Opening New Channels

The Issue: Urban Flooding

During heavy storms, water can flood city streets and surge over riverbanks. Safety measures will become more important if climate change causes more intense storms.

The Strategy: Underground Tunnels

Thanks to a series of massive underground tunnels and tanks, no one worries when the rivers near Tokyo swell with rain. The subterranean drainage system, know as "G-Cans," stretches 6.4 kilometers (4 miles) and lies 50 meters (165 feet) below ground.

Over 12 million people live in Tokyo, with more in the surrounding region. G-Cans is based in the Tokyo suburb of Saitama Prefecture. There, excess water from rivers including the Nakagawa and Tone is diverted into the tunnels and eventually into a main tank. Turbines then pump out the water at a rate of 200 cubic meters (52,000 gallons) per second into the Edo River, which flows into Tokyo Bay. Begun in 1992, the enormous flood-diversion tunnels of the G-Cans project are now partially in use, and construction was completed in 2009.