If you live in Kathmandu, you might have noticed how quickly water accumulates on the streets these days whenever it rains, especially in busy areas like Tinkune, Jamal, and Putalisadak. Just 20–30 minutes of rainfall is enough to make people worry not only about getting soaked but also about their safety.
We grew up hearing and watching news reports of deaths and property losses caused by massive floods in the southern plains, or the Terai, during the rainy season, and somehow those disasters seemed distant and normal. But have we ever wondered why floods have suddenly become our new and frequent companions right here in the heart of Kathmandu and Bhaktapur?
Cities around the world are facing a surge in flash floods. In 2024 alone, a major flash flood in Spain killed nearly 224 people. As the name suggests, these floods appear suddenly following intense rainfall.
To understand why this is happening, we need to revisit the basic hydrological cycle. Normally, when rain falls on the ground, the soil acts like a sponge. It absorbs water, feeding springs, rivers, and deep underground aquifers, while the remainder evaporates, forms clouds, and eventually returns as rain. This million-year-old cycle makes our entire ecosystem possible.
In our rush to import glass-and-concrete infrastructure from the West, we have also imported many of the urban problems that accompanied it, including flash flooding.
Scientists often point out that every drop of water we have on Earth today is the same water that has always existed on Earth. However, modern cities and towns are increasingly covered with concrete. Urbanization, including the construction of buildings and the paving of land for roads, parking lots, and other infrastructure, has disrupted the natural water cycle.
With less exposed soil available, rainwater can no longer infiltrate the ground effectively. Instead, it remains on the surface, rushing across asphalt roads and overwhelming everything in its path within minutes. While light showers may not create major problems, heavy downpours can quickly turn into disasters such as flash floods.
To make matters worse, scientists predict that rainfall will become more intense and frequent in the coming years due to climate change. This means flash floods are likely to become even more common and dangerous.
According to a satellite-tracking study published in 2017 in the journal Environments, urban areas in the Kathmandu Valley have increased by 412 percent over the past three decades, wiping out nearly 31 percent of the valley’s fertile agricultural land. Due to weak governance and unchecked urban planning, many wetlands, ponds, and open-soil areas that once acted as natural sponges have been sealed beneath concrete.
In Kathmandu’s case, the flood problem is compounded by several other systemic failures, including settlements built directly on floodplains, excessive groundwater extraction, and a weak drainage system. As people migrated to the valley in search of better opportunities, the natural floodplains of the Bagmati, Bishnumati, Hanumante, and Dhobikhola rivers were heavily encroached upon and transformed into dense settlements.
Today, an estimated 15 to 35 percent of the valley’s built-up area lies within high- to very high-risk flood zones. This vulnerability exists largely because we continue to expand settlements on floodplains while simultaneously increasing concrete coverage.
The catastrophic floods of September 2024 in Kathmandu, which killed 56 people within the valley, were a tragic manifestation of this double disaster.
When conventional drainage infrastructure proved inadequate for managing flash floods, cities such as Copenhagen, Wuhan, Shenzhen, Philadelphia, Manchester, and London adopted a new approach to water management known as the Sustainable Urban Drainage System (SuDS). This approach is also referred to as the Sponge City Concept or Water Sensitive Urban Design (WSUD).
These systems operate at the micro level, integrating natural elements such as plants, soil, and water features to reconnect urban environments with the underlying earth. This makes infiltration—the process by which soil absorbs water—and the safe storage of stormwater possible.
For example, instead of using asphalt or concrete for parking lots and sidewalks, many cities use permeable pavements. These are specially designed paving materials with gaps that allow rainwater to filter through and reach the soil beneath.
Similarly, roadside rain gardens are constructed to capture, slow, and absorb runoff before it reaches drainage systems. These shallow depressions are planted with native vegetation and filled with deep soil that helps retain water naturally.
Cities also create retention ponds and artificial wetlands connected to urban drainage networks. These features can safely store millions of liters of stormwater and release it gradually over several days rather than allowing it to flood streets within minutes.
Such techniques are often used individually or strategically combined to give cities a flood-resilient, sponge-like quality. Although the primary objective is flood mitigation, SuDS also address multiple urban challenges simultaneously by integrating blue infrastructure (water) and green infrastructure (vegetation).
SuDS help cool local climates, improve air and water quality, and enhance biodiversity. Interestingly, urban designers and drainage engineers have also found ways to make these systems both functional and enjoyable.
One popular example is Benthemplein Water Square in Rotterdam, completed in 2013. On dry days, it functions as a public square featuring basketball courts and recreational spaces. During heavy rainfall, however, it transforms into a temporary water-storage basin.
But this is where the discussion becomes particularly relevant for Nepal.
What many Western cities have only recently embraced as an effective solution for managing urban water, our ancestors mastered centuries ago. Kathmandu’s traditional Hiti (stone spout) system, supplied through a network of Rajkulos, was not merely a drinking-water system but also a sophisticated water-management network resembling modern SuDS principles.
The underlying logic of the sponge city was never foreign to us; it is woven into our history. For Kathmandu, the sponge city was never merely a concept. It was once a reality.
The key to this system lay in the thousands of interconnected ponds, or Pukhuris, scattered across the valley. Some functioned much like modern retention ponds, storing excess monsoon runoff and recharging underground aquifers.
Likewise, the brick-paved Bahals, Chowks, streets, and narrow alleys acted as natural permeable surfaces, allowing the earth beneath to breathe and absorb water. In many ways, our ancestors built a literal sponge city from terracotta and stone, protecting Kathmandu from flooding for centuries.
Unfortunately, these traditional water-management systems have gradually been neglected and undermined. Of the valley’s 233 historic ponds, only 193 remain, and many of those are either dysfunctional or significantly reduced in size. Likewise, traditional brick-paved roads and alleys in historic urban cores such as Kathmandu, Bhaktapur, and Lalitpur have largely disappeared.
In our rush to import glass-and-concrete infrastructure from the West, we have also imported many of the urban problems that accompanied it, including flash flooding.
At the same time, it is unrealistic to expect Kathmandu to dismantle its modern built environment and return to a medieval urban form. Nor can we ignore the needs of a rapidly growing city. The encouraging news, however, is that Kathmandu does not need to start from scratch, and SuDS do not necessarily require large amounts of space.
While localized SuDS alone cannot solve Kathmandu’s complex flood challenges, they can significantly reduce the immediate pressure on streets and drainage systems. Implementing SuDS can be as straightforward as replacing concrete parking lots with permeable pavements, installing rainwater-harvesting barrels, or retrofitting roadsides with engineered rain gardens designed to slow and filter surface runoff.
Therefore, the way forward is to restore as much of our traditional water wisdom—including Hitis and Pukhuris—as possible, while simultaneously adopting modern SuDS solutions tailored to Kathmandu’s unique soils, streets, and urban context.
The underlying logic of the sponge city was never foreign to us; it is woven into our history. For Kathmandu, the sponge city was never merely a concept. It was once a reality.
And it can be again.
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