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NDTV
08-08-2025
- Climate
- NDTV
What Caused Uttarkashi Flash Floods? ISRO Images, Expert Papers Reveal Clues
The devastating mudslide that killed five people and left over 50 people missing as it swept through the Dharali area in Uttarkashi has left the region reeling. As rescue and relief operations continue, a new debate has emerged among experts and glaciologists: what triggered this deadly natural disaster? The answer, it seems, lies in a combination of the region's unique geology and the growing impacts of climate change. According to a detailed report, the village of Dharali is built on an 'alluvial fan,' a cone-shaped landform created by debris from past floods at the confluence of the Kheer Ganga stream and the Bhagirathi River. Despite the village being 200 metres from the river, commercial development has encroached directly onto this unstable landform, increasing its vulnerability. The Landslide Theory: A Reawakened Threat One of the leading theories is that the mudslide was a reactivated landslide. A glaciologist has pointed to a previous landslide that occurred 2 kilometres uphill from Dharali, suggesting it may have been triggered again. This idea is backed by findings from a paper, Bhambri R et al, High Mountain Hazards in Uttarakhand, which provides a historical context. The paper notes that following heavy rainfall in the Bhagirathi Basin in June 2013, a staggering 533 existing landslides were reactivated. The areas closest to Bhatwari, Pilang, Dharali, and the Tehri dam sites were hit the hardest. The paper explains that this region is prone to such events due to its geology, steep terrain, and unpredictable weather. Glaciers and a "Breached" Lake: A New Threat? Another powerful hypothesis points to a potential glacial lake outburst. Using satellite imagery and terrain analysis, glaciologists found two glaciers with huge crevasses approximately 7 kilometres above Dharali. The presence of these crevasses, they say, could be speeding up the melting process. Below these glaciers lie seven smaller lakes. Experts believe that rising temperatures could have caused one of these lakes to breach, sending a torrent of water, rock, and debris-known as moraines-downstream. The sheer destructive power of this debris-filled flow could be the reason for the extensive damage. ISRO Images Capture the Aftermath Adding weight to the analysis, ISRO has released satellite images that show the scale of the destruction, which show Dharali Village both before and after the event. The image from August 7, 2025, specifically reveals a significant deposition of sediments and mudflows near the Bhagirathi and Khera gad Rivers, painting a clear picture of the aftermath of the cloudburst. The Orographic Twist: Rain, Not Snow A third, less-widely accepted theory, introduces the concept of an "orographic barrier." A scientist suggested that a mountain peak nearly 5,700 metres high could have blocked moisture-laden clouds. This would have caused the clouds to condense and release heavy rain in the upper areas, rather than snow. The scientist argued that this phenomenon could have caused floods in Kheer Ganga, Harsil, and Sukhi top. This theory challenges the glacial breach hypothesis by suggesting that a breach would have only affected the Kheer Ganga catchment. While the debate continues, what is clear is that the tragedy in Uttarkashi is a stark reminder of the fragile ecosystem of the Himalayas and the urgent need to understand and address the risks posed by climate change and unsustainable development.
Indian Express
07-08-2025
- Indian Express
What caused the Uttarkashi flash floods? Some theories
While the exact cause of the flash floods in Uttarakhand's Dharali remains unclear, experts and glaciologists have proposed various hypotheses to explain the massive mudslide that ravaged the area. The affected area in Dharali is located on an alluvial fan, a geological feature formed by debris from past flood events. Specifically, earlier floods at the confluence of the Kheer Ganga stream and the Bhagirathi River led to sediment deposition at the river mouth, shaping this fan. Although the village is situated around 200 metres away from the river, increasing commercialisation in recent years has led to the development of several establishments directly on the alluvial fan, experts say, pointing to the growth of tourism-related activities and the construction of the Dharasu-Gangotri highway through the village. A glaciologist said that a landslide had struck 2 kilometres uphill from Dharali earlier, which might have been reactivated. In a paper by Bhambri R et al, High Mountain Hazards in Uttarakhand, covering hazards, including glacier lake outburst floods, rainfall-induced landslides, and flash floods, and their influence on downstream areas in Uttarakhand, the authors identified 2,772 landslides over an area of 15.9 sq km. Following the June 2013 rainfall in the Bhagirathi Basin, 1,434 new landslides and 533 existing landslides were activated. The locations closest to the Bhatwari, Pilang, Dharali, and Tehri dam sites observed the most landslides. New ones covered 32% of all landslides, compared to reactivated landslides, which accounted for 39% of all, the paper says. With the help of satellite imagery and terrain analysis, the glaciologist said that further above this slope, around 7 km above Dharali, there are two glaciers with huge crevasses, which cause expedited melting. 'They are also avalanche-feeded glaciers. Below these are seven small lakes. These glaciers could have breached, went downstream, broke out as the temperature is higher and the water is moving downwards, bringing with it sedimentary deposits and moraines, rock and debris transported from the glaciers. From what we gather from videos, debris per cent is higher in the flow that struck, and debris erosion has destructive potential,' he said. While IMD data shows moderate rainfall, the scientist said that the orographic barrier must have played a part in decreased precipitation in areas in lower elevation. 'Some heights block moisture. Due to the height of a particular peak, which is around 5,700 metres above sea level, it did not let this cloud pass it, resulting in the accumulation of moisture. Due to increased condensation of the cloud as well as a rise in temperature, the cloud precipitated in the form of rainfall and not snow. Kheer Ganga, Harsil, and Sukhi top could have flooded because of this phenomenon,' he said, adding that this rules out glacier breach as that could have flooded just Kheer Ganga, without causing floods in Harsil and Sukhi top. These three tributaries joining Bhagirathi river downstream have different catchments. 'Orographic barrier and condensation led to lower elevation receiving less rainfall in comparison to the upper areas,' he said. Aiswarya Raj is a correspondent with The Indian Express who covers South Haryana. An alumna of Asian College of Journalism and the University of Kerala, she started her career at The Indian Express as a sub-editor in the Delhi city team. In her current position, she reports from Gurgaon and covers the neighbouring districts. She likes to tell stories of people and hopes to find moorings in narrative journalism. ... Read More
