THIRUVANANTHAPURAM: At a time when there is a debate over whether human interventions led to the twin landslides in Wayanad, an expert study on the first massive landslide at Mundakkai in 1984 had categorically stated that unfavourable geological, climatological and geotechnical factors were responsible for the natural disaster.
The study, done by Dr P Basak and NB Narasimha Prasad from Ground Water Division, Centre for Water Resources Development and Management, Kozhikode, had recommended the installation of a warning system to minimize loss of life and movable property.
The first landslide at Mundakkai happened on July 1, 1984 at 2 pm. Field investigation revealed that the landslide originated in the Western Ghat hills. Even though only 17 people lost their lives, the actual death toll would have been much more but for the fact that the landslide area was in the reserved forest and majority of the field estate workers were away to attend a festival. However, the landslide extended around 80 acres. A total volume of 9.5 lakh cubic metres of earth mass was shaved off.
Monthly and daily rainfall records of the data of the area during 1983-1984 reveals that monthly rainfall during June-July of 1984 was as high as 1400 mm and on July 1, the day of the landslide rainfall was 340 mm in 24 hours. "The geological formations (very thick soft lateritic deposit, presence of viscous plastic clay and highly weathered and fractured rock basement) steep slope, presence of tension cracks, usually heavy rainfall on the fateful day, extreme saturation (within 80 percent of the liquid limit), loss of shear strength of the overburden coupled with possible tampering of natural drainage system at the top of the sliding mass, all have at various proportions triggered the landslide at Mundakkai," said the study.
"Geological map of Wayanad shows that the district is covered by gneissic rock, a commonly widely distributed rock of precambrian age (2500 million years). The cross-section of the exposed faces of the slided area at its origin indicates 3-4 metres of loose brown lateritic deposit overlying micaceous kaolinitic plastic clay bed around 3 metre thick which turn rests on highly weathered and joined gneissic rock around 15-20 metres. Such a high degree of lateralization and weathered gneiss in a relatively unweathered formation is due to extreme high annual rainfall of more than 3500 mm," it added.
The scientists said that "the liquified clay must have entered in all the fracture planes of the weathered gneiss below thus reducing the fractional resistance needed to resist the landslide further". They pointed out that the deforestation and subsequent replacing of plantation crops might have also contributed to it. The study also warned that the postmortem analysis of the Mundakkai landslide indicates the probable danger ahead in similar areas of the Western Ghats.
