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A whirlpool of flood water forms around a manhole without a cover near the Daechi Intersection in the early hours of Aug. 9. (Yonhap News)

An unexpected downpour in downtown Seoul turned manhole covers into deadly weapons. Despite weighing dozens of kilograms, the cast-iron covers were blown into the air by intense hydraulic pressure, causing injuries and damaging vehicles. Some people were even swept into open manholes.

On Monday evening, when the Korean capital region was seeing record-setting rainfall, two siblings fell into a manhole in Seoul’s Seocho District. Around 3 pm on Wednesday, three days after the search began, the missing man’s body was found inside a manhole about 1.5 kilometers from the site of the siblings’ disappearance.

Experts advise people to refrain from walking around outside when the water is too deep to see the ground. If you do have to move through the water, it’s best to stick as close as possible to buildings.

When the siblings fell into the manhole, the water had already risen to their knees. They were presumably unaware that the heavy rain had blown open the manhole.

At the time of the accident, rainfall exceeded 120 millimeters (4.7 inches) per hour.

In an earlier experiment in which the National Disaster Management Research Institute recreated road conditions at Gangnam Station in Seoul, a 40 kg (88 lb) manhole cover was blown 26 cm (10 in) into the air after just 41 seconds of precipitation at a rate of 50 mm an hour.

The manhole cover was launched 15-16 cm into the air after 1 minute of precipitation at 40 mm an hour and 1 minute and 25 seconds of precipitation at 30 mm an hour.

Heavy rainfall on Aug. 8 led to manhole covers coming off in some places around Seoul. (still from social media)

Even when rain was falling at 20 mm an hour — just one-sixth of the rate during the downpour on Monday — the manhole opened after 4 minutes and 4 seconds.

Manhole covers are blown into the air with considerable force. The covers were knocked off at precipitation at 30 mm an hour even when a 30 kg child, or 50 kg or 70 kg adult were standing on them.

Some signs that a manhole cover is about to be blown off include air whistling or the cover rattling. Experts recommend leaving the area immediately if you notice such signs.

Another thing to avoid is watching sewer water bubbling up from a manhole like a fountain. That’s another sign that the manhole cover is about to fly up.

The hydraulic shock that knocks manholes open is powerful enough to shake a city bus. For that reason, drivers should avoid parking or idling near manholes during a heavy downpour.

In an experiment involving a subcompact car (weighing 1,105 kgs), a manhole opened under a downpour of 40 mm an hour even when the car’s front tire or back tire was on the manhole cover.

There are a total of 276,923 manholes connected with the Seoul sewer system. Manholes vary in weight from 40 to 100 kg.

“Even if you can’t see the ground in heavy rain, you can spot an open manhole by the way flowing water bends toward it. To prevent injury, you need to get as far away as possible. Whatever you do, don’t hang around for a better look,” said Cho Won-cheol, professor emeritus of civil and environmental engineering at Yonsei University, on Wednesday.

By Park Su-ji, staff reporter

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