**2. Study Area**

The study area was the Samdrup Jongkar–Trashigang (S-T) highway, which is a 180 km stretch of road located in the eastern part of Bhutan, which covers 1880 km2 (Figure 1). The region was selected as it connects eight districts (known as "dzongkhags" in Bhutanese) and is a major route for the people residing around the highway. The highway is critical as it is the only transportation network connecting eight dzongkhags in East Bhutan, and it is a lifeline for the people residing in these areas. The transportation corridor has a history of severe slope failures in the form of frequent landslides, rockfalls, and mudflow. The region falls in a sub-tropical zone, where heavy rainfall is frequent. The region receives its maximum rainfall during the monsoon seasons (June–September). The rainfall pattern in this region can be described as low-intensity and long-duration with occasional intermittent outbursts.

**Figure 1.** Location of (**a**) Bhutan; (**b**) spatial distribution of landslides in the study area considered in this analysis. The zones were categorized based on spatial coverage of rain gauges, rain gauge location, and elevation difference.

The area includes five different geological groups, which are Baxa Group, Daling Shumar Group, and the Greater, Lesser, and Sub-Himalayan zones. The geology varies from the Baxa Group in the south to the Greater Himalayan Zone in the north. The rocks found in the region are predominantly dark grey to green fine-grained phyllites, slate varying from dark brown to black, and fine-grained, mediumto thick-bedded quartzite with thin to very thin grey-black fine-grained phyllite interbeds [34,35]. The average thickness of the quartzite is about 100 m, but the individual bands of quartzite range from 10 cm to 2 m. The orientation of the latter is 48◦ NW, with an average dip 40◦ towards the slope direction of the slide. The quartzite in the landslide area undergoes brittle deformation with many irregular joints [36]. Figure 2 depicts examples of the damage caused by landslides along the highway.

(**a**)

(**b**)

**Figure 2.** Landslide damage: (**a**) landslide at 68.1 km along the Samdrup Jongkar–Trashigang (S-T) road (N 26.903◦, E 91.505◦) (5 July 2016); (**b**) landslide at 93.8 km along the S-T road (N 27.112◦, E 91.544◦) (17 July 2016) [36].
