Location-Specific Rainfall Threshold for Landslides in Select Micro-Watersheds in Coonoor Taluk, Tamil Nadu, India

Landslides are a common geomorphic hazard in most regions of the Western Ghats in India. A number of environmental, geotechnical, geological and geomorphological factors contribute to the occurrence of landslides in this region but rainfall often is the most common and important triggering factor. The relation between rainfall and landslides is not clearly understood in this region. An attempt has been made to study the relationship between the various rainfall parameters like daily rainfall, cumulative rainfall, antecedent rainfall and calibrated antecedent rainfall and landslide occurrences. A part of Coonoor Taluk in the Western Ghats (i.e.) three micro-watersheds were selected for the study taking into account the landslide density and its socio-economic importance. Major landslide events and multiple landslides that were triggered by rainfall were considered for the study. Study results indicate that the rainfall threshold equation based on the intensity (I)–duration (D) parameters for the selected micro-watersheds was I = 46.486 * D−0.364. The study shows a clear bias towards the antecedent rainfall in initiating a landslide. The rainfall threshold determined from the study will be useful input for developing a location-specific early warning system.