Uncertainty prevails over the cause of Uttarkashi flash floods

The Hindu

5 days ago

Days after heavy rains triggered a flash flood and an avalanche of debris that deluged Dharali in Uttarkashi, Uttarakhand, uncertainty prevails over what caused the disaster.
As of Thursday (August 7, 2025), rescue and relief operations in Uttarkashi to extract those stranded, and accounting for those missing, has meant that scientific investigations into the cause of the disaster have not yet begun.
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'We are planning a reconnaissance but, currently, accessing the sites is difficult. Rainfall data that we have from the India Meteorological Department (IMD) suggests that this was not a cloudburst,' Vineet Kumar Gahalaut, Director, Wadia Institute of Himalayan Geology, Dehradun, said. 'It is possible that there is a glacier lake overflow or heavy rain triggering a landslide of accumulated silt and debris,' he said.
The IMD too has characterised 'extremely heavy rainfall' from August 3-5 as a probable cause, but not a cloudburst.
A 'cloudburst' is a more extreme form of rainfall, defined by the IMD as exceeding 10 cm in an hour over a 10 sq. km. grid, and it results from a build-up of moisture forming large cumulonimbus clouds that suddenly release all their water.
These causes matter as rising instances of Himalayan disaster precipitated by extreme rainfall and changing climate help better understand potential causes, and design warning systems.
While cloudbursts aren't unexpected given the prevailing monsoon conditions, they are challenging to forecast, and confirming them requires specific instruments, including the Doppler Weather Radar (DWR) that can image them, as well as Automatic Weather Stations (AWS) that can provide hourly data on the rainfall, temperature, wind, and humidity. There are three DWR (DWRs) in Uttarakhand but even between them, they leave vast stretches of the Upper Himalayas unmapped. Data from the AWSs is not publicly shared.
Rainfall data from the IMD that is publicly shared only indicates eight-hourly or 24-hour rainfall data. On August 5, the day of the Uttarkashi deluge, rainfall data for Uttarakhand showed, for instance, that Haridwar (Haridwar district) reported 30 cm of rain over the previous 24 hours; Narendranagar (Garhwal Tehri district) reported 17 cm; Rishikesh (Dehradun district) 14 cm; Jolly Grant (Dehradun district) 13 cm; and Kotdwara (Garhwal Pauri district) 12 cm. While this is significant rain, it isn't known whether this was concentrated over a short duration or averaged over a day.
'While the data we have so far does not indicate a cloudburst, I cannot entirely rule it out,' M. Mohapatra, Director-General, IMD, said. 'It is theoretically possible there was a cloudburst in the upper reaches [of the Himalayas] where we do not have instruments. Satellite imagery and other remote sensing could provide some answers, and I believe that the National Disaster Management Agency is coordinating some investigation on it.'
Noida-based satellite imagery company Suhora Technologies said that a 'cloudburst' was probably a cause as their analysis showed there were no glacier-fed lakes or large water bodies that had spilled over. 'We have used synthetic aperture radar (SAR) imaging from the ICEYE satellite and checked upstream of Dharali village. There is no glacier lake or water body. We have a fairly extensive database of existing glacier lakes in the Himalayas. It does look like rainfall was a trigger,' Subhajit Bera of Suhora Technologies said, adding, 'However, we don't yet have images from the upper stretches and so don't know the exact causes yet.'
'The steep terrain around Dharali village, where the tragedy occurred, acted like a chute, accelerating mud and debris into a deadly rush downhill. Just like the 2024 monsoon tragedy in Wayanad in Kerala's Western Ghats, the Himalayas too are becoming monsoon graveyards. Climate change is intensifying these extremes, since monsoon storms are now forming in a warmer atmosphere that holds and dumps far more moisture in short bursts, causing flash floods and landslides. What's worse is that many of these regions lack real-time weather surveillance and effective early warning systems,' Akshay Deoras, research scientist at the University of Reading, said in a statement.
In February 2021, an ice-rock avalanche north of Joshimath in Uttarakhand, claimed 72 lives, with hundreds missing, and two hydropower projects destroyed or damaged. A glacier lake burst in north Sikkim in October 2023 damaged the Teesta-3 hydropower project in the State, claiming at least 40 lives.