According to a preliminary geological study, a huge chunk of unstable glacial sediment came tumbling down the slopes, possibly triggered by days of heavy rainfall. As reported by TOI, early estimates suggest nearly 360 million cubic metres of glacial material came crashing down, equivalent to filling 1.4 lakh Olympic-sized swimming pools with rocks, mud, and icy debris.
‘Not a typical cloudburst’
“This wasn’t a typical cloudburst,” said Imran Khan, a geologist heading the geology division at Bhutan’s Punatsangchhu-I hydel project. Speaking to TOI, Khan said satellite imagery revealed that the origin of the disaster lay nearly 7 km upstream at an elevation of 6,700m, where unstable glacial deposits were perched on a fragile slope.
“The event likely involved a massive detachment of unconsolidated glacial deposits. Heavy rainfall may have acted as the trigger, but the disaster was waiting to happen,” Khan added.
The debris travelled down a narrow stream called Kheer Gad, striking Dharali village in seconds. Over 20 structures were flattened, and at least four lives were lost, as per official figures.
Nature’s trap waiting to spring
The dangerous mix of steep slopes, narrow stream channels, and unconsolidated material made the region highly vulnerable. The glacial deposit that collapsed spanned over 1.1 sq km, with an estimated vertical thickness of 300 metres. It sat in what experts call a hanging trough, an unstable geological formation highly prone to collapse.As per TOI, Khan explained that the Kheer Gad stream has “a high longitudinal gradient, limited lateral confinement, and sharp incision paths, all of which contribute to rapid debris flow mobilisation.” With days of intense rain loosening the sediment, the slope finally gave way.
Speed and destruction
At speeds of 6 to 7 metres per second, the debris flow would have been impossible to outrun. Rajiv Saran Ahluwalia, a geologist at Doon University, told TOI, “Debris-laden flows at that velocity are capable of destroying any structure in their path. And if the velocity doubles, the debris-carrying capacity increases by a factor of 64.”
Experts also suspect that glacial melt, driven by recent high temperatures, may have added to the instability.
A senior glaciologist told TOI that exact causes will be clearer only after real-time satellite images are analysed or a team manages to survey the collapse site directly. “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,” the glaciologist said.
The bigger warning
Experts have long warned against construction in fragile zones, especially near steep nalas and debris paths, often seen in pilgrimage routes like Gangotri.
As one of the geologists involved in the assessment put it to TOI, “We can no longer afford to ignore hidden upstream hazards, especially in zones with growing human and pilgrimage footprints.”
Inputs from TOI
