“Water has memory, it remembers the paths it used to once flow along,” said Dr K Soman, about the destructive path that water and debris took immediately after the landslide in Vellarimala in Kerala’s Wayanad on Tuesday, July 30. The massive landslide swept away large parts of Mundakkai and Chooralmala in Meppadi grama panchayat.
Soman, a retired scientist and former head of the Resources Analysis Division of the National Centre for Earth Science Studies (NCESS), explained to TNM the geological factors that caused the landslide and the land use patterns that aggravated its impact.
Several factors are at play in a landslide, such as slope, soil thickness, nature of soil and rocks, and rainfall. However, differences in soil type and rock formations can mean that factors contributing to landslides are different in different locations, Soman said.
“Soil piping (formation of underground tunnels due to subsurface soil erosion) is considered a factor contributing to landslides, but that is applicable in places with alkaline soil, like the Himalayas, not in Kerala, which has acidic soil. Instead, landslides along the Western Ghats in Wayanad can be attributed to fractured rocks,” he said.
Fracturing of rocks is a natural geological phenomenon by which they are divided into two or more pieces by joints or faults. “If one examines the topography of Vellarimala, you can see that the landslide appears to have originated from a saddle (a low point between two areas of higher ground). Saddles can be formed in two ways – either by fracturing or by erosion. Saddles formed by erosion usually occur in limestone, which is not the case in this location. The one in Vellarimala was caused by fracturing,” Soman said.
According to him, water accumulates in the faults of fractured rocks, and when the soil reaches saturation, the water bursts forth, taking with it soil, rocks, and vegetation. “The Vellarimala landslide itself is a natural phenomenon, even though one has to admit that the rains were excessive and would have led to greater water accumulation within a short time.”
Moreover, the difference in altitude between Vellarimala, where the landslide originated, and the towns that it ravaged heightened the impact of the debris flow. “Vellarimala is approximately 2,000 m above mean sea level, while Mundakkai and Chooralmala are 900-1,000 m above mean sea level. That is a drop of almost 1,000 m, but in a very short distance of a few kilometres. This means that the landslide debris must have descended upon Mundakkai and Chooralmala with great force in no time, sweeping away everything on its course,” Soman said.
He distinguished the Wayanad landslide from the one in Shirur, Karnataka on July 16. “The Shirur landslide was caused only due to human activity. It was solely the unscientific construction of the highway that caused it,” Soman said.
At the same time, Soman attributed the large-scale destruction that happened in Mundakkai and Chooralmala to inordinate human activity. The scale of the Wayanad tragedy is the direct result of unscientific land use patterns in the affected areas, he said, explaining the role of tea plantations in the tragedy. “When the British set up tea plantations in areas of high altitude, they levelled the small gullies through which water flowed downstream, and settled their workers along the river terraces (levelled surfaces formed by sediments deposited by the river). Later, towns developed along these areas.”
“The greatest losses have been caused downstream from Vellarimala in Mundakkai and Chooralmala. In both places, the houses and buildings were situated on the river terrace. The Vellarmala school too stood on one such river terrace formed by a turning in the river’s course.”
“The river used to once course through these terraces and must have been diverted to its present course either by a previous landslide or due to reduced water. However, when the landslide struck on July 30, it reclaimed its course, thus destroying hundreds of houses and other buildings along its path,” Soman explained.
“Water has memory, it remembers its course, even centuries after it was diverted. It’s dangerous to attempt to trick nature by occupying the path of a river,” he said.
Soman also pointed to another reason why Chooralmala and Mundakkai were unsuitable for human habitation. The area had experienced landslides of lesser intensity in 1984 and 2020. Soman said that the angular rocks point to the possibility that there had been other landslides too in the past century or before. “The recent landslide has exposed angular rock masses on the river bank in Chooralmala. These rocks could have been brought there only by a previous landslide.”
“It is unfortunate that despite a precedent of landslide in this location, people were still allowed to live in these two affected areas.”
