Estimation of rainfall threshold for the early warning of shallow landslides along National Highway-10 in Darjeeling Himalayas

Abstract

The Darjeeling Himalayan region receives heavy rainfall during the monsoon and as a result is known for having many shallow landslides. These monsoon-induced landslides cause massive disruption to traffic on the National Highway-10 (NH-10), which serves as a life-line for the area. Developing an early warning system for landslide occurrence can contribute to minimize the risk. In the present study, an attempt is made to estimate the rainfall threshold for initiation of landslides which may help in developing an early warning system for rainfall-induced shallow landslides. To work out the threshold for rainfall in triggering landslides, a rainfall intensity-duration model has been developed for the study area. Daily rainfall data were obtained from the India Meteorological Department (IMD) rain gauge stations and adjacent tea gardens along NH-10, whereas landslide event-related information was obtained from the Public Works Department (PWD), newspapers, and earlier publications. The study was carried out separately on four lithostratigraphic units along NH-10 between 2011 and 2018. Over this time, 288 rainfall events were reported which are responsible behind triggering 681 landslides along this road corridor. The overall threshold for the entire NH-10 has been estimated as I = (20.10 ± 1.84)D(−0.45 ± 0.05), where I is the rainfall intensity (mm/d) and D is the event duration (d). The result shows that the area with a slate lithology is more susceptible to landslides than in areas with other rock types. Along the NH-10 corridor in general, over a cumulative rainfall event of 3 or 5 days before the failure, the daily rainfall on the final day is effective in triggering about 17% and 8% of landslides, respectively.