**2. Study Area**

Bhutan is situated in the eastern section of the Himalaya with an area of 38,394 km<sup>2</sup> (Figure 1). The nation is enclosed with the Tibetan Plateau in the north and Indian states Arunachal Pradesh in the east, Bengal and Assam in the south, and the Darjeeling–Sikkim Himalayas in the west. Bhutan is divided into 20 Dzongkhags and has elevations varying from 150 m to 7570 m [30]. The elevation of the present study area Chukha Dzongkhag ranges from 1000 m to 4200 m. Most of the people in the region depend on agriculture and livestock for their livelihood.

(**b**)

**Figure 1.** Location of (**a**) Bhutan; and (**b**) elevation map of Chukha Dzongkhag depicting rain gauge and landslide locations.

Geologically, the Chukha region belongs to the Lesser Himalayan formation which includes a wide variety of sedimentary and low-grade metamorphic rocks. The study area is mainly comprised of tectonically active metasedimentary rocks such as phyllite, schist, quartzite, and limestone. The northern part belongs to the Higher Himalayan crystalline rocks comprising mainly of garnetiferous mica-shist, quartzite, and gneiss. The soils in the region are mostly moderate to high weathered and are comprised of weaker fracture phyllites, which make the soil texture very fine, and slopes are very unstable [25]. Based on the practical experiments, the soil can be classified as poorly graded sand (SP) mixed with pebbles. The geology of the region described in [31]. The tectonic setting of the region is very similar to Nepal and Indian Himalayas [25]. The morphology of the unstable slopes leading to failures are complex and managed by several factors including lithology and rock type.

The region receives yearly rainfall of 4000–6000 mm with heavy bursts reaching up to 800 mm/day [18]. More than three-quarters of the annual rainfall occurs during the monsoon period of June to September [25]. Figure 1b depicts the landslide occurrences between 2004–2014 and the rain gauge coordinates used for the analysis. The monsoon rainfall leads to erosion of rocks causing widespread slope instability and mass movements. The increase in anthropogenic activities has escalated deforestation leading to slope instability. The landslide typology in the region can be described as rock fall, rockslide, debris flow, debris slide, and earth slide [25]. The major effects due to this monsoonal rainfall of this region are roadblocks. As mentioned earlier, the Phuentsholing-Thimphu highway (also known as Asian Highway) situated in Chukha Dzongkhag is one of the major road connections of Bhutan, which is key to the trade, passes through these landslides affected areas. The landslide events along the highway pose serious logistic problems as it affects the economy of the region. Figure 2a,b illustrates the damage caused by landslides along the highway.

**Figure 2.** Landslide damages in Chukha Dzongkhag along Phuentsholing-Thimphu Highway after (**a**) 2016 and (**b**) 2017 monsoon.
