There is a need to have a landslide inventory study and mapping done for Thimphu to identify risk-prone areas and prevent landslides.
Urban Planner of Thimphu thromde, Kezang Dorji said that landslides are a natural phenomenon but earthquakes, floods and human activities could also trigger it. “Most areas where there is development in Thimphu are on the alluvial fan deposit site, formed by several layers of debris containing different elements such as twigs, mud and sand. It is risky when there is heavy rainfall as these kinds of land are more suitable for agricultural purposes.”
He said that slump, flow and torrent are some of the common types of landslide that occur in the country. Slump and flow are common in Thimphu.
He said that landslides are classified into rock falls, which are immediate, and slide-type landslides that are slow. He added that site-specific condition needs to be assessed (stability and foundation study) and construction should be approved only after conducting proper study of the area.
A study was conducted in five specific areas – Dechenphodrang, Changangkha, Chubachu area, Olakha (Bhutan Power Corporation) area and TechPark.
He said that in Dechenphodrang, landslide occurred after building a hostel and kitchen. A report from geology department stated that there were moderately weathered granites. “Stones which were about to be turned into soil were present which are fragile.” He said that the area has already met water saturation, which makes the soil unstable.
In Changangkha area above the Lhakhang, there are settlements on slope, which is about 28 degree. “After 30 degree, development is not encouraged as slopes are steep which is risky when it comes to soil stability.” He added that the area is risky for constructing structures as active soil erosion were also revealed by the geology report.
Department of geology and mines found that the topsoil of Chubachu area is about 25cm alluvial and loose soil were found a metre below the topsoil.
Above Olakha, he said that when the BPC colony was about to be constructed, a test pit done found a crack formed between the two layers of soils. “If an earthquake occurs, there are chances of the crack widening and causing landslide.”
He added that the crack expansion would affect nearby settlement and that the soil stability is compromised. Recently, the soil in the area near the TechPark sank and there was significant seepage of water loosing the soil, he said.
Kezang Dorji said that no study has been conducted on landslide and mapping for the capital. “There needs to be a study conducted so that we can use mitigation measures to avoid landslides and its impacts.”
He said that specific mitigation is required for specific areas as nature of slope varies.
To identify landslide prone areas, it was important to know where accessibility could be affected such as the Thimphu and Phuentsholing highway in case of a major event, which could trigger landslides.
The findings included relating landslide with settlement pattern, where the prioritisation of landslides could be rated based on the number of population that could be affected in the event of a major landslide.
Thrompon Kinlay Dorjee said that the land in Thimphu is fertile and before the constructions began, people cultivated crops, which was a major source of livelihood. “Now, people buy a plot of land and they wish to build five-storey buildings without knowing the consequences.”
He said that without permission, thromde would not allow people to construct structures wherever they wish to avoid damages due to landslides.