𝐂𝐚𝐭𝐚𝐬𝐭𝐫𝐨𝐩𝐡𝐢𝐜 𝐂𝐨𝐧𝐟𝐥𝐮𝐞𝐧𝐜𝐞 𝐨𝐟 𝐍𝐚𝐭𝐮𝐫𝐚𝐥 𝐅𝐚𝐜𝐭𝐨𝐫𝐬 𝐔𝐧𝐥𝐞𝐚𝐬𝐡 𝐭𝐡𝐞 𝐃𝐞𝐜𝐡𝐞𝐧𝐜𝐡𝐨𝐥𝐢𝐧𝐠 𝐅𝐥𝐚𝐬𝐡 𝐅𝐥𝐨𝐨𝐝

…𝑹𝒆𝒑𝒐𝒓𝒕 𝑯𝒊𝒈𝒉𝒍𝒊𝒈𝒉𝒕𝒔 𝑺𝒆𝒗𝒆𝒓𝒆 𝑮𝒆𝒐𝒍𝒐𝒈𝒊𝒄 𝒂𝒏𝒅 𝑯𝒚𝒅𝒓𝒐𝒍𝒐𝒈𝒊𝒄 𝑽𝒖𝒍𝒏𝒆𝒓𝒂𝒃𝒊𝒍𝒊𝒕𝒊𝒆𝒔 𝒊𝒏 𝒕𝒉𝒆 𝑹𝒆𝒈𝒊𝒐𝒏

By Yeshey Lhadon
Thimphu

The flash flood that ravaged Dechencholing on August 10, 2024, has been attributed to a deadly combination of severe weather, geological instability, and challenging topography. This conclusion comes from an extensive assessment released by the National Centre for Hydrology and Meteorology (NCHM) and the Department of Geology and Mines (DGM), Ministry of Energy and Natural Resources (MoENR). The report provides a detailed analysis of the causes and impacts of the disaster, emphasizing the urgent need for improved monitoring and mitigation strategies in this increasingly vulnerable region.

The serene landscape of Dechencholing was transformed into a scene of devastation on the afternoon of August 10, 2024, when a flash flood tore through the area, leaving destruction in its wake. This catastrophe was not the result of a single cause but rather a confluence of natural factors, each contributing to the flood’s ferocity and the resulting damage. Today, a joint report from the National Centre for Hydrology and Meteorology (NCHM) and the Department of Geology and Mines (DGM) sheds light on the complex interplay of weather and geological conditions that led to this disaster.

The report, which was the result of an extensive investigation by two technical teams comprising seven experts, presents a grim picture of the vulnerabilities in the Dechencholing region. Their investigation involved an on-site visit to the watershed along the Dechencholing Chhu, the stream at the heart of the disaster. In addition to fieldwork, the experts analyzed satellite images and weather data from the day of the flood, striving to understand the sequence of events that culminated in the tragedy.

One of the most significant findings of the report is the role played by a convective thunderstorm—a type of weather phenomenon that is both common in mountainous regions like Bhutan and notoriously difficult to predict. “The intense heat in the morning, combined with favorable wind conditions, led to the formation of the convective thunderstorm,” explained one of the assessment team members.

Convective thunderstorms are known for their sudden onset and the heavy rainfall they bring, often accompanied by strong winds and sometimes hailstones. According to the NCHM, the amount of rainfall in the region this year has been within the usual range. However, the report notes a concerning rise in temperature, which likely contributed to the intensity of the storm. “We presume that it wasn’t a cloudburst but rather a convective thunderstorm. During the incident, the temperature reached a peak, which may have fueled the storm,” the team member elaborated.

Meteorology Officer Dechen Lhamo Gyeltshen from the NCHM added further insight, explaining the dynamics of such storms. “Convective thunderstorms are particularly hazardous because they are driven by strong heating from the sun during the day. This heating causes air to rise, condense, and form thunderclouds. These clouds are often tall and laden with moisture, leading to intense rainfall in a very short period,” she said.

The consequences of this meteorological event were evident in the hours that followed. The Dechencholing Chhu, which drains a catchment area of approximately 17 square kilometers, swelled rapidly as the storm’s rainfall poured into its tributaries. Although the stream is typically fed by rainwater and lacks significant lakes upstream to regulate its flow, the storm’s intensity was more than enough to overwhelm the system.

The topography of the Dechencholing region, characterized by steep slopes and deep valleys, further exacerbated the situation. The report highlights that the steep terrain significantly increased the velocity of water flow in the Dechencholing Chhu. While such topographical features are typical of mountainous regions, they also enhance the stream’s capacity to transport large boulders and coarse sediment. This characteristic, combined with the intense rainfall, created a perfect storm that unleashed destructive flooding.

The geological instability of the area was another critical factor. The slopes surrounding the Dechencholing Chhu are highly prone to erosion, particularly along the riverbanks. Over time, the river has carved deep valleys and gorges into the landscape, further destabilizing the slopes and making the area more vulnerable to landslides. Field investigations by the technical teams revealed numerous instances of slope instability, with toe erosion, where the base of a slope is undermined by river action, being particularly problematic.

Rinzin Wangmo, a geologist from the DGM, described one of the more significant findings: “At the confluence of two major tributaries, we observed a substantial landslide that had carried loose debris, including boulders and logs, into the main river. This material temporarily blocked the river’s flow, creating an artificial dam. When this dam eventually gave way, it unleashed a powerful surge of water downstream, leading to the widespread destruction we witnessed.”

This phenomenon of temporary damming was a recurring theme in the report. Several landslides occurred at key confluence points along the Dechencholing Chhu, each contributing to the flash flood’s overall severity. One tributary, marked as stream 3 in the report, was identified as a significant source of debris, while another, stream 1, exhibited high turbidity, indicating that it was likely the primary source of the floodwaters.

The hydrological analysis conducted by NCHM revealed that under normal conditions, the Dechencholing Chhu experiences an average lean flow of 0.134 cubic meters per second (cumecs). However, on August 11, this flow increased dramatically, reaching 0.5 cumecs due to sustained monsoon rains. At the height of the flood, the peak discharge was estimated at 120 cumecs, an increase that was corroborated by aerial imagery and preliminary hydraulic modeling.

This dramatic surge in discharge was primarily driven by the convective thunderstorm on August 10, which brought sudden and heavy rains to the region. The report emphasizes that the preceding monsoon rains had already saturated the soil, making the area particularly susceptible to flooding and landslides. High water marks observed on all tributaries indicate that the combination of intense rainfall and saturated soil conditions was enough to trigger widespread landslides, which in turn contributed to the flash flood. There stream has approximately five tributaries.

However, the lack of hydro-meteorological observatories in the catchment area meant that the exact amount and duration of the rainfall leading to the flood could not be precisely determined. Local accounts suggest that the heavy rainfall began around 2:30 PM on August 10 and lasted for 45 to 60 minutes.

The team relied on the hydro-met station in Babesa to assess real-time data on rainfall, river flow, and other critical variables. Having hydro-met observatories will enable more accurate forecasts and early warnings, reducing the impact of natural disasters.Although the team recommends the establishment of hydro-meteorological stations in the Dechencholing catchment in future, the sustainability of the station is questionable with limited human resource.

On August 11, NCHM also conducted an aerial survey of the Dechencholing area using drones. This survey was crucial for assessing the full extent of the flood damage and identifying the areas most severely affected. The aerial images provided critical data on the flow patterns of the floodwaters and pinpointed the locations where temporary dams had formed.

According to the report, the flash flood was likely initiated by a significant landslide at the confluence of two major streams. This landslide involved a large amount of loose debris and boulders, which temporarily blocked the river’s flow, creating an artificial dam. When this dam eventually gave way, it unleashed a powerful surge of water downstream, resulting in widespread destruction.

The report concludes that the Dechencholing flash flood was primarily caused by a combination of intense rainfall from a convective thunderstorm, steep topography, and unstable geological conditions. The presence of loose debris in the area, which formed temporary dams during the flood, significantly increased the disaster’s magnitude.

The report by NCHM and DGM is not just an account of what happened, it also offers a clear roadmap for mitigating the risks associated with such disasters in the future. One of the most urgent recommendations is the installation of a manual early warning system with a siren, particularly for communities living along small streams and rivers. “Luckily, the recent flood occurred during the daytime. If it had happened at night, the devastation would have been far greater,” said Karma Dupchu, Director of NCHM. “Our team strongly recommends the installation of sirens to provide early warning.”

In addition to early warning systems, the report also suggests the establishment of hydro-meteorological observatories in the Dechencholing catchment area. These observatories would provide real-time data on rainfall, river flow, and other critical variables, enabling more accurate forecasts and early warnings. However, the report acknowledges the challenges associated with this recommendation, particularly the limited human resources available to maintain such stations.

Sangay Dorji, Chief of the Meteorological Services Division, emphasized the importance of vigilance, particularly during the monsoon season. “People need to be vigilant. With consistent rainfall over the past week, there is a high likelihood of further landslides and the formation of artificial lakes, which could lead to more flash floods,” he warned.

The broader context of climate change also looms large in the report’s findings. Bhutan’s first glaciologist, Toeb Karma from NCHM, hinted at climate change as a possible factor behind the recent flood. He pointed out that global climate change is expected to increase both temperatures and precipitation levels, leading to more frequent and severe weather events. “We can only expect floods to become more frequent in the future as a result of climate change,” he said. “People need to be prepared and adapt to these changes, including rethinking settlement patterns and infrastructure development plans.”

As the residents of Dechencholing and other vulnerable regions rebuild in the wake of this disaster, the lessons learned from the flood will be crucial in shaping future preparedness and response efforts. The Dechencholing flash flood serves as a stark reminder of the challenges posed by Bhutan’s mountainous terrain and the increasing frequency of extreme weather events due to climate change.

The report by NCHM and DGM provides a detailed understanding of the causes behind the disaster and offers practical recommendations to prevent such occurrences in the future. However, the success of these recommendations will depend on prompt action and sustained efforts to enhance the resilience of communities in vulnerable regions.