On October 4th, 2023, a major disaster of a geologic origin struck high within the Himalayas
of India.
As, people who lived within numerous villages were suddenly alerted to a loud roaring sound.
Not long after hearing this, a wall of water appeared that towered as much as 60 feet above
the height of the Teesta River, even carrying with it large blocks of concrete.
The concrete in question partially originated due to the destruction of a large dam, specifically
the Teesta III dam 45 kilometers to the north.
Once this massive flood had ended, more than 70 people were confirmed dead with a dozen
still missing, more than 1,000 structures were destroyed, and a trail of damage was
left that was more than 150 kilometers in length.
Although there is not a firm damage assessment yet, the destroyed dam on its own had cost
1.5 billion dollars to build.
The origin of this disaster can be found 65 kilometers upstream of the Teesta III dam,
located at a high elevation of 17,300 feet above sea level.
There, glaciers cover wide swaths of the landscape, but these have largely been retreating in
recent years since their peak extent at the end of the little ice age in 1850.
As these glaciers retreated, they left behind large amounts of loosely consolidated rock
in what is known as a moraine.
The South Lhonak Glacier specifically created three such moraine types, with the terminal
moraine forming the very end of the glacial lake that would develop, acting as a natural
dam.
Meanwhile, rock particles carried on the sides of this glacier as it carved away against
the mountainsides were left behind as what is known as a lateral moraine.
These lateral moraines were highly unstable, existing as material up to 100 meters thick
on a 45 degree slope.
As the South Lhonak Glacier retreated, it no longer held up sections of the lateral
moraine that it previously kept in place, while at the same time causing the South Lhonak
Lake to grow larger and larger.
Water in this lake then slowly began soaking into each of the lateral moraines, destabilizing
them.
Once the glaciers no longer held up a highly prone section, a 16.7 million cubic meter
landslide began at a rate of a few meters a year eventually increasing to moving 10
meters a year.
Then, after the adjacent glacier lost an ever increasing amount of length in 2023, a threshold
was reached on the hillside.
Finally, on October 4th, a 14 hectare or 34.6 acre mass of rock totaling 16.7 million cubic
meters of material collapsed into the lake all at once as a massive landslide, displacing
much of the 85 million cubic meters of water in it, and generating what may have been a
megatsunami 100 meters in height.
As this tsunami raced towards the east, it reached an unstable mass of sediment that
formed a sort of natural dam only 60 seconds later.
Upon impact, it caused a large section of this terminal moraine to be removed, causing
63% of the lake's water to rapidly wash away, gaining speed as it was carried downwards
by gravity from 17,300 feet to 5,100 feet elevation.
This wall of water then impacted the Teesta III dam, causing it to quickly fail as it
was not meant to contain such large volumes of water.
As this failed, additional water ponded behind the dam was added to the floodwaters, with
flood waters now likely containing what I estimated to be 100 million cubic meters of
water.
On its own, this disaster might seem like an unlikely tragedy.
However, scientists had been warning for years that the South Lhonak Lake could overflow
its natural dam, possibly leading to severe damage downslope via an outburst flood.
Yet, these reports were presumably ignored.
And, a similar disaster, also caused by a large landslide had occurred on the slopes
of Nanda Ghunti only two years earlier.
But most importantly, the disaster which was witnessed on October 4th 2023 could potentially
occur at any of India's more than 500 glacial lakes.
The problem is that as glaciers continue to retreat across northern India, these events
will in my opinion become far more common, including at certain other glacial lakes,
although these landslides could be potentially detected early on from satellite.
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