An investigation carried out in Idukki by a team led by S. Sreekumar, Associate Professor with the Department of Geology and Environmental Science of Christ College, Irinjalakuda, indicates the possibility of fresh landslides being triggered in areas near where they occurred recently.
Dr. Sreekumar had earlier been Principal Investigator in a three-year project on long-term mitigation strategies in landslide-prone areas in the Western Ghats region of Kerala.
“If heavy downpour persists, landslides may be triggered in more locations. Different types of hill slope processes coexist in Idukki,” he said.
Rainfall measurement at Kulamavu rain-gauge station before the recent landslide was 245.2 mm.
A report submitted to the Kerala State Council for Science, Technology and Environment in 1994 indicated that 180 mm rainfall for a few days should be considered a threshold value to alert the civic administration to initiate disaster management measures, including evacuation of people from critical areas identified in the Landslide Hazard Zonation Map prepared by various research institutions, said Dr. Sreekumar.
Whenever extreme dry season followed heavy monsoon, the cohesive strength of soil and weathered rocks overlying the hard compact rock was reduced, he said.
Unscientific quarrying practices, traffic congestion on the narrow Ghat roads and slope modification had rendered the hill ranges conducive to landslides, he added.
Shift to Idukki
Landslides, landslips, and heavy flow of debris were reported in 2004, 2007, 2009 and 2012 in the northern districts of Kerala. During the period, only isolated slope failures were reported in the hill ranges of Idukki, he pointed out.
The pattern has changed.
“The reasons for the slope failure are found to be site-specific. Landslips along the road-cut hill slopes and adjoining valleys of Cheeyapara along the National Highway connecting Aluva and Munnar were triggered by the vibration of heavy traffic or constant movement of heavy vehicles.
The road-cut, 60-to-70 degree slopes are overlaid with oversaturated thick weathered rock-soil mixture and perched boulders. Afforestation programmes are a must in such areas,” he said.
Much of the debris flow in Kunjithanni, Malayinji, Methotti, Peringassery and Moolakkad was reported near first-order streamlets.
“This could have been averted if precautions had been taken on the basis of an earlier report that recommended avoidance of settlements near natural drainage. The vertical subsidence of ground and formation of depression (5.3 metre deep with a radius of 3.2 metre), and opening of numerous horizontal underground tunnels 150 metre downhill from the depression at Upputhara Fourth Mile resulted from soil piping (erosion of soil). Several settlements in this area are facing imminent danger from unstable slopes,” he observed.
The team spotted new horizontal cavities in highly-weathered rocks of road-cut slopes on the Kanjaar-Pullikaanam route.
“The National Highway between Cheruthoni and Kulamavu is under threat of collapse. Traffic along the route should be regulated. The pore pressure induced by an increase in water level in the Kulamavu Dam and less coherent, highly permeable texture of soil in the area worsen the problem. An 80-metre stretch of road near Cheruthoni has caved in because of the weight of a cliff-top building and significant reduction in lateral support. Quarrying must be banned in critical areas during monsoon, as it was done this year,” he stated.
Maintenance of roads
He said officials should pay more attention to maintenance of and flow of traffic along the Ghat roads.
He supported a recommendation of the Madhav Gadgil committee that cultivation of crops such as paddy and tapioca should be discouraged on slopes of more than 30 degree. “Mono-cropping (agricultural practice of growing a single crop year after year on the same land) should be discouraged on steep slopes. People should be made aware of the threat of constructing houses near ephemeral drainage lines,” he said.
The other members of the investigation team included Arish Aslam, K.P. Mithun and P.G. Gopakumar.