東京を洪水から守る「地下神殿」　世界最大級の放水路

【春日部市ＡＦＰ＝時事】「地下神殿」の異名を持つ埼玉県春日部市の首都圏外郭放水路は、壊滅的な洪水の被害から首都圏を守っている。一方で専門家らは、気候変動の進行に伴い、洪水のリスクの高まりを指摘している。（写真は首都圏外郭放水路の調圧水槽。埼玉県春日部市で）
　地上では世界最大級の地下放水路を造っている工学技術の粋はほとんど分からないが、巨大な地下構造は、米ニューヨークの自由の女神像がすっぽり入る深さの立て坑もあり、豪雨や台風であふれた川の水の一部を他の川に流すことで近隣地域および首都を守っている。
　１本５００トンの柱が何本もそびえ立ち、サッカー場２面分の広さがあるむき出しのコンクリートの調圧水槽を支えている。
　現場責任者は、この地下放水路のおかげで、近隣河川流域で水害を受ける家屋数を約９０％減らすことができたと説明した。
　１００本以上の河川がある東京には、これ以外に地下調節池が１０施設、地下トンネル式調節池が３施設ある。さらにいくつかの治水施設の建設が進められている。
　大阪でも、春日部の放水路に似た治水施設を３６６０億円かけて建設中で、完成は２０４４年を予定している。
　しかし、地球温暖化の影響でかつては１００年に１度の規模と思われていた暴風雨が多発し、壊滅的被害をもたらしているため、専門家らは、水害へのさらなる対応が必要ではないかと警鐘を鳴らしている。【翻訳編集ＡＦＰＢＢＮｅｗｓ】


〔ＡＦＰ＝時事〕（2020/10/22-15:49）

It has been called Japan's underground Parthenon, a cavernous complex charged with protecting Tokyo and surrounding areas from catastrophic flooding -- a risk experts warn is growing as climate change advances.
Above ground, there is little to give away the cathedral-like feat of engineering that forms the main reservoir of the Kasukabe flood tank, the largest facility of its kind in the world.
The immense structure -- deep enough in some parts to hold the Statue of Liberty -- funnels away and redirects excess water from storms and typhoons, protecting one of the globe's most populous capitals.
Soaring pillars weighing 500 tonnes each support the main reservoir, a bare concrete tank the length of two football fields.
Staff at the facility in Saitama, north of Tokyo, are on constant alert, especially during Japan's rainy and typhoon seasons from June to late October.
In this area, torrential rain, typhoons and even daily rainfall can cause damage by submerging houses and roads, the site's chief, Nobuyuki Akiyama, told AFP.
The reservoir has helped reduce the number of homes affected by water damage in nearby areas by around 90 percent, he said.
In Tokyo alone -- a city cut through by more than 100 rivers -- there are 10 other underground reservoirs and three flood tunnels, and more flood-protection structures are being built.
And in western Japan's Osaka, a flood facility similar to the Kasukabe reservoir is being built at a cost of 366 billion yen ($3.5 billion). Construction is scheduled to finish in 2044.
But experts warn more may be needed, as global warming makes what were previously once-a-century storms increasingly common, and catastrophic.
Japan... has a climate in which floods and heavy rain tend to occur frequently, says Kei Yoshimura, professor of meteorology at the University of Tokyo and expert on river flooding.
But on top of that, now global warming is advancing, Yoshimura told AFP.
- More typhoons -
In recent years, the rainy and typhoon seasons have brought regular destruction.
Massive flooding and landslides killed more than 80 people in west Japan this July, and a typhoon last year killed nearly one hundred people in the country's east.
Japan's Meteorological Agency says the number of typhoons a year that threaten Tokyo has jumped 1.5 times in the last four decades.
The Kasukabe reservoir is connected to a 6.3-kilometre (four-mile) tunnel and the system can release accumulated water into the nearby Edogawa river at a rate equivalent to discharging a 25-metre (83-foot) swimming pool every second, with the power of a jumbo-jet engine.
Built in 2006, at a cost of 230 billion yen ($2.2 billion), the facility swings into action around seven times a year.
Excess water flows in automatically, and operators pump it out from the main tank when it approaches capacity, Akiyama said.
This year it had already been used seven times by September, with water discharged twice after an unusually long rainy season, he added.
Official studies credit the single facility with having saved 148 billion yen in disaster clean-up costs so far.
Japan's anti-flood systems are considered world-class, with the country having learnt bitter lessons from several major disasters after World War II.
But experts including Yoshimura say that infrastructure alone isn't enough, especially with advancing climate change, and Japanese authorities have stepped up efforts in recent years to remind citizens to evacuate homes early when requested.
The Kasukabe system accepts visitors when it isn't in use, in part to promote the importance of disaster management.
This underground facility is great but it's only one defence measure, Toru Tamai, a 79-year-old pensioner who attended a recent tour, told AFP.
I live on low ground, so floods are a clearer and more present danger than any other natural disasters, he added.
In the end, you can't count on anyone but yourself.