After deluge, climate change fears make South Korea prioritise Seoul flood defences

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SEOUL – The heaviest rain in Seoul in 115 years has spurred the South Korean capital to revive a $1.15 billion (S$1.6 billion) plan to improve drainage after floods exposed how even the affluent Gangnam district is vulnerable to climate change-driven extreme weather.

Experts say the city's capacity to drain water is far behind what's needed to handle a deluge like the one suffered this week.

That has disastrous implications for low-lying areas like Gangnam, as these bouts of extreme weather are becoming increasingly common.

This week's torrential rain killed at least 11 people across the northern part of the country, as of Thursday (Aug 11) morning.

The downpour, which began on Monday and shifted southwards on Wednesday, knocked out power, caused landslides and flooded roads and subways.

Monetary estimates of the damage were still being compiled.

In the wake of the downpour, Seoul Mayor Oh Se-hoon announced on Wednesday the city will spend 1.5 trillion won (S$1.6 billion) in the next decade to build six massive underground tunnels to store and release rainwater to prevent flooding.

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"The damage from this record rainfall shows that there are limits with short-term water control measures when unusual weather conditions due to global warming have become common," Mayor Oh said, vowing to establish a city-wide system capable of handling 100mm rainfall an hour from the current 95mm.

The city's development meant increased pavement and impermeable surfaces, leading to higher runoff and more flooding.

More than 50 per cent of Seoul's land areas are impermeable, with the figure much higher in the affluent Gangnam district with wide boulevards and office buildings, experts said.

"It's always a see-saw game between cost and safety," said Moon Young-il, a professor of civil engineering at University of Seoul.

"We need to find a balance point and 100mm seems reasonable enough."

Seoul had lacked any detailed plan for water control as it grew from a city of two to three million people in the 1960s to one with over 10 million by the 1990s, Moon said.

The underground tunnels were originally proposed in 2011 after heavy rains and landslides killed 16 people, many of them in Gangnam. But the plan was put on hold amid decreased precipitation and budget issues in the following years.

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The Seoul city also plans to ban basement or lower ground apartments after three family members including a woman with developmental disabilities drowned in their home on Monday. 

The calamitous wet weather prompted President Yoon Suk-yeol to hold a series of meetings with officials this week, to find fundamental ways to improve South Korea's preparedness against similar climate change-induced disasters.

Warmer weather increases moisture levels in the air, leading to more intense rainfall.

So while there has been little change in the annual precipitation over the past four decades, the frequency of heavy rains in Seoul has increased by 27 per cent since the 2000s, according to a 2021 report by the Seoul Institute.

"It was indeed an extreme weather. But we can no longer call this kind of weather event unusual," President Yoon told a meeting on Wednesday. "The largest, highest record can be broken at any time."