Bangkok's Flood: A Recap and What to Watch Next

Summary of this round of Bangkok flooding, plus what to watch for from here.

My best wishes to everyone who has been affected. This time I narrowly missed the worst of the flooding, because my appointment with Nakhon Ratchasima Municipality was moved to Friday afternoon (to discuss the project assessing risks from climate change and infrastructure disasters in Korat municipality), so I wasn’t stuck in Bangkok when the rain hit. A strangely lucky coincidence.

I’ve seen many people asking whether it will be like 2011, so here is a short summary in 4 points.

1. This time it’s rainwater, not water from the north.

In 2011 it was a huge mass of water from the North flowing down the Chao Phraya. Right now, the flow at Nakhon Sawan (C.2) is about 1,795 m³/s, compared with 4,254 in 2011, and the large dams can still take a lot more water. The problem this time is that the rain falling on Bangkok itself is too much to drain in time.

2. The rate of water flow, not just the amount of water.

Think of a kitchen sink. If you open the tap harder than the drain can take, the water overflows even though the drain is still working. Bangkok’s drainage system is designed for about 50 to 80 mm of rain, but this time some places accumulated nearly 300 mm, falling continuously for more than 30 hours. Every canal is full, and the high tide is like someone covering half of the sink’s drain with a hand.

3. The rain clouds stayed in the same place.

Normally, monsoon rain in Bangkok is a heavy downpour that stops after a short while. But this time a strong low-pressure area moved in from Cambodia and sat right along the monsoon trough across the central region. On top of that, a strong southwest monsoon kept bringing moisture from the Andaman Sea and the Gulf of Thailand to feed it. So the rain kept circling here, with nothing to push it away.

Another thing I’d like us to think about: warmer air can hold more water vapor, about 7% per 1 degree of warming (Clausius-Clapeyron). 7% may not sound like much, but it accumulates, and each rainfall gets heavier. We can’t say this particular rain was caused directly by climate change, but trends like this will definitely be seen more often, and drainage systems designed from past rainfall statistics will be overwhelmed more and more often.

4. Reading the forecast (though now there may not be much to wait for, because it has already fallen).

“Rain over 70 to 80% of the area” doesn’t mean there is a 70 to 80% chance of rain at your house. It means rain will cover almost the whole city. The Thai Meteorological Department expects heavy rain to continue until September 28, then ease to 30 to 40% on September 29 to 30. Bangkok estimates it will take another 2 to 3 days to drain after the rain stops.

I think there are 3 variables to watch: rain, sea level, and runoff from the north. If they come together, it will likely be severe (compound flooding).

• Rain: the Meteorological Department expects heavy rain to last until September 28, then ease to 30 to 40% of the area on September 29 to 30. If that holds, Bangkok estimates it will take another 2 to 3 days to drain after the rain stops.
• Sea level: today (September 26) is a full moon, so this is a spring tide period. The sea rises higher than normal and water drains out more slowly. It will likely still have an effect for several days, especially in the early evening.
• Northern runoff: the Royal Irrigation Department expects the water at Nakhon Sawan to peak around October 2 at about 2,000 m³/s. That is still far below 2011, but people along the Chao Phraya outside the flood barriers (Ayutthaya, Pathum Thani, Nonthaburi) may want to prepare a little.

Signs that the situation is starting to improve are: 1 to 2 days of continuous dry weather, canal levels dropping below critical levels, and the water at Nakhon Sawan starting to fall after October 2. October is also normally a period when storms can enter Thailand, so keep following Meteorological Department announcements.

Finally, what I went to discuss in Korat yesterday and what happened in Bangkok today are the same story. The drainage systems, roads, and bridges that we designed from past rainfall statistics are meeting a new kind of rain that is heavier than before. Cities that start assessing risk and adapting first will probably be affected less.

Take care of yourselves.

Pai