Why southern floods turned catastrophic as monsoon trough triggers heaviest rainfall in 300 years
The Royal Irrigation Department says a powerful monsoon trough caused record rainfall of 630 mm in three days, triggering severe floods across ten southern provinces
Three-day rainfall hits 630 mm, breaking the 2010 record.
The Royal Irrigation Department (RID) says an intense monsoon trough has caused catastrophic flooding across ten southern provinces, with Hat Yai experiencing its most severe deluge in three centuries.
The Smart Water Operation Centre (SWOC) of the Royal Irrigation Department explained that the heavy flooding across the South since November 19, 2025 has been caused by the combined influence of a strong monsoon trough and a low-pressure cell covering the southern and lower southern regions. This has triggered heavy to very heavy rainfall from Chumphon downwards, resulting in major flooding in ten provinces:
- Chumphon
- Surat Thani
- Nakhon Si Thammarat
- Satun
- Songkhla
- Phatthalung
- Trang
- Narathiwat
- Pattani
- Yala
Accumulated 24-hour rainfall in many areas exceeded 300–500 mm.
Hat Yai hit by its heaviest rainfall in 300 years
The three-day accumulated rainfall from November 19–21 reached 630 mm, surpassing the historic 2010 Hat Yai flood, which recorded 428 mm over the same period.
Flooding in Hat Yai Municipality saw water levels ranging from 0.50 to 2.50 metres.
Rivers and canals surged rapidly, causing widespread overflow
Water levels in major and secondary rivers — as well as multiple canals — increased sharply, leading to overflow in many areas, including:
- Khlong Wad
- Khlong U-Taphao
- Khlong Tam
- Khlong Wah
- Khlong Phuminat Damri (Drainage Canal R.1) — a newly constructed canal designed to divert water from Khlong U-Taphao into Songkhla Lake
Khlong R.1 has a drainage capacity of 1,200 cubic metres per second, significantly reducing the volume of water flowing into Hat Yai Municipality.
Although rainfall continues in some areas, it is showing a downward trend. If no new heavy rain occurs, officials expect the situation to ease and return to normal within 3–5 days.
RID steps up drainage operations
The Royal Irrigation Department has deployed:
- 32 water pumps, and
- 14 water-propulsion machines
to accelerate drainage from the worst-hit zones.
Without Khlong R.1, Hat Yai would have been much worse
The RID noted that the rainfall this time exceeded the design capacity of Khlong Phuminat Damri (Khlong R.1).
However, the canal still played a crucial role in reducing the volume of water flowing into Hat Yai district. Officials emphasised that without R.1, the flooding in Hat Yai would likely have been far more severe than what occurred.
