[H-GM03] Geomorphology

[HGM03-08] Vulnerability to Geomorphic Disasters in the Thimphu Valley, Capital of Bhutan

*Takashi Nakata¹, Hiroshi Inoue², Karma Namgay³ (1.Hiroshima Univ., 2.Kyoto Univ., 3.DGM Bhutan)

Keywords: Bhutan, micro-landform, geomorphic disaster

Based on interpretation of the anaglyph image created from the ALOS 30 DSM and the large-scale aerial photograph about 1/5000 taken in 2004 with short-term field survey, we prepared a micro-topographical classification map in order to evaluate the topographical disaster environment of the Timphu basin. The mountains surrounding the Thimphu basin also show evidence of glacial and periglacial erosion, and the gentle slopes are predominant. Such slopes are dissected to form steep and deep valleys, which develop debris flow fans at the down-stream. The debris flows that make up the city central of Thimphu can be divided into two levels: the old debris flow in the upstream and the new debris flow in the downstream that flowed down through the valley that dissected the old one. In addition, several large debris flow developed along the left bank (east bank) of the Thimphu River. On the other hand, around the Indian House in the northern part of Thimphu, a fluvial fan has developed near the confluence of the Samtelingchu that also originates its headwaters in the glacier area and Thimphu rivers. In addition, along the river, small river terraces develop in places. Furthermore, on the gently sloping surface of the paddy fields around Dzon (county office) in the northern part of the city, lacustrine sediment consisting of sand and silt layers is widely recognized. It suggests that an extensive lake was once formed by dammed-up probably due to debris flow in downstream. The altitude of the old shoreline of the lake is about 2350m, which is about 30m higher than the surrounding current riverbed (2320m). At the outcrops at the exit of a small valley on the opposite bank of the Farmers Market on the Thimphu River, a debris flow deposit appears around 2350 m that is more than 30 m higher than the riverbed (2315 m), and covers cover well-rounded gravel layer of the Thimphu River. Therefore, the debris flow deposits from the branch blocked the Thimphu River and formed a temporary lake reaching Dzong and beyond. Along the current river channel, floodplain with abandoned channel is found around the royal palace and on the eastern edge of the city, indicating that the river shifted its channel within a limited range.
Regarding natural disasters around Thimphu, there is a report on the evaluation of flood danger (Merzet al., 2007). The report points out the possibility of flooding of the royal palace, but overlooks the fact that the royal palace is located in an abandoned channel lower than the surroundings, and underestimates the inundation depth. This report also points out that the Chubachu flowing through the city center of Thimphu may cause also heavy damage to the minister's collective settlement by flooding but not by debris flow. Therefore the risk estimation by only flood is fundamentally wrong. In the city of Thimphu, debris flow boulders are found extensively. Multiple units of debris flow deposits are found at the outcrops of excavation work, and so it is clear that debris flow occurred repeatedly. In addition, the Diet Building is located at the exit of the debris flow mountain stream, and the risk is extremely high. In the Thimphu basin, relatively thick unconsolidated sediments are distributed. Large-scale development is progressing around the Dzong, and there is concern about damage to houses due to amplified seismic motion and surface failure in the area of the lake deposits.