The modern "sponge city" concept follows a simple idea: rainwater should be absorbed, stored, slowed, and reused before being removed immediately through drains, with drainage systems primarily handling excess water
Urban flooding has been the story of every monsoon in Kathmandu. Flood arrives following a rainfall, and every commuter struggles with severe waterlogging. Then the season changes, water disappears along with the public attention until the next monsoon brings the same problem. Only 20-30 minutes of rainfall is enough to turn the roads of Teku, Anamnagar, Tinkune, and Maitighar into rivers and ponds. But is Kathmandu's urban flooding really a "too much rain" problem?
Not necessarily. When rain falls, some soaks into the ground, some is temporarily stored in ponds, wetlands, or pits, some eventually evaporates, and the rest the rest flows away as runoff into drains or rivers. In a healthy landscape, all these processes share some burden. But in urban areas like Kathmandu, urban flooding begins when we disturb that balance. Very little rainwater gets to soak into the ground, and too much water is forced to run off at once. And when this runoff reaches the drainage system faster than the drains and rivers can carry, the streets themselves become the drainage channels.
Kathmandu has changed drastically over the last few decades. Green fields have become housing colonies; open spaces are paved, and roads are widened and covered with asphalt. Places where rainwater had ample time to seep underground now shed it within minutes. Recent studies show that urban expansion in the Kathmandu Valley has increased by over 400% in the last three decades. Roofs, roads, paved surface, and other hard surfaces are impermeable, which prevents rainwater from soaking naturally into the soil. When such surfaces dominate a city, even a short burst of intense rainfall can generate large volumes of runoff and overwhelm the drainage system.
Because less water is absorbed, more runoff reaches Kathmandu's rivers. However, in many places, we have also reduced the space available for these rivers to carry water. Rivers like the Bagmati, Vishnumati, Dhobi Khola, Tukucha, and Hanumante have faced increasing encroachment as the city expands. Concrete embankments along the riverbanks also confine rivers and disconnect them from their natural floodplains, reducing the space available to spread during heavy downpours. As a result, water levels rise more rapidly, increasing the risk of flash floods along river corridors.
As Kathmandu grew, we increasingly depended on the drainage system to carry water away. But the drainage system did not grow at the same pace as the city. In many places, drains are undersized, clogged with sediment, garbage, and construction debris, or poorly connected. This can cause a local drain to fail even when it is functioning properly, particularly when the downstream channel is already full. We are asking an old, fragmented, and undersized system to handle far more water than it was designed to carry. During intense rainfall, drains quickly fill, and excess water runs down the streets like rivers, turning intersections into ponds.
Urbanisation and poor drainage are not the only parts of the problem. Climate change is also affecting rainfall patterns. Seasonal total rainfall may remain similar, but intense rainfall over a few hours can make the impact very different. The ground, drains, and rivers have less time to absorb, store, and safely convey the water. Climate change then acts as a risk multiplier. It does not create the problem by itself, but because of Kathmandu's existing urban conditions, it can make the consequences much more severe. This then raises the question of how we have designed the city.
Ironically, many ideas now promoted globally under the "sponge city" concept are not entirely new to Kathmandu. The Valley's traditional landscape included ponds, rajkulos, and hitis that collected and distributed water and helped recharge groundwater. Agricultural fields and open spaces allowed water to soak into the ground. These systems followed a simple hydrological principle of not forcing every drop of rainwater out of the city as quickly as possible but giving water time to stay, seep, circulate, and recharge the ground.
As Kathmandu urbanized, many of these spaces and systems were built over or neglected. While trying to modernize the city, we removed some of the very features that once helped manage water naturally. Kathmandu may therefore not need to look entirely outside the Valley for answers. Some of the principles required for a more flood-resilient future can already be found in the Valley's own history.
The modern "sponge city" concept follows a simple idea: rainwater should be absorbed, stored, slowed, and reused before being removed immediately through drains, with drainage systems primarily handling excess water. Measures such as permeable pavements, green spaces, restored ponds and wetlands, infiltration trenches, and temporary detention areas can help. None of these measures will solve Kathmandu's flooding problem alone, but they can help reduce the volume and speed of runoff reaching drains and rivers during intense rainfall.
The contrast is simple. In traditional Kathmandu, rain could fall on soil, collect in ponds, recharge groundwater, and move gradually through the landscape. In today's concrete Kathmandu, rain falls on roofs and roads, becomes runoff almost immediately, and is pushed towards drains that are already under pressure. A future sponge Kathmandu would try to restore some of that lost balance by capturing water where it falls, allowing more of it to infiltrate, storing some temporarily, reusing what can be reused, and draining only what remains.
The solution, therefore, may not be to build bigger drains but to make the city capable of holding water again.
Dhakal is a geomatics engineer working as a remote sensing and geospatial associate at ICIMOD