Why Bangkok Flooded This Time (the Short Version)

Why did Bangkok flood this time? (the short version, from a physics lecturer’s point of view)

My best wishes to everyone who has been affected. This time I narrowly missed it, 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 “will it 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. It’s about the “rate,” not just the “amount.”

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. Why the rain stayed in the same place.

A low-pressure area sat right on the monsoon trough that runs across the central region, with the southwest monsoon bringing moisture to feed it all the time. On top of that, warmer air can hold about 7% more water vapor per 1 degree (Clausius-Clapeyron), so each rainfall tends to get heavier and heavier.

4. Reading the forecast correctly.

“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.

What to watch next week

I think there are 3 variables to watch: rain, sea level, and runoff from the north. If they come together, it’s severe (the academic term is 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) should prepare.

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.

For now: move your car to higher ground (the MRT is offering free parking until October 2), raise power strips off the floor, cut the power first if water enters your home, and follow information directly from the Meteorological Department (1182) and Bangkok Metropolitan Administration.

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 hurt less.

Take care of yourselves.

Pai