The Tokio Sewer system is an impressive series of vast underground chambers and vaults. The climatic conditions require enormous sewers to drain the huge amounts of water rainfall might bring. The eastern part of Saitama Prefecture has always been prone to floods. The flood control system is actually separated from the normal sewers and does not contain waste water. It is intended to swallow floods, store them for some time and then release the water controlled into rivers. It also allows the transort of huge amounts of water from one part of the city to another.
The operator of the whole sewer system is the government, or better the Edogawa River Office under the Land, Infrastructure and Transport Ministry. They now offer regular public tours into the flood control tunnel in Kasukabe, Saitama Prefecture. The 6.3km long conduit is located 50 meters below national road 16 and connects the Otoshifurutone and Edo rivers. If the Otoshifurutone River or the nearby Naka River flood, the excess water may be channeled into the tunnel from five different locations. It is stored in an underground chamber and released into the Edo River over the next days.