One district in Seoul dug itself out of floods

Social Affairs

Published: 11 Aug. 2022, 16:39 Updated: 11 Aug. 2022, 18:10

One district in Seoul dug itself out of floods

A deep underground rainwater tunnel in Sinwol-dong in Yangcheon District, southwestern Seoul is under construction in 2018. The tunnel, completed in 2020, was built by Hyundai Engineering & Construction and can be used for 100 years to store and drain water in case of flooding. [JOONGANG ILBO]

Yangcheon District in southwestern Seoul came through the unprecedented rains of this week better than most parts of Seoul.

The reason: a deep underground rainwater tunnel located in Sinwol-dong, near the Mok-dong Rainwater Pumping Station in Yangcheon District, completed only a couple of years ago.

On Monday and Tuesday, Gangnam District and surrounding areas in southern Seoul saw some of the worst flooding following the heaviest rainfall in around a century.

Kim Myeong-bo, head of the water management department at the Yangcheon District Office, said Wednesday that Yangcheon District and neighboring Gangseo District suffered less damage than Gangnam because of the giant rainwater tunnel in the Mok-dong reservoir.

Water from sewers in the neighborhood drains into the underground tunnel, also known as the Sinwol Rainwater Storage Facility, situated 40 meters underground.

A 4.7 kilometer-long drainpipe, 10 meters in diameter, extends from Hwagok-dong in Gangseo District to Sinwol-dong.

The water eventually flows to the Anyangcheon, a stream that stretches southward from the Han River, through Yangcheon District and toward Gyeonggi.

In the case of major flooding, excess water can be stored in a 7.5-meter-diameter hole connected to a tunnel in the bottom of the Mok-dong reservoir, which itself is the size of three football fields.

Such underground tunnels are seen as one solution to prevent massive flooding in the Seoul metropolitan area as climate change brings more precipitation to Korea.

The entrance to a deep underground rainwater tunnel located in Sinwol-dong, near the Mok-dong Rainwater Pumping Station in Yangcheon District, seen on Wednesday after record-breaking rainfall earlier this week. [PYUN GWANG-HYUN]

Total precipitation in Yangcheon from Monday to Wednesday was 291 millimeters. Over Monday and Tuesday, 225,728 tons of water entered and exited the tunnel, and only 53 percent of its capacity was used.

But the tunnel, which can handle up to 100 millimeters of rain per hour and accommodate 320,000 tons of water, has never overflowed since it was completed in May 2020.

It took over seven years to build and cost around 139 billion won ($106 million). Over 600,000 tons of rainwater has passed through the Sinwol tunnel over the past two years.

"It's thanks to the rainwater tunnel that Sinwol-dong, which used to be a habitually flooded area, did not suffer damage from the heavy rain this time around," said Kang Jong-gu, head of the Yangcheon District Office's drainage facility team,

Sinwol-dong is a relatively low-lying area especially vulnerable to flooding.

Construction of the tunnel began after heavy rainfall in September 2010 — with a precipitation rate of 90 millimeters per hour — flooded 6,000 buildings in Yangcheon District.

On Wednesday, Seoul Mayor Oh Se-hoon announced the city would spend 1.5 trillion won over the next decade to build six large-scale rainwater storage and drainage facilities, reviving a plan from his earlier term.

This is the core of its medium- and long-term flooding prevention measures.

In the first phase, the city will build underground rainwater storage tunnels in three areas by 2027: Gangnam Station, Dorimcheon, a stream in Gwanak District, southern Seoul, and Gwanghwamun in central Seoul.

The tunnels are expected to have diameters of around 10 meters and will be situated 40 to 50 meters underground.

In July 2011, during an earlier mayoral term, Oh pushed a deep underground rainwater tunnel construction project. Seoul planned to invest 5 trillion won over 10 years and 17 trillion won in the long term into this project. But the project was later shelved due to budgetary issues by Seoul Mayor Park Won-soon, who took office in November 2011, and only the Sinwol underground tunnel was completed.

Yangcheon District's Kang said, "The Sinwol rainwater tunnel can withstand heavy precipitation of 100 millimeters per hour. The annual maintenance cost is about 600 million won, but that's less than the amount that can be caused by damage from flooding."

Some experts warn that underground tunnels are not a panacea because of the irregularity of precipitation and where rain falls most heavily.

"Despite investing a huge budget for dealing with rainwater, flooding continues because there is more precipitation than the drainage system capacity," said Byun Byung-seol, a professor of public administration at Inha University. "A good countermeasure would also be to install a large rain gutter and water discharge system in Gangnam, a habitually flooded area."

Han Moo-young, a professor at the Seoul National University's Department of Civil and Environmental Engineering, said, "Even after construction that costs a lot of money, it may not rain in that particular area."

BY PYUN GWANG-HYUN, SARAH KIM [kim.sarah@joongang.co.kr]