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'Backwater phenomenon' linked to deadly flood in Okayama Pref.: experts


A massive flood that killed scores of residents in the western Japan prefecture of Okayama occurred due possibly to a "backwater phenomenon," according to experts.

When such a situation occurs, the flow of a tributary rises too high and breaches its banks because it gets blocked by its mainstream filled with far more water than usual from heavy rains upstream.

Okayama University professor Shiro Maeno, a specialist in river engineering, thinks that was exactly what happened to the Oda River in the Mabicho district of the city of Kurashiki in Okayama Prefecture in the early hours of July 7. The river overflowed at a point about 3.4 kilometers west of the confluence point with the Takahashi River. The flood killed at least 48 people in the district, and submerged about 12 square kilometers, or about 27 percent of its land area.

Maeno, a member of a committee investigating the cause of the flood organized by the Ministry of Land, Infrastructure, Transportation and Tourism (MLIT), was visiting the site of the breach on the left bank of the Oda River on July 10. He revealed that the committee noticed another overflow on the Takamagawa River, a tributary of the Oda River. Maeno said that it was possible that the Takamagawa River overflow triggered other overflows downstream, creating a domino effect of backwater phenomena.

Masato Sekine, a Waseda University professor of river engineering, said, "This can happen anywhere across the country because there are many river systems with similar conditions."

According to the MLIT Okayama River Management Office, major flooding occurred near the confluence point of the Oda and Takahashi rivers in 1972 and 1976. The central government has plans to start moving the convergence point five kilometers downstream in the next fiscal year to make the merging go more smoothly. An official with the office said that the impact of the latest flood could have been eased if the engineering work had been completed.

Meanwhile, the high number of human casualties that has topped 150 due to the latest torrential rains, floods and mudslides has highlighted the need for authorities to improve their evacuation plans.

In the Mabicho district of Kurashiki, a 62-year-old woman told the Mainichi that she ended up stranded in her home during the flood because she was not able to make the decision to evacuate. "I thought that things would not get this tough," she told the Mainichi Shimbun. The first floor of her second-story house was flooded, and she was rescued by a boat.

An evacuation order was issued for the hardest-hit portion of the Mabicho district at around 1:30 a.m. on July 7. Half an hour earlier, the woman went to bed after checking the road in front of her house, which looked normal. She realized that the water was rising at around 3 a.m. and moved up to the second floor to wait for help.

Almost all of the 24 evacuation centers in the low-lying Mabicho area face the risk of flooding or landslides, and 19 of them could not be used during the latest disaster. The remaining three facilities accepted evacuees but some of them were inundated by more fleeing residents than the local government had expected and experienced some confusion.

In the city of Seiyo in the western Japan prefecture of Ehime, the Hijikawa River flooded at a point about 3 kilometers downstream of the Nomura Dam. The flooding killed at least seven people in the city and the neighboring city of Ozu.

According to multiple residents, the river swelled rapidly at around 6:30 a.m. on July 7. Shortly before that, the Nomura Dam's operator, the MLIT Shikoku Regional Development Bureau, had opened the floodgate because the dam had reached its capacity due to continuing torrential rains. The downstream flow grew to four times the normal current, but the operator said it did warn officials of downstream municipalities about the increased flow.

A man in his 70s said the flow was below the banks at around 6 a.m., but overflowed later, and the water level around his house was as high as the ceiling on the first floor at around 7 a.m., and reached the second floor at around 7:30 a.m.

The municipal government issued evacuation orders at around 5:10 a.m. using radio equipment installed at each household and local public address systems but some residents complained that they could not hear the message due to the loud sound of the downpour. In some areas local firefighters visited each resident to wake them up and evacuate.

A 70-year-old man living along the river said angrily, "The dam has been around for 36 years but we never had a flood like this before. They released too much water." He added that local residents had felt too secure because of the dam, which was installed in part for flood control.

Local governments issue hazard maps to warn about the risk of flooding. But professor Norio Tanaka of Saitama University, a river engineering specialist, said, "Making people realize the importance of such maps is not easy in areas that have not experience a natural disaster for years." Efforts are necessary to make residents aware of the danger and their required response in times of disaster, such as creating evacuation timelines based on existing hazard maps, the professor said.

(Japanese original by Yuki Takahashi and Nana Hayashida, Okayama Bureau; Yuichi Nakagawa, Matsuyama Bureau; Masakatsu Yamasaki, Osaka City News Department; Tsuyoshi Goto, Norihito Hanamure and Kazuki Mogami, City News Department)

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