Today, 26 September 2026, the Department of Highways lists 109 flooded points on its roads in 19 provinces. Eight cannot be passed. One is Highway 1 near Takhli in Nakhon Sawan. Five are in Chonburi and Chachoengsao, on the Eastern Seaboard. The day before, the Office of the National Water Resources warned that the release through the Chao Phraya Dam would reach about 2,000 cubic metres a second, and named the low-lying districts it expects to flood from Chai Nat down to Nonthaburi.
None of that came as a surprise. Floods on Thai roads follow a calendar, follow the rivers, and mostly give notice. This article shows which roads tend to go first, which public numbers warn you, and how to turn the warning into a date on which to move stock.
Three flood seasons, not one
Thailand does not have one flood season. It has three, in order.
| Region | When the risk peaks | What kind of flood |
|---|---|---|
| North, and the hills of the upper Central region | August and September | Flash floods and landslides. Fast, local, little warning |
| Central plain, the Chao Phraya and its tributaries | September to November | River floods. Slow, wide, announced days ahead |
| South, east coast (Songkhla, Pattani, Narathiwat, Nakhon Si Thammarat) | November and December | Monsoon rain on flat coastal land. Days of warning, days of closure |
The Thai Meteorological Department puts the rainfall peak for upper Thailand in August or September and for the southern east coast in November. Its count of tropical storms and depressions entering Thailand over 75 years makes the same point more sharply: of 207, 55 came in September, 56 in October and 32 in November. That is 69% of all storms in three months. Add August and it is 81%.
So a lane from Bangkok to Chiang Rai carries its risk in August and September. A lane from the Eastern Seaboard to Hat Yai carries it in November and December. A lane through Ayutthaya carries it from September until the river goes down, which in 2011 kept industrial estates under water into November. If your contingency plan covers "the rainy season" as one block, it is aimed at the wrong months for at least one of your lanes.
Which roads go first
The Department of Highways publishes a daily list of flooded points during an event, with highway number, kilometre and whether traffic can pass. Read several seasons of these lists and a pattern appears.
Most closures are on the smaller roads. In the Department's numbering, one- and two-digit highways are the main routes and three- and four-digit numbers are the secondary and connecting roads. Of today's eight impassable points, one is on Highway 1. Two are on three-digit roads and five are on four-digit ones. In a bigger event the trunk roads do go too: on 27 September 2021, during that year's storm floods, 6 of the 28 impassable routes listed were one- or two-digit highways, including Highways 1, 11, 12 and 32. But even then, most of the list was the smaller roads. The same shape shows in the 2011 World Bank assessment of that year's flood: some main roads had reopened while the secondary roads to factories were still under up to 1.5 metres of water.
For a shipper, that means the last few kilometres are the risk, not the trunk route. A factory that is 2 km off a highway that stays dry is still closed if those 2 km flood.
The same places come back. A few sections appear in different years:
- Sukhothai. Highways 101 and 1195 were impassable in 2021 and again in 2024, when 21 km of Highway 101 at Si Samrong was on the list. Highway 12 was impassable at Sukhothai in 2021.
- Ang Thong, Sing Buri and Ayutthaya. Highway 32 was impassable at Ang Thong in 2021. In October 2022 Highway 311 was under up to a metre of water at Sing Buri and In Buri, and Highway 32 was the recommended detour. Highway 3412 from Ayutthaya to Bang Ban was on the same list.
- Hat Yai and the deep south. Highway 4 at Hat Yai was impassable in November 2024, and in November 2025 the regional highway office again listed 79 flooded points and 11 impassable routes around Hat Yai, Satun and Narathiwat. The November 2024 list had 35 impassable points across five provinces, including Highway 42 and the roads to the Prakop and Sungai Kolok border crossings.
- Chiang Rai. In September 2024 landslides closed Highways 1389, 1334 and 1314, and the Department's own list marked them as having no alternative route. The approach road to the second Mae Sai friendship bridge was also impassable.
Two things in that list matter for planning.
- A detour can be on the same river. Highway 32 was the detour for Highway 311 in 2022, and was itself impassable at Ang Thong the year before. Both run through the low land along the Chao Phraya. When the river is high enough, the detour floods too. A second route is only a second route if it does not share the water.
- In the hills there may be no second route. A landslide on a mountain road is not a detour problem. It is a wait.
The number that warns you on the central plain
The central plain is where much of the country's manufacturing sits, and it has the best warning system. It is worth learning to read.
Water from the north meets at Nakhon Sawan, where the Royal Irrigation Department measures the flow at station C.2. Downstream, the Chao Phraya Dam at Chai Nat decides how much of that water goes on towards Ayutthaya and Bangkok. The dam release is the number to watch, because the department publishes which areas flood at each level.
| Release through the Chao Phraya Dam | Low-lying areas outside the dikes that flood |
|---|---|
| 700 to 2,000 m³/s | Khlong Phong Pheng (Ang Thong), Khlong Bang Ban (Ayutthaya), communities along the Noi River at Sena and Phak Hai |
| 2,000 to 2,200 m³/s | Adds Wat Sing (In Buri), Mueang Sing Buri, Phrom Buri, Wat Chaiyo (Ang Thong) |
| 2,200 to 2,400 m³/s | Adds Pa Mok (Ang Thong), Wat Suea Kham (In Buri), Pho Nang Dam (Chai Nat) |
| Above 2,400 m³/s | Adds Thewarat (Chaiyo), In Buri, Ban Tha Sai (Chai Nat) |
Note the words outside the dikes. These are the areas between the river and its flood banks, and the list names riverside places, not industrial estates. Read it against your own site, your customers' sites and the roads that reach them.
The release also has a built-in warning time. The same Royal Irrigation Department diagram sets who may approve each level. Above 1,500 m³/s the department must ask a sub-committee three days ahead. Above 2,000 it needs permission from the National Water Resources Committee, also three days ahead. A large release has to be approved days before the water arrives, and the public warnings follow from it.
This season shows how it plays out:
- 21 September 2026. The Office of the National Water Resources warns that the release will be about 1,500 m³/s.
- 25 September 2026. It raises that to about 2,000 m³/s and lists the districts at risk, including Bang Pa-in, Bang Sai, Sena and Phak Hai in Ayutthaya, and Sam Khok and Mueang in Pathum Thani. The department says it will hold the release at 1,750 m³/s or less for as long as it can, by pushing water into canals and harvested rice fields above the dam.
- 26 September 2026. Flow at C.2 is 1,827 m³/s and rising. The river at Ayutthaya is 8 centimetres below its bank.
For scale, the 2011 peak at C.2 was 4,686 m³/s on 13 October, against a channel capacity the department now gives as 3,660. That was not a record: 2006 peaked at 5,960. What made 2011 different was duration. The river at C.2 went over its banks on 16 September and stayed there until 29 October, 44 days, longer than in any other year on record. Watch how long the flow stays high, not only how high it gets.
Why the road is not the whole story
The 2011 flood cost THB 1.43 trillion in damage and losses by the World Bank's count, and about 70% of it fell on manufacturing. Transport damage and losses were about THB 30 billion, a small part of the total. Production at flooded plants stopped for two to three months at smaller firms and three to five months at large ones.
The lesson for a transport plan is that the long stoppage was at the plants, not on the highway. Main roads reopened while the roads into factories were still under water, and the World Bank's assessment records that some plants stopped production altogether for that reason alone: they could not get raw materials in or finished goods out. So a flood plan built only around detours is solving the smaller problem. The larger one is stock in the wrong place when a site is cut off.
Turning a warning into a date
The warning is only useful if you know how much notice you need. That comes from your own numbers, not from the government's.
Take a plant in a flood-risk district that ships 6 truckloads a day to a customer. Say you judge the access road could be shut for 10 days. To cover that, 60 loads need to be at the customer, or at a dry site near it, before the road closes.
Now ask how fast you can move them. Say you can get 8 extra trucks, each doing 2 turns a day. That is 16 extra loads a day, so 60 loads take 3.75 days, call it 4. On top of the normal 6 loads a day, which still have to run.
Four days is longer than the three-day notice built into the release rules. So in this example, waiting for the official release decision is already too late. The trigger has to come earlier: the first warning that names your province, or C.2 rising past a level you set. Your notice need is volume to move divided by the capacity you can add. If that is longer than the warning, move the trigger, not the plan.
Two more numbers make the difference between a plan and a hope.
- Extra trucks are hardest to find exactly when you need them. Every shipper in the same districts reads the same announcement on the same day. Book the extra capacity against the trigger in advance, not after it fires.
- Detours cost hours, not just kilometres. A detour that adds two hours can push a trip past a driver's legal day and turn one trip into two. How transit time is really built up is covered in how long inland transport really takes.
What a flood does to the contract
Most road freight contracts do not say what happens when a road is shut. The law fills the gap, and it is less simple than "floods are force majeure".
Section 616 of the Civil and Commercial Code makes a carrier liable for loss, damage or late delivery unless it proves one of three things: force majeure, the nature of the goods themselves, or the fault of the sender or the consignee. Section 8 defines force majeure, read in Thai, as something that could not be prevented even though the person took the care reasonably expected of someone in that position.
That test is about care, not about water. A flash flood on a hill road at night may well pass it. A truck sent at noon down a road that was on the Department of Highways' impassable list at eight that morning is a much weaker case. The same logic runs the other way: a shipper who loads a truck for a district named in a warning without telling the carrier has made the problem partly their own.
What that means for your cover, and for whose policy responds when a load is lost to water, is covered in what insurance actually covers on a Thai road. For the contract, write down four things before the season starts:
- Who decides to hold, detour or go. And that a hold on a road on the official impassable list is not a service failure.
- How a detour is paid. Per extra kilometre, per extra hour, or a fixed amount by lane.
- When waiting becomes cancellation. After how many hours or days either side may stop the job without penalty.
- Who tells whom, and how fast. A carrier that knows a road is shut and does not say so has lost the argument that it took reasonable care.
Where to look, and how often
All of these are free, and all are primary.
- Department of Highways: hotline 1586, free and 24 hours, and the HDMS disaster site at hdms.doh.go.th, which lists each flooded point by highway number, kilometre and whether traffic can pass. The daily press release is a summary, and the live list can be longer.
- Royal Irrigation Department, Central Region Hydrology Center: a daily report at 06:00 with flow at C.2, the release below the Chao Phraya Dam, and each station's level against its bank.
- Office of the National Water Resources: numbered flood watch announcements that name provinces and districts. These are the ones that give days of notice.
- Thai Meteorological Department: storm tracks and heavy rain warnings, the only early signal for the north and the hills.
In season, the daily check takes five minutes. What changes on the lists from one day to the next matters more than any single reading. Using the live picture to run a day that goes wrong is covered in running the day when the plan meets the road.
What to do before the next season
- Put each lane in its season. North in August and September, central plain from September to November, the southern east coast in November and December. Some lanes cross two.
- Check the last kilometres of every site you ship from or to. Find the road number. A three- or four-digit highway or a local road near a river is the one to watch. Check whether the district has appeared in past warnings.
- Name a second route that does not share the water, and cost it in hours as well as kilometres. For hill lanes, accept that the fallback may be waiting, and plan stock for it.
- Work out your own notice need as volume to move divided by the capacity you can add. Set your trigger to give that many days, which on the central plain probably means the first warning that names your province, not the release decision.
- Agree extra capacity against the trigger now, in writing, before every other shipper in the district wants it.
- Write the four flood terms into the contract: who decides, how a detour is paid, when waiting becomes cancellation, and how fast each side tells the other.
- Watch the four public sources daily in season, and record what you did and when. That record is what settles an argument about reasonable care later.
The one line worth keeping
Floods on Thai roads come in a known order, close the small roads before the big ones, and on the central plain are announced days ahead. The plan that works starts from how many days you need, and makes the trigger early enough to give them.