Heavy rain is falling over Colombo, bringing a familiar anxiety to a City that knows how quickly roads can disappear beneath water. For anyone whose home or business has flooded before, every prolonged downpour carries an unsettling question: Will it happen again? As the rain continues, drains, canals and wetlands become more than features on a planning map. They become the difference between getting home safely and finding that the water has arrived first.
Across Asia however, parts of that story have been rewritten. In Singapore, a river has been given space to spread through a public park. In Bangkok, Thailand, valuable urban land has become a landscape that holds stormwater. China has experimented with cities designed to absorb rain, while Japan has built vast underground passages for floodwater. These are different responses to a shared predicament: urban development has changed where water can go, while the consequences arrive at somebody’s doorstep.
Singapore: Flood management through the source of runoff
Singapore’s approach begins before water reaches a drain. Its National Water Agency, PUB, describes flood management through the source of runoff, the pathway that it follows and the places that it can damage. That encompasses detention within developments, improvements to drainage networks and protection at vulnerable buildings. New developments and redevelopments of at least 0.2 hectares are required to incorporate measures that slow surface runoff and reduce peak flows into public drains. Responsibility for stormwater begins within the development itself.
The distinction matters because a City is also a surface. Roofs, roads and paved compounds influence how quickly rainfall becomes runoff. When water leaves many sites simultaneously, the pressure accumulates downstream. Detention tanks and ponds interrupt that timing by holding water temporarily and releasing it more slowly. Their contribution is partly invisible: less water arriving at the same place at the same moment. Flood management, in this sense, involves managing time as much as managing space.
At Singapore’s Bishan–Ang Mo Kio Park, this thinking becomes visible. A concrete canal was transformed into a naturalised, meandering river running through Parkland. Grassy banks and surrounding open space accommodate changing water levels. During heavy rainfall, the river occupies more of the landscape. Monitoring and warning arrangements help manage the danger to visitors. The space offers recreation in ordinary weather while performing a different function when the water rises. Its appearance changes because its purpose changes.
That challenges an assumption embedded in many urban landscapes: that water outside a narrow channel automatically signifies failure. In a designed floodplain, temporary inundation can be part of the intended operation. The important distinction concerns where the water spreads and what is exposed. A lawn that temporarily holds water presents a different situation from an occupied home doing the same. The photograph alone can obscure that difference. Two flooded spaces can tell entirely different stories about a City’s preparedness.
Bangkok’s Chulalongkorn University retains water within a place that people use every day
Bangkok’s Chulalongkorn University Centenary Park offers another expression of this relationship. Opened in 2017, the Park includes a green roof, constructed wetlands, detention lawns and a retention pond. Its gently sloping terrain directs runoff through a connected landscape. According to the project designers, it can hold up to one million gallons of water during heavy rainfall. Beneath the green roof, underground tanks store rainwater that can later irrigate the Park. Water is retained within a place that people use every day.
The design makes gravity part of the infrastructure. Water moves from higher ground through planted areas towards the pond. During severe flooding, storage can expand into the main lawn. Wetlands help slow and treat runoff along the way. These elements form a working landscape whose drainage function is integrated with public recreation. The Park occupies 11 acres, a modest footprint beside the scale of Bangkok’s flood exposure. Its significance lies in what that footprint demonstrates about urban land.
A Park is easily understood as an amenity: somewhere to walk, exercise or escape the heat. A place that stores stormwater carries another kind of value, extending beyond the people inside its gates. Yet, that service becomes most visible during weather that discourages visitors. Much of the time, the infrastructure looks like greenery. Its usefulness can therefore be underestimated precisely because it does not resemble the structures commonly associated with protection: walls, concrete channels or machinery.
China’s Sponge City Initiative
China’s Sponge City Initiative, introduced in 2014, brought similar ideas into a broader urban programme. Permeable paving, rain gardens, green roofs and wetlands are combined with conventional infrastructure to retain, filter and slow rainfall. The sponge metaphor describes a City with more opportunities to hold water before discharging it. The approach distributes water management across multiple locations. A planted depression or absorbent surface contributes one part of a system whose effects accumulate across the catchment.
The metaphor has limits. Saturated soil cannot keep absorbing indefinitely, and storage eventually fills. Extreme rainfall can overwhelm both planted landscapes and engineered systems. The World Bank’s (WB) discussion of China’s experience stresses the combination of natural features, structural measures, forecasting and early warning. The existence of sponge City projects does not mean that a City has become immune to floods. Their role is to reduce particular risks within a larger system, whose performance still depends on the scale and character of the event.
Japan’s underground discharge channel takes floodwater from several rivers
Japan provides a striking example of that larger system’s engineered side. The metropolitan outer area underground discharge channel takes floodwater from several rivers through underground infrastructure towards the Edogawa River. Its pump station contains four large pumps, each capable of moving 50 cubic metres (cm2) per second. Together, their maximum capacity reaches 200 cm2 per second. Weather and river information are monitored centrally, supporting coordinated control of gates and pumps as conditions change above ground.
The underground structures attract attention for their scale, but their operation reveals another dimension of flood protection. Water has to enter the system, travel through it and reach a receiving river. Pumps and controls connect those stages. The visible structure is only part of an operational network. Unlike a Park that changes appearance during rain, much of this activity happens beyond ordinary view. Residents encounter the outcome above ground without necessarily seeing the processes beneath them.
Set beside one another, these examples complicate the familiar division between engineering and nature. Singapore’s river Park depends on designed landforms and warning systems. Bangkok’s planted landscape includes underground storage. China’s Sponge approach integrates greenery with conventional infrastructure. Japan’s pumps operate within a wider geography of rivers. The differences concern scale, location and function. Each intervention deals with water moving through a larger landscape, and none exists independently of what happens upstream or downstream.
Colombo already belongs within this conversation. The WB has documented its urban wetlands storing floodwater and releasing it as water levels recede, alongside pumping stations and underground tunnels. This is a City where ecological systems and built infrastructure share the work of drainage. Wetlands also support biodiversity and help regulate the urban environment. Their appearance as open, sometimes waterlogged land can conceal how much activity depends on the space that they leave available for water.
There is an uncomfortable imbalance in how that space is valued. A filled plot produces something immediately visible. The lost capacity to hold water may remain unnoticed until rain exposes its absence. The consequences can appear elsewhere, beyond the boundary of the development and long after construction has finished.
Nothing about that transfer is obvious on a sunny afternoon. Buildings stand complete, roads look usable, and the missing wetland leaves no warning on the skyline. The City appears settled. The unanswered question lies in what happens when the rain returns.
In the next downpour, some Asian Cities will direct water towards landscapes prepared to receive it. Beneath others, pumps will begin turning in chambers that most residents never see. And somewhere, on a street that looked perfectly ordinary that morning, the water will reach a doorstep. Behind it, someone will be waiting to discover whether it stops there.
The writer is an electronic engineer with a background in information technology and sustainability
--------------------------------
The views and opinions expressed in this column are those of the writer, and do not necessarily reflect those of this publication