Was it a cloudburst or glacial lake burst in Dharali? Know the difference

On Tuesday (August 5), a sudden flash flood struck Dharali village in Uttarkashi district, Uttarakhand. A huge amount of water, mud, and debris swept through the area burying half the village, including hotels, homes, and shops, as seen in multiple  viral videos circulating on social media. Multiple three- and four-storey buildings collapsed like a house of cards, trapping an estimated 60-70 people. At least four people were killed, and dozens remain missing. As we publish this story, rescue operations are still underway. However, rescuers are facing increasing difficulties due to blocked roads, disrupted communications, and uneven terrain.

 

What was the initial cloud burst theory? 

Early reports attributed the deluge at Dharali village to a cloudburst, a localised, intense rainfall event defined as rainfall exceeding 100 mm per hour. Some outlets reported precipitation of over 210 mm in 24 hours, supporting this theory at first glance. However, the data from the India Meteorological Department (IMD) has disputed the claim of the natural disaster as one being caused due to sudden cloudburst. 

 

The IMD has said that the area recorded minimal rainfall: only 6.5 mm in Harsil on Tuesday, 9 mm over 24 hours in Harsil, and 11 mm in Bhatwari. Even the highest district-level rainfall was just 27 mm which is well below the thresholds which is typical of cloudburst events.

 

What are experts, IMD scientists saying? 

As more data has come in, both geologists and IMD scientists have shifted focus away from the cloudburst narrative. The IMD stressed that rainfall levels were too low to explain the scale of devastation.

 

Meanwhile, geologists, survey teams, and government agencies now believe the flood was triggered by glacial activity, especially a glacier collapse or ‘glacial lake outburst’. An emerging hypothesis involves a massive snout detachment at heights above 6,000 meters, which may have unleashed debris and water downstream. Satellite imagery shows large glaciers and at least two glacial lakes directly upstream of the disaster site, particularly above the Kheer Gad stream, The Times of India reported.

 

What is a glacial lake burst? 

A glacial lake outburst flood (GLOF) occurs when the accumulated water, often above or near a glacier, suddenly breaks through its moraine dam [a natural dam formed by glacial moraine, which is the accumulation of glacial debris (rocks, gravel, and sediment) left behind by a retreating glacier], unleashing a super-fast flow downstream. These can be triggered by rising temperatures, avalanches into the lake, seismic tremors, or unstable dam structures.

 

How is it different from a cloud burst? 

A cloudburst is described as a purely meteorological event which is a rapid, intense downpour of rain over a small area, usually exceeding 100 mm in an hour, often due to orographic uplift [process where air is forced to rise over a mountainous barrier]. Its effects are immediate and are driven by rainfall alone. 

A cloudburst is a meteorological event which is a rapid, intense downpour of rain over a small area whereas a glacial lake burst stems from geological triggers that suddenly releases stored water. (Photo| AI generated)

 

In contrast, a glacial lake burst stems from geological triggers: dam failure or glacier collapse that suddenly releases stored water. GLOFs may occur with little to no warning and can generate far more destructive force and they are not connected to rainfall intensity.

 

What triggers a glacial lake burst?

 

Several mechanisms can cause a glacial lake burst:

 

>Moraine-dammed lakes held in place by loose glacial debris can fail due to internal erosion (piping), heavy rain, or landslides.

 

>Ice avalanches, landslides, seismic activity, or rapid melt that raises water volume.

 

>Climate change is exacerbating these risks, causing glacier retreat, lake expansion, and destabilised terrain.

 

What are some examples of glacial lake bursts in India?

 

>2013 Kedarnath (Chorabari Lake) remains one of the most devastating glacial outburst events in close memory. On 17th June 2013, the moraine dam of Chorabari Lake broke and around 262 million liters of water came rushing downstream in a matter of minutes, causing widespread destruction. 

 

>2023 Sikkim (South Lhonak Lake): In October 2023, South Lhonak Lake burst due to significant rain events, breaching the Teesta III dam, washing away bridges and destroying towns all the way downstream.

 

>2021 Chamoli Disaster (Ronti Gad): A rock-ice avalanche caused large debris flow into the Ronti Gad stream causing flooding, infrastructure loss, and many casualties.