CASE HISTORY Automatic calculation of rainfall thresholds for landslide occurrence in Chukha Dzongkhag, Bhutan Stefano Luigi Gariano 1 & Raju Sarkar 2,3 & Abhirup Dikshit 2 & Kelzang Dorji 2 & Maria Teresa Brunetti 1 & Silvia Peruccacci 1 & Massimo Melillo 1 Received: 17 July 2018 /Accepted: 20 October 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018 Abstract Bhutan is highly prone to landslides, particularly during the monsoon season. Several landslides often occur along the Phuentsholing–Thimphu highway, a very important infrastructure for the country. Worldwide, empirical rainfall thresholds represent a widely used tool to predict the occurrence of rainfall-induced landslides. Nevertheless, no thresholds are currently designed and proposed for any region in Bhutan. In this work, we define empirical cumulated event rainfall–rainfall duration thresholds for the possible initiation of landslides using information on 269 landslides that occurred between 1998 and 2015 along the 90-km highway stretch between the towns of Phuentsholing and Chukha, in southwestern Bhutan, and daily rainfall measurements obtained from three rain gauges. For this purpose, we apply a consolidated frequentist method and use an automatic tool that identifies the rainfall conditions responsible for the failures and calculates thresholds at different exceedance probabilities and the uncertainties associated with them. Analyzing rainfall and landslide data, we exclude from the analysis all the landslides for which the rainfall trigger is not certain, so we reduce the number of landslides from 269 to 43. The calculated thresholds are useful to identify the triggering conditions of rainfall-induced landslides and to predict the occurrence of the failures in the study area, which is, to date, poorly studied. These rainfall thresholds might be implemented in an early warning system, in order to reduce the risk posed by these phenomena to the population living and traveling along the investigated road stretch. Keywords Landslide prediction . Rainfall events . Road . Triggering rainfall . Himalaya Introduction Worldwide, landslides are widespread and hazardous phenome- na, threatening population, infrastructures, and properties (Petley 2012; Haque et al. 2016; Froude and Petley 2018). Rainfall is the main trigger of landslides; therefore, defining the rainfall condi- tions responsible for landslide occurrence in a given area is an important scientific and social issue. Rainfall thresholds are the most commonly adopted tools to define quantitatively the rainfall conditions that, when reached or exceeded, are likely to trigger landslides. There is a huge amount of literature on the statistical analysis of the rainfall conditions that have triggered landslides and on the methods used to calculate rainfall thresholds all over the world (e.g., Brunetti et al. 2010; Cepeda et al. 2010; Tiranti and Rabuffetti 2010; Saito et al. 2010; Peruccacci et al. 2012; Staley et al. 2013; Chen and Wang 2014; Zêzere et al. 2015; Rosi et al. 2016; Lainas et al. 2016; Li et al. 2017; Soto et al. 2017; Marjanović et al. 2018; Martinović et al. 2018; Valenzuela et al. 2018). Diverse procedures and methods were presented to select rainfall parameters, to collect landslide data, and to define, cali- brate, and validate rainfall thresholds. Overall, the widely used empirical rainfall thresholds are relationships linking rainfall con- ditions or variables to landslide occurrences. Most of the thresh- olds present in the scientific literature link the duration of the rainfall (D, usually expressed in h) to the cumulated event rainfall (E, in mm) or to the mean rainfall intensity (I, in mm∙ h − 1 ) of known rainfall events that have triggered landslides (Guzzetti * Stefano Luigi Gariano stefano.gariano@irpi.cnr.it 1 Research Institute for Geo-Hydrological Protection, Italian National Research Council (CNR-IRPI), Perugia, Italy 2 Center for Disaster Risk Reduction and Community Development Studies, Royal University of Bhutan, Rinchending, Bhutan 3 Delhi Technological University, Shahbad Daulatpur, Delhi, India Bulletin of Engineering Geology and the Environment https://doi.org/10.1007/s10064-018-1415-2