Thinley Namgay
The National Centre for Hydrology and Meteorology (NCHM) has concluded that the flash flood in Dechencholing on August 10 was most likely caused by a convective thunderstorm.
This type of thunderstorm, typically seen during summer and spring, results from a combination of moisture, instability, and lifting. For a convective thunderstorm to develop, several conditions must be met – a surface dew point of at least 55 degrees Fahrenheit, atmospheric instability, and sufficient lifting to overcome low-level convective inhibition.
Instability occurs when a parcel of air is warmer than the surrounding environmental air and rises on its own, while lifting enables air in the lower levels of the troposphere to overcome low-level convective inhibition.
A technical team from the NCHM, and the Department of Geology and Mines, inspected the affected watershed on August 13. Their investigation revealed that intense rainfall had saturated the soil, leading to landslides. These landslides, along with temporary damming caused by debris and landslide-induced blockages in tributaries, significantly worsened the flood’s impact.
However, according to the NCHM, the magnitude of the flood was aggravated due to temporary damming caused by landslides and large debris dislodged into the main river from at least one of the tributaries.
The NCHM observed that there are five tributaries located on the left side of the main stream and two on the right. There were also signs of flooding in most of the tributaries.
Contrary to local rumors suggesting the flood was caused by a lake outburst, the NCHM confirmed that aerial images showed no such lake upstream. The catchment area, covering 17 square kilometres, is well-forested.
Dechencholing is situated on an alluvial fan of the Dechencholing chhu, and there was evidence of floods having occurred in the past at Dechencholing, according to the NCHM.
NCHM Director Karma Dupchu announced plans to install a manual early warning system upstream of Dechencholing to provide quick information to the residents in the future.
The specialist and head of Cryosphere Service Division of the NCHM, Toeb Karma, said that flash floods occurring in the country should be understood within the context of climate change.
He pointed out that Bhutan’s vulnerability is heightened by the fact that many settlements are located near streams. He added that the natural course of river flows should not be disturbed by developmental activities.
“While there is little we can do to prevent flash floods, we must learn to adapt by integrating science-based information into developmental activities,” Toeb Karma said. “Understanding and preparing for these events with accurate scientific data is crucial for minimising future impacts.”