Nepal's complex chain of flood hazards - expert comment
26 August 2026
Following the flash flooding tragedy on the Nepal-Tibet border, experts from the University of Reading have provided regular comment on the factors behind the tragedy to media and others requesting expert comment. This page was updated with new comments as they became available from Wednesday 26 August to Friday 28 August 2026.
UPDATED COMMENTS from FRIDAY, 28 August 2026
Professor Maria Shahgedanova, a climate scientist researching climate impacts on mountain glaciers, Department of Geography and Environmental Sciences, University of Reading, said:
"We now believe that the Rasuwa flood was triggered by a large rock and ice avalanche at around 5,200m above sea level, when a huge section of glacier and the underlying bedrock, estimated to be between 1 and 1.3km wide, detached from the Lirung massif.
"The avalanche rapidly carried large volumes of ice, rock and sediment downslope. On reaching the valley floor, it interacted with deposits from a previous flood, the 2025 Purepu glacial lake outburst flood, temporarily damming the river. The failure of this temporary blockage released water and debris as a powerful flood wave.
"Researchers from the HiRisk project, which investigates cryosphere-related hazards in High Mountain Asia, suggest that longer-term glacier retreat and permafrost degradation may have contributed to slope destabilisation. The failed glacier retreated by approximately 450m between 1990 and 2020, potentially reducing the mechanical support provided by the glacier to the underlying rock slope. Permafrost degradation may have further contributed to instability. The warmer than average temperatures observed in the area this summer might have contributed to the glacier collapse too, although observations at these elevations remain limited.
"The event demonstrates how climate-driven changes in the cryosphere can change conditions that make hazards in high-mountain regions even more risky for people in the area. Such events cannot be prevented, but their impacts can be reduced through systematic hazard assessment and monitoring. High-resolution satellite observations and InSAR, a type of satellite-based radar that detects small changes in the earth's surface, can identify changes in glaciers and slope stability and contribute to early-warning systems, particularly in remote mountain regions where conventional monitoring is limited. While the initial avalanche at Rasuwa was too rapid to provide substantial warning, longer warning times downstream could potentially have reduced loss of life."
Prof Liz Stephens, Professor in Climate Risks and Resilience, Department of Meteorology, University of Reading, said:
“The barrier lake has formed behind a pile of debris that was left behind by the initial flood of water, rock and debris, with this debris dam blocking the normal flow of the river.
“Chinese engineers are monitoring the levels of the lake. The higher the water levels get the higher the pressure that builds up behind it, increasing the likelihood that it could fail.
“This is not an uncommon engineering problem in this part of the world. Chinese and Nepalese engineers know what to do, but they will be challenged by the inaccessibility of the landscape high up in the mountains. This is a very new debris dam so it will be very unstable in itself. It will be a difficult and dangerous environment to work in for the engineers on site in the mountains.
“It appears this morning that water levels have started falling. This fall in lake levels suggests some water is now escaping naturally through the dam. That’s very promising, it reduces the imminent threat and may reduce the need to intervene with an engineering solution, to drain the lake artificially. However, this does not mean the risk is over, the debris dam will remain unstable, and the authorities will need to be on the lookout for rain upstream, which could add to the challenge.
“It is important to be aware that this might not be the only lake that forms. The initial flood was so big that it will have had enormous erosional power. It may well have undercut the steep slopes as it went down through the valley into Nepal, leaving behind a much more unstable landscape. Any monsoon rains could threaten that stability further. Monitoring for further landslides is critical, the risks to people in the area, including those involved in rescue and recovery efforts, are definitely not over yet.”
Dr Fatima M. Pillosu, hydrometeorologist and flash floods researcher from the Department of Meteorology, University of Reading, said:
“A barrier lake forms when a landslide or flood dumps enough debris into a valley blocking a river. Hence, water ponds behind a natural dam of loose rock and mud.
“Unlike an engineered dam, this barrier has no spillway and no solid core. So, dams of this kind often fail, many within days or weeks of forming. The most common type of failure is due to rising water spilling over the crest (overtopping) and the flow begins to cut down through the loose debris. If that erosion accelerates, much of the stored water is released as a single flood wave that carries mud and boulders down the valley at high speed and gives people very little time to react.
“The exact moment of failure cannot be predicted, because it depends on the internal structure of a dam. However, close monitoring of the water flowing in, the lake level and the seepage through the barrier can show whether conditions are stabilising or deteriorating, and can give hours of warning if a breach begins.”
Dr Jeff Da Costa, Researcher in Hydrometeorological Hazards, Early Warning & Disaster Risk, University of Reading, said:
“The immediate challenge is managing uncertainty. We may not be able to predict exactly when or how much water will be released. Monitoring can show how conditions are changing, modelling can estimate where the water could go and how quickly, and engineers can assess options to reduce the risk. That information can then be used to warn communities, move people away from exposed areas and keep rescue teams out of danger.
“The first disaster has changed the risk downstream. Roads and bridges have been destroyed, communications disrupted and rescue operations are still underway. An evacuation route that was available before the flood may no longer exist. A second flood does not have to be as large as the first to have serious consequences, so warnings now need to reflect what has changed on the ground.
“This is a transboundary river system, and information about the barrier lake is being shared between China and Nepal. Observations upstream can give people downstream valuable time, but they become useful when they are connected to modelling, warnings and practical decisions on the ground. With a rapidly developing flood, even a short period of warning can make a real difference.
“We still do not know the full warning timeline for the original disaster, and we should be careful about judging the warning system from the death toll. Warnings and precautionary action may have saved many lives. We need to establish what was detected, when information was shared, who received it and what action followed. That reconstruction matters because it tells us what worked as well as what can be improved.”
Comments from THURSDAY, 27 August 2026
Professor Liz Stephens, Professor in Climate Risks and Resilience, Department of Meteorology, University of Reading, said:
"Initial analysis of before and after satellite images now suggests that part of a glacier collapsed high in the mountains. This huge ice-rock avalanche appears to have blocked the river, and the failure of that debris dam led to catastrophic flooding downstream. This avalanche of debris will have picked up more material as it travelled down the mountain.
"The resulting flooding is likely to have caused considerable erosion, undercutting slopes as it travelled downstream, picking up more debris and leaving behind a very unstable landscape. Scientists in the region will be concerned that the initial event could lead to more dangerous landslides and debris dams over the coming days.
"The exact trigger for the glacier collapse and the precise chain of events may not be known for some time. In-situ assessments will be extremely difficult, and what we can see from satellites is hampered by monsoon clouds.
"Climate change is rapidly altering high-mountain environments. The IPCC report has high confidence that glacier retreat and permafrost thaw are reducing the stability of mountain slopes and increasing the potential for complex cascading events such as this. However, the exact contribution of climate change to this particular event may be difficult to determine.”
Professor Hannah Cloke OBE, Regius Professor of Meteorology and Climate Science, Department of Meteorology, University of Reading, said:
“When a disaster unfolds on this scale, keeping people safe from harm is a complex task, and forecasting the hazard is only part of the challenge. The chain of tragic events along this river show that there were three distinct zones of risk, each with its own lessons for early warning systems.
“Highest up the valley near the source of the flood, the floodwaters arrived within minutes. In these environments, there is often very little time between the triggering event and the impact, making warnings exceptionally difficult, no matter how advanced the technology.
“Further downstream, many miles away, the situation was different. While the floodwaters caused severe damage to property, there were several hours between the first warning of a flood and the arrival of the torrent. According to Nepal's forecasting authorities, those warnings helped save many hundreds of lives, demonstrating the value of effective forecasting and communication.
“The greatest challenge lay in between. This was a flood that arrived beneath clear skies. What came down the valley was not simply water, but a devastating torrent of water, mud and debris. In these circumstances, very often many people struggle to believe the severity of a risk because there was no local rainfall. For many, a flood is something that follows a storm. When a warning describes a threat that does not match people's mental picture of danger, it can be easy to dismiss.
“This highlights a critical but sometimes overlooked weakness in early warning systems. Success is not determined solely by the accuracy of forecasts, the quality of sensors, or the reach of communication networks. It also depends on how people interpret, trust and act upon the information they receive.
“This imagination gap can come between people who are in danger hearing a warning and truly understanding what it means. If people cannot visualise a danger they have never experienced before, they may underestimate the risk, even when the warning is accurate and timely.
“As forecasting science and technology continue to improve, we will become increasingly capable of detecting and predicting extreme events. But better predictions alone will not eliminate disaster risk. Early warning systems must be designed around human behaviour as well as physical hazards. They need to anticipate how people perceive risk, how they make decisions under uncertainty, and how they respond to unfamiliar threats.
“The most effective warning is not one that is simply heard. It is one that is understood, believed and acted upon. Unless we close the imagination gap between issuing a warning and inspiring a response, we will continue to lose lives even when the information needed to prevent those tragedies is available.”
Dr Jeff Da Costa, researcher in flood early warning and disaster risk, University of Reading, said:
"The reports of new barrier lakes are particularly concerning because conditions upstream are still changing. Close monitoring and immediate information sharing between China and Nepal could buy precious time for communities on both sides of the border.
"Barrier lakes can form behind obstructions across a valley, blocking the regular flow of water downstream. In deep, steep-sided valleys such as those in the Himalayas, they can quickly build up an enormous quantity of water, creating huge pressure on the newly-formed barrier dam in front of it . The risk is that the weight of water, or another event such as further fall of rock or ice, can cause the dam to break and cause another sudden surge of water downstream within seconds.
“In steep Himalayan valleys, that can leave communities with very little time to get out of harm’s way.
"The transboundary aspect really matters here. Communities in China and Nepal share the same mountain and river system, and there have been terrible impacts on both sides of the border. Warning information has to move through that system more quickly than the hazard does. A flood can travel downstream and across a border in minutes, so the warning needs to get there first.
"This is why we need to look closely at the warning timeline. What was detected, when was it detected, who knew about it, and how quickly did that information reach people who could act? Those are the questions that will tell us what can be improved before the next event.”
"Barrier lakes can form behind obstructions across a valley, blocking the regular flow of water downstream. In deep, steep-sided valleys such as those in the Himalayas, they can quickly build up an enormous quantity of water, creating huge pressure on the newly-formed barrier dam in front of it . The risk is that the weight of water, or another event such as further fall of rock or ice, can cause the dam to break and cause another sudden surge of water downstream within seconds.
“In steep Himalayan valleys, that can leave communities with very little time to get out of harm’s way.
"The transboundary aspect really matters here. Communities in China and Nepal share the same mountain and river system, and there have been terrible impacts on both sides of the border. Warning information has to move through that system more quickly than the hazard does. A flood can travel downstream and across a border in minutes, so the warning needs to get there first.
"This is why we need to look closely at the warning timeline. What was detected, when was it detected, who knew about it, and how quickly did that information reach people who could act? Those are the questions that will tell us what can be improved before the next event.”
Comments from WEDNESDAY, 26 August 2026
Professor Liz Stephens, Professor in Climate Risks and Resilience, Department of Meteorology, University of Reading, said:
"Most people think of flash floods as being caused by heavy rainfall, but in Nepal and Tibet, as in many other high-mountain regions, they can also result from complex chains of hazards, including landslides, avalanches and glacial lake outburst floods. This complexity makes early warning especially difficult.
"Initial reports from Nepal suggest that an earthquake occurred around the same time as a landslide or avalanche, which then led to the catastrophic flooding. One way this can happen in steep mountain valleys is when a landslide blocks a river, allowing water to build up behind it. If that landslide dam then fails, it can cause a sudden, devastating release of water and sediment downstream.”
Dr Jeff Da Costa, researcher in flood early warning and disaster risk, University of Reading, said:
“For early warning, you do not always need to know exactly what has happened upstream before taking action downstream. A sudden and unusual change in the river can be enough to warn communities while the cause is being established.
“This is particularly important because the same river corridor experienced a major flood originating in Tibet in July 2025. After that event, Nepalese flood forecasting officials said they had received no advance warning from China and that there was no mechanism in place for sharing that information. Nepal subsequently committed to strengthening coordination with China. We do not yet know what information was available before today’s flood, when it became available or how it moved across the border. Establishing that timeline will be important, including what changed after the 2025 event and how much time communities had to act.
“Early warning in transboundary river basins depends on international cooperation. Monitoring can take place in one country while communities at risk are across the border. Rapid information sharing and trusted communication between neighbouring countries are part of the warning system. This has wider relevance. Many communities around the world depend on environmental information from across a border. Where relations are unstable or cooperation is limited, maintaining those channels becomes critical.
“Events like this can be extremely difficult to predict precisely. Warning systems need to work under that uncertainty by recognising danger quickly, sharing information and having agreed actions that can begin before the exact cause is known.”
Dr Fatima M. Pillosu, hydrometeorologist and flash floods researcher from the Department of Meteorology, University of Reading, said:
"Everyone in a mountain valley faces the same hazards, but tourists are more vulnerable because they may not recognise the danger or understand local warnings. Visitors from regions where such events are rare may not appreciate how fast a mountain river can rise, nor that these valleys also face avalanches, landslides and floods from melting glaciers, even in dry weather. Knowing in advance what can happen, and what to do if it does, is one of the most useful protections.
"A flood thick with mud and rock can be even more lethal than a rain-fed flash flood. Both can strike with great force and speed and carry boulders and debris along. Fast flowing water can easily pick up anything in its way and carry it off downstream. Mud simply buries people. The moving mass is much heavier than water, and it grows heavier as it travels, scouring soil, gravel and boulders from the riverbed and banks and carrying them along.
"The boulders it carries can destroy buildings and bridges that a water flood might have left standing. When the flow stops, the mud and rock settle into a dense mass that is hard to dig through, making rescue even slower and more difficult."
Dorothy Heinrich, extreme weather researcher, Department of Meteorology, University of Reading, said:
"The sheer speed and scale of the mudslides and floods in Nepal was catastrophic and would have left little to no time for early warning and for people evacuate or protect themselves in other ways.
"The causes of this particular event are complex and still unclear. In these high mountain environments, various combinations of extreme rainfall, avalanches, landslides, and glacial lake outbursts can all change or block the normal path of a river, causing it to swell extremely rapidly downstream and bring along devastating ice, rock, and debris-filled torrents.
"With critical response activities ongoing, it is essential that quality information about any current risks reaches the first responders, communities, and tourists who need it."
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