National

Flooding of Tama River put capital on the brink of crisis during Typhoon Hagibis

by Reiji Yoshida

Staff Writer

Tokyo faced crisis last Saturday, with water levels in the Tama River quickly climbing as heavy rains and winds from Typhoon Hagibis inundated the Kanto region on an unprecedented scale.

Hakone, in Kanagawa Prefecture, saw a staggering 922.5 mm of rain that day alone — three times as much as the total for the month of October of an average year.

Levees all along the Tama, which stretches over 138 kilometers between Tokyo and Kanagawa Prefecture, were designed to withstand precipitation levels seen only once in every 200 years. But at the Ishihara observation station in the capital’s Chofu area, water levels had hit their highest-ever record of 6.24 meters by 11 p.m. on Saturday, far exceeding the 5.9 meters threshold the levees were built to withstand.

Since the levees were designed to have a safety margin of 1.5 meters, making their total height 7.4 meters at the Ishihara observation station, the riverbanks withstood the storm, but only barely.

Any failure of levees along the Tama River could have brought devastating flooding to areas of Tokyo and Kanagawa. For the first time ever, the city of Kawasaki issued an urgent warning, for 915,770 local residents to evacuate by 7 p.m. that night.

“Yes, the situation was very tense,” said Kenichi Ito, who heads the initial crisis management response team at Kawasaki Municipal Government.

In the age of climate change

That tense night for Tokyo and Kanagawa residents has underscored the risks Japan faces in the age of climate change, predicted to increase the number of powerful typhoons like Hagibis.

“This time, the (levees of the) Tama River withstood the typhoon well,” said Nobuyuki Tsuchiya, a senior civil engineering expert for the Tokyo-based Japan Riverfront Research Center.

But given the progression of climate change, stronger typhoons are more likely to strike Tokyo and the metropolitan area, which are “not in any way ready yet (to handle such storms),” he said.

In March 2018, the Tokyo Metropolitan Government released the results of a flood simulation based on a worst-case scenario involving a massive typhoon simultaneously causing heavy rains and tidal flooding.

The results were shocking: Waters would submerge about one-third of the 23 wards of central Tokyo, including 90 percent of Sumida, Katsushika and Edogawa wards, as well as parts of the Marunouchi, Shimbashi and Ginza downtown business districts — the heart of the nation’s capital.

The three wards in eastern Tokyo are particularly vulnerable because many of them are so-called “zero-meter zones,” meaning they are lower than sea level.

According to the metropolitan government, the simulation was based on a worst-case scenario that could happen only once every 1,000 to 5,000 years. But experts warn that powerful typhoons are likely to hit Tokyo more frequently than in the past as the climate continues to warm.

Last Saturday, the Arakawa River also rose to an alarming level, prompting the Edogawa Ward Office to issue an advisory for 432,000 local residents to evacuate.

Damage along the Tama

Luckily, Hagibis didn’t inflict any major damage to Edogawa or any other zero-meter zones. The typhoon did wreak significant havoc, however, in areas along the Tama River in western Tokyo.

Such riverside areas included Futako-Tamagawa, in the Setagaya Ward of Tokyo, and Musashikosugi in Kawasaki, both popular residential areas that are home to relatively young, high-income households.

Damage to Musashikosugi has drawn particular media attention because it is a newly-developed area with a number of high-rise, quake-resistant condominium towers particularly popular with young and affluent families.

Heavy rains brought by Hagibis caused flooding as deep as 1 meter in some areas near Musashikosugi Station.

It also submerged the ground floor levels of some high-rise condominium towers, knocking out power and water-supply systems for at least two of them. As of Wednesday, many residents were reportedly still unable to use their toilets.

Many residents came to realize the flood risks to Musashikosugi only after the disaster hit, but it was common knowledge for experts and businesses involved in developing the area, said Tsuchiya of the Japan Riverfront Research Center said.

“Musashikosugi was a low-lying area with many water channels. You can’t see them now because they have been converted into covered conduits and they now look like ordinary roads,” he said.

In fact, old maps of Musashikosugi show that it had many creeks connected to the Tama River. The area was not considered an ideal residential district because of its flood risks.

Official hazard maps published by the city of Kawasaki clearly show that some areas around Musashikosugi Station are so low that they are all marked in dark purple, meaning that water would recede only two to four weeks after a flood — the worst category in the city’s flood risk scale of six levels.

On Saturday, the Tama River didn’t overflow its banks near Musashikosugi as had initially been feared by Kawasaki officials, but massive amounts of water did flow back into the area through underground drainage channels connected to the river.

The back-streaming flooded some areas of the Kami-Maruko Sannocho district, located east of Musashikosugi Station and closer to the Tama.

“We had to evacuate our 94-year-old grandmother to a nearby three-story building,” said a woman who runs a barbershop in the district who gave only her family name of Asai. “Around 9:30 p.m. (on Saturday), the water was already waist-high for an adult.”

On that night, Asai said she saw water gushing out of manholes in her neighborhood and from a drain outlet inside her barbershop.

Electricity was soon disrupted as the typhoon approached, which left her and her family unable to learn what exactly was happening in her neighborhood.

“We couldn’t even watch TV. Some local offices said they sent out some information on Twitter, but elderly people can’t use something like that,” she said.

Asai initially tried to bail the water out of her shop, but soon gave up, as the water level quickly rose to about one meter inside the building.

The waters receded the following day, but the flood left large amounts of mud and dirt in her barbershop.

“I don’t know if we can continue the business, but we can live here, anyway,” she said.

The cause of the damage

Small-scale flooding took place on Tokyo’s side of the Tama River as well, damaging more than 40 houses in the Tamagawa, Tamazutsumi and Noge districts of Setagaya Ward.

One location, the Futako-Tamagawa area, has been redeveloped in recent years, again with some high-rise condominium towers, several popular shopping malls and a luxury department store.

There, though, and in other areas of Setagaya Ward , the situation was rather different to that of Musashikosugi.

Over decades, river banks have been built up all along the Tama River to protect adjacent residential areas, except for in one section stretching for 500 meters southwest of Futako-Tamagawa Station.

Around 10 p.m. on Saturday night, the Tama River started overflowing from that low-lying bank, flooding several houses and condominium buildings facing the waterfront of Futako-Tamagawa.

The cause of the flooding in Futako-Tamagawa probably goes back at least a century, experts say.

About 100 years ago, Futako-Tamagawa was a ferry point with several ryōtei style Japanese restaurants, where travelers could enjoy dining on river fish while viewing the waterfront scenery.

In 1918, local authorities planned to construct a riverbank to prevent flooding, but those restaurant owners refused to relocate, saying the river view was more important for them than avoiding being hit by rising waters.

Authorities acquiesced to their requests and, instead, built a levee behind the restaurants.

But after World War II, the restaurants went out of business and in their place many residential houses were built. In recent years, some residents had also opposed a plan to increase the height of the riverbank — again prioritizing the view. As a result, the 500-meter section was left untouched, although raised banks were built to protect Futako-Tamagawa Station.

“The state government has organized many meetings with locals to form consensus and it was about to soon start construction,” Tsuchiya said. “It’s a regrettable such a disaster happened now.”

A local lady in her 70s, who declined to be named, said she still remembers the beautiful river view from when she was a child.

That view has been blocked now by a newly-built levee near the station, she said. But many elderly residents don’t regret the decision, because they still remember the Futako-Tamagawa area being hit by flooding numerous times in the past, she said.