Rain set off landslide in Wayanad, change in land use pattern made it lethal: K K Ramachandran

The criticality of the terrain is the static slope, and rainfall makes it dynamic.

Scientist and consultant with the National Centre for Earth Science Studies (NCESS), K K Ramachandran, said the prime culprit of Wayanad landslides was the torrential rain that battered the area

KOCHI: While social media is blaming the destruction of the Western Ghats as the primary reason for the massive landslide at Mundakkai in Wayanad that claimed over 350 lives, scientist and consultant with the National Centre for Earth Science Studies (NCESS), K K Ramachandran, said the prime culprit was the torrential rain that battered the area. The region received 572 mm of rainfall in 48 hours.

“Rain is the key dynamic agent that triggered the debris flow. Changes in land use patterns and disturbed soil in the valley are semi-dynamic factors that contributed to the massive devastation. The crest of the peak is around 1,600m high and till around 750m from the top, there is an 80% slope on the terrain. Any terrain with over 20-degree slope receiving 12 cm rainfall in 24 hours is susceptible to landslide. If the slope is disturbed and the land use pattern has changed, it is sure to witness a debris flow,” he explained.

The criticality of the terrain is the static slope, and rainfall makes it dynamic. The NCESS conducted a landslide susceptibility study and prepared a hazard zonation map of the area in 2009. The map was provided to all district collectors years ago. In 2016-17, the NCESS prepared a criticality model based on the rainfall received in the area.

“The same slope in Mundakkai has witnessed a landslide in the past. The western side of the peak is very steep, with a hard and rocky terrain. However, the eastern side was disturbed by the previous landslide. The steep flow of water from the crest disturbed the already-soaked surface located around 500m below which failed and started flowing. As the flow gained momentum, the rocks underneath started falling. The debris flow gained momentum as it moved down the slope, making it deadly,” said Ramachandran.

He said the landslide was not caused by piping phenomenon, but was a typical debris flow. “Piping is caused by rainwater that seeps in and liquifies the subsurface soil. But the landslide in Mundakkai was a surface phenomenon. The piping phenomenon is witnessed in areas with laterite soil like Malappuram and Kozhikode. In Mundakkai, it was the momentum gained during the 8km course downstream that caused massive destruction,” explained Ramachandran.

Massive destruction

• Mundakkai received 572 mm rainfall in 48 hours

• Debris flow caused massive destruction to a distance of 8 km 86,000 sq m of land destroyed

• NCESS had provided hazard zonation map to district collectors a decade ago