About 14 storeys beneath the bustling city of Tokyo, or 42 m underground, lies a cavern of tunnels and vaults that temporarily stores swelling rainwater to prevent flooding above ground. Under a 6.6 trillion yen (S$66 billion) plan, the capital of Japan is planning to strengthen its resilience against flooding, alongside plans to raise coastal sea walls and river embankments.
The city started out building conventional in-ground reservoirs after land scarcity and sky-high land prices drove urban planners to dig deep. The current system of 28 so-called “regulating reservoirs”, with a capacity enough for 1,056 Olympic-size pools, comprises 16 in-ground reservoirs, nine underground vaults, and three underground tunnels.
Plans are now under way to further grow this operational reservoir capacity to 3.6 million cubic metres – from the current 2.64 million cubic metres – by the end of this decade, with the construction of more underground facilities. An additional 1.5 million cubic metres of reservoir storage is expected to be under construction in 2030, with the city aiming for a total planned capacity of 5.1 million cubic metres, which is enough for 2,040 Olympic-size pools.
Tokyo is sharing its expertise of using underground space to manage flooding risks with regional capitals such as Kuala Lumpur and Jakarta. Singapore’s national water agency PUB is also exploring the use of underground space to combat inland and coastal flooding.
The buttressing of anti-flooding measures is one of five pillars under the Tokyo Resilience Project announced by the city’s Governor Yuriko Koike in December 2022.