Tokyo was once called the "Venice of the East" because it was seen as a water city with an abundance of water resources that supported people's lives. However, that image of Tokyo as a water city gradually evaporated as boat transportation gave way to land transportation.

Today, Tokyo, like other parts of the world, has its own share of water shortages. In fact, Tokyo has one of the most severe water shortages in the world with the city relying on neighboring prefectures for much of its domestic and industrial water requirements. At the same time, the city receives high precipitation and is being called upon to take urgent measures against intensifying disasters caused by super typhoons, torrential rains, and other extreme weather events. Tokyo must also consider how to respond to droughts caused by global warming and water supply risks in the event of natural disasters.

Against that background, the Obayashi Project Team came up with a future-oriented vision for Smart Water City Tokyo, involving the restoration of the city's beautiful former waterscape and effective utilization and management of water resources, which, given their finite nature, can only increase in value going forward. The project is built around the following three central themes.

1) A smart water network that stores large volumes of urban rainwater for recycling.

2) A revival of the canal system by invigorating the flow of water to tributaries and urban rivers and developing a water transportation network.

3) The building of Tokyo Welcome Gate, an offshore landmark befitting a water city.

Under this new concept, rainwater that currently is hardly used before being discharged into Tokyo Bay would be put to effective use as a new water source.

The most important decision surrounding the effective use of rainwater is how to store it. Within the designated area, any rainwater collected from Smart Water Network (SWN) buildings would be stored in SWN building systems and the Waters Ring, a huge water storage facility built deep underground. The water would be used as general service water and shared within the network to also be utilized in responding to drought, sudden torrential rains, and heat island effects.

The Smart Water Network would be equipped with systems for analyzing weather forecasts by modifying weather information and converting it into big data, as well as for monitoring water storage within the designated region and constantly controlling the water in the entire network. In the event of a drought, water would be pumped from the Waters Ring into water tanks in SWN buildings through the network as general service water. Also if torrential rains are expected in the center of Tokyo, the Waters Ring would release water in advance into inner and outer moat of the Imperial Palace, the Kanda River, and other waterways, and the system would be readied to receive large amounts of rainwater.

While Tokyo is fortunate to experience abundant precipitation, the city must also prepare for torrential rains and other downpours that see large amounts of rainfall in a short period of time. Tokyo needs a future-oriented system such as the Smart Water Network--, a huge mechanism for storing and circulating rainwater and controlling water flows.

The Waters Ring would be at the heart of the Smart Water Network. A huge, double-ringed water storage facility with a circumference spanning approximately 14 kilometers, the ring would be built 50 meters underground in the center of Tokyo.

The water storage capacity of the Waters Ring would be 2.3 million cubic meters in normal times, with a maximum capacity of 4.6 million cubic meters. In the event of a disaster, the Waters Ring could also be used as a source of emergency water. It would contain enough water to serve all the residents of Tokyo's 23 special wards for a total of 6.5 days. If the city experiences torrential rains and a large amount of rainwater, emergency inlets installed throughout the city center could be used in addition to the SWN buildings' underground water reservoir network.

The Waters Ring, which stores large amounts of rainwater under the city center, would also serve as a new transportation network. People could use amphibious vehicles to bypass traffic jams on the ground above, making trips to key spots in the city center much quicker. The Waters Ring would circulate water as a new water source for the city center. It would make Tokyo more resistant to water shortages and water-related disasters and could serve as valuable infrastructure for a modern city complete with an ultimate network of deep underground transportation waterways.

In the Edo period (1603-1868), the city's outer and inner moats boasted smooth flowing water and inspired a vibrant atmosphere befitting a water city, with people coming to relax by the water's edge and ships coming and going on the waterways. Using rainwater to create a water circulation system in the center of the city could help revive that vibrant Edo-style waterscape.

Currently, around 20% of the water for domestic use in the Tokyo city center is supplied through the intake weir on Tama River. Using rainwater as an alternative source for city center general service water would enable a portion of the upstream water intake to be discharged into the Tama River waterway and channeled into the outer and inner moats. If the waterways were to be restored to their former Edo period glory and water flowed into the Sanada Moat at the highest point, the water could then flow naturally down to moats at lower elevations and fill the entire outer and inner moats.