DNA Explainer: What is 'Glacial outburst' and why does it occur?

Nearly 150 people were are missing and several are feared dead in Uttarakhand after a Himalayan glacier broke and crashed into a dam early on Sunday, with floods forcing the evacuation of villages downstream.

A part of the Nanda Devi glacier broke off at Joshimath in Uttarkhand near the Rishi Ganga power project leading to a massive flood in Dhauli Ganga river. 

The incident occurred in Rishi Ganga at around 10.45 am.

What is glacial burst?

A glacial burst is a rare instance that occurs when water dammed by a glacier or moraine is released. The glacier containing the body of water melts and overflows the glacier.

Why does a glacier break?

Glacier breaks can happen due to erosion, a buildup of water pressure, an avalanche of rock or heavy snow, an earthquake or cryoseism, or volcanic eruptions under the ice. 

It can also happen after a massive displacement of water in a glacial lake when a large portion of an adjacent glacier collapses into it.

The direct causes of glacial lake outbursts are earthquake, heavy rainfall/snowmelt, long-term dam degradation, etc.

What is Rishi Ganga Power Project?

A 13.2 MW hydroelectric project on the Rishiganga river has been completely destroyed due to the glacial burst. Owned by a private firm, the Power Project near Rani village at the confluence of Rishiganga and Dhauli Ganga has suffered major damage when the river took a severe form in the Dholganga Valley and Alaknanda region due to floods.

Work on the power generation project was underway for more than 10 years, which is named Rishi Ganga Power Project. Earlier, this project was strongly opposed. Social workers working for the protection of the environment had knocked on the door of the court to stop the project, but this could not happen.

Meanwhile, the Indian Air Force said an Advanced Light Helicopter (ALH) deployed at Joshimath has undertaken recce of affected areas.

"Two IAF C-130 aircraft from Air Force Station Hindan have positioned NDRF teams at Dehradun, where Mi-17 and ALH helicopters are deployed for onward deployment to Joshimath. An additional ALH deployed at Joshimath has undertaken recce of affected areas," IAF tweeted.

It added that humanitarian assistance disaster relief (HADR) operations will continue.