What are glacial lake outburst floods and why do they occur?

Chamoli: A massive glacial burst in Uttarakhand's Chamoli district led to massive flooding and avalanche along the Alaknanda and Dhauliganga rivers on Sunday morning.

Reports suggest flooding in areas around the Dhauliganga river, following an avalanche near Rishiganga Power Project in the Chamoli district on Sunday.

Emergency evacuation of thousands from the surrounding areas have been ordered after the floods damaged houses and the nearby Rishiganga power project.

The flooding due to cloudburst or breaching of a reservoir has destroyed many river bankside houses. Hundreds of ITBP personnel have rushed for the rescue of feared casualties, the ITBP said.

"Some water bodies flooded and destroyed many river bank-side houses due to cloudburst or breaching of the reservoir. Casualties are feared and hundreds of ITBP personnel rushed for rescue," ITBP officials said.

The Chamoli district magistrate has instructed officials to evacuate people living in villages on the bank of the Dhauliganga river. The district magistrate and the superintendent of police have left for the spot.

According to reports, five NDRF trams and two ITBP teams have already been deployed to carry out search and rescue operations. In addition, around 200 SDRF (State Disaster Response Force) have also been deployed.

Following the glacial burst, the Uttarakhand Chief Minister Trivendra Singh Rawat said: "A disaster has been reported from Chamoli district. The district administration, police, and disaster management departments have been directed to deal with the situation. Do not pay attention to any kind of rumours. Government is taking all necessary steps."

He even issued a helpline number and requested netizens not to spread rumours.  “If you are stuck in the affected area, if you need any kind of help, please contact 1070 or 9557444486. Please do not spread rumors,” the CM said in a tweet.

To understand the situation better, we need to first know why a glacial disaster of this nature happens in the first place and what are the different types of glacial disasters

What triggered the floods?

According to a statement by Chamoli police, the Rishiganga Power Project was damaged after a breach of a glacier in the Tapovan area. The statement said that people living on the bank of the Alaknanda river were advised to move to safer places.

What is glacial lake outburst flood (GLOF)?

A glacial lake outburst is a type of outburst flood that occurs when water dammed by a glacier or moraine is released. In other words, it's an outburst that happens when a dam containing a glacial lake fails.

When and why a glacier breaks?

Failure can happen due to multiple reasons like erosion, the build-up of water pressure, an avalanche of snow or rocks, and also an earthquake under the ice.

It can also happen if there is a massive displacement of water in the glacial area when a large portion of an adjacent glacier collapses.

Glacier lakes can vary in volume but are known to hold millions to hundreds of millions of cubic metres of water. Failure to contain ice or glacial sediment can result in water being released for minutes, hours, or even days.

A glacial outburst may occur due to several factors. According to experts, the direct causes of glacial lake outbursts resulting in floods are heavy rainfall/snowmelt, earthquake, cascading process, long term dam degradation, and rapid slope movement into the lake.