NO STRANGER TO FLOODING
This sudden and extreme downpour was caused by a low-pressure system that stayed over the Bangkok region for about three days. In a low-pressure system, air rises and moisture in the air condenses to form clouds and rain. If a low-pressure system stalls, it can dump rain over the same location for days, leading to severe flooding.
In the nearby Gulf of Thailand, sea surface temperatures are sitting close to 30°C, several degrees above the local average. These warm conditions provide ample moisture and energy for the storm to persist.
Thailand is no stranger to flooding. In 2011, more than 800 people died when high rainfall, multiple tropical storms, high sea levels and an early start to the monsoon season collided. This triggered widespread flooding that lasted months. World Bank figures show these floods caused an estimated US$46.5 billion in damage, with the local manufacturing industry worst hit.
Bangkok sits on a floodplain near the mouth of the Chao Phraya river, which drains into the Gulf of Thailand. The city is largely flat, without the natural topography to slow or redirect vast volumes of water.
Bangkok has also been sinking for decades. Extensive groundwater extraction in the mid-20th century caused the ground beneath Bangkok to progressively sink, dropping by more than 10cm in 1988 alone.
Policy changes, such as groundwater pricing and extensive monitoring, have reduced this rate of sinking to about 2cm to 3cm per year. But as sea levels rise, this sinking coastal city is less able to drain floodwater into the Gulf of Thailand, heightening the risk of prolonged floods.
Bangkok is also rapidly becoming more urban, characterised by high rises and a booming population. Concrete is replacing spongy soils across much of the city, meaning the ground absorbs less water when it rains. The past 30 years of urbanisation in Bangkok have increased surface water runoff by 25 per cent during bouts of extreme rainfall.
