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If there is one thing that Japan and the Netherlands share, it is their efforts to control water. However, the two countries take dramatically different approaches to flood prevention. 

In the Netherlands, which is about one-third below sea level, authorities constructed water control measures mainly above ground, installing dikes, pumps and canals across the country. The Japanese went mainly underground, at least in the Tokyo area.

In Tokyo, where urban sprawl leaves little space for surface water run-off, municipal leaders decided that massive tunnels deep below the surface were the best way to safeguard against flooding from typhoons and heavy rains. On February 28, members of the Netherlands Chamber of Commerce in Japan got the chance to see the Tokyo tunnel system up close, thanks to an exclusive guided tour at the Kanda River Underground Regulating Reservoir. 

“It’s an incredible facility,” says NCCJ member Jim Fink, of Halifax Associates. “Considering there are at least three of them in Tokyo and they’re building more, it’s quite a remarkable undertaking.”

The reservoir lies up to ten stories (43 meters) below Tokyo’s central Loop Road 7, and stretches about 4.5 kilometers. Designed to handle run-off from flooded rivers in the Kanda River basin, the reservoir includes massive tunnels, roughly 12.5 meters in diameter. 

More than half a million (540,000) cubic meters of flood water from the Kanda and Zenpukuji Rivers can be stored in the reservoir. But, amazingly, following a flood, massive pumps can have the tunnels drained in about 48 hours. 

The entire process is monitored from a control station in west Tokyo.

“I remember in the early 90’s when Tokyo used to flood heavily during typhoons and things like that,” says Fink. “It just doesn’t happen anymore. Now, I get a much better understanding of why.” 

The reservoir project was completed in two stages over twenty years, with the first stage finished in 1998, and the second stage in 2008. However, Stage 1 was actually pressed into service early, taking in river run-off in the spring of 1997. Over the past two decades, the system has proven its effectiveness, being called into service an average of twice a year. 

In fact, the reservoir has been employed 42 times since it was first built, according to assistant manager Takashi Ito, of the Tokyo Metropolitan Government’s construction department. Ito gave NCCJ members a brief lecture about the reservoir before guiding the underground tour. He says 2013 was a particularly bad year for rains, forcing the use of the reservoir 4-5 times. He adds that a third stage of the project is now underway, and will be finished in about seven years.

To judge the effectiveness of the tunnels, one can compare home loss statistics from two recent typhoons. In 1993, when a storm dumped more than 280 millimeters of rain on the Tokyo area, more than 3,100 homes were damaged or destroyed. During a similar typhoon in 2004, only 46 houses were lost or damaged. 

According to Ito, Kuala Lampour has a similar tunnel system. However, few other jurisdictions have such an extensive underground reservoir. That has made Tokyo’s system attractive to leaders from other countries, who might be interested in learning from Japan’s flood response measures. 

“This facility has been visited by so many countries, including Denmark, Sweden and Korea,” says NCCJ board member Naoki Shirakawa, who organized the tour for the chamber. “So the place has been seen as special interest for delegations from other countries.”  The visit was certainly of special interest for members of the NCCJ!
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