Tokyo Metropolitan Area Underground Drainage Canal

The Tokyo Metropolitan Area Underground Drainage Canal is one of the largest underground canals in the world and was built to prevent flooding. When medium and small rivers in northern Tokyo are close to overflowing, the system diverts some of the water to the Edo River, which has a greater capacity to handle this excess through a 6.3-km tunnel that runs 50 meters below the ground. Construction began in March of 1993 and was completed in 2006 using Japanese civil engineering technologies. 

These smaller rivers flow into the Naka and Ayase River basins, located in the Tokyo metropolitan area, mainly in Saitama Prefecture. The area has predominantly flat, low-lying geography, with relief that resembles the shape of a plate — a depression that retains water rather than allowing it to drain naturally. 

The low slope of the rivers also makes it difficult for water to drain into the sea, causing levels to remain high during heavy rains. In addition, recent rapid urbanization and the resulting soil sealing have diminished the region's ability to absorb rainwater, making it susceptible to flash flooding.

To address this problem, the Tokyo Metropolitan Area Underground Drainage Canal was implemented: its structure began with the catchment facilities and vertical wells that receive excess water from each river. The collected water is directed to the underground wells, and this flow is then conducted through a tunnel towards the Edo River – where the drainage and pumping station is located. It passes through the pressure regulation tank, where the force of the water is reduced in order to stabilize the operation of the pump that draws the water accumulated in the pressure adjustment water tank from the underground to the surface, using gas-powered turbines to discharge the water to the drainage locks. Finally, a drainage station pumps water out of the ground and sends it to the Edo River via drainage locks.

The completion of this work has reduced the population's vulnerability, decreasing the number of homes and the extent of the areas affected by floods.