Photo/Illutration Water from the Kumagawa river and a tributary in Kumamoto Prefecture inundate the Senjuen nursing home, lower center. (Minako Yoshimoto)

The tsunami-scale wave of water that engulfed a nursing home in the southern prefecture of Kumamoto, claiming the lives of 14 residents, was a tragedy waiting to happen.

Several factors converged to trigger the July 4 disaster: hours of torrential rains, the geographic contour of the Kumagawa river and the location of the Senjuen nursing home in the Watari district of Kuma.

Experts said the steady rise and flow of river water had become so intense that it overflowed and also caused a backwater phenomenon in a tributary, which found release by bursting its banks and smashing into the nursing home as a giant wall of water.

Haruhiko Yamamoto, a professor of environmental disaster management at Yamaguchi University, explained the circumstances that brought about the disaster.

He noted that the Kumagawa river curves around the area close to Senjuen and narrows further downriver, geographic features that ensured disastrous flooding was inevitable.

The relentless downpour caused a huge volume of water to flow from upriver. As a result, water levels rose close to 10 meters between midnight of July 3 and 7:30 a.m. on July 4, according to a water level meter in the Watari district. The water level continued to rise and exceeded the ability of the meter to record the levels.

A tributary of the Kumagawa known as the Ogawa river flows next to where the Senjuen stood. But the sudden increase in the water level of the Kumagawa stopped water from the tributary from joining the main river. This backwater phenomenon likely contributed to the flooding of Senjuen.

“Water overflowing from the Kumagawa gathered in the narrow space along the riverbed and moved upriver like a tsunami,” Yamamoto said. “More water accumulated from the flow of the Ogawa.”

Senjuen was located about 5 meters lower than a neighboring road. Aerial photos taken by the Geospatial Information Authority of Japan showed that flooding in the area reached between three and four meters above ground.

That meant the first floor of the Senjuen, where many of the residents were taking shelter, was completely submerged.

There have been other incidents of flooding of facilities located where a river and its tributary converge.

A tributary of the Tamagawa river in Kanagawa Prefecture flooded when Typhoon No. 19 hit the Kanto region in October 2019. Heavy rains that pounded western Japan in July 2018 caused extensive damage to the Mabi district of Kurashiki, Okayama Prefecture.

Yasuo Nihei, a professor of hydraulic civil engineering at Tokyo University of Science, noted that the backwater phenomenon can occur anywhere.

“Even a river that ordinarily has very little water flow could flood if the water level of the main river rises due to heavy rains,” Nihei said.

There has also been a tendency in recent years to construct facilities in low-lying areas near rivers so as to provide an area for a parking lot.

The latest revision by the central government of expected flooding in vulnerable areas found that some could be inundated by more than 10 meters of water.

“When heavy rain is forecast, it is important to have plans in place to move to a higher floor or evacuate quickly to another location,” Nihei said. “There may also be a need for stricter land use regulations.”

(This article was written by Yu Fujinami and Takahiro Takenouchi.)

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