By NATSUKI EDOGAWA/ Staff Writer
October 16, 2019 at 18:20 JST
Pressure-adjusting tanks for the Metropolitan Outer Area Underground Discharge Channel in Kasukabe, Saitama Prefecture, on Oct. 14 after water from five rivers flowed in due to Typhoon No. 19 (Provided by Edogawa River Office)
Deep below areas around Tokyo, an unsung hero has been hard at work preventing rivers from flooding due to Typhoon No. 19 that lashed eastern Japan over the weekend.
The cavernous Metropolitan Outer Area Underground Discharge Channel beneath Kasukabe, Saitama Prefecture, started operating at full capacity when record amounts of rain were dumped in the natural disaster.
The 50-meter-deep, 6.3-kilometer-long system stores water in pressure-adjusting tanks when levels rise at five vicinal rivers. The water is then pumped out to the wider Edogawa river that runs through Tokyo, Saitama, Ibaraki and Chiba prefectures.
According to the land ministry’s Edogawa River Office, water started flowing into the system from the No. 18 Channel at 11:30 a.m. on Oct. 12, about seven hours before the typhoon made landfall.
By 6 p.m. the same day, water from the other four rivers--Nakagawa, Kuramatsugawa, Ootoshifurutonegawa and Komatsugawa--was flowing into the underground discharge channel.
Officials said it is rare for water levels of all five rivers to rise past the intake threshold.
At 7:10 p.m., officials started pumping water into the Edogawa river. However, toward the morning of Oct. 13, the water level of the river also started rising, although it later subsided.
“It made me nervous,” an official recalled.
The channel system, dubbed “the underground temple” for its cavernous appearance and monumental columns, was completed in 2006 and is among the largest in the world.
The only other time it ran at full capacity was in September 2015, when Typhoon No. 18 struck the Kanto and Tohoku regions, flooding the Kinugawa river in Ibaraki Prefecture.
The underground channel has a storage capacity of 670,000 cubic meters of water, about the same volume as the Sunshine 60 shopping and entertainment complex in Tokyo’s Ikebukuro district.
The channel discharged 15 times that amount, or 10 million tons of water, into the Edogawa river during the period from Oct. 12 to the morning of Oct. 14.
During the 2015 disaster, the channel discharged 19 million tons of water to prevent flooding.
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