The devastating collapse of the Nanda Devi glacier in India’s Uttarakhand state on 7 February that killed hundreds should be a warning for Nepal to be prepared for similar disasters in future.
A PlanetLabs image seems to confirm that a rock-ice slope detached completely from near the summit of the 7,816m high peak in the Garhwal Himalaya near Nepal’s western border, crashed on to the glacier below, and bulldozed the ice and debris into the Dhauliganga and the Alkananda Rivers.
There are tell-tale signs of new brown dust on the western side of the valley in satellite images. The ice melted as it reached a warmer, lower altitude and unleashed the flood that raced downstream. Two hydroelectric projects on the rivers were completely destroyed, highways were washed away and settlements buried under metres of water and debris.
The Nanda Devi disaster is almost identical to the Seti River flash flood of 2012 north of Pokhara that killed nearly 80 people. A massive chunk of rock near the summit of Annapurna IV broke off, fell on the glacier and the ensuing flood barrelled down the Seti, causing the fatalities.
Another flash flood on the Arun River in eastern Nepal in 2017 was traced to slope failure on a mountain on the Nepal-China border that fell on a glacial lake, triggering a flash flood that washed down so much debris that it dammed the main river.
In all three cases, disasters that at first thought to be a Glacial Lake Outburst Floods (GLOF) turned out to be rock-ice avalanches that led to a glacier collapse. The water and debris then cascaded down the valleys in a deadly paste obliterating everything in its path.
Whereas the increased risk of GLOFs in the Himalaya is linked directly to the climate crisis melting the ice and expanding moraine-impounded lakes, this does not appear to be the direct cause of the Nanda Devi event—just as it was not in the Seti and Arun. Rockfalls that lead to the collapse of glaciers, squeezing the water out of ponds in them and melting of the ice as it is pushed down to a warmer altitude.
Some of these glaciers may have lakes that top over when an avalanche or rockfall hit their surface, or global warming may have melted the ice within debris-covered glaciers. We do not know if these events are becoming more frequent due to a Himalayan thaw caused by the climate crisis.
Whatever the case, it is clear that the countries that share the Himalaya from Afghanistan to Burma have to be prepared for more frequent disasters of this type. The risk of GLOFs and flash floods have to be factored into planning infrastructure and human settlements downstream.
Although GLOFs and glacial collapse get all the attention, the Himalaya are also prone to landslide-blocked dams during the rainy season. The mountains are relatively young, they are seismically active and their slopes very unstable. Monsoon rains have been known to trigger landslides the block major rivers, as happened on the Bhote Kosi in Jure in 2014.
Fortunately, the river found a way around the blockage. But there have been cases when the impounded lake is large enough to cause catastrophic bishyari floods downstream when the landslide dams collapse.
The flat terraces where the city of Pokhara is situated today was probably formed, geologists say, when a landslide blocked the Seti upstream some 700 years ago. It gave way, unleashing a tsunami at least 150m high. Pokhara is located in the debris fan of this flood which dammed side rivers, forming Phewa, Begnas and other lakes.
To warn that similar catastrophes can strike in the Himalaya is not panic-mongering. It is a call to be prepared. And let us hope the worst-case scenario of a mega-quake causing multiple glacial lake outburst floods does not happen any time soon.