Rainfall threshold for prediction of shallow landslides in the Garhwal Himalaya, India

Abstract

Rainfall is a significant triggering factor for landslides, after tectonics and structurally vulnerable lithology— particularly in Himalayan Range. Globally, extensive efforts have been undertaken to determine the specific rainfall threshold conditions that lead to the initiation of a slide on a regional scale. Rainfall-induced landslides disrupt life and cause extensive damage to properties in the Himalayan region. This study has a two-fold objective; to determine the relationship between the occurrence of landslides and a significant triggering factor, namely rainfall, based on intensity-duration (I-D) and antecedent rainfall methods, and also to determine the best fit distribution for rainfall data based on goodness of fit tests within the four western-most districts of the Garhwal Himalaya, in Dehradun, Rudraprayag, Tehri Garhwal, and Uttarkashi. The rainfall patterns of these four districts conform to the log-logistic (3P) distribution, and the rainfall threshold has been fitted over a power-law equation with the lower boundary demarcated by a quantile regression that is presented as a threshold with an established relationship of y(I)=1.38D−0.126 (I=rainfall intensity, D=duration). The results suggest that a rainfall intensity of 0.45-0.50 mm per hour over short durations (48 h) has the potential to trigger landslides in this region. Antecedent rainfall of around 80 mm in the 15 days prior to a landslide event significantly raises the landslide risk. Further, lithologies like mud and sandstones are highly susceptible to landslides and can be triggered by rainfall of 10–20 mm occurring, consecutively, over a 5-day period.