Dhaka's next flood is predictable. Are we prepared?

In September 2004, some 839 mm of rain fell in Dhaka. As astounding as it may sound, in just one month, an estimated 256 million cubic metres of water fell in the city, roughly the volume of over 100,000 Olympic swimming pools. What followed was a catastrophe: roads vanished, homes flooded, people died. What was perhaps the most unfortunate was that none of it managed to inspire a flood management strategy that works in the capital city of Bangladesh.

A newly completed analysis of 40 years of monthly rainfall data in Dhaka, from January 1985 to December 2025, forces us to confront a truth that our planning institutions have spent decades politely ignoring. Dhaka is not experiencing freak weather, but a statistically predictable, mathematically quantifiable deluge that this city was never designed to handle. Over the past 40 years, about 29 “extreme rainfall months” (months with more than 500 mm of rain) were identified. This translates to about one major event every 16 months on average. The worst of these events was in September 2004 (839 mm), July 2007 (753 mm), September 1991 (692 mm), and September 1986 (687 mm). These are household tragedies for millions of Dhaka residents, but they are also planning data as they define the upper boundary of what this city must be able to withstand.

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Dhaka WASA’s pump station network, operating at full capacity 24 hours a day, can drain approximately 8.6 million cubic metres of water per day. Against the 256 million cubic metres of a peak event, that is a drainage capacity of 3.4 percent. The remaining 96.6 percent ends up on our roads, alleyways, and houses. And this is the part of this story that demands the most honest accounting, as the natural systems that once protected Dhaka from precisely this kind of flooding—the canals, haors, beels, and eastern floodplains—were not lost to climate change but to ourselves.

The greater Dhaka metropolitan area experienced a significant loss of wetland by more than 50 percent between 1990 and 2020. From 2000 to 2010, the water body area alone was reported to have decreased by 13 percent in remote sensing studies. The Begunbari Khal, which historically moved stormwater from central Dhaka to the Balu River, was reduced to a concrete-lined trickle. The eastern lowlands, the city’s primary natural flood buffer, were converted into the Purbachal and Bashundhara developments, built directly atop functioning floodplains.

The estimated loss of natural water storage in the urban hydrological system is 2,500-6,500 cubic metres per hectare of wetland lost. Dhaka has already lost 5,000 hectares of functional wetland since 1985, and a conservative estimate (based on published research) has shown that Dhaka has lost between 125 and 325 million cubic metres of natural flood storage capacity. That’s just five percent of the peak event volume of 256 million cubic metres. The city itself has been dismantling its own flood protection measures even as rainfall extremes have become more severe.

The reactive cycle that keeps failing us

Bangladesh’s urban water governance has a predictable rhythm of its own. A catastrophic flood arrives. Politicians make commitments. Donors pledge funding. Drainage projects are partially implemented. Then the next flood overwhelms the improved system, and the cycle begins again.

The 1988 floods—when both June and July exceeded 500 mm and over 60 percent of Bangladesh was inundated—led to the Flood Action Plan of the early 1990s. Then September 1991 delivered 692 mm. The 1998 floods, the worst in recorded national history with consecutive extreme months, produced a revised metropolitan development plan and renewed World Bank engagement. Then, in September 2004, 839 mm was recorded. Post-2004 World Bank drainage projects were implemented, and in July 2007, 753 mm was recorded.

A planning system that designs for the last disaster is perpetually one monsoon season behind. We are not learning from our data. We are surprised by it.

Could Dhaka become a sponge city?

The “sponge city” framework does not ask us to stop building drains. It makes us think that we need not pretend that drainage is a solution to a problem that can only be resolved with landscape. This is an important transformation from the evacuation of water to the absorption of water: keep it where it falls, let it out slowly, and embed that management in all land-use planning.

Here are the five most important things to do for Dhaka.

First, on-site retention must be mandatory, requiring any new development greater than 1,000 square metres of impervious surface to retain the first 100 mm of water from any rainfall event, through green roofs, bioretention cells, permeable paving, or underground cisterns.

Second, canal restoration must be the primary objective of infrastructure investment. Restoring a kilometre of khal to functional drainage is significantly more cost-effective than the equivalent expansion of grey infrastructure, which delivers no co-benefits for biodiversity, urban cooling, or groundwater recharge. The Begunbari Khal corridor alone, restored and buffered, would return significant stormwater attenuation capacity to central Dhaka.

Third, preservation of every remaining wetland east of the Demra-Narsingdi Road corridor must be legally binding. This needs a permanent zoning reclassification to ecological reserve, with all pending development approvals within that belt cancelled immediately.

Fourth, a network of 200-400 small retention basins—each between half a hectare and two hectares—distributed across the city’s ward areas should be built, connected by swale systems to the main khal network.

Fifth, the city’s capital expenditure for water management should be rebalanced. Over the next 10 years, shift the budget from its current 85-90 percent grey infrastructure allocation to a 50-50 split between grey and blue-green infrastructure. Maintaining the city’s pipes and pumps is also essential.

The question that politicians and planners in this city must answer is not whether Dhaka can afford to invest in its wetlands and canals. It is whether Dhaka can afford another September 2004 or July 2007. The last extreme month was August 2024. Records show extreme months return every 16 months on average. We are now past that interval. The 2026 monsoon has begun. Unfortunately, when the next extreme month arrives, Dhaka will not be ready. The data has been saying so for 40 years; the question is whether anyone in power is listening.

Masudur Rashied is an urban planner and water management policy and technology professional.

Views expressed in this article are the author's own. 

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