Landslide, a frequently occurring natural hazard in the hilly terrains of India, shows preponderance of activity during the monsoon period from July to September and after the snow fall from January to March. The strong earthquakes also cause triggering of landslide, particularly in regions marked by critically disposed and unstable slopes. On a rough estimate, nearly 15% of India’s landmass or 0.49 million sq km area is prone to landslide hazard. This includes 0.098 million sq km of the North Eastern Region, comprising the Arakan Yoma ranges, and 0.392 million sq km of parts of the Himalaya, Nilgiri, Ranchi Plateau and Eastern & Western Ghats. As many as 20 States of India are affected by different degrees of landslide hazard. Of these, the States of Sikkim and Mizoram have been assessed to be falling under very high to severe hazard classes. Most of the districts of the States of Jammu & Kashmir, Himachal Pradesh, Uttarakhand, Arunachal Pradesh, Nagaland and Manipur come under high to very high landslide hazard classes. In the Peninsular Region, the hilly tracts of States like Karnataka, Andhra Pradesh, Tamil Nadu, Maharashtra, Goa, Madhya Pradesh and Kerala constitute low to moderate hazard prone zones.
The Geological Survey of India, one of the oldest surveys in the world, has been engaged in the study of this natural hazard for the last 125 years.Some of the landslide investigations of earlier times include :
Study of problem of slope stability in Nainital by Sir R.D. Oldham - 1880
C. S. Middlemiss Re-study of the Nainital area in 1890.
Gohana landslide that blocked the river Birehiganga in UP in 1893 was studied by T. H. Holland in detail.
Many such landslides blocking the courses of rivers including the recent landslide blocking the course of river Pare Chu in Chinese territory and posing danger to population inhabiting the downstream areas in Satluj valley in HP have been studied by the officers of the Survey during a period of more than a century.
The study of landslide hazard carried out by the Geological Survey of India can be divided into two broad categories:
Pre Disaster Studies
Identification of vulnerable slopes through landslide hazard zonation (LHZ) mapping on various scales or studying the critical slopes individually and determine their status as far as their stability is concerned.
Post Disaster Studies
Detailed analysis of landslides that have occurred, determine the causes responsible for failure and suggest treatment measures required to stabilize the slopes.
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