On the surface, almost nothing betrays the underground presence of the cathedral-like main reservoir of the Kasukabe site, in Saitama (north of Tokyo), the largest of its kind in the world. The soil is damp and the lighting is poor.
As long as two football fields and supported by 500-ton pillars, the huge structure is deep enough in places to contain the Statue of Liberty.
The installation makes it possible to channel and redirect excess water from storms, thus protecting one of the most populous metropolises on the planet.
Site staff are constantly on alert, especially during the rainy season and tropical cyclones (typhoons) in Japan, which generally runs from June to the end of October.
"We are in an area where torrential rains and even the usual precipitation can overwhelm houses and flood roads," Nobuyuki Akiyama, Kasukabe's infrastructure director, told AFP.
World class systems
Completed in 2006 after ten years of work, at a cost of 230 billion yen (nearly two billion euros), the installation is used on average seven times a year.
The excess water is routed there automatically and operators pump it out of the main tank as it approaches maximum capacity, Akiyama says.
The reservoir is connected to a tunnel 6,3 kilometers long and equipped with a system that can discharge the equivalent of a 25-meter swimming pool into the nearby Edogawa River every second.
According to Akiyama, the reservoir can reduce the number of homes affected by flooding in surrounding areas by 90%.
Official studies also estimate that it has so far saved 148 billion yen (1,2 billion euros) in disaster cleanup costs.
Japan's flood control systems are among the best in the world, the country having learned the lessons of several disasters after World War II, especially the massive Typhoon Vera (1959), which killed more than 5.000 people in the center and The west of the country.
This typhoon, the deadliest in recent Japanese history, sparked national awareness and encouraged large infrastructure projects to reduce risks.
In Tokyo alone, a city crisscrossed by more than 100 rivers, there are ten other underground reservoirs and three flood protection tunnels.
Others are under construction, such as in Osaka (west), where a facility similar to the Kasukabe reservoir is due to be completed in 2044, at a cost estimated at 366 billion yen (2,9 billion euros).
More frequent threats
Experts warn, however, that more must be done. Because global warming makes more and more frequent natural disasters that previously only happened once a century.
According to the Japan Meteorological Agency, the number of typhoons that threaten Tokyo each year has increased 1,5 times over the past XNUMX years.
Kei Yoshimura, professor of meteorology at the University of Tokyo and expert in flooding, is helping to develop an early warning system to identify particularly exposed areas.
"It is clear that infrastructure alone is not sufficient in the face of natural disasters," he said.
The Japanese authorities regularly make the population aware of the need to follow evacuation instructions.
The Kasukabe Reservoir is thus open to visitors when not in use, to emphasize the importance of natural disaster management.
Toru Tamai, a 79-year-old retiree who took part in a recent visit, said he was impressed by the size of the facility, "but it is only a defensive measure," he told AFP.
As an inhabitant of low-lying terrain, for him "floods are a clearer and more present danger than any other natural disaster". Ultimately, "you can only rely on yourself".
