Bangkok's canals, sinking ground and flood walls: what visitors see

Claims that ten Bangkok districts are at risk of going under water come from old studies, according to the city's own drainage department. Yet the city is not flood-proof either: it still lists hundreds of places where water gathers after a storm.

This guide explains how the river walls, canals and tunnels work, what the official record says about land subsidence and groundwater, what the 2011 flood showed, and what all of it means if you visit in the rainy season. The figures come from the Bangkok Metropolitan Administration (BMA), a joint World Bank and Thai government flood assessment, and the Groundwater Act.

Walls along the river

The Chao Phraya is lined with flood walls on both banks. The BMA's Drainage and Sewerage Department (สำนักการระบายน้ำ) says the riverbank it has to protect, including Khlong Bangkok Noi (คลองบางกอกน้อย) and Khlong Maha Sawat (คลองมหาสวัสดิ์), runs to 88 km.

Of that, 80 km are BMA walls and 3.65 km belong to private owners or other agencies. The remaining 4.35 km are what the department calls แนวฟันหลอ, literally "gap-tooth lines": stretches where there is no effective wall. By August 2025, 2.60 km of those gaps had been closed and the city expected 3.20 km, or 75%, to be closed before the September high-water season.

The walls are measured against mean sea level (MSL). The department gives their height as +2.80 m to +3.50 m above MSL, and says temporary walls, made up with sandbags where the permanent wall is low or weak, stand at +2.40 m to +2.70 m.

The river normally stays well below that. The same department says the Chao Phraya's level in Bangkok usually moves between -1.00 m and +1.00 m above MSL, and that the highest level on record was +2.53 m in 2011. After that year it raised the walls to an average of about +3.00 m, and in the October–November high-water period it adds sandbags of about 0.50 m.

The weak points are known and counted. Reviewing the 2022 high-water season, the department found 120 problem spots: 76 leaking sections of BMA wall totalling 12.20 km, 12 private or government sections totalling 2.15 km, and 32 gaps totalling 4.35 km. By November 2024 it reported 45 leaking BMA sections repaired and 9 under construction.

The river can take a lot of water from upstream. The BMA says the channel through Bangkok can carry 2,500 to 3,000 cubic metres a second of northern runoff without generally overtopping its banks or the walls.

Upstream releases are the signal to watch. In August 2025 the Royal Irrigation Department expected to release 1,200 to 1,500 cubic metres a second below the Chao Phraya Dam, and the BMA warned that low-lying riverside land outside the flood walls in Ang Thong and Ayutthaya provinces could see water rise by 0.20 to 1 m.

Not everyone lives behind the wall. The department counts 14 communities, 635 households in all, outside the flood line in seven districts: Dusit, Phra Nakhon, Samphanthawong, Bang Kho Laem, Yan Nawa, Bangkok Noi and Khlong San. The department says it posts staff to watch these communities when northern water and high tides coincide.

Canals, pumps and drainage tunnels

Rain inside the walls has to be pumped out. The BMA describes the route as surface water into drains, then minor canals, main canals, pumping stations, gates and tunnels, and finally into the Chao Phraya.

The scale of that system, as the city listed it in 2026, is below. The two lists were issued ten weeks apart and do not match exactly.

In August 2025 the department listed 47 pumping stations on the Phra Nakhon (east) bank and 49 on the Thonburi (west) bank, with a combined capacity of 1,230.55 cubic metres a second.

Drain cleaning is planned each year. In the 2024 fiscal year the city's drains totalled 6,816 km, 2,083 km looked after by the Drainage and Sewerage Department and 4,733 km by district offices, and the department reported cleaning its planned 4,309.6 km in full. The plan for the 2025 fiscal year was 3,803.6 km.

Before a storm, the canals are deliberately lowered. The BMA says it draws down water in main and minor canals, retention ponds known as แก้มลิง ("monkey cheeks") and "Water Bank" storage so there is room for the rain, and it has 50 district flood centres ready during the season.

Backup matters when the power fails. In August 2026 the BMA listed 12 mobile pump trucks with cranes, spare pumps and mobile generators, 200 engine-driven pumps handed to district offices, and work with the Metropolitan Electricity Authority to trim trees near power lines so that pumping stations and tunnels keep running. Rail and other construction sites were told to keep drainage open and have their own pumps ready.

The newest tunnel is in the east of the city. In April 2026 the governor described the Nong Bon tunnel as a 9.4 km "ทางด่วนน้ำ", a water expressway, taking water from Bueng Nong Bon, Khlong Khet, Khlong Bang Na and Khlong Prawet Burirom straight to an outfall at the mouth of Khlong Bang O on the Chao Phraya. Its structure was complete and its six pumps at Bang O were being tested.

That tunnel was delayed by the ground itself. The governor said work fell behind after "เหตุการณ์อุโมงค์และสะพานทรุดตัวเมื่อปี 2565", the sinking of the tunnel and a bridge in 2022, which forced a new, higher alignment between two intake shafts.

More tunnels are under way. The BMA said in August 2025 that boring was finished on a 2.03 km tunnel under Khlong Thawi Watthana on the Thonburi side, due in service in July 2026 with a capacity of 40 cubic metres a second, and that a 13.6 km tunnel along Khlong Premprachakorn in the north was expected to be complete in 2029.

The Thawi Watthana tunnel also has a role beyond local rain. The BMA says the government asked the city to improve drainage on the Thonburi side so that, if upstream floods are very large, about 75 cubic metres a second of floodwater can pass through that side of Bangkok to the sea.

The Premprachakorn tunnel shows how canals and tunnels work together. It is 5.70 m across, serves about 109 square kilometres in the northern districts of Don Mueang, Lak Si, Sai Mai, Bang Khen and Chatuchak, and includes a pumping station and outlet of 60 cubic metres a second. In the dry season the same tunnel can pump river water back up to dilute wastewater in Khlong Premprachakorn, Khlong Bang Khen, Khlong Lak Si and Khlong Bang Bua. Houses built on planned intake sites held up construction, and in August 2025 one site was still under negotiation.

If you want to use the canals rather than worry about them, the guide to river and canal boats covers the passenger services on the Chao Phraya and the khlongs.

What the record says about subsidence

Land subsidence is real in Bangkok, but the official numbers do not agree on how fast it is happening now. In February 2025 the Drainage and Sewerage Department said that, according to current Department of Mineral Resources data, "กทม. มีค่าการทรุดตัวเฉลี่ยที่ 0.35 มม. ต่อปี": Bangkok's average subsidence is 0.35 mm a year.

A much larger figure circulates too. In September 2025 the Department of Mineral Resources' own website reposted a newspaper article whose headline says Bangkok's land is sinking by "เฉลี่ยปีละ 3 ซม.", an average of 3 cm a year. That is a press report, not a department dataset, and the article itself sits on the newspaper's site; the data behind it could not be checked.

The two figures differ by a factor of about 85. No official subsidence map or district-by-district rate was found for this guide, so the only official rate found is the BMA's 0.35 mm a year; the 3 cm figure is a newspaper's.

The BMA names the places it regards as low. In the same February 2025 statement it listed areas close to sea level such as Ramkhamhaeng Road, Bang Kapi and Bang Na.

It also rejected a specific scare. The statement was issued to answer a social-media claim that ten districts were at risk of going under water; the department said the claim came from old studies carried out with the World Bank, the Department of Groundwater Resources and the Land Department, and that the city had since upgraded its defences.

Groundwater and the law

The main tool against subsidence is a limit on pumping groundwater. The BMA said in February 2025 that, with the Department of Groundwater Resources' support, groundwater pumping in Bangkok has been stopped or banned ("หยุด/ห้ามการสูบน้ำบาดาล") to reduce subsidence.

The legal basis is the Groundwater Act of 1977, amended most recently in 2003. An unofficial English translation prepared for the Office of the Council of State defines Critical Groundwater Areas as places where more groundwater is drawn than flows back in, which "may cause land subsidence, infiltration of seawater into the groundwater layers, lowering of water level in the groundwater layers, or other significant environmental impacts".

The Act gives the minister two powers that matter here. Where pumping may cause harm "or land subsidence", section 5 lets the minister designate an area "in which the draw of groundwater is prohibited", by notice in the Government Gazette.

Money is the second lever. Since the 2003 amendment, section 7 allows a groundwater conservation fee to be "additionally collected from groundwater license holders in Groundwater Critical Areas", on top of the ordinary usage fee. The rates are set by ministerial regulation and were not read for this guide.

The translation carries a disclaimer that it has no legal force and that the Thai text prevails. The Department of Groundwater Resources' own website could not be reached, so the current list of prohibited and critical areas in Bangkok is not reproduced here.

Lessons from the 2011 flood

The 2011 flood is the benchmark every Bangkok flood plan refers back to. A rapid assessment carried out by the Royal Thai Government with the World Bank in November 2011 says the floods "inundated more than six million hectares of land in 66 of the country’s 77 provinces, and affected more than 13 million people from July through December 2011".

The human and economic toll was heavy. The assessment says the floods "resulted in more than 680 deaths" and that total damage and losses came to "THB 1.43 trillion (USD 46.5 billion)", with losses making up 56% of the total.

The causes were a sequence rather than one storm. The report describes heavy rain from successive monsoons and tropical storms, followed by breaches in 10 major flood-control structures, and says that by November more than 5.5% of Thailand's land area had been inundated.

Industry bore most of it. The report says manufacturing accounted for roughly 70% of damage and losses because six industrial estates in Ayutthaya and Pathum Thani were flooded from mid-October to November 2011, and it records "record-high flooding, including the Bangkok metropolitan area and its surrounding areas".

The city's response is written into its river walls. The Drainage and Sewerage Department says it raised the walls along the Chao Phraya, Khlong Bangkok Noi and Khlong Maha Sawat after 2011, to an average of about +3.00 m above MSL, against the 2011 peak of +2.53 m.

Rain, not rivers, for visitors

Most of the flood-risk points the city lists are streets where rain collects, not riverbanks. When the department listed Bangkok's 737 flood-risk points in November 2024, 120 were linked to northern water and high tides, while 144 were on main roads and 473 on minor roads, where the problem is heavy rain.

Progress on those points is partial. The governor's three-year report in May 2025 said 383 had been fixed, 133 partly fixed and 221 were still being worked on. Elsewhere the same report says more than 516 of the 737 points had been fixed, a figure that equals the fixed and partly fixed points added together.

Smaller works target the streets themselves. The department said in February 2025 that it was enlarging drains, adding more inlets along roads, building U-shaped and O-shaped gutters to make up for ground that no longer soaks up water, building embankments along and dredging main canals, adding retention lakes and building underground detention tanks. It also said that forecasting work with JICA and Weathernews Japan aims to warn residents of heavy rain three hours ahead.

The design standard is also being raised. At a meeting with JICA in September 2026, the BMA said the city's current design rainfall is about 76 mm an hour, that it could rise to 80–97 mm an hour in 2046–2065, and that planning will use a target of 100 mm an hour. The same meeting discussed making at least half of the open space required around new buildings permeable, and using roads, pavements, parks, schools and public buildings to hold rainwater.

The comparison below sets out the two kinds of flooding the BMA plans for.

For timing and weather patterns across the season, the guide to planning around the rainy season covers what changes month by month.

Canal terms decoded

The Thai words below appear on BMA notices and in the city's flood alerts. Thai script is given where a BMA source uses it.

What this guide does not cover

No official count of Bangkok's canals was found, so the size of the khlong network is not given. No official subsidence map, per-district rate or time series was found either; the only official figure is the BMA's 0.35 mm a year average citing the Department of Mineral Resources, and the 3 cm a year figure is a press headline whose data could not be checked.

The Department of Groundwater Resources' website could not be reached, so the current list of prohibited and critical groundwater areas, and the fee rates, are not given. The Drainage and Sewerage Department's own website could not be opened, so this guide points to its tide tables without reproducing them.

The names of the 737 flood-risk points are not published in the sources used, so this guide cannot tell you whether a particular hotel street floods. Insurance, property risk and long-term sea-level projections are also outside its scope.