Wayanad: The landslide that wiped out Mundakkai and Chooralmala villages in Wayanad moved at an extremely rapid pace; its rate of movement was of the highest degree and travelled for a distance of up to approximately 7 km, according to the first information report (FIR) of the Wayanad landslides prepared by the Geological Survey of India (GSI).

The rate of movement of a landslide is categorised as extremely slow, very slow, slow, moderate, rapid, very rapid and the highest one extremely rapid. It is calculated in velocity and the extremely rapid category has a velocity of 10ft/sec. An official who did the field investigation for the preliminary report said that what hit Mundakkai was faster than 10ft/sec. A rapid modelling done by NRSC has estimated that the flow velocity in Wayanad was 57m/sec.

The report notes that multiple landslides (debris flow) occurred as a succession of the first landslide in the Mundakkai area, which almost devastated the village. The probable source of Mundakkai debris flow is the same as the source area of Punapuzha, a tributary of the Chaliyar river and flows towards Chooralmala town in the east. The debris flow was initiated as a debris slide at the source and followed the path of the stream carrying tons of overburden (weathered rock and soil) mixed with water-formed slurry and washed away the path through which it flowed. Thus, altogether, this havoc made the Mundakkai and Chooralmala villages a graveyard. Heavy downpours and an enormous volume of landslide material forced the river Punapuzha to change its course, the report notes.

''It was a deadly combo of heavy incessant rainfall and massive debris. In such a situation, extremely heavy materials like boulders gain speed every 100-200 metres. With double speed, the force, the stress of impact would be so much more and hence the extent and scale of devastation,'' an official who was part of the field investigation team said.

GSI also did an analysis of Google Earth images to study the source area. It says that the actual initiation of the landslide in the source area occurred during the monsoon of 2018. During the successive years, the area of the slide seems to increase, and also observed minor slides developed along both the flanks of the stream. ''The source area has remained active since 2018. The main slide happened in 2019 here and every monsoon, the area got reactivated and this time, the slide was really devastating,'' the GSI official said.

All four affected zones; Chooralmala, Mundakkai, Vellaramala and Attamala were marked as Moderate Susceptibility Zones (MSZ) as per the susceptibility map prepared under the National Landslide Susceptibility Mapping (NLSM) Programme. Hilly areas around these affected areas are highly susceptible according to the NLSM map.

The probable source of the Mundakkai debris flow has an elevation of 1544 m and it is approximately 3 km and 5 km from Mundakkai and Chooralmala villages respectively. The loose and unconsolidated nature of overburden material resulting in rising excess pore pressure due to incessant rain is assessed to be the triggering factor for all these incidences. While the upslopes and midslopes in and around the source area are covered by thick forests, the affected areas include tea plantations and mixed crops, the FIR notes.