to the river, where there are no dams.

By absorbing and accommodating 1000 cumecs, the Idukki dam saved the Periyar plains from getting further inundated. The engineers and other staff at KSEBL, therefore, deserve our gratitude and applause and not the brickbats being hurled at them.

(The views expressed in the article are the author’s own.)

This article first appeared in National Herald Sunday.

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Published: 30 Aug 2018, 5:00 PM