Bangkok Post – Capital girds for big rains

As Thailand officially enters the 2025 monsoon season, concerns about prolonged heavy rainfall and potential flooding are rising, particularly in Bangkok.

Already, fears abound that this year will bring severe floods after early rainfall levels appear unusually high. Meteorologists, however, warn that climate conditions remain unpredictable, requiring close monitoring throughout the season.

The Bangkok Metropolitan Administration (BMA) has announced a raft of measures to cope this year, following data indicating rainfall in April and May exceeded the 30-year average (1992–2020) for the same period.

Authorities have also confirmed the city’s current drainage infrastructure can accommodate rainfall of up to 80 millimetres within a three-hour period without suffering significant flooding. However, where rainfall exceeds this, local floods remain a risk, although even these are expected to be resolved within two hours.

The BMA has assured the public that it is actively monitoring weather patterns and has contingency plans in place to mitigate flood risks and minimise disruption to daily life.

Jessada Chandraprabha, director of the BMA’s Drainage and Sewerage Department, last week provided an update on the city’s preparations for what may be an unusually wet season.

Above average rainfall

According to data published on the department’s website, between Jan 1 and May 29, Bangkok endured 493.5 millimetres (mm) in rainfall.

This figure surpasses the 30-year average of 398.4mm as well as that of the past three — 341.0mm in 2022, 221.0mm in 2023 and 300.5mm in 2024, which suggests the capital may experience above-average rainfall in 2025, prompting heightened preparedness measures.

However, 2022 recorded the highest rainfall in three decades at 2,355.5mm, during an abnormally wet year.

Mr Jessada cautioned that this year’s early rain pattern — characterised by intense and localised downpours, often referred to as “rain bombs” — has already posed challenges for water drainage.

He said that when rain falls in a broadly constant manner across the city, drainage can be managed more efficiently which is in stark contrast to the chaos caused by a so-called “rain bomb” where heavy downpours in concentrated areas overwhelm the system.

In general, rainfall should not exceed 60mm per hour to avoid waterlogging as the city only has a limited capacity to handle heavy rainfall, particularly during peak downpours.

To address this, the city has begun widening drainage pipe diameters to 1.5 metres in selected high-risk areas.

“Meteorological forecasts indicate the city may receive up to 1,800mm of rainfall this year –well above the annual average,” Mr Jessada said.

“Bangkok cannot afford to be complacent. Given current trends and past data, we must prepare accordingly,” he said.

He said that last month, individual daily rainfall reached up to 146mm, with several other days exceeding 100mm. These intense rain events have led to temporary flooding in multiple locations, including Bang Khen Roundabout, Soi Ram Inthra 5 and Chaeng Watthana area.

Despite these incidents, Mr Jessada said that flood-caused problems in the most affected areas were cleared within three hours, thanks to the city’s enhanced drainage infrastructure and rapid-response teams.

Mr Jessada has identified 737 flood-risk locations across the capital, encompassing both public and private areas.

Three-phase strategy

He said the BMA has launched a three-phase strategy to manage the seasonal weather and reduce flood risks across the capital.

Phase 1 known as pre-rainfall preparation includes comprehensive inspection and maintenance of critical water management infrastructure.

Over 200 pumping stations, 243 water gates, 349 stormwater pumping wells, and 11 pipe jacking systems have been serviced and prepared for peak performance.

The pre-rainfall phase also includes inspection and maintenance of all four major drainage tunnels to ensure full functionality, dredging of 263 kilometres of canals to improve water flow, clearing and opening of 1,355 kilometres of waterways, cleaning of over 3,800 kilometres of drainage pipes across the city and deployment of mobile water pumps at identified flood-prone locations for rapid response.

In phase 2 (during rainfall), the BMA’s Flood Prevention and Control Centre acts as the central hub for monitoring the situation in real time. It tracks rainfall conditions and receives reports of waterlogging incidents throughout the city. The centre coordinates closely with district-level flood prevention and mitigation operations centres to mobilise assistance for affected communities promptly.

In phase 3 (post-rainfall), after rainfall subsides, efforts focus on accelerating water drainage in critical areas to restore normal conditions as quickly as possible. Priority zones include major roadways, economic districts, schools, and hospitals to minimise disruption.

“Currently, Bangkok’s drainage system can handle up to 80mm of cumulative rainfall per day, typically concentrated over about three hours. Should rainfall exceed this level, temporary flooding may occur; however, on average, water is drained within two hours.” he said.

Monitoring run-off

Mr Jessada also provided an update on the city’s preparedness to manage flood risks from run-off from the North and tidal surges along the Chao Phraya River.

Currently, flood embankments along the riverbank can contain water levels ranging from 2.8 to 3.5 metres.

The embankments upstream — near Rama VII Bridge — are constructed higher than downstream sections, reflecting the natural flow of water that originates upstream and moves downstream.

This variable design approach has been adopted to optimise resource use and reduce construction costs at different points along the river.

To increase flood absorption capacity, the BMA has lowered the water level in front of pumping stations by one metre (relative to mean sea level), creating additional space in canals for water storage.

These adjustments remain flexible and subject to change according to evolving water conditions, he said.

The BMA continues to closely monitor run-off from the North, focusing on the volumes held in the four main reservoirs: Bhumibol Dam, Sirikit Dam, Kwae Noi Bamrung Dan Dam and Pasak Chonlasit Dam.

As of now, reservoir levels are below 40% capacity.

Meanwhile, rainfall downstream of these dams has been insignificant, and water flow measurements at the Bang Sai monitoring station in Ayutthaya province indicate no cause for concern, he said.

Meanwhile, the BMA has allocated nearly 8 billion baht from its 90-billion-baht flood-control budget for 2025 toward improving drainage systems, installing additional pumps, and expanding retention areas.

Flood command centres are now operational 24 hours a day in all city districts, supported by mobile pumps and emergency teams.

Residents can report flooding through the Traffy Fondue app or by calling the 1555 hotline.

Yet in the Rong-Si community, located on Rama III Road, a longstanding riverside neighbourhood located at the mouth of the bay and one of the few areas outside Bangkok’s main flood barriers, residents remain wary.

Here, the merging of tidal surges and city runoff regularly results in severe flooding, exacerbated by floating trash that blocks drainage pathways.

“When the tide rolls in at the same time it rains, water can remain waist-deep for days,” says Ms Somkid, (last name withheld) who has run a local noodle stall for over 20 years.

“High tide is when the sea level here at the bay’s mouth rises to match or exceed the the level of the river, preventing freshwater from flowing out and forcing seawater back into the Chao Phraya,” she said.

“City pumps help, but once that debris is here, it has nowhere to go, so when it floods the water just pours straight into our homes,” adds Uncle Nu, who runs a local neighbourhood grocery store.