Extreme rain is testing Japan’s flood defences

Matsudo, Chiba Pref. – When Shuji Nishizawa and his wife bought their semi-basement apartment in 2003, the tiled garden terrace was the main draw. On Aug. 13, it became the floodwater’s way in.

Now, pointing to muddy lines that run across the walls 1.8 meters above the floor of his apartment, Nishizawa, 60, explains how the water rapidly rose during the torrential downpour that struck his neighborhood that evening.

Nishizawa, a self-employed web designer, was in the common area of Sanctus Shinmatsudo, a 10-story apartment complex in Matsudo, Chiba Prefecture, when his wife rushed in to tell him that rainwater had begun gushing down the steps from ground level and pooling on their terrace.

He ran back to his apartment, joining the elder of his two adult daughters and family dog in the sunken living room. His wife and their other daughter threw them towels from the main entryway to prop against the glass sliding doors. In 20 to 30 seconds the flow surged, rising from below the balcony railing to halfway up the glass panels. The glass shattered, and water poured into the living room.

“Run!” he shouted. His wife and younger daughter fled out the front door.

Shuji Nishizawa stands beside a broken patio door separating his living room from the tiled terrace at his home in Matsudo, Chiba Pref. | ALEX K.T. MARTIN

However, Nishizawa, his daughter and the dog could not follow. The wooden door dividing the living room and entryway had closed and would not open due to the pressure or the water. They were trapped as the water rose until the wooden door splintered, leaving a hole just large enough for them to squeeze through. His daughter cut her foot on the broken wood and was later taken to the hospital, but all three escaped, leaving their possessions behind.

“It was like a tsunami,” Nishizawa says. “The water came rushing in all at once.”

The flood swamped all eight first-floor units and mechanical equipment in the basement, cutting off the building’s water supply and knocking out its elevators and mechanical parking system. Damage to electrical equipment in the common area left most residents without television or internet. The drainage system has since been restored, but the water supply and other essential services remain disrupted two months after the flood.

“There are older people living on higher floors who have to make multiple trips up and down the stairs during the day carrying things,” Nishizawa says, noting the elevators remain out of service.

Local residents walk through a flooded area after heavy rain in Oami-Shirasato, Chiba Pref., in August. | REUTERS

People walk through a flooded area in Oami-Shirasato, Chiba Pref., on Aug. 14. A Weathernews survey found that 55.7% of respondents who experienced flooding during the August downpour lived outside predicted flood zones. | REUTERS

What happened at Sanctus Shinmatsudo was an extreme example of a problem that has besieged Chiba since August — water arriving faster and in greater volume than the land and infrastructure could handle. The prefecture has faced it three times: the record rains of Aug. 13; rain again on Sept. 6 and 7; and most recently, Typhoon Dujuan, the 25th named storm of the year, on Sept. 20 and 21. The August flooding killed 13 people, and the typhoon killed 15 across Japan, 10 of them in Chiba.

The pattern extends well beyond Chiba, a summer marked by rain, rain and more rain. Sudden, localized downpours and typhoons have inundated vast stretches of the country, including the city of Nagoya and portions of Ishikawa, Toyama and Fukui prefectures, as several forces have converged: a warming climate making intense rain more frequent, unusual atmospheric and ocean conditions, and vulnerabilities in infrastructure and terrain that can turn heavy rain into disaster.

The repeated flooding has also underscored how preparedness cannot be left to governments and drainage systems alone.

“Responding to urban flooding, which can rapidly become dangerous, requires a societal shift,” said Chiba Gov. Toshihito Kumagai during a press conference on Sept. 11. “Not only the government but businesses and organizations, local communities and individual residents need to recognize flood risk as something that concerns them personally, and prepare and act accordingly.”

Rain beyond the old assumptions

For one hour on Aug. 13, 115 millimeters of rain fell at the Japan Meteorological Agency’s observation station in Chiba City’s Chuo Ward. Over the following 24 hours, the total reached 367 millimeters. Both were station records. Other parts of the prefecture received more than 300 millimeters.

The streets filled first. More than 3,000 vehicles were stranded along roads across the prefecture, about 2,500 of them on Chiba City’s municipal roads alone. Based on vehicle insurance claims, at least 15,000 vehicles are believed to have been damaged across the prefecture.

A car is seen partially submerged in floodwaters after Typhoon Dujuan brought torrential rain to Sakura, Chiba Pref., in September. | REUTERS

Kenji Kajiwara, a professor at Aikoku Gakuen University in Chiba who studies river administration and the history of flooding, says the climate has changed less like a gentle slope and more like a steep staircase. The average temperature in Japan rose 1.44 degrees Celsius over the past century, but not gradually: It rose markedly from the Showa Era (1926-89) to the Heisei Era (1989-2019), and Kajiwara sees 2023, a year marked by a strong El Nino climate pattern, as a new dividing line. Now, with another El Nino intensifying into a very strong event, “this feels like we’ve moved up another step,” he says.

The first typhoon to make landfall in Japan this year came unusually early, on June 3. Japan recorded 36 instances of temperatures reaching 40 C or higher this summer, the most since the current observation method began in 2010. Before the war, Kajiwara says, such extreme heat came perhaps once a decade; now it can feel almost daily.

He cautions against pinning the August rain on any single cause. El Nino was one factor, but warming and other atmospheric and oceanic conditions were also involved, and he sees them as intertwined. “It’s not just that the rain is getting stronger,” he says. “The extremes are becoming more pronounced.”

A woman crosses a street in heavy rain as a result of Typhoon Dujuan in Tokyo, in September. | REUTERS

A man takes a photo of a toppled tree amid rain from Typhoon Jangmi in Tokyo, in June. | REUTERS

While Chiba flooded, parts of Kyushu and western Japan suffered their worst drought in years. Meanwhile, central Tokyo recorded rainfall for 37 consecutive days through Oct. 2, the longest such streak since records began in 1886.

Kajiwara expects the current El Nino to be intense and long-lasting, with a high peak and a wide base extending into the new year, and he fears what 2027 could bring.

But the flooding was not simply a matter of too much rain in too short a time. Where the rain fell and what it met on the ground mattered just as much. In cases of inland flooding, water pools in streets and neighborhoods without necessarily spilling over from a major river. Urban drainage is built around assumptions on how much rain it can handle, and once those limits are exceeded, water piles up faster than it can be carried away.

Upgrading those systems is possible but slow. Sewer pipes can be enlarged when replaced and underground storage built beneath parks or in new developments, but large projects take years. Kajiwara favors smaller storage facilities scattered across cities rather than a few big ones. The difficulty, he says, is that no one can predict where the next extreme rainfall will strike.

A signboard announcing suspended train services caused by heavy rain at Narita International Airport in August. | REUTERS

Still, the recent pattern has made one thing increasingly clear: Flood defenses designed around past assumptions may be tested more often than they were built to withstand. They were tested yet again on Sept. 22, when Typhoon Dujuan drove the water level in Lake Inba to a record 5.04 meters, well above the 4.25-meter threshold for issuing a flood risk alert. About 100 meters of the lake’s northern embankment collapsed near Narita — the first such breach since a land reclamation project was completed in 1969.

When safe spaces flood

Mayumi Tomita watched from her window in Inzai, Chiba Prefecture, as an intersection near her local train station disappeared beneath the water on Aug. 13. Several cars stalled while other drivers turned back, abandoning their attempts to get through. The next day, she found the sidewalk caked in mud. “I saw it as evidence that the flooding had reached well beyond the roadway,” she says.

Her home was not damaged, but the scene showed how quickly intense rain can overwhelm places people would not expect to flood. Ramen Mitake, a restaurant in Ōami-Shirasato, flooded during the Aug. 13 rains, reopened in early September and was flooded again during the Sept. 6-7 rains. Typhoon Dujuan brought another round of flooding. “We sincerely apologize to everyone who has been eagerly awaiting our reopening for the repeated closures and the concern and inconvenience they have caused,” staff posted on the restaurant’s Instagram page. Ramen Mitake reopened on Oct. 5.

Firefighters carry an elderly woman after transporting her by boat from a flooded area after Typhoon Dujuan brought torrential rain to Sakura, Chiba Pref., in September. | REUTERS

Analyzing roughly 20,000 survey responses and weather reports on the Aug. 13 event, Weathernews found that 55.7% of respondents who experienced flooding were outside predicted flood zones. The company’s data, however, is not based on official government damage statistics.

A man uses a broken umbrella in the rain after Typhoon Bavi passed through Ishigaki, Okinawa Pref., in July. | REUTERS

Farther into the countryside, Pierre Andries saw a different version of the same problem. The Frenchman owns a traditional Japanese house on a slope above rice paddies in Nagara, a rural town in Chiba’s central Boso hills, where he and his family have spent weekends and holidays since buying the property in 2011.

The heavy rains in early September were manageable: a few small trees came down, and Andries cleared them within a day. Then Typhoon Dujuan arrived, triggering landslides that cut off the roads to his house. After the storm, he spent hours trying to get through before just parking hundreds of meters away and walking the rest of the distance.

When he reached the house, he found the property transformed. Windows in one of the rooms were broken, water and silt had seeped inside, and parts of the garden were damaged. The house itself could probably be repaired, Andries says. The harder question was whether the landscape his family had come to cherish could ever be restored to what it had been.

“There was another typhoon seven years ago, but our neighbors said this one was unprecedented in scale,” he says. “You try to protect yourself, but you can’t protect everything.”

The last mile

Two days after the Aug. 13 downpour swept across Chiba, Yoshiharu Yokoyama returned to his home in Matsudo after visiting relatives for the Bon holiday. He missed the worst of the storm, but its aftermath was evident. Water had risen about 30 centimeters on nearby streets, enough to strand vehicles. “There was a car right in front of my neighbor’s place that had just given up,” he says. A familiar underpass near his home also flooded, as it often does after heavy rain.

A resident walks near a 7-Eleven in a flooded area after heavy rain fell in Oami-Shirasato, Chiba Pref., on Aug. 14. | REUTERS

A partially submerged truck stands in a flooded road after Typhoon Dujuan brought torrential rain to Sakura, Chiba Pref., in September. | REUTERS

Yokoyama, a geotechnical disaster-prevention consultant at Sakura Office, saw a reminder that the damage from extreme rain depends not only on how much falls, but on where it falls. An underpass is a low point, he says. A valley collects water. Roads and railway embankments can act like dams, and paved surfaces send water to lower ground instead of letting it soak in.

“It’s like catching a cold,” Yokoyama says. “Being prone to colds doesn’t mean you always catch one — you catch it when your body’s defenses are down.”

Public infrastructure cannot eliminate every risk. That leaves what Yokoyama calls the “last mile,” where individual households need to make decisions.

“You need to know whether your own house is going to flood, and how deep the water is going to be,” he says. “That’s the starting point.”

The site of a landslide after Typhoon Dujuan brought torrential rain to Sakura, Chiba Pref., in September. | REUTERS

And where is the nearest safe high ground? Hazard maps help, but they are not a complete answer. Inland flooding maps have only recently become widely available, and the risk to any given home depends on its elevation, foundation height and number of floors.

The answers can help residents determine what physical protection they need, including flood barriers or sandbags around doors and garages, as well as measures to keep sewage from backing up through toilets and drains. Being in a marked risk area does not necessarily mean going to a shelter; sometimes moving to a higher floor is safer than traveling through floodwater.

A car drives through water on a flooded road after heavy rain in Oami-Shirasato, Chiba Pref., in August. | REUTERS

Once water is rising, the advice is simpler: Do not drive into it. Floodwater can conceal underpasses, sudden drops and open manholes, and strong currents can sweep vehicles away.

After the water recedes, residents should photograph the damage — including water marks, appliances and vehicles — before starting cleanup. Such records can support applications for disaster damage certificates, insurance and other assistance.

For Nishizawa and his neighbors at Sanctus Shinmatsudo, that work has already begun. They have formed a committee to investigate how the water reached their complex and how to prevent a repeat, including studying runoff from surrounding land and ways to protect the building. Nishizawa also launched a website this week detailing the difficulties they are facing and calling for financial support.

Pedestrians cross a street in heavy rain as a result of Typhoon Dujuan in Tokyo in September. | REUTERS

Meanwhile, since the August flooding, he and his family have moved three times — from an evacuation center to temporary housing in the neighboring city of Noda, then to a vacant apartment provided by Matsudo. Another move into private rental housing is on the horizon, under a government-run emergency-housing program.

“We need to start by scientifically understanding what happened,” Nishizawa says. “Once we know how the water flowed, we can think about what measures will actually work to stop it.”