Kerala experienced its worst floods in recent history during the third week of August 2018. It was similar to the one that occurred in 1924, known as the “Great Deluge of 99”, the figure “99” denoting the year 1099 as per the Malayalam calendar. In the latest instance, extreme flooding affected 13 of Kerala’s 14 districts. The State has 1,564 villages, and just about half of them, 774 to be precise, were inundated. Out of a population of about 3.48 crore, more than 54 lakh people—or one sixth of the population—were affected by this deluge.

According to the India Meteorological Department (IMD), Kerala received 2,346.6 millimetres of rainfall between June 1 (traditionally the date of onset of the monsoon in Kerala) and August 19. This is 42 per cent above the normal rainfall of 1,649.5 mm. Further, the rainfall over Kerala during June, July and between August 1 and 9 was 15 per cent, 18 per cent and 164 per cent above normal, respectively.

But this comparison will not suffice to understand why this extreme precipitation created the kind of flooding Kerala witnessed during August 15-17. We must understand how “heavy rainfall”, “very heavy rainfall” and “extreme rainfall” events test, in varying degrees, the resilience of Kerala’s reservoirs, river channels, riverbanks, backwater lakes and the exit mechanisms that discharge the freshet, or flood from rivers, to the sea and lead to different flooding intensities.

Heavy rainfall

There was heavy rainfall between July 15 and 20, which inundated the Kuttanad region, an area very susceptible to flooding whenever the Vembanad lake and the rivers draining into it get such heavy rainfall. During August 8-9, very heavy rainfall occurred at several places in the State. On August 9, rainfall of 398 mm, 305 mm, 255 mm and 214 mm was recorded at Nilambur in Malappuram district, Mananthavady in Wayanad district, Peermade in Idukki district, and Palakkad in Palakkad district, respectively, as per IMD data. This heavy storm resulted in severe flooding at several places in the Malabar areas, especially Nilambur in Malappuram district and Malampuzha in Palakkad district during August 8-10. This heavy spell was then followed by an extreme rainfall event, which started on August 14 and continued up to August 19. The peak of this event was observed between August 15 and 17.

Most of the reservoir systems of Kerala swelled during the first heavy spell during the third week of July, and the water level in several major reservoirs rose above 90 per cent of their storage capacity. But the heavy rainfall and extreme rainfall event in the August 8-19 period forced the authorities to open the spillways of most of the reservoirs in order to release the excess water, safeguard those structures and ensure public safety.

These spillway releases triggered a heated debate over whether the recent deluge was a man-made disaster caused by the discharges from the dams. Sweeping statements were made by responsible persons alleging that the dam managers were to be blamed for it, on the basis of inferences drawn without analysing the hard data or examining the structural limitations of the river basins.

What we must look at first is whether this year’s event is a replica of the one in 1924, and second, whether the spillway releases from the dams were the major cause of the recent deluge.

The three-day storm on July 16-18 of 1924 struck the entire Kerala land mass, recording 897 mm rainfall at Munnar, 754 mm at Peermade, 777 mm at Vythiri and 540 mm at Kannur. A geographic information system-based (GIS) run-off analysis of the above historical data from the IMD shows that this storm resulted in a total run-off of about 12.6 billion cubic metres (BCM), or 446 thousand million cubic feet (tmc feet), over the entire State. The eye of that storm was centred at Munnar and Devikulam and the core zone of heavy rainfall was in the Periyar river basin.

The three-day storm of (August 15-17, with its eye centred at Peermade, between the Periyar and Pamba river basins, recorded 818 mm rainfall at Peermade, followed by more than 700 mm at the Idukki rain gauge stations maintained by the IMD. The extreme zone was shared by both the Periyar and Pamba basins unlike in 1924. For this analysis, rainfall records of 67 rain gauge stations of the IMD spread over the entire State were considered. (To maintain data consistency, the rainfall data logs at dam sites were not considered even though they have recorded higher rainfall than those recorded by the above IMD stations.)

The total run-off generated by the event in August was equivalent to 12.3 BCM (433 tmc ft), which was just 3 per cent less than the run-off of 12.6 BCM (446 tmc ft) in 1924. It is important to note that this quantity of a three-day spell is approximately 16 per cent of the average annual run-off through Kerala’s 44 rivers.

Storage capacity

How much of this quantity of rain could have been contained by the reservoir systems of Kerala? Kerala has 57 large dams (including four dams operated by the Tamil Nadu government), but the combined live storage capacity of these dams is only 5,806 million cubic metres (MCM), or 5.8 BCM, or 205 tmc ft. Only 13 reservoirs have more than 100 MCM live storage capacity and they represent 88 per cent of total live storage capacity. Moreover, only seven reservoirs have more than 200 MCM live storage capacity. The Idukki and Idamalayar reservoirs are the only ones with a live capacity of more than 1,000 MCM.

With a total live storage capacity of 5.8 BCM, the 57 large dams can barely store 7.4 per cent of the average annual run-off of all 44 rivers, which is 78 BCM as per Water Resources of Kerala, 1974. (Also, 0.8 per cent of this water is exclusively for the use of Tamil Nadu.) During heavy flooding, the capacity of the reservoirs to manage the huge influx by effecting controlled releases or accommodating the flood within the storage area becomes redundant and they have no choice but to release the inflow downstream. The ratio of live storage capacity to the annual runoff also shrinks by about 3.5-4.5 per cent, as the spate swells to 70-100 per cent more than the average flow in most rivers.

Flood cushion

Many experts have said that if Kerala had kept a flood cushion of at least 20 per cent, this disaster could have been averted. However, even if the State decided to create a flood storage cushion of around 20 per cent in the reservoirs that have a capacity of 200 MCM or more, that quantity would only be 0.858 BCM. Smaller reservoirs are not suitable for creating such a flood cushion as such a provision would take away much of their utility value; also, even if a flood cushion were created, it would be exhausted in a couple of hours. This capacity of 0.858 BCM can barely absorb 7 per cent of the total run-off of 12.3 BCM witnessed during the three-day storm.

In this context, there is no merit in discussing whether opening the dams early enough would have mitigated this disaster. We must also keep in mind that the storages help the State meet its drinking water and irrigation demands and peak power needs during summer months. So, civil society and policymakers in the State must brainstorm whether leaving out 20 per cent storage capacity is the right thing to do, especially when the State struggles with the existing storage capacity during lean years.

Let us examine the case of the biggest storage reservoir, Idukki. Even if the authorities had started releasing water from Idukki when its storage capacity crossed 80 per cent—from July 19 until August 9, the date of actual opening of spillway—at a rate of 50 cubic metres per second (cumecs), they would have created an additional space of only 90.72 MCM. In fact, the inflow into the dam was getting reduced gradually in the above-mentioned intervening period. Compare this with the huge inflow of 929.65 MCM that came into the dam in the subsequent August 10-20 period and the spill discharge of 759.93 MCM during the same period. So, any flood storage cushion created by the early discharge would have been negated within six to seven hours on the very first day of the storm. It would not have made any qualitative difference to the peak flood discharges from this reservoir. The situation in the case of other reservoirs would have been worse, considering their lower storage capacities.

Now, let us look at whether the spill releases from these reservoirs were the major reason for the flooding. While Idukki and Idamalayar released a peak discharge of 1,500 cumecs each, the gauging and discharge (G&D) site of the Central Water Commission (CWC) at Neeleswaram, near Kalady, shows a peak discharge of 8,800 cumecs. This shows that the extreme flooding downstream of these dams had very little to do with the flood discharges from the reservoirs; rather, the majority of the freshets were contributed by the unrestricted catchment areas.

Also, during that period, the Idukki reservoir absorbed more than 1,000 cumecs from the peak inflow of 2,532 cumecs. (It discharged about 1,500 cumecs into the Periyar river.) This 11.4 per cent absorption of 1,000 cumecs against the peak discharge of 8,800 cumecs observed at Neeleswaram is very important in cases of extreme floods. If you are in neck-deep water, any further addition will worsen the flooding and cause many more thousands to drown.

Take the case of four rivers draining into the south of the Vembanad lake: Pamba, Achankovil, Manimala and Meenachil. Among these, only Pamba has some dams, but their storage capacity is limited to 10.5 per cent of the average annual run-off. The other three have no storages. The Pamba basin has eight dams and one barrage, but only Kakki reservoir with a 447 MCM capacity (92 per cent total capacity) is the major reservoir in it. The Kakki reservoir was close to full reservoir level in July itself, but water could not be released from it to reduce the level because, at that time, the below-mean sea level areas in the Kuttanad region downstream were already experiencing heavy inundation and any release would have added to the misery of the people living in that region.

While the combined peak discharge from the four rivers was around 5,500 cumecs during the extreme three-day-long storm, the Thottappally spillway’s limited discharging capacity of 630 cumecs and the Thanneermukkom barrage’s available discharging capacity of about 1,706 cumecs resulted in a huge “backwater effect” in the upstream areas of the Vembanad lake.

The run-off generated from the Pamba, Manimala, Achankovil and Meenachil rivers during the August 15-17 period was about 1.63 BCM. The carrying capacity of the Vembanad lake has come down to an abysmal 0.6 BCM from its original capacity of 2.4 BCM.

Hence, it can be concluded that only about 0.605 BCM runoff could possibly have been drained from the Vembanad lake through both its outlets. The remaining run-off volume of about 1 BCM resulted in a rise in the water level in the lake and low-lying areas of its upper reaches. Along with this, the breaching of the riverbanks from the heavy spate worsened the situation in many places in the downstream catchment areas of these rivers in Kottayam, Pathanamthitta, and Alappuzha districts.

Banasura Sagar release

Another controversial case of release of floodwaters was from the Banasura Sagar reservoir in Wayanad district. The reservoir reached its full reservoir level as early as July 16. After that, whatever inflow that came into the reservoir was released through the spillway considering the very safety of this earthen dam, which does not permit an overflow of water over the dam even for a moment. On August 8 and 9, record rainfall of 278 mm and 443 mm, respectively, occurred at the dam site. The average spillway release from this dam on those days was 131 cumecs and 228 cumecs, respectively, while the observed discharge at the downstream CWC G&D site was about 2,100 cumecs and 2,200 cumecs, respectively. Hence, it is evident that the major run-off was generated from the untapped catchments of the Kabini river.

Similar was the case of extreme run-off and management of floodwaters at the Chalakudy and Bharathapuzha river basins. The extreme rainfall event affected all rivers more or less in the same manner and they struggled to handle the deluge.

Flooding, often, is essentially an issue of rate of discharge—it is all about how much water can be drained out as it comes in. For example, if 10,000 cumecs is received and the channel capacity is 8,000 cumecs, 2,000 cumecs will be breaching the banks. Then, if the exits to the sea from the backwater lakes are able to drain only 5,000 cumecs, there will be a backwater surge towards upstream areas.

It is important to understand the nuances of water flow during extreme rainfall events. Most of the reservoirs in Kerala take just three to four hours to reach full reservoir level because of their limited storage capabilities. When the spate reached its peak, the shrunken river channels swelled and breached the riverbanks. This was compounded by the reduced storage capacity of our backwater lakes and the deficiency of the exit systems opening out to the sea.

IMD predictions

The next question is whether the IMD predicted this extreme rainfall event in advance or not. The IMD describes rainfall greater than 244.5 mm as “extreme rainfall”, between 124.5 and 244.4 mm as “very heavy rainfall” and between 64.5 and 124.4 mm as “heavy rainfall”. Its long-range forecast and halfway monsoon prediction in August only spoke about normal monsoon and did not say anything about a flood year. The authorities use these predictions for long-term planning for the whole water year. Also, the district-level forecast until August 14 only mentioned “heavy” to “very heavy” rainfall events at some places; nowhere did the IMD predict an “extreme rainfall” event.

It must be understood that the reservoirs and river channels of Kerala are able to negotiate “heavy” to “very heavy” rainfall events in all normal flood years, with moderate flooding. Moreover, to let out water from a dam, the authorities need a reliable forecast of the storm at least three to four weeks before they can take action because the gradual releases from reservoir systems require a long duration of a couple of weeks—otherwise there will be chaos in the downstream areas.

“Monsoon prediction is a tough proposition. We don’t know many things... Beyond 3-4 days we can’t predict exactly,” said M. Rajeevan, Secretary in the Ministry of Earth Sciences (as reported in LiveMint in June). It is strange to see the same person contradicting himself within a couple of months. Also, it is strange that comparisons are made between (responses to) cyclone predictions in the east coast (which can be done 7-10 days in advance with around 90 per cent accuracy) and monsoon storm predictions in the west coast (where the storms may take about four to five hours to intensify) in order to blame the State administration for an extreme storm the IMD had failed to predict.

The IMD’s prediction of heavy rainfall events a couple of days in advance would have enabled the authorities to operate their reservoir systems to avoid overtopping of the dams. The State Disaster Management Authority (SDMA) has now pointed out that the Doppler radars of the IMD at Thiruvananthapuram and Kochi were not functional on August 8 and 9 and the IMD fixed them only after the National Disaster Management Authority (NDMA) intervened at the SDMA’s insistence.

The Hydrology (South) Directorate of the CWC places this storm’s “intensity return period” as a once-in-500-years event, which means that the Annual Exceedance Probability (AEP) of this type of storm is 0.2 per cent every year, or 18.4 per cent over a span of 100 years.

Flood narrative

It is important to review the entire narrative about this flood in the light of the hard facts presented above. Reducing it all to ill-timed opening of the spillways of the dams is only a sign of selective blindness or an act of damning at any cost, without regard to the relevant facts. In a State that has been water-deficient in every lean season, serious thought must be given to the question of how much space must be provided in major reservoirs for “flood storage” for one-off, abnormal events such as the latest episode of flooding, without seriously affecting the lean season demands.

How can we create a sustainable flood management system? It is good to remember that many flood-prone countries have now adopted strategies to “live with the flood”. Loss of life has to be avoided at all costs. But there is a limit to what man can do to contain nature’s fury.

James Wilson is the Special Officer, Inter State Waters, Government of Kerala. Views expressed here are personal.