Estimation of rainfall thresholds for landslide occurrences in Kalimpong, India

The Indian Himalayan locale has been essentially influenced by the increase in the frequency of landslide events. Out of 0.42 million km(2) of India's landmass prone to landslides, 42% falls in the North East Himalaya, especially Darjeeling and Sikkim. The harm due to landslides is massive, causing loss of life, property and agricultural land, thus initiating a dire need for formulating strategies to minimize its impact. There have been many attempts to establish rainfall thresholds on global, regional and local scales which compare analysis at various levels. Rainfall thresholds anticipate landslide occurrence and help in issuing a warning to civil authorities and the general population. However, empirical relations defining the relationship between landslide occurrences and rainfall events in Kalimpong remain unattended. In this paper, rainfall thresholds for landslide occurrence have been ascertained for Kalimpong area of Darjeeling Himalayas, in the Indian province of West Bengal. A threshold for landslide occurrences which describes intensity-duration threshold was estimated using the power law equation. The relationship for the study area is I = 3.52 D-0.41 (I is rainfall intensity (mm/h) and D is duration (h)). Results show that events with a rainfall intensity of 0.95 mm/h with a duration of 24 h have a high risk of slide initiation in this region. It also demonstrates that for 10- and 20-day antecedent rainfall, an intensity of 88.37 and 133.5 mm is required for landslide occurrence in this region. Such data would help in implementing early warning systems that focus on rainfall thresholds and forecasting rainfall measurements. Rainfall thresholds for landslide initiation in Kalimpong can be enhanced with more precipitation and landslide data as and when available.