Sir,
This is in response to the letter by Geologist Mrinmoy Chakraborty (SOM dated Aug.11, 2019) defending the views of the Geological Survey of India (GSI) published in SOM dated Apr.25. I write this to bring to light a few facts that go to show that Chakraborty has hastened to defend the warnings given by GSI a bit too early and to furnish some clarifications regarding the extreme events that wreaked havoc in Kodagu this August and perceptions arising out of them.
The series of landslides that devastated Kodagu in 2018 occurred mostly during the period 15th, 16th and 17th August. Some intriguing data on rainfall pertaining to this period tells the story. This data is from the rain gauge at Kudigana, a place near the border of the catchment of Harangi, which witnessed the worst kind of disasters in 2018.
Record rainfall
Till 6th August, the station had recorded about 5,500 mm of rain, and then on rainfall on the successive days read (in mm): 81, 250, 269, 90, 115, 121, 262, 295, 320, 461 (16th), 394, 197, 164, 95, the total for the fortnight amounting to 3,114 mm (Mysuru’s annual rainfall is 780 mm). Probably the 5-day extreme rainfall recorded then at Kudigana (1,686 mm) is the highest ever recorded in the rain gauging history of Kodagu (which extends for over 120 years now), the earlier highest being 1,462 mm in 1964 pertaining to the station of Bhagamandala.
On the other hand, the records at Kudigana for this year read thus: Total rainfall up to 1st August – 2,041 mm; then on – 30, 0, 16, 136, 183, 254, 312, 276, 298, 158, 90, 46, 35, 100 (15th), during the next 14 days – this is the period when the floods ravaged the district. The extremes here are just about 75% of last year’s and the total for season up to 20th Aug. is not even half of last year’s (nearly 4,090 and 8,614 mm respectively).
Once in 10-12 years
The highest one-day rainfall recorded this year has been reported from Bhagamandala, places around which contribute to flow in Cauvery and stands at 400 mm. The seven-day extreme for this year here has been 1,142 mm. Strangely, these values have return periods of 12 years and 10 years respectively.
This means, the type of flood-causing rain that fell this year occurs, according to data available, nearly once in 10-12 years. Even the flood magnitudes in the four rivers of the basin that were reported this year have return periods of 15 – 20 years.
On the other hand, the rainfall of 2018 that caused havoc in areas around Madikeri and in the catchment of Harangi shows a return period of over 50 years! This is the reason why even though Kodagu experienced severe floods this August, only just a couple of incidents of severe landslides have occurred.
Technical data
Very often, blockage of roads due to mud falls from tall and nearly vertical cuttings (Plate 1), many times triggered by uprooting of trees and washing away of the valley side road formations, are also termed as landslides, while the landslides of 2018 were removal of unimaginably large portions of hills, mostly in the valleys.
A large number of the over 150 landslides that occurred then got initiated far away from the highways and inhabitations (Plate 2), in total contrast to the common belief that tampering with nature and cutting land indiscreetly is the root-cause of landslides.
Many of these slides carried tens of thousands of cubic meters (like 200 m x 100 m x 2 m) of earth submerging hundreds of hectares of land downstream in mud (Plate 3 and 4). However, even earth falls of volume of a few hundred cubic meters (20 m x 10 m x 2 m) block roads and cause damage to houses — only one landslide of the former nature has been reported this year. It is not strange that extremes not quantified by measurements are blown out of proportion, painted colourfully and projected as catastrophes in this era of rampant use of mass and social media.
Predictions fail
Dr. H.S.M. Prakash, Director General of GSI, is reported to have explained (SOM, 25th April) thus: “There are indications of excessive downpour happening in five phases just like last year …”; “likewise, three to four more similar downpours are likely to be experienced… The rainfall in August is likely to result in floods”. Yes, one spell of rainfall has resulted in floods. But what about the rest? It is a common experience that any astrologer, once approached, would point out a number of cases in which his predictions have gone perfectly right, hiding others! The fact is that rainfall in Kodagu this year has been very poor except for that one spell — even after that week of deluge, the total seasonal rainfall for the year remains 25-40% lower than the normal (except in small pockets of Virajpet), while the last year it was about 25-50% higher than the normal.
Strong low pressure
It is claimed by Chakraborty (SOM, 11th Aug.) that the predictions for this year have been those based on “evidences”. Dr. Prakash had explained: “There is a possibility of heavy rain being caused due to volcanic eruptions in Hawaii and Mauritius. And, as if an indication, it has already started raining in Kodagu (in April)”.
It is utterly sad that an agency with a legacy of 200 years has fished out evidences for the floods in August, in the excessive rainfall of April! It is well established that the pre-monsoon rains of April and May are a phenomenon occurring due to local convections, while the monsoon rains in Kodagu are due to the global circulation, aided by strong low pressures in North/ Central India and the local orography (high rising mountains) of the Sahyadris.
It is now popularly known that the excessive rainfall of the ten days of August this year has been due to a strong low pressure that developed in the Odisha coast, and experts from GSI claim it to be due to excessive heat of magma in Mauritius!!
Better to err on safer side
Being half correct about events not known of is one hundred perfect in Statistics — probably the messengers, the people at the helm of affairs, believe that it is better to err on the safer side than on the other. Sweeping and arbitrary long-range predictions are not only unwarranted, but are also dangerous.
Rather, there shall be healthy introspection and debate on why even extreme events of smaller magnitude have been resulting in excessive damages in recent days. Further, very little of the Hydrology of this region has been understood, much remains to be done. It is hoped that the agencies concerned take a more pragmatic and purposeful look at the recent events and use them to learn about the region.
– Yadupathi Putty, Mysuru, 3.9.2019
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source: http://www.starofmysore.com / Star of Mysore / Home> Voice of the Reader / September 16th, 2019
