KOCHI: The high magnitude devastating landslide at Kokkayar that claimed the lives of six is the second such occurrence in the area in a little over a year. According to geologists, the landslide on October 16th is a reactivated extension of the one that happened last year.
Dr K R Baiju, director, School of Environmental Sciences, Mahatma Gandhi University, said they had reported a landslide on September 6, 2020 at Ilamkad-Moopanmala in Kokkayar panchayat in Idukki district. "The landslide in 2020 went unnoticed since the magnitude and intensity of landslides are measured in terms of the death toll involved. Moreover, usually, landslides that occur in remote areas never attract the attention of the authorities and the public," he said.
Dr Baiju said, "Our team of researchers had presented a report on the 2020 landslide at the national conference on Disaster Management with focus on geohazards organised by the National Institute of Disaster Management (NIDM) under the Ministry of Home Affairs." The conference was held on September 23 and 24 this year. "The 2020 landslide was initiated at the upstream part of Manimala river basin and occurred in the drainage divide, which is an elevated part of the Kokkayar tributary. The landslide occurred on the southwest part of Peerumedu plateau," said Dr Baiju.
According to Dr Anish A U, assistant professor of Geology, Govt College, Kottayam, since the rocks in the head portion of these river basins are highly weathered and fractured, they are susceptible to rock slides. "The rainfall data in 2020 showed that the area received 106mm of rain. This heavy rainfall activated the topple failure which is a common mode of instability in layered and blocky slopes. This resulted in the subsequent debris flow," he said.
"Field studies carried out on 4th October 2020, revealed that the rocks in the upstream portion of the drainage basin are characterized by multiple joint sets or fractures. The presence of clay layers between the fractures in rocks made them unstable," said Dr Anish.
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Considering the fracture planes running parallel and sub-parallel to the direction of slope, during the heavy rainfall events, water will try to flow through the weak planes, generating high water pressure, he said. "This further weakens the fracture planes in rocks and results in the development of failure planes and initiating a rock slide."
The study revealed that the highly weathered rocks in the upstream part of the Manimala drainage basin got toppled and flowed under the influence of heavy rain. So the present landslide event that occurred on Saturday can be considered an extension of the 2020 landslide pertaining to the initiation of drainage divide migration in the drainage basin, said Dr Baiju.