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Natural disasters

'Underground temple' saves Tokyo from typhoon flood

Flood-control system keeps capital from drowning after October hit from Hagibis

Excess river water flows through the Metropolitan Outer Area Underground Discharge Channel, dubbed "underground temple." (Courtesy of the MLIT Edogawa River Office)

TOKYO -- Typhoon Hagibis caused widespread damage in Japan, leaving more than 80 people dead as rivers breached levees and overflowed banks. But the capital was largely spared thanks to an elaborate network of reservoirs and drainage basins that worked overtime to minimize flooding.

Nonetheless, it was a close call. Just before noon on Oct. 12 when Typhoon Hagibis began to slam the Greater Tokyo area, river water began flowing into a cavernous channel -- dubbed "underground temple" -- in Saitama Prefecture, bordering Tokyo.

Water from the Nakagawa, Kuramatsu and Komatsu rivers -- all tributaries of the Tone River, the largest river in the metropolitan area -- were diverted to the Metropolitan Outer Area Underground Discharge Channel to be pumped into the more accommodating Edogawa River. By 3 p.m. on Oct. 15, a massive 11.5 million tons of water had been redirected.

During the storm, the discharge channel filled to capacity, only the second time it has done so since its completion in 2006.

Despite the channel, there was some damage. According to the Ministry of Land, Infrastructure, Transport and Tourism, 1,200 houses bordering the Nakagawa and Ayase rivers experienced floods. Still, this was less than 5% of the number that were flooded during Typhoon Judy in 1982, which saw about the same amount of rainfall.

 

A man takes a picture of the Tama River in Tokyo, which caused flooding in the downstream city of Kawasaki after Typhoon Hagibis hit in mid-October. Flood-control infrastructure saved most of the capital, but officials feel more is needed.   © Reuters

Other flood-control infrastructure also helped during the storm. The Watarase Retarding Basin -- Japan's largest stormwater basin that runs through Tochigi, Saitama, Gunma and Ibaraki prefectures -- has a capacity of about 170 million tons of water. Typhoon Hagibis dropped a record 160 million tons into it, filling it to 95% capacity.

Numerous other reservoirs did their part too, with 35 million tons of water flowing into a Saitama reservoir along the Arakawa River and pushing it to 90% capacity. Yamba Dam in Gunma Prefecture, which began storage tests in early October, also nearly topped out. Tokyo has 28 regulating reservoirs, 21 of which played important roles in mitigating the recent deluge.

Rivers overflowed some banks of the Arakawa and Tone tributaries, while rising waters of the Tama River caused flooding in upscale residential areas in Tokyo's Setagaya Ward as well as the neighboring city of Kawasaki. But the defensive measures held up, barely.

"If the torrential rains had changed course a little bit, the reservoirs could have overflown," said a land ministry official. "In some areas downstream of the regulating reservoirs, water levels reached alarming levels that could have triggered [more] flooding. The capital was very close to becoming submerged."

Rain from Typhoon Hagibis flooded some roads in Tokyo's Setagaya Ward, but for the most part, flood-control measures held up.   © Kyodo

In 2012, the metropolitan government improved its flood-control system to handle up to 75 mm of rain per hour. It now plans to beef this up by increasing the storage capacity of its regulating reservoirs to 8 million cu. meters from the current 2.56 million. There are, however, hurdles.

"Acquiring [reservoir] sites in central Tokyo is difficult, so we have to build them deep underground," said a Tokyo government official. "This requires enormous amounts of money and time."

The central government plans to add three more regulating reservoirs to the existing one in Saitama, but building the second and third will cost more than 160 billion yen ($1.47 billion). Construction is expected to be completed by fiscal 2030.

The Kanto region where Tokyo is located contains many big rivers and has repeatedly suffered severe floods. In 1947, Typhoon Kathleen caused rivers to overflow their banks, flooding about 90,000 homes in the capital and about 300,000 homes overall, while Typhoon Ida in 1958 flooded more than 40,000 houses along two rivers in the region.

Meanwhile, construction of flood-control infrastructure, including levees and regulating reservoirs, is gradually progressing, with both the central and the metropolitan governments devoting funds to various projects.

But construction requires vast amounts of money and time, leaving the capital -- some of it below sea level -- still vulnerable to flooding in the event of another large typhoon.

Additional reporting by Yosuke Kurabe

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