How do landslides occur and what triggers them?

From Uttarakhand's Chamoli to Himachal Pradesh's Kinnaur, Maharashtra's Satara and now Kerala's Idukki and Kottayam districts, India has witnessed an increase in incidents of landslides lately.

The recent disaster in Kerala following heavy rains in the state has killed 35 individuals so far and left several missing. A number of experts have attributed the cause for the latest landslide to varying factors from a mini cloudburst to an interaction between two low-pressure weather systems.

But how exactly do landslides occur?

Landslides, in short, are the movement of rocks, debris or earth down a slope. They can happen in a different number of ways. An illustration shared on Twitter by the National Disaster Management Association (NDMA) explains it best.

Most landslides, barring submarine (underwater) occur as a result of heavy rain or flooding (often caused by heavy rain). The diagram above mentions seven ways in which a landslide can occur.

In more mountainous terrain, landslides often occur due to the falling of rocks; or rockfall, which leads to either blocking of roads, or worse, its destruction. Poorly constructed roads, ie. those that are built on an eroding slope also cause landfalls.

A house in Kottayam district's Mundakayam that was washed away by an overflowing river is an example of a landslide due to a riverbank collapse. This tends to happen when buildings are constructed alongside a river. When the force holding the sediments of a riverbank together is unable to, in this case, withstand the river currents, the building gets washed away.

Similarly, when debris, be it rocks, earth or buildings, falls down and finds its way to a stream or a river, that too can result in a landslide.

While the previous instances of landslides in Maharashtra, Uttarakhand and Himachal Pradesh suggest human intervention as a possible cause, it remains to be seen if that is the case in Kerala. However, many, including former Union Environment Minister Jairam Ramesh have pointed to the non-implementation of a report published by the Madhav Gadgil panel in 2011 as a cause for the state's woes.

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