A sudden surge of water, mud, boulders and debris swept through settlements along Nepal’s Budhi Gandaki river in Gorkha district early on October 11, causing damage to infrastructure.Early reports pointed to a landslide above Siuribas that blocked the Chumke Khola stream. Officials also suspected that a temporary blockage near Chekampar may have broken, releasing a sudden surge downstream.The fresh Nepal incident has renewed concerns about sudden floods across the Himalaya, where retreating glaciers, unstable mountain slopes and large quantities of loose sediment can combine to produce destructive flows.The Indian Express spoke to experts to understand what caused the Fresh floods in NepalWhat caused the Gorkha flash flood?The immediate trigger appears to have involved a sudden release of water and debris, but the precise mechanism has not been conclusively established.Speaking to The Indian Express, Manish Mehta, a glaciologist at the Wadia Institute of Himalayan Geology in Uttarakhand’s Dehradun, said his initial assessment was based on the available visuals and reports. He said the event appeared consistent with a possible glacial lake outburst flood or an avalanche involving ice and rock, but this was not a confirmed finding about the flood’s origin.Why do these floods carry so much mud and rock?Story continues below this adMehta explained that high-altitude Himalayan terrain contains large quantities of loose material shaped by erosion and past glacial activity. Glaciers transport and deposit sediment, while weathering and erosion continue to break down mountain slopes.“Warming is happening, snow is melting. When snow melts, the cohesion between rock and ice ends,” he said.He explained that loose material in upper mountain valleys can be mobilised by rainfall or a sudden release of water.As the surge moves down the valley, it picks up additional sediment, boulders and vegetation, turning into a dense slurry.Story continues below this adP D Pant, director of the School of Earth and Environmental Science at Uttarakhand Open University, also highlighted the sediment content visible in footage of the flood going around on social media.Speaking to The Indian Express, Pant said “The amount of gravel and sediment coming in is very high. That is causing more damage,”In simple terms, the danger is not determined by water volume alone. A flood carrying heavy rocks and dense sediment can exert enormous force on houses, bridges and roads.How does the Gorkha flood compare with Nepal’s disaster last month and why should India pay attention?Mehta said that, based on the initial videos, the current event did not appear as catastrophic as the earlier one. He suggested that the width of the valley and the location of settlements could help explain the apparent difference.Story continues below this ad“The valley the last time was very narrow and concise, you were sitting on the river bed,” he said. He contrasted this with the current location, saying that the wider valley could have influenced how the flood spread.The same broad categories of hazards exist across the Indian Himalaya, including Uttarakhand, Himachal Pradesh, Ladakh, Sikkim and Arunachal Pradesh. However, the precise mechanisms differ from one disaster to another.The 2013 Kedarnath disaster involved exceptionally heavy rainfall, flooding and the mobilisation of sediment and debris. It is commonly discussed in the context of extreme rainfall and cloudburst-related flooding. A cloudburst is an intense, localised rainfall event; it is not the same as a GLOF or a landslide-dam breach.The 2021 Chamoli disaster involved a massive rock-and-ice mass movement in the Rishiganga valley that generated a destructive downstream surge. It demonstrated how a high-altitude slope failure can rapidly transform into a major disaster, affecting river infrastructure and hydropower projects.Story continues below this adPant said “similar hazards could occur across the Himalayan arc, from Kashmir to Arunachal Pradesh, including Uttarakhand. The broad risk associated with glaciers, unstable slopes and sudden floods was not confined to one part of the Himalaya.”Why are such floods occurring repeatedly? How much is climate change responsible?Climate change is altering the conditions in which Himalayan hazards develop, but it does not establish the cause of any individual flood.As temperatures rise, glaciers lose mass and retreat. The space left behind can form depressions where water accumulates. Some of these lakes are held back by moraines — deposits of sand, silt, gravel and boulders left by glaciers.Story continues below this adPant explained that when a glacier retreats, a lake can form in the space left behind. He also explained that a moraine is not solid bedrock but a barrier composed of loose material.Because such barriers can be vulnerable to erosion and failure, a sudden influx of water, overtopping or a breach can release a large volume of water downstream.Pant also pointed to the role of prolonged or intense rainfall. If water accumulates in a lake or behind a landslide obstruction, it can contribute to a sudden release.How does warming affect mountain slopes?In areas where ice helps bind material together, changes in temperature and melting can make slopes more susceptible to failure.Story continues below this adPant said climate change was the biggest factor behind retreating glaciers.Scientific research, including assessments by the Intergovernmental Panel on Climate Change, has documented glacier loss across the Hindu Kush Himalaya and other mountain regions. This supports the conclusion that warming is changing the physical environment in which mountain hazards occur.What is the way forward?For Uttarakhand and the other Himalayan states in India, the challenge is to identify vulnerable lakes and unstable slopes, monitor changes and ensure that information reaches downstream settlements quickly enough for people to act.Satellite observations can help map lakes, track glacier retreat and identify changes in the landscape. But satellite monitoring alone may not provide the real-time information needed to detect a rapidly rising lake or an impending failure of a natural dam.Story continues below this adThis is where ground-based sensors can help. Mehta emphasised the need for early-warning systems across the Himalayan terrain.“Basically, it is essential to install or establish an early warning system in the Himalayan terrain,” he said adding that the need was especially pressing because of the difficulty of protecting settlements once a destructive surge had begun.Pant similarly said sensors could, where conditions allow, provide warnings hours in advance, giving authorities time to evacuate vulnerable settlements. “At least we can inform downstream settlements several hours in advance. We can evacuate them,” he said.