Dharali flash flood: 360 million cubic metres of glacial debris came tumbling down, experts estimate

Time of India

17 hours ago

DEHRADUN: It wasn't just the rain. A preliminary geological assessment conducted after Tuesday's devastating flash flood in Dharali village, suggests a far more complex and violent trigger - a massive collapse of glacial sedimentary deposits, possibly caused by a retrogressive slope failure high up in the mountains.
Initial estimates made by experts, based on satellite data and terrain analysis, point to an avalanche of nearly 360 million cubic metres of debris. For perspective, a quick, back-of-the-envelope calculation reveals the figure is roughly equivalent to over 1.4 lakh Olympic-sized swimming pools filled with mud, rocks and glacial debris simultaneously hitting the village at high speed. With such an impact, it's not surprising that Dharali did not have a chance.
The avalanche was mobilised by a sudden release of unstable morainic and glacio-fluvial material-thundering down the Kheer Gad stream into Dharali. The surge reached the village within seconds, flattening over 20 structures and claiming lives of at least four people (as per official figures).
Imran Khan, geologist and head of the geology division at Bhutan's Punatsangchhu-I hydel project, who studied satellite imagery of the area, said "this wasn't a typical cloudburst".
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"The event likely involved a massive detachment of unconsolidated glacial deposits, perched roughly 7 km upstream at an elevation of 6,700m. Heavy rainfall may have acted as the trigger-but the disaster was waiting to happen."
The glacial deposit in question, spanning over 1.1 sqkm with an estimated vertical thickness of 300m, is located in a hanging trough - an inherently unstable geological feature. The steep terrain, combined with narrow, pre-incised channels in the Kheer Gad stream, allowed the debris to accelerate at terrifying speeds, leaving virtually no time for residents to escape.
"The stream draining this area has a high longitudinal gradient, limited lateral confinement, and sharp incision paths-all of which contribute to rapid debris flow mobilisation," Khan wrote.
He added surface runoff and percolation, following days of intense rain, may have loosened the sediment, triggering the collapse.
Rajiv Saran Ahluwalia, geologist at Doon University, told TOI that at velocities of 6-7m per second, debris-laden flows are capable of destroying any structure in their path.
"And if the velocity doubles, the debris-carrying capacity increases by a factor of 64," he said. Experts believe glacial melt, intensified by elevated temperatures in the days preceding the disaster, may have compounded the instability.
A senior glaciologist said the precise cause will only be known once real-time satellite imagery - captured just before and after the event - is analysed or a ground team conducts a survey upstream.
"It appears the flash flood originated across three narrow valleys, with the most violent surge occurring in the Kheer Gad. Something extraordinary happened up there. It needs urgent investigation."
Geologists have for long been calling for a ban on construction along steep nala corridors and debris flow paths, particularly in pilgrimage-centric regions like Gangotri. "We can no longer afford to ignore hidden upstream hazards - especially in zones with growing human and pilgrimage footprints," said a geologist involved in the assessment.