The city of Tokyo has over 13 million inhabitants and spans the floodplain of three major rivers – the Edogawa, Arakawa and Sumida Rivers. During the annual rain and typhoon season (June-August), the area receives a large amount of precipitation and is subject to devastating flash floods and tidal surges. “Shutoken Gaikaku Housui Ro,” or the Metropolitan Area Outer Underground Discharge Tunnel, completed in 2009, is the world’s largest drain built to alleviate the flooding and water surge problem. The drains, tunnels, reservoir and reservoir pumps are engineering marvels. Large Drain Holes located near the rivers are connected by tunnels to a large central reservoir storage space that is emptied into the river upstream from the city.
“Shutoken” means metropolitan, and “Gaikaku” means outline and “Housiro” means Tailrace. Located at Kasukabe city in Saitama prefecture, 1 hour north of downtown Tokyo. It consists of five concrete containment silos (70 m high and diameters of 32 m) that collect river water and that then flows through buried tunnels (6.4km long). The water is then funneled into to a huge water tank (177 m long and 78 m width and 22 m below ground.), connected to pumps that can discharge up to 200 tons of water into the Edo River.
There is an Underground Exploration Museum connected to the Metropolitan Area Outer Underground Discharge Channel that is called “Ryukyukan.” The museum details the mechanics, and the science associated with this gigantic and complex engineering feat that protects Metropolitan Tokyo from heavy rains or typhoons. By reservation, a visitor can view, the major museum’s attraction, the huge reservoir underground tank. The mysterious tank is supported on 59 pillars and the tank resembles an underground shrine.
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