This massive underground complex protects Tokyo from floods

In what looks like a set piece out of the James Bond film You Only Live Twice lies a civil engineering masterpiece that protects Tokyo from floods. It's official title is the Metropolitan Area Outer Underground Discharge Channel, and its job is to direct excessive rainfall away from the metro area into underground shafts and eventually into rivers.

Five vertical shafts, each wide and tall enough to fit a Space Shuttle, takes in excess runoff. Each shaft is connected by a 10-meter diameter, 6.3 km long channel that ends in a pressure adjusting tank (commonly referred to as the "Underground Parthenon".

The tank is designed to slow the flow of water and maintain water pressure in the system - which could change if a water pump breaks down or there is a sudden surge of water. At 177 meters long and 78 meters wide, and lying about 22 meters below ground, the water tank is larger than a soccer field. The ceiling of the water tank is supported by 59 pillars which are 18 meters tall and weigh 500 tons each.

Large pumps control the water level (and therefore water pressure) in the system and are capable of discharging up to 200 cubic meters of water per second - that's one swimming pool per second.

Controlling not only the volume of water, but also the flow rate, is an important job for a metropolitan area like Tokyo. Flooding is exacerbated in urban areas because the amount of impervious ground cover - that is, soil that would normally absorb water - is drastically lower than undisturbed countryside. So when a heavy rain event occurs, that water is funneled from streets into gutters and into sewers.


Scientific American
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