Down to Earth spoke to Kalachand Sain, director, Wadia Institute of Himalayan Geology, Dehradun, to understand the causes of the Sikkim glacial lake outburst flood
Photo: @Laxmanacharya54 / X, formerly Twitter
Several questions are being asked in the aftermath of the South Lhonak Lake in North Sikkim bursting in the form of a Glacial Lake Outburst Flood (GLOF). But there is near unanimity that climate change is a major cause of this natural disaster, and the risk of glacial lakes forming and bursting in the future is increasing.
South Lhonak Lake is at an altitude of about 5,200 metres above sea level. Usually, there is no rain but snowfall at an altitude of more than 4,500 metres. Was there a cloudburst over the lake, which suddenly increased the amount of water in it?
The mean global temperature for September 2023 was 0.93 degrees Celsius more than the average of 1991-2020. The Indian Space Research Organisation (ISRO)’s satellite data shows that the size of the South Lhonak Lake increased in September. Did the melting of glaciers increase the water in the lake?
Experts have said tremors may have also caused the bursting of the lake.
In the last one year, there have been three major earthquakes in the Himalayan region. On the night of October 3, a 6-magnitude earthquake in western Nepal and eastern Uttarakhand also affected the Sikkim Himalayas, about 700 km away.
Should an early warning alert system be developed to monitor and prevent damage to pro-glacial lakes (whose embankments are made up of moraines) in the Himalayan glacier region, considered vulnerable to danger?
Down to Earth spoke to Kalachand Sain, director of the Wadia Institute of Himalayan Geology, Dehradun, on the disaster and its possible causes. Edited excerpts:
Varsha Singh (VS): What is the reason for the South Lhonak Lake in Sikkim bursting on October 4, 2023?
Kalachand Sain (KS): ISRO’s satellite images show that on September 17, 2023, the size of the South Lhonak Lake was 162.7 hectares. On September 28, the lake increased to 167.5 hectares and after the lake burst on October 4, its size decreased to 60.4 hectares. That is, during that time, water from more than 100 hectares of the lake came downslope.
South Lhonak Lake also has moraine embankments. The water from the lake came down to the Chungthang Hydro Dam on the Teesta river, about 50 kilometres below. The size of the lake is more than six times the reservoir of this hydro power project.
Cloudbursts and extremely heavy rains were also reported at that time. The dam reservoir could not withstand this pressure of water, due to which it broke down and a large amount of water moved downstream.
India Meteorological Department information shows that the heat was intense in the last week of September. One hypothesis is that the water in the lake may have increased due to the melting of a glacier. Then, there are reports of cloudbursts and heavy rains. Due to over-filling of water in the lake, its moraine embankments were broken and the lake burst.
A lot of weather irregularities are happening due to climate change. The Wadia Institute does not study weather events. But there is snowfall at an altitude of 4,500 metres and above sea level, not rain. The cloudburst incident over the South Lhonak Lake at an altitude of about 5,200 metres also needs to be investigated.
Apart from this, we are also trying to understand whether an earthquake of magnitude more than 6 at 2:51 pm near Pithoragarh on the border of western Nepal and eastern Uttarakhand, may have had an impact on the night of October 3.
About 25 minutes before this earthquake, a small earthquake of magnitude 4.3 occurred. Even after the earthquake of 2:51 pm, aftershocks was felt six times till around 9 pm. All of them had a magnitude greater than 5 on the Richter scale.
Earlier, earthquakes of magnitude 5.4 and 5.8 were felt on November 12, 2022 and on January 24, 2023. We are trying to understand whether these earthquakes triggered the already sensitive South Lhonak Lake and whether geological factors could have caused the lake to break.
VS: Before Sikkim, a major accident occurred due to an avalanche at Chamoli, Uttarakhand in February 2021. The heat at that time too was recorded to be very high. Are global warming or extreme weather events currently increasing the risk of glacial lakes forming and bursting?
KS: There are more than 10,000 glaciers and more than 20,000 glacial lakes in the Indian Himalayan region. Uttarakhand alone has 1,250 glaciers and about 350 moraine glacial lakes. Lakes continue to form and burst in the glacial region. Not all lakes are dangerous. But moraine lakes are.
For example, in the Bhilangana basin, a tributary of the Bhagirathi basin in Uttarakhand, there is a lake on the Bhilangana glacier. We have been monitoring this lake for the last several years and we are getting to know that this lake is continuously getting bigger due to the melting of glaciers.
Based on satellite images from 1976, the size of the lake is zero. In current satellite images, it is spread over an area of about 400,000 square metres.
Right now, we do not have an idea of the depth of the lake, so it is difficult to assess the exact amount of water in it. But it could be a threat in the future. We have also submitted a report on this to the state government.
Due to the rise in temperature, glaciers are melting everywhere and the number of glacial lakes is increasing. In such a situation, if there is an extreme weather event such as very heavy rain or an earthquake, the possibility of disaster also increases.
VS: Are we able to monitor these dangerously sensitive lakes?
KS: The Wadia Institute or any other institution cannot monitor all the lakes. We have selected some glacial lakes according to our capacity and resources.
As of now, we are monitoring 10 glacial lakes of Uttarakhand including the lake of Dhauliganga glacier area after the Chamoli disaster as well as the Bhilangana Lake.
Apart from this, two lakes in Himachal Pradesh and one glacier in the Karakoram range of Ladakh are being monitored.
VS: In both, Chamoli 2021 and Sikkim 2023, no early warning system seems to have worked. Is there an early warning system for glacial lakes?
KS: In Sikkim, the state government as well as many institutions were monitoring the South Lhonak Lake. The Wadia Institute had also studied it. We knew the lake could become a threat anytime.
Similarly, we have given a report to the Uttarakhand state government regarding the possibility of danger from Bhilangana Lake. The government has to make arrangements and preparations for rescue in the event of a disaster.
But events such as sudden heavy rains or earthquakes can increase the risk of the collapse of proglacial lakes. We are trying to improve the alert system with the help of artificial intelligence.
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