Can 'sponge cities' help Bangladesh beat urban flooding?

The images have become all too familiar. Hours of heavy rain are enough to bring Chattogram to a standstill, with roads turning into rivers, homes and businesses inundated, and the country's busiest seaport struggling to keep cargo moving.

Dhaka faced much the same situation after overnight heavy rain on Sunday, with major thoroughfares submerged.

Many cities around the world experience equally intense downpours, yet they do not remain underwater for days. The difference lies not only in how much rain falls, but also in how effectively cities drain, absorb and manage stormwater.

Here lies a paradox. Both Dhaka and Chattogram are inundated by heavy rain, yet both are also witnessing alarming declines in groundwater levels, the primary source of their drinking water. 

While neither city has managed to prevent chronic waterlogging, both continue to channel almost all rainwater into drains instead of retaining at least some of it to recharge the aquifers that sustain their water supply.

This is where new approaches are needed to address both waterlogging and growing water scarcity.

Grey infrastructure has reached its limits

Rapid urbanisation, shrinking wetlands, encroached canals and climate-driven extreme rainfall are making urban flooding an increasingly serious challenge worldwide. While many cities continue to invest in drains, pumping stations and embankments, a growing number are complementing these "grey" infrastructure projects with nature-based solutions that allow rainwater to soak into the ground instead of overwhelming drainage networks.

The latest flooding in Chattogram and Dhaka has once again exposed the limits of relying primarily on conventional infrastructure. The cities have hardly lacked plans, projects or investment focused on engineering solutions to chronic waterlogging. Chattogram alone has undertaken a Tk11,000-crore waterlogging mitigation project, scheduled to run until June 2026. 

Despite canal excavation, retaining walls, sluice gates and drainage upgrades under the project, residents experienced severe waterlogging twice within three months, raising questions about whether expanding concrete infrastructure alone can solve a problem that has become increasingly complex.

The experience of cities elsewhere suggests it cannot. Around the world, urban centres are increasingly combining conventional drainage with "Sponge City" measures such as permeable pavements, bioswales, rain gardens, retention ponds and restored wetlands. These green solutions slow surface runoff, store excess rainwater and allow it to infiltrate naturally, reducing pressure on drainage systems during intense storms.

For a fast-growing coastal city like Chattogram, such approaches may no longer be optional but essential.

Canals, ponds and wetlands as urban assets

Urban planner Muhammad Rashidul Hasan agrees, saying that canals, retention areas and green zones around the city featured prominently in Chattogram's 1995–2015 master plan. It proposed excavating four canals to improve the city's natural drainage while protecting open spaces, hills and water bodies such as ponds.

The same priorities have also been incorporated into the new master plan currently being prepared, he said.

The new plan is based on the "one city, two towns" concept, expanding the city on both sides of the Karnaphuli River, now connected by underwater tunnels. It envisages 11 satellite towns and six industrial zones with integrated drainage networks, flood control, river management and water supply systems, which still rely largely on groundwater extraction.

"We have insisted that low-cost green solutions must be prioritised in the new master plan. If the priorities of the previous plan, such as recovering four canals and preserving ponds, had been implemented, the city would be in a much better position today," said Rashidul Hasan, Dean of the Faculty of Architecture and Planning at Chittagong University of Engineering and Technology (CUET).

Hasan said Chattogram's unique geology compounds the problem. Soft hill soils are easily washed away during heavy rain, carrying sediment into canals and drains. Combined with thousands of tonnes of unmanaged solid waste, the sediment frequently blocks drainage inlets. Routine desilting, improved waste management and stricter enforcement against indiscriminate dumping are therefore just as important as expanding drainage infrastructure.

Cities around the world are increasingly combining conventional drainage with nature-based solutions. China has introduced the Sponge City programme in around 80 cities, aiming to retain up to 70% of rainfall for reuse through wetlands, ponds, permeable pavements and green spaces. Shanghai has developed sponge parks and artificial lakes that have helped the city cope with recent typhoon-related downpours.

Hasan welcomed progress on the city authorities' latest projects to expand drainage and construct canal walls and sluice gates, while pointing to legal grey areas surrounding responsibility and funding for operating and maintaining the newly built infrastructure.

Dhaka faces the same dilemma. A six-hour downpour that brought 76 mm of rain on Sunday inundated roads across the capital and paralysed traffic. Yet the city continues to lose groundwater at an alarming rate. While heavy rain disrupts public life by submerging large parts of the city, the rainwater ultimately rushes into drains instead of soaking into parks, ponds and wetlands. This represents another missed opportunity to replenish the aquifers on which millions depend.

Many canals that once criss-crossed Dhaka have disappeared and are unlikely to be restored. Protecting these remaining waterways from further encroachment should be an urgent priority.

Besides collecting runoff from surrounding areas, canals retain water that can help recharge groundwater. The re-excavation and redevelopment of Hatirjheel and parts of the Begunbari Canal demonstrated how Dhaka's stormwater detention capacity could be enhanced. Further development could transform these water bodies into valuable freshwater reservoirs.

Similar initiatives could be undertaken for canals and water bodies, including those at Kalyanpur, Dholaikhal, Rayerbazar and the area between Uttara and Dakshinkhan, according to a 2017 study by Sadia Subrina of BRAC University and Farahnaz Khadiza Chowdhury of the State University of Bangladesh.

In their paper, Urban Dynamics: An Undervalued Issue for Waterlogging Disaster Risk Management in the Case of Dhaka City, the researchers argued that Dhaka should move away from expensive engineering-heavy projects towards solutions requiring fewer resources, greater community participation and stronger enforcement of existing laws, which could still protect many of the city's remaining water bodies.

They also highlighted easily adoptable measures such as rainwater harvesting, water plazas, rain gardens and grassed swales at both community and household levels. Widening canal banks with pedestrian walkways, green belts and permeable surfaces would cost relatively little while significantly strengthening the city's ability to combat waterlogging.

What Bangladesh can learn from Sponge Cities

Conventional, or "grey", infrastructure—including drains, pumping stations, embankments and sluice gates—will always remain essential. However, these systems are designed primarily to move stormwater away as quickly as possible. During increasingly frequent cloudbursts, their capacity is easily overwhelmed.

Nature-based, or "green", infrastructure works differently. Wetlands, ponds, parks, permeable pavements, rain gardens and bioswales temporarily store rainwater, allowing it to seep into the ground, easing pressure on drainage systems while replenishing groundwater.

Cities around the world are increasingly combining conventional drainage with nature-based solutions. China has introduced the Sponge City programme in around 80 cities, aiming to retain up to 70% of rainfall for reuse through wetlands, ponds, permeable pavements and green spaces. Shanghai has developed sponge parks and artificial lakes that have helped the city cope with recent typhoon-related downpours.

Jakarta is pursuing its "Live with Water" strategy to improve flood resilience without displacing riverside communities. Chennai has restored wetlands and incorporated bioswales and retention basins into its urban planning to slow runoff while recharging groundwater.

Towards cities that absorb rain, not just drain it

Bangladesh's cities will continue to need larger drains, stronger embankments and better pumping systems. But engineering alone cannot keep pace with rapid urbanisation and the increasingly erratic rainfall driven by climate change.

The next generation of urban planning must allow cities to behave less like concrete basins and more like natural landscapes that absorb, store and gradually release water.

Shanghai's Sponge City project reportedly cost about $230 million. Could Chattogram, which has spent around five times that amount on four mega projects over the past decade, incorporate Sponge City principles into its two upcoming waterlogging mitigation projects worth Tk3,871 crore?

Urban planner Rashidul Hasan has suggested offering incentives to property owners to preserve ponds within the city, alongside stronger enforcement of laws protecting wetlands and other water bodies.

Can institutions in Dhaka, Chattogram, Khulna, Rajshahi and Sylhet allow their open spaces, ponds and wetlands to serve as nature-based solutions to the waterlogging that recurs almost every monsoon?