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Entire area along major northeastern Japan river flooded as typhoon path matched flow

Areas in the Miyagi Prefecture town of Marumori are seen submerged in water that overflowed from the Abukuma River, left in the photo, on Oct. 14, 2019. (Mainichi)

TOKYO -- Almost the entire area along the Abukuma River in northeastern Japan was flooded after Typhoon Hagibis moved north along the river's stream, dumping huge amounts of rain that caused it to overflow, central and local government information has shown.

A so-called "backwater phenomenon" occurred at many locations of the river system in Fukushima and Miyagi prefectures, causing dikes of the river and its branches to burst at 41 locations. The phenomenon occurs when the level of a river rises too high and blocks water from its tributaries. This in turn causes the water levels of the tributaries to rise and breach their dikes.

According to experts, it is extremely rare for almost all areas along such a major river -- designated by the central government as a class A river -- to be flooded.

A total of 45 people lost their lives to the typhoon in Fukushima and Miyagi prefectures, accounting for roughly 60% of all victims of the disaster. Floods from the Abukuma River contributed to this massive damage.

The upper reaches of the Abukuma River are managed by prefectural and municipal governments while its middle reaches and downstream area are managed by the central government.

The Mainichi Shimbun asked the Land, Infrastructure, Transport and Tourism Ministry about the outcome of a survey it conducted on flooding by taking photos of the Abukuma River basin from a helicopter on Oct. 13, the day after the typhoon lashed the region. The Mainichi then interviewed officials of local bodies along the river about the details of the floods. It emerged that almost all areas along the 239-kilometer river were flooded.

According to the ministry, approximately 12,600 hectares along sections of the river and its branches managed by the central government were submerged.

Osamu Itagaki, head of the Flood Disaster Prevention Division of the ministry's National Institute for Land and Infrastructure Management, said, "It has been rare in recent years, when flood-control measures have been implemented, for areas extending from the upper to lower reaches of a river to be flooded."

Experts pointed out that almost all areas along the river were flooded because the direction of the typhoon was almost the same as that of the river stream. The Abukuma River flows from south to north, and Typhoon Hagibis moved along the river in the same direction.

When torrential rain causes river levels to rise, the water level in the upper reaches usually rises first, and the level downstream rises later. In the latest disaster, the water level downstream remained high because of torrential rain brought by the typhoon. This and the massive amount of water from the upper reaches apparently resulted in levee breaches at many locations. The river's dike burst at seven locations in Fukushima Prefecture.

The swelling of the Abukuma River also caused the dikes of its tributaries to burst. At points where branches merge with the Abukuma River, the branches' water levels rose because their flow was blocked by the swollen river's main current. The dikes of 16 branches were unable to withstand the pressure and collapsed at 34 locations.

Water also apparently overflowed from the river and its branches at other locations, causing floods in extensive areas. Seven people died in Motomiya, Fukushima Prefecture, after a massive amount of water overflowed from the Adatara River, a branch of the Abukuma River. At least two people were confirmed dead in the Miyagi Prefecture town of Marumori, where the dikes of two branches -- the Shinkawa and Uchikawa rivers -- burst.

According to the ministry, about 30% of locations where dikes collapsed as a result of Typhoon Hagibis are situated along the Abukuma River and its branches.

Yoshiyuki Yokoo, associate professor at Fukushima University, pointed out, "Record-breaking rain fell along the river's stream that flows from south to west, and water levels (in the Abukuma River and its branches) remained at their peaks."

(Japanese original by Haruna Okuyama and Ikuko Ando, City News Department)

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