The Underground Kandagawa River / Ring Road No. 7 Storm Water Reservoir

Inside a storm water reservoir. When full, the system can hold 540,000 tons of floodwater.

Ring Road No. 7 loops around Tokyo’s 23 wards; directly under part of the road, about 40 meters below the subway tracks, lies a huge tunnel-like structure generally forgotten by the people above. The structure, extending north-south 4.5 kilometers, is called the Kandagawa River / Ring Road No. 7 Storm Water Reservoir.

The Kandagawa River used to be called Kanda Josui, indicating its role in supplying water to the city of Edo. Residents today are certainly fond of the river, although in low-lying areas many used to face flooding when it overflowed its banks from time to time. The storm water reservoir was proposed to solve that problem. Today, when the river swells, water is drained off and stored temporarily in the reservoir. Engineers began drawing water from the river for part of the system in 1997, and they have done so 36 times thus far, dramatically reducing flooding downstream.

Similar systems have been constructed in several other places in Japan, although Tokyo’s is the first and the most impressive. Recent trends show an increase in torrential rains in the country, so storm water reservoirs could soon play an even more critical role in saving communities from floods.