Bangkok’s 48-hour deluge exposes limits of city drainage system

More than 300 millimetres of accumulated rain over Bangkok during September 25–26, 2026 has exposed the limits of the capital’s drainage infrastructure and prompted experts to call for a fundamental reassessment of how the city prepares for extreme rainfall.

The prolonged downpour lasted for more than two days and inundated major roads even though there was no significant influx of upstream water into Bangkok.

Conditions in the inner Gulf of Thailand added to the problem, with strong monsoon winds and high sea levels making it more difficult to drain water from the capital towards the sea.

Academics and disaster specialists say the episode illustrates the need to move beyond simply responding to flooding and instead design the city around worst-case scenarios, covering drainage capacity, early warnings and long-term infrastructure investment.

They also argue that historical rainfall records should no longer be treated as fixed limits when assessing future climate risks.

Experts call for worst-case warning systems

Assistant Professor Dr Thon Thamrongnawasawat, a marine and environmental specialist at Kasetsart University’s Faculty of Fisheries, described the event as prolonged or “stationary” rainfall, with rain-bearing cloud systems repeatedly passing over the same areas for more than 30 hours.

He attributed the intensity to the combined effects of what he described as Super El Niño conditions and rising global temperatures, which allow the atmosphere to hold more moisture.

Thon noted that, as a general physical principle, the atmosphere can hold around 7% more water vapour for every 1C increase in temperature, increasing the potential for heavier rainfall.

He said conditions were compounded by monsoon winds over the inner Gulf of Thailand exceeding 40 kilometres per hour, generating waves of more than one metre and contributing to elevated sea levels that restricted Bangkok’s ability to discharge floodwater.

Thon also criticised the timing of emergency warnings.

He said a Cell Broadcast message advising residents to move belongings to higher ground was issued at around 10am, even though flooding had already affected areas including Ekkamai from around 1am–2am.

Post-disaster assistance could also be delayed by legal and administrative constraints, he added.

“The phrase ‘unexpected’ can no longer be used in an era of global warming, because it means using the past to predict a future that is no longer the same,” Thon said.

He argued that authorities should prepare for the worst plausible outcome in advance, even if forecasts ultimately prove more severe than what occurs.

Thailand, he added, needed to acknowledge that its warning and emergency-response systems were still insufficient and should be strengthened nationwide, particularly ahead of future climate-related risks.

Experts question reliance on major drainage infrastructure

Associate Professor Dr Seree Supratid, director of the Climate Change and Disaster Center at Rangsit University and an expert involved with Intergovernmental Panel on Climate Change work, said the flooding also raised questions over Bangkok’s existing drainage strategy.

Following the major floods of 2011, Bangkok invested heavily in large-scale drainage infrastructure, including several major drainage tunnels, largely to cope with water flowing into the capital from upstream areas.

Seree said the latest event challenged assumptions that these investments alone would be enough.

He pointed out that cumulative rainfall exceeding 300mm overwhelmed parts of Bangkok even without significant upstream run-off.

“I used to believe that the 2011 upstream flood led Bangkok to build more than four large drainage tunnels, with several more planned at enormous cost,” he said.

“That belief has been washed away by this year’s flood. Even with more than 300mm of accumulated rain falling locally, without upstream water, the system still could not cope.”

Seree also warned that heavy accumulated rainfall remained a concern in western and eastern Thailand.

He called for a wider reassessment of flood risks rather than continued reliance on conventional drainage infrastructure alone.

Three areas identified for reform

The two academics pointed towards three broad areas requiring changes if Bangkok is to cope better with future extreme rainfall.

First, early-warning systems need to be strengthened. Cell Broadcast and other emergency alerts should operate in real time and reach residents before flooding occurs rather than after roads and communities are already inundated.

Second, infrastructure planning needs a broader approach. Bangkok should not rely solely on large drainage tunnels, but should integrate urban drainage with coastal conditions, monsoon winds, sea levels and systematic water diversion.

Third, disaster policy should become more proactive, using worst-case assumptions when preparing personnel, equipment and compensation measures so that assistance can be deployed before extreme weather causes greater damage.

Bangkok increasingly relies on data and AI

The Bangkok Metropolitan Administration (BMA) has already been shifting towards data-driven water management, using a central command centre to collect information from closed-circuit television cameras and telemetry systems before directing teams to problem areas.

Artificial intelligence is also being used alongside C-band and X-band weather radar to analyse the formation and movement of rain clouds.

According to the supplied information, the system can forecast rainfall up to three hours ahead with an accuracy of around 80–85%, allowing officials to lower canal levels and position machinery before heavy rain arrives.

Bangkok is also focusing on what it describes as the drainage system’s “capillaries” — roadside and community drains that carry water towards the city’s larger “arteries”, including major drainage tunnels.

Measures include dredging drains according to local conditions, repairing roads and introducing new drainage channels.

These include O-Gutters, designed with long, narrow openings to reduce rubbish blockages, and larger steel-covered U-Gutters, intended to improve water flow in low-lying areas.

Bangkok’s current drainage infrastructure

The BMA has installed automated pump-control sensors at 431 pumping pits, along with backflow-prevention gates, and has added 1,107 pumps across Bangkok.

Its current drainage infrastructure includes:

• 1,980 canals, with a combined length of 2,745 kilometres
• 38 retention areas and water banks, with combined capacity of 13.71 million cubic metres
• 207 pumping stations
• 244 water-control gates
• Five major drainage tunnels
• 6,854 kilometres of drainage pipes

The city also has 12 mobile units, 46 water-propelling machines and 24 BEST rapid-response teams available around the clock.

Despite those resources, experts say the September rainfall demonstrates that Bangkok’s future flood planning will need to account for increasingly extreme combinations of rainfall, sea conditions and urban constraints rather than relying on historical assumptions alone.

Source: Krungthep Turakij

 

For those who wish to help our fellow citizens suffering and affected by the floods, you can make a donation through the following accounts:

Nation Foundation

Bank of Ayudhya (Krungsri): Account No. 333-0-01399-6

Kasikorn Bank: Account No. 192-3-59782-0

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