Characteristics of Debris Flow that Crosses Bhutan’s Mountain Roads and Its Countermeasures

Most of the National Highways in Bhutan pass through mountainous areas and they are extremely fragile. At the point where the highway crosses the valley, protection or drainage facilities have not been sufficiently installed. During the monsoon season, debris flows cause road closes lasting several hours or days had a significant impact on the stability of traffic. JICA Bhutan Office is implementing a project to strengthen the capacity of the Department of Road of Bhutan for slope protection works. In this project, the authors organized the procedures and methods to introduce countermeasure facilities with reference to the countermeasure methods of Japan for debris flows. First, desk study was conducted to identify landslides, slope failure, and other landforms indicating unstable slope, and next step, fieldwork to investigate the volume of unstable sediment and the diameters of numerous boulders. Based on these studies and rainfall observation data, it was expected that the volume of debris flow, peak flow rate, and maximum diameter of boulders in debris flow flowing down from the target valley. With these results, the authors considered about the scale of countermeasures to construct safely against such debris flow, and culverts constructed at the pilot site “Bjee” in Trongsa. At the results of these studies, “Guidelines for Countermeasures against Debris Flows” were developed to mitigate road disasters in Bhutan for sustainable construction works.