Tokyo’s Underground Flood Nightmare: When 30cm of Water Can Trap You

The fear of underground flooding. Plunged into darkness by a power outage

This year, the Japanese archipelago has been battered by water-related disasters. Just as the torrential rain that struck Chiba Prefecture on August 13 appeared to have passed, a linear rainband formed over the Hokuriku region, flooding numerous homes. Heavy rains then struck Aomori and Kyushu one after another. It is becoming increasingly difficult to feel safe anywhere in Japan. In fact, on August 22, a “Level 4 Heavy Rain Danger Alert” was issued for the Tokyo region, while a “Level 4 Flooding Danger Alert” was issued for the Zenpukuji River. That day, rainfall of more than 120 mm per hour was recorded around Itabashi Ward. Tokyo was also hit by torrential rain of around 100 mm per hour in August 2024, causing flooding at four stations, including Ichigaya Station and Azabu-juban Station.

“In an underground facility, it is already too late to try to escape after water starts flowing in. It is important to notice warnings from the facility and any unusual changes in the surrounding area as early as possible and move to a safe location before the water arrives.”

So explains Koji Matsumoto, a certified weather forecaster and disaster prevention specialist.

“Stairways can become channels for the water, allowing it to rapidly flow into underground areas. If flooding affects electrical equipment and other infrastructure, it could cause not only power outages but also problems with multiple functions, including lighting, elevators, communications, and drainage.” (Koji Matsumoto; same below)

Is there no way to prevent water from flowing inside? What about flood barriers and watertight doors?

“We do not know whether these have been installed at every entrance and exit. Even if they have been installed, closing all entrances and exits with flood barriers or watertight doors could end up blocking evacuation routes.”

As the water gets deeper, water pressure can make doors difficult to open, potentially rendering evacuation routes unusable. According to materials from the Ministry of Land, Infrastructure, Transport and Tourism, a flood depth of 30 cm is considered an important benchmark for evacuation in underground spaces, with the risk that walking becomes difficult and doors become harder to open because of water pressure. On stairways where water is rushing down forcefully, a water depth of 30 cm can make walking difficult.

Even if you are lucky enough to reach an emergency exit and think, “If I open this door, I can get outside!” you could find yourself unable to open it, leaving you powerless as the water level continues to rise.

In 2021, Zhengzhou, Henan Province, in central China, experienced record-breaking heavy rainfall that flooded subway stations and tunnels, leaving hundreds of people trapped inside trains. The cars were plunged into darkness, and oxygen levels fell inside the confined spaces. Speaking to Reuters, one woman recalled, “The most frightening thing wasn’t the water; it was the air running out.”

The sound of water can drown out announcements

In fact, the flooding of underground areas in Tokyo is not limited to the 2024 incidents involving four stations, including Ichigaya Station. In 1981, torrential rain reaching a maximum of 75 mm per hour caused urban flooding, with about 30 cm of water accumulating in Shinjuku Subnade. In 1999, torrential rain reaching a maximum of 99 mm per hour flooded the Shibuya underground shopping district and areas around the station to a depth of about 20 cm.

“Tokyo has experienced repeated incidents of underground flooding in the past as well.”

The area beneath the Ōgakudo overpass in Shinjuku has a basin-like topography, with the road surface there about 1.5 to 2 meters lower than the surrounding area. The central part of Kabukicho is also one of the lowest points. The area around Shibuya Station likewise sits at the bottom of a basin-like depression.

Because of this terrain, the Tokyo Metropolitan Government has established Underground Shopping District Flooding Countermeasure Councils in 12 areas, including Shinjuku West, Shinjuku East, and Shibuya, working with relevant businesses to implement flood-prevention measures.

“When a facility issues instructions to evacuate or relocate, it is important to follow those instructions.”

Of course, as a general rule, following instructions from the facility is paramount. However, during a panic or a power outage caused by flooding, you should also be prepared for the worst-case scenario in which the announcements themselves cannot be heard or are no longer functioning. In an actual emergency, the roar of water rushing down can drown out announcements. If you are wearing earphones, the situation could be even worse.

Moreover, there have been reports during heavy rains in Chiba and elsewhere that the water rose in no time. If you wait for an announcement, you may end up unable to escape.

Both Shinjuku Station and Shibuya Station are currently undergoing redevelopment, turning parts of the stations into mazelike environments. If flooding strikes an underground shopping district where people can already get lost under normal circumstances, how are they supposed to find their way out in complete darkness?

As mentioned earlier, when water is rushing forcefully down a staircase, even a depth of just 30 cm can make walking extremely difficult.

You may not even be able to see your feet. You may have no idea which way to go. In such an extreme situation, if you panic and are pushed over by a crowd, you could potentially drown.

“The worst thing you can do is panic. If flooding has already begun, do not try to force your way through against the incoming water. It is important to follow announcements from the facility and instructions from staff and move to a safe route or location that has not been flooded. Some people even stop to film water rushing down stairways like waterfalls and post it on social media, but doing something like that is essentially putting your life on the line.”

Treat a train suspension as a signal to evacuate

The key to underground disaster preparedness is to evacuate before the water arrives, not after it starts flowing in.

If heavy rain is forecast, check the latest information frequently. Before the danger becomes imminent, follow instructions from the facility and move to a safe location. There is one other thing you should also pay attention to:

“Train service information. During the heavy rains in 2024, the Tokaido Shinkansen stopped operating between Tokyo and Shin-Yokohama at 6:38 p.m., and the Yamanote Line was suspended across the entire line at 6:50 p.m. During that period, roads became flooded and subway stations also experienced flooding.”

The Record-Breaking Short-Duration Heavy Rain Information was not issued until 7:07 p.m. A railway service suspension does not itself constitute an evacuation alert, but since service information is also displayed on electronic signs, it appears advisable to check those as well.

“No one knows exactly what an unprecedented torrential downpour will look like or what will happen underground when one occurs. With localized heavy rain, conditions can change extremely rapidly, so if you wait until danger is right in front of you before taking action, it may already be too late.

When heavy rain is forecast, avoid going out if possible. If you do go out, check the weather conditions frequently. Carry a whistle and a small flashlight in case of an emergency. These kinds of self-help measures are important.”