The massive rock slope failure and its consequences on the morning of August 26th on the north side of the 7234m Langtang-Lirung mountain is accurately described by the Sanskrit term pralaya. The word translates into English as apocalypse, the former carrying a Vedantic and the latter a Biblical meaning – total destruction and cosmic dissolution. Langtang-Lirung is the 99th highest peak in the world visible from Kathmandu slightly left of Shivapuri.
Avalanche with 150 km per hour speed
The rock face with ice pack on top within Nepal went down, first on a free-fall from 5200m to 3800m where it bulldozed ages of debris on the steep slopes to come crashing down another 1000m into the small Lhende khola that forms Nepal’s border with China. That entire debris mass with unimaginable kinetic energy then went barreling down the gorge at speeds estimated to be over 150 km/hour to hit Chinese and Nepali border check posts within minutes.
Monsoon helps the havoc
They were located much further down at 1800m where Lhende merges into Trisuli river, an important tributary of the Narayani/Sapta Gandaki system. It being the height of the monsoon season, the churning mass of rocks and boulders got further lubricated by the waters in the rivers to create havoc for another 50 km downstream sweeping away towns, villages, bridges, roads, hydropower stations and all lives human and animal therein that lay on their way.
Disaster development discourse
A serious scientific symposium was organized by Everest Engineering College together with the Society of Hydrologists and Meteorologists SOHAM in Sanepa, Lalitpur on 1st September to understand what really happened and what it means for Nepal’s overall development pathway.
Very sensitively, before starting cold, objective scientific discussions, the meeting began with a minute of silence for the victims of this inconceivable disaster. It was a very emotional moment, including for this writer whose brilliant research assistant doing her PhD on high altitude water quality is still not sure what has happened to her uncontactable engineer husband working in a hydropower project on the Trisuli. It is difficult to grasp how Nepali and foreign families of the thousands still missing are coping – and will have to cope in the years ahead – with this instant meat-grinding tragedy that, in many cases, will have left not a trace of the remains of their loved ones.
The deliberations began with hard earth science, led by two of Nepal’s top experts in this field. Cryosphere expert Prof. Rijan Bhakta Kayastha began by showing remote sensing pictures of where and how the slope failure occurred. Engineering geologist Prof. Ranjan Dahal followed, describing the event as a rock-ice avalanche of debris and ground-dust that became a proper flood water flow only after Syabrubesi at 1500m some 16 kms below the border check posts. Despite less-than-satisfactory state of scientific collaboration on these issues between government agencies of China and Nepal (or for that matter Nepal and India!), these professors were in good communication with their Chinese colleagues in the past as well as after the instant catastrophic event. They were able to share actual photos and other data provided by their Chinese counterparts. ICIMOD’s geoinformatics specialist Birendra Bajracharya discussed difficulties of accessing, analyzing and sharing such information primarily due to expense.
A failure of inherently unstable Himalayan slopes
One blunt fact emerged clearly: this was a phenomenon of mass wasting, i.e. failure of inherently unstable Himalayan slopes that occurs frequently at different scales with or without ice or glaciers on top of them. In decades gone by, the now-debunked Theory of Himalayan Degradation held sway. It held that poor, fecund Nepali farmers cutting trees were responsible for sedimentation in the Ganga that caused flooding in Bangladesh.
It was debunked in 1986 in New York’s Mohonk Conference which showed that Himalaya are still rising and its unstable slopes eroding at almost the same pace; and its mass wasting would continue even if all of Nepal was a fully covered canopy of trees with no one living there!
Climate change linkage claim needs nuanced analysis
While geological instability is the pervasive underlying reason, what triggers such failures in particular cases is open to hypothesizing. Last year, there was a similar event in the same place but the slope failure was smaller at a much lower level at 2900m and hence was not as destructive as this year’s event.
Knee-jerk reactions point to climate change as a lazy explanation, but even that needs much more nuanced analysis. Instructive is a similar, much bigger pralayakari event that happened some 800 years ago when a chunk of Annapurna-IV fell into the Seti river with that event creating the flat, debris-filled Pokhara valley of today. There was no climate change then to blame!
Seismic tremors piggyback unstable terrain
The first culprit is seismic tremors with delayed impact even years later. Prof. Dahal showed me satellite-mapped land shifts of several millimeters in the region in different directions that indicated increasing slope instability. Even small magnitude earthquakes can change mountain tilts by ever so little, making what is inherently unstable even more so, to say nothing of big ones in the recent past such as the 2015 earthquake.
That event brought down on the same Langtang-Lirung slopes an avalanche of estimated 40 million tons of rock destroying Langtang village. Similar events happened in recent years in Thame, Melamchi and many other places. The media uproar in the immediate aftermath dies down after a few months, with no real lessons learned, and no institutional memory surviving.
Earth tremor-induced impact is just as bad in the lower altitude mid-hill river valleys where there are no glaciers or snow, where regular landslides block rivers, create an unstable lake behind them which eventually burst, sweeping villages and lives on its way. Our ancestors that lived and survived on these lands had invented a word for it: bishyari. It is a landslide-dammed lake outburst flood (LaDLOF).
Any hill slope above a river can slip anywhere; triggering a bulldozer effect
Unlike glacial lake outburst floods (GLOFs) made very “sexy” by international researchers and ICIMOD (make no mistake, they are important global indicators of climate change), LaDLOFs are far more uncertain in that they cannot be identified by remote sensing satellite. Any hill slope above a river can slip anywhere; and once they do, depending upon the size of the landslide and the stage of the river, an unstable lake forms behind that eventually overtops the earthen dam moving everything downstream like a massive bulldozer.
If it is daytime, villagers notice the drop in water level and sometimes fish kill and head uphill to save themselves: if it is night, the disaster sweeps them away.
Unlike GLOFs, LaDLOFs happen in heavily populated mid-hills with bigger disaster consequences. Unfortunately, Nepal’s official institutional memory regarding bishyaris is practically absent.
A normal bishyari can happen even without immediate rain
Furthermore, it is not very distinguishing of different types of floods. What happened in the upper Bagmati basin in 1993 was a massive cloudburst that dumped about 450mm of rain in about 9 hours resulting in what may be termed a major flood. It was similar to what happened in August 2022 in Pakistan’s Sindh and Baluchistan provinces (semi-arid desert, one might add) where several years of rain was dumped in a few days!
A normal bishyari happens even when there is no immediate rain. The August 2008 Kosi embankment breach affected 65 thousand families in Nepal and 3.5 million in Bihar, but it was NOT a flood since the river was at only 60% average flow level for the month of August! The causes were elsewhere: politician-bureaucrat-contractor iron triangle corruption leading to substandard engineering.
Current Rasuwagadhi bishyari too should not be labeled a flood which is a latter downstream consequence, not the main cause of the disaster. Nor should one easily jump to blaming climate change, which only allows the whitewashing of mal-developments in our midst. There is little doubt that climate change will exacerbate the bad consequences of mal-developments; however, as Nepalis, our mitigation efforts regarding fossil fuel use (negligible globally) will have little impact on reversing it. We should focus on reversing mal-developments and giving up fossil fuel use for economic reasons.
Mal-development and societal weakness play their role
As the South Asian network for an alternative perspective on disasters Duryog Nivaran has argued, natural events do not necessarily become disasters: they do so when compounded with societal weaknesses and mal-development. And in Nepal we have both aplenty.
With the advent of modernity, we, aided and abetted by donor agencies, opted for fossil-fuel guzzling vehicle transport that destabilize fragile hillslopes. We should have taken up ropeways technology that is fuelled by clean Nepali hydropower. Compared to hill roads, it is three times cheaper to build, eight times quicker to install, twice as energy efficient and has, practically no environmental or social costs. But we sidelined it.
Modernity and market have brought people close to the rivers
It gets worse: traditionally Nepalis built houses and settlements on ridges leaving valley bottom land for rice farming. With modernity, our main highways were built alongside rivers, creating better economic opportunities around them, drawing people away from hill villages to encroach on flood plains.
That, as panelist Tara Nath Sapkota of IPPAN described, also has attracted hydro developers out for quick profit over longer-term safety. Bishyaris including this recent one, have been telling everyone every so often that these drainage corridors for water and matter belong to them, not to big towns like Melamchi and Betrawati Bazaars built on them. The lesson they ask us to take away is to rethink our development away from mal-development to something more suited to the nature of our mountains.
Dipak Gyawali has remained attached with Nepal’s water sector for about 50 years; a former water resources minister of Nepal, an engineer and frontline regional water expert
This article was first published in Spotlight News, Nepal ( https://www.spotlightnepal.com/2026/09/03/26-august-2026-pralayakari-bishyari-lessons-be-learned/ ) and The Plurals has just changed the headings, sub-heads and photograph