The landslide triggered by excess precipitation at Mundakkai in Mepadi panchayat, Wayanad, on Tuesday bore close resemblance to a dam break, C . Muraleedharan, Deputy Director General (retired), Geological Survey of India (GSI), told The Hindu. He added that this was his first experience of a landslide of such magnitude, spewing debris and water as far as six to seven kilometres down the valley.
In normal landslides, the debris flow would be limited to 1–1.5 km, except for some rare occasions in the Himalayan regions. Here, the supersaturated soil with semi-liquid conditions, triggered by the excess downpour in the region, blasted as if a dam were breaking, sweeping away the villages down the valley through which a portion of the debris flowed into the Chaliyar river. Already, the area is categorised as a high-susceptibility zone by the GSI after the landslide in 2019 at Puthumala, which was hardly 5 km away from the current spot.
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Puthumala tragedy
In the Puthumala landslip too, which claimed 12 lives and left five people missing, the trigger was excessive rainfall. According to a report compiled under the supervision of Mr. Muraleedharan after the 2019 tragedy, the realised 24-hour rainfall on August 7 and 8 was 259 mm in Mananthavady and 168 mm in Vythiri, which were the main triggers for the Puthumala landslip.
In the present case as well, 200 mm of rainfall occurred in the first 24 hours and 372 mm of rainfall in the subsequent 24 hours, a total of 572 mm of rain in just 48 hours. Geomorphologically, it is a rugged terrain with ridge and valley topography resting over the Wayanad plateau, which has a relative relief (the difference between the highest and lowest points) of around 1,000 metres.
According to the post-disaster report in 2019, the debris flow was then initiated from the fringe slope of Vellarimala at an elevation of 1,218 metres in the Thollayiram estate. The first-order streams, originating from the Vellarimala, flowed along the Puthumala valley and joined the Iruvanjipuzha, a major tributary of the Chaliyar river, also known as the Chullika river. In both tragedies, it is assumed that debris from several flows on the right and left flanks of the valley joined the major flow, increasing the quantity of total debris and water in the flow path.
Elevation of 2,000 m
The huge mass that rapidly encroached on the valley caused total devastation in the settlements. The huge force of the debris flow from an elevation of around 2,000 metres seems to have increased the magnitude of the disaster, said Mr. Muraleedhran. A detailed study is required to assess the exact factors that contributed to the worst landslide in the history of Kerala, said Mr. Muraleedharan.
After the 2019 landslide, the GSI maintained in its report that the people in the affected Puthumala area should be relocated to a stable location. Any further modification to the affected slope should be avoided, and the natural flow path of the streams should be maintained.
