The land ministry knew the Kinugawa River was prone to flooding and created a detailed simulation of watershed damage 10 years ago. The city of Joso, Ibaraki Prefecture, where riverbanks collapsed and houses were swept away Thursday, also had a flood hazard map for residents ready.

Despite all the preparations, this week’s typhoon-triggered downpours and resulting landslides saw hundreds of people failing to evacuate, leaving many stranded on rooftops or displaying SOS signs from the balconies of their homes. One man clung to a utility pole for several hours.

Authorities did alert residents in high-risk areas in advance. The Meteorological Agency issued a tokubetsu keiho for heavy rain, the highest-level special warning, for all of Tochigi Prefecture, at 12:20 a.m. on Thursday and for the whole of Ibaraki Prefecture at 7:45 a.m. the same day. The banks of the Kinugawa River in Ibaraki broke at 12:50 p.m.

The tokubetsu keiho system was introduced in 2013 after a late response was blamed for casualties in the typhoon-related floods and landslides in Wakayama Prefecture and in northern Kyushu, in 2011 and 2012, respectively.

In addition, the Joso Municipal Government issued an evacuation order to residents along the Kinugawa River at 2:20 a.m., more than five hours before the agency’s special warning.

Kei Yoshimura, an associate professor of hydrology at the University of Tokyo’s Atmosphere and Ocean Research Institute, said that, given all the warnings, everyone along the river should have evacuated to safer ground long before the banks collapsed.

“One probable reason some people didn’t evacuate early is that rain had subsided in the area at around noon, when the banks collapsed,” Yoshimura said. “But flooding always occurs (several hours to days) after heavy rainfall. It’s also impossible to predict where exactly the banks will break.”

Hirotada Hirose, professor emeritus of disaster psychology at Tokyo Woman’s Christian University, said the city of Joso’s decision to order evacuations ahead of the meteorological agency’s warning was commendable, even though it came in the early hours of the morning.

“Joso’s evacuation order, reflecting the bitter lessons of belated warnings in the 2013 landslides of Izu-Oshima Island and last year’s Hiroshima mudslides, was swift enough, and it saved lives,” he said.

Meanwhile, the land ministry’s flood simulation data (jtim.es/S4Ggn) and the city’s hazard map were little known to the public, he said, urging prefectural authorities to assist resource-poor municipal governments with evacuations.

Residents should also pay more attention to flood risks, Hirose said.

“Of all disasters, people in Japan worry most about earthquakes,” he said. “But their risk awareness for floods has been extremely low.”

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