Photo/Illutration An area near the conjunction of the Natsuigawa river, in the background, and its tributary, the Yoshimagawa river, was inundated by floodwaters in Iwaki, Fukushima Prefecture, on Oct. 13. (Satoru Semba)

Eighty percent of sites where embankments collapsed due to flooding from last month's typhoon were within 1 kilometer of river junctions, indicating a higher risk to residents near such areas.

"People living near the confluence of main waterways and tributaries should be aware that their community is vulnerable to flooding," said Masato Sekine, a river engineering professor at Waseda University.

In a survey by The Asahi Shimbun, locations along 71 rivers at 140 embankments that collapsed in the storm striking eastern Japan on Oct. 12-13 were identified.

The sites were identified based on information released by the land ministry as well as Miyagi, Fukushima, Tochigi, Ibaraki, Saitama, Nagano and Niigata prefectures, which were affected by the disaster.

The survey also studied the connection between junctions of tributaries and main streams of the stricken rivers and the sites of bank breakages.

The findings showed that there were 35 bank collapses on 28 branch rivers within 1 km from the point where they joined the main waterway.

Sekine attributed the breaches to a "backwater phenomenon," whereby surging waters in a main river prevent a branch from flowing in, causing its levels to rise and resulting in embankment collapses.

He also noted that the angle at which a tributary joins the main stream and whether there is a bridge nearby can also be factors.

Such a phenomenon occurred in Marumori, Miyagi Prefecture, for example, where seven bank breaches occurred near where the Abukumagawa river is joined by the Gofukuyagawa and Shinkawa rivers, leading to widespread inundation of the town.

With regard to breaches of embankments on the main streams of rivers, the survey found that 77 collapses on 38 rivers occurred in the vicinity of their junctions.

Sekine said that the likelihood of a bank collapse on a main waterway increases when a swollen tributary joins the main flow or when it meets the main stream at an angle close to 90 degrees.

In Fukushima Prefecture, 12 banks on the Yashirogawa river were breached, causing floods in Shirakawa and three other municipalities. Of these, 10 occurred near the confluence of the river and its tributaries.

Sekine noted that anti-flooding measures such as the construction of higher embankments near such meeting points or having a tributary run close to parallel with the main flow would likely not be enough to prevent similar breaches in the future.

"Local authorities should consider developing city plans from a long-term perspective on the assumption that areas near river junctions are particularly vulnerable," he said.

(This article was written by Yosuke Watanabe and Shun Niekawa.)