What can we learn from flash floods?
As devastating flash floods become increasingly common across South Asia, they raise urgent questions about how we build our cities and settlements
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Last month, the tragedy in Nepal left the world stunned with its intensity and destruction across Rasuwa and downstream areas. It further exposed various aspects of developmental strategies at once – early warning systems, glacier monitoring, and urban planning, among them. Most importantly, it raises a serious question about where and how we build near highly uncertain river systems, as in the Himalayan region.
Satellite analysis confirmed that the disaster was triggered by the collapse of an approximately 600-meter-wide section of glacier and rock, which fell nearly 1,200 meters to the valley floor. The collapse generated a seismic signal equivalent to magnitude 5.2 and triggered a powerful surge of water and debris that travelled through the Lhende Khola into the Bhotekoshi and Trishuli river systems. These waves swept away whatever came their way: markets, schools, villages, bridges, vehicles, and people.
Satellite and aerial imagery captured before-and-after conditions along the river plain, showing how development has confined the river with increasingly narrow boundaries. As of this writing, the disaster has claimed over 1400 lives, with around 6,100 still missing. China and India have also reported hundreds of missing citizens, underscoring the disaster’s cross-border impacts.
Traditional Himalayan settlement practices, evolved through generations of living in fragile terrain, favoured elevated slopes and ridges over exposed river channels. Contemporary development, by comparison, has inclined towards the assurance of early warning systems and engineering stability to justify building settlements directly on floodplains, thereby reducing the room for rivers to flow. The floodplains are being built upon, leaving less space for rising water. The imagery provides a glimpse of this aspect, in which development has confined the river within increasingly constrained boundaries.
A Regional Pattern
This is not unique to Nepal. Bangladesh, a country defined by its rivers, is closely familiar with floods – yet land use and settlement expansion continue to alter floodplains and the ecological systems they support. The more that alteration accelerates, the more conservation effort is needed to offset it. The Chattogram Hill Tracts, which include Rangamati, Bandarban, and Khagrachhari, are among Bangladesh’s most vulnerable regions and have become focal points for climate resilience and watershed management. But the discussion does not end with climate action alone. Tourism and infrastructure growth in these same areas have driven displacement and land use change.
Rapid urbanization near Cox’s Bazar paints a similar story, leading to compounding exposure to cyclones, coastal flooding, and tsunamis. Despite these risks, urbanization is inevitable – it offers people an opportunity for a better life. Therefore, the real question is not whether these regions will develop, but to what extent they can absorb the risks and at what cost.
A similar pattern is also visible in India. Examples from Bengaluru, Gurugram, and Chennai illustrate how encroachment, loss of wetlands and natural drainage corridors, and inadequate stormwater planning can increase urban flood risk. Recent evidence from the Aravalli region suggests that altering natural drainage channels and runoff pathways for development is significantly increasing flood risk in downstream urban areas.
These rapid, unplanned, and nature-insensitive forms of urban development need to be revisited. Importantly, this does not mean stopping development, but rather reconsidering how development is planned in relation to natural water systems – an approach which several countries have already adopted, and from which flood-prone regions elsewhere can learn.
Learning from Elsewhere
While these events serve as a stark reminder of nature's power, there are also instances in which societies have built themselves through careful, thorough consideration of this aspect. Denmark implemented the Cloudburst Management Plan after the 2011 flash floods in Copenhagen, focusing on green spaces, permeable pavement, and wetlands to create a distributed system for water retention and ease pressure on conventional systems.
The Netherlands has pioneered the “Room for the River” program, which helps reduce flood risk by giving rivers space to accommodate high flows through floodplain restoration, water buffers, and side channels. Similarly, Singapore replaced the concrete channel in Bishan-Ang Mo Kio Park with a naturalized river and floodplain landscape, creating a space that focused on flood control while providing recreation and ecosystem services. South Korea completely removed the Cheonggyecheon Highway, which had previously been built by covering a stream. After the elevated highway was removed, the stream was daylighted and restored to create a multifunctional blue-green corridor through the city center, supporting ecological and urban restoration.
Rebuilding with the River
Flash floods of this scale are not confined to any one region. Kathmandu experienced severe flooding in September 2024 from sustained heavy rainfall. India recorded three comparable events between 2021 and 2024 – Uttarakhand (2021), Sikkim (2023), and Kerala (2024) – driven by glacial lake outbursts, heavy rainfall, and landslides. Valencia’s flash floods killed over 200 people in its urban core. Bangladesh has also experienced floods of comparable scale in recent years. And in 2022, extreme rainfall in northeastern India, particularly Meghalaya and Assam, combined with heavy rainfall in Bangladesh, produced one of the most severe floods recorded in the Sylhet region in more than a century.
The recent Bhotekoshi River disaster is not the first and will not be the last of its kind. It is a warning that is all too likely to be delivered again wherever rivers are treated as obstacles to development rather than as systems with which people can coexist. For the Himalayan region and its downstream neighbours, the lesson to be learned is that construction near rivers must be approached with the same gravity that traditional settlement practices had towards fragile terrain. Early warning systems and monitoring will always remain essential, but there are no substitutes for intact floodplains, wetlands, and settlements built with nature’s uncertainty in question. The cost of relearning this lesson is measured in more than infrastructure – something that is unmistakably familiar to every Nepali citizen.
Shashwat Sharma is researching nature-based solutions for water security in the Kathmandu Valley and is a Doctoral Researcher at the United Nations University's Institute for Integrated Management of Material Fluxes and of Resources (UNU Resource Nexus) and Dresden University of Technology (TUD).
Vaibhav Aggarwal is a Doctoral Researcher at the United Nations University's Institute for Integrated Management of Material Fluxes and of Resources (UNU Resource Nexus) and Dresden University of Technology (TUD). He is also an alumnus of the Indian Institute of Technology Roorkee (IIT Roorkee), India.
Alavy Kifait Reza is a Doctoral Researcher at the United Nations University's Institute for Integrated Management of Material Fluxes and of Resources (UNU Resource Nexus), Leibniz Institute of Ecological Urban and Regional Development (IOER), and Dresden University of Technology (TUD).