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Tinley Nangai
The National Centre for Hydrometeorology (NCHM) concluded that the flash floods in Deqin Choling on August 10 were most likely caused by convective thunderstorms.
These thunderstorms typically occur in the summer and spring and are the result of a combination of moisture, instability, and lift. Several conditions must be met for a convective thunderstorm to form: a surface dew point of at least 55 degrees Fahrenheit, atmospheric instability, and enough lift to overcome low-level convective suppression.
Instability occurs when a parcel of air is warmer than the surrounding air and rises on its own, and the lift allows air in the lower troposphere to overcome low-level convective suppression.
On August 13, a technical team from the Ministry of Hydrogeology and Mines inspected the affected watersheds. Their survey revealed that heavy rainfall saturated the soil, leading to landslides. These landslides, as well as temporary dams created by debris and blockage of tributaries caused by landslides, significantly exacerbated the impact of the floods.
However, the flood situation was made worse by landslides and the creation of temporary dams due to large amounts of debris from at least one tributary flowing into the main river channel, according to the National Hydrogeological Survey.
NCHM observed that there were five tributaries on the left side of the main stream and two tributaries on the right side. Most of the tributaries also showed signs of flooding.
Contrary to local rumors that the floods were caused by a lake outburst, the National Hydrogeological Survey confirmed that aerial images showed no such lake upstream. The catchment area is 17 square kilometers and is densely forested.
According to the NCHM, Dechen Choling is located on the alluvial fan of the Dechen Choling River and there is evidence that Dechen Choling has experienced flooding in the past.
NCHM director Karma Dupchu announced plans to install a manual warning system upstream of Dechencholing to provide quick information to residents in future.
Toeb Karma, head of the NCHM Cryosphere Service, an expert said that flash floods in the country should be understood in the context of climate change.
He noted that many of Bhutan’s settlements are located near rivers, which increases the country’s vulnerability, adding that the natural flow of rivers should not be disturbed by development activities.
“While there is little we can do to prevent flash floods, we must learn to adapt by incorporating scientific information into development activities,” Tob-Karma said. “Understanding and preventing these events using accurate scientific data is critical to minimizing future impacts.”
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