It’s just a matter of time before Tokyo is struck by the same magnitude of flooding that devastated parts of the northern Kanto region this month, and should the capital remain unprepared it will most likely be “annihilated,” followed by an unprecedented death toll and economic damage, experts warn.

With the effects of global warming becoming increasingly obvious, the climatic conditions that triggered torrential rain in Ibaraki and Tochigi prefectures two weeks ago is no longer a rarity, and the odds are “100 percent” that similar downpours will hit Tokyo, says Nobuyuki Tsuchiya, a civil engineering expert and author of the 2014 book “Shuto Suibotsu” (“The Capital Submerged”).

The downpours that led to the Kinugawa River overflowing its banks in Ibaraki were mainly caused by a concentration of thunderclouds that formed a lengthy band of rain, a phenomenon that was also responsible for torrential rain that killed 74 people in the city of Hiroshima in summer 2014.

Although previously considered very rare, the abnormal weather condition is becoming increasingly prevalent due to the growing impact of global warming, Tsuchiya said.

“It so happened that the rain zone moved (northeast) after striking Tokyo and stayed over the Kinugawa River. But think what a disaster it may have been if the band of rain had moved about 50 km westward and struck the Tone and Arakawa rivers instead.

“There is no way that such downpours won’t happen again,” Tsuchiya said.

The rupture of the Tone and Arakawa rivers would cause “far more severe devastation” than that of the Kinugawa deluge, he said.

With the Arakawa, for example, boasting one of the densest populations in its surrounding areas of any river in Japan, extensive flooding would lead to unprecedented fatalities and an economic catastrophe that would send shock waves around the world, Tsuchiya said.

Akira Kawamura, a professor of engineering at Tokyo Metropolitan University, agrees.

Should the banks of the Arakawa River break, the damage would be so extensive that it could annihilate the entire capital, he said, with the below-sea-level areas in eastern Tokyo, including Koto and Edogawa wards, likely to be hit the hardest.

Indeed, a 2010 government report released by a panel of outside disaster-prevention experts calculated several possible death tolls in the event that the Tone and Arakawa rivers rupture. The deadliest scenario was if the Tone River broke its banks near the cities of Koga and Bando in western Ibaraki, in which case the death toll could rise to as many as 6,300, the report said.

Tsuchiya said, however, that Tokyo should brace for an even more apocalyptic scenario, noting the amount of rain that entered the Kinugawa River was far larger than that anticipated by the report.

“If Tokyo is struck by the same level of downpours that hit the Kinugawa, I’d say the damage would be far more disastrous.”

Kawamura of Tokyo Metropolitan University also said that the recent urbanization of wider parts of Tokyo, where more land surface is covered with asphalt, and the ever-expanding subway network that stretches all the way from Saitama Prefecture to Kanagawa Prefecture, mean a faster reach of flood water in Tokyo.

“It used to be that, when the Tone or Arakawa rivers flooded, water from the ruptured areas used to take four or five days to reach central Tokyo,” he said.

“But now, the subways and underground structures have become so developed that there is nothing to absorb the river water before it arrives in Tokyo. . . .The subways would function like water drainage pipes in times of flooding disasters.”

For its part, Tokyo is trying to make itself flood-resistant and, by widening sewer pipes and digging out riverbeds to make rivers deeper, it is currently nearly ready to cope with rainfall of 50 mm per hour, Kawamura said.

The Tokyo Metropolitan Government also has plans to increase the system’s capacity to accommodate precipitation amounting to 75 mm per hour by increasing the number of holding pools both on the surface and underground, but it will take some 20 years for the project to be completed, he said.

Tokyo said in a 2014 report on handling flooding and downpours that rainfall of 75 mm per hour tends to hit areas such as Nerima and Nakano wards.

Other wards and cities in Tokyo susceptible to downpours include Suginami Ward and the cities of Mitaka and Musashino, author Tsuchiya said, adding that people who live or work in these areas should be particularly keen to check hazard maps compiled by the local municipalities because rivers that may have run underneath some parts of their neighborhoods in days gone by may now have been converted into culverts.

Tsuchiya also said kanji characters used in place names are handy pointers about how vulnerable a neighborhood is to flooding and other water-related disasters.

Districts with such water-related kanji as tani (valley), ike (lake) and numa (pond) often tend to be prone to flooding and downpours. Shibuya, for example, whose name includes a kanji for valley, suffered during torrential rainfall in July.

That particular rainstorm resulted in Tokyu Shibuya Station being inundated to the point that one of its underground ticket gates temporarily shut down.

The best way to prepare for an onslaught of flooding, both Tsuchiya and Kawamura emphasized, is to check hazard maps made available by the various municipalities.

Kawamura, for one, urged residents to check flood hazard maps before buying or renting property, saying real estate agents tend not to share negative information.

“Residents should also be determined and think it’s up to them to protect their own lives,” he said, noting that when hit by torrential rain, it’s probably safer to escape to higher floors of the building they are in than trying to rush to an evacuation center as heavy rain continues to fall.

Tsuchiya added that people should ideally invest in emergency devices such as life jackets, but as a last resort Styrofoam blocks are also helpful.

“Don’t count on water rings. As was the case with the Kinugawa disaster, they will easily puncture when hit by rubble,” he said.

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