Can Bangkok’s ageing drainage system defeat La Niña flood threat?
With La Niña expected to bring unusually high rainfall over the next few months, Bangkok is again at risk of serious flooding.
Urgent measures to combat flooding are hampered, however, by an expanding population that has outgrown the capital’s ageing drainage pipes and infrastructure.
“If plans drawn up by the Bangkok Metropolitan Administration [BMA] fail, this year’s flooding will be worse than the crisis we witnessed back in 2022,” said Sonthi Kotchawat, a lecturer in environmental health at seven state universities.
Two years ago, low-lying Bangkok was one of 37 provinces inundated as total rainfall in October surged 23% higher than average.
Bangkok’s drainage system can handle up to 60 millimetres of rainfall per hour. Anything beyond this and the capital starts to submerge beneath flash floods.
If rainfall exceeds 100mm per hour, large parts of the capital become paralysed and Bangkokians must wait at least two hours for the water to subside.
The BMA’s Department of Drainage and Sewerage has promised to drain floodwater within hours if rainfall is above 100mm/hour, and do so overnight if it ranges between 140 and 150mm.
Better plan this time?
Deputy Bangkok Governor Wisanu Subsompon has sought to reassure residents amid growing flood concerns, explaining that City Hall has applied lessons learned from flooding two years ago.
“Statistics from Traffy Fondue, the popular online platform that accepts and responds to complaints from Bangkokians, has identified the flood-prone areas,” he said.
He was referring to 727 areas of the capital that flooded in 2022. Of them, 617 were inundated after rainstorms while the rest were submerged by runoff from the swollen Chao Phraya River and other waterways.
The BMA said it has strengthened flood defences in 179 of these at-risk areas and is working to protect the remaining 190 before this year’s flood season.
“In all, we have already addressed problems at half of the flood-prone spots,” Wisanu said.
Other preparations this fiscal year include the dredging of 4,280 kilometres of drainage pipes, as well as the capital’s canal system. Over 60% of target pipes and 70% of target canals have been cleared so far to boost drainage capacity, according to the BMA.
It said the efficiency of pumping stations, water gates, drainage tunnels, and overspill ponds is also being maximised, while more pumps are being installed in flood-prone areas.
“We are confident that our drainage performance will improve this year,” Wisanu said.
Bangkok currently has four giant drainage tunnels, with construction underway on four more. Another three are at the planning stage. Once completed, the network of giant tunnels will cover almost all of the capital.
Higher rainfall forecast
Assoc Prof Dr Sucharit Koontanakulvong, director of the Spearhead Research Programme on Water Management at Chulalongkorn University’s Faculty of Engineering, says rainfall will be higher than average this year in Thailand.
“La Niña is intensifying,” he said. “As a result, rainfall levels during the last two weeks of next month will surge. We need to manage water resources carefully.”
He estimates that total rainfall will increase by 5% to 10% nationwide this year compared to 2023. “Following storms, run-off will flow into rivers and flow southwards downstream,” Sucharit said.
Bangkok will be at risk of being flooded along with other provinces in the Chao Phraya Basin, upper North, and upper Northeast, he warned.
Flood risk in other zones
Assoc Prof Dr Seree Suparathit, who heads Rangsit University’s Climate Change and Environmental Centre, said Thailand has experienced unusually high rainfall since mid-July this year, especially in the central, eastern, and southern regions.
But heavier rainfall is expected in August and October as monsoon conditions intensify.
“That means we must closely monitor the flood situation for the rest of the year,” Seree said.
The Central region is particularly prone since it receives huge volumes of runoff from areas upstream, he added.
Some observers suggest the capital risks a repeat of the 2011 flood disaster, which killed 815 people nationwide and cost the economy an estimated 1.425 trillion baht (US$46.5 billion).
Seree is more optimistic, predicting this year’s flooding in Bangkok will be far less serious, and certainly no worse than two years ago.
Recent satellite images show approximately 147,512 rai (236 square kilometres) of land in the country’s northeastern and eastern regions is submerged by floodwater.
Drought risks
Despite the forecast of higher rainfall and flooding, Thailand also faces the threat of water shortages. Drought could hit some parts of the country due to the low volume of usable water in major dams.
As of August 11, the Bhumibol Dam in Tak had dropped to 19% of its capacity with just 5,000 million cubic metres of water. In Uttaradit, the Sirikit Dam had fallen to 37% at just 5,300 million cubic metres. The situation is even worse in the drought-prone Northeast, where Khon Kaen’s Ubol Ratana Dam holds only 684 million cubic metres of water – just 5.5% of its total capacity.
Meanwhile, La Niña’s drought-inducing twin is looming on the horizon.
“The ongoing La Niña will end around the middle of 2025. That means Thailand will face El Niño and perhaps serious drought in 2026,” Seree warned.
Water strategy
The Office of the National Water Resources (ONWR) predicts a high likelihood of flooding this year in Central provinces of the lower Chao Phraya River Basin, including Bangkok, due to rain and runoff.
“Rainfall will be 20% higher than average in this zone between August and September,” said Surasri Kittimonthon, ONWR’s secretary-general.
Surasri says his agency is working hard with other authorities to curb water-related threats as much as possible.
Local empowerment key?
Prof Seree urges the government to revise its approach to water management, insisting that local bodies have the best understanding of local contexts.
“The central government should empower local authorities and organisations by providing them with knowledge, tools and budget to manage disaster risks in their areas efficiently,” he said.
He said centralised management means people living in many at-risk areas of Thailand fail to receive the early warnings required to prepare for floods.
“Even if warnings do arrive, they often come too late,” Seree said.
