In the torrential rains that caused extensive damage across western Japan last weekend, many of the areas where the disaster claimed heavy tolls had previously experienced large-scale landslides and flooding due to massive rainfalls. In the Mabicho district of Kurashiki, Okayama Prefecture, where dozens of residents either drowned or remain unaccounted for after a third of its area was submerged, the extent of the flooding roughly matched what was envisioned in a hazard map that had been prepared by the city to warn residents of the flood risk and guide them on evacuation shelters and routes. But the disaster’s toll was still high. To learn from this deadly outcome and save more lives in the future we need to look into whether and how the disaster risk information was put to use.

The risk of catastrophic inundation in the Mabicho district — which is surrounded by rivers on its east and south sides and mountains to its northeast — has been known since the district experienced large-scale flooding in the 1970s. In the downpours last weekend, the rivers breached their embankments and roughly half of the houses in the area were inundated by water, at one point leaving more than 1,000 residents stranded in top floors or on rooftops, desperately awaiting rescue. Dozens of people have since been found drowned, while the search continues for others still missing.

Since 2005, the government has required municipalities crossed by rivers that are at risk of flooding to create a hazard map showing the areas that could be flooded, the anticipated scale of flooding, the location of evacuation shelters and evacuation routes, and to share the information with residents. As of March last year, 98 percent of the roughly 1,300 municipalities across Japan subject to the obligation are known to have created such a map.

The map created last year by the Kurashiki Municipal Government of the potential hazards in Mabicho showed that if total rainfall in areas along the river that runs south of the district were to reach 225 millimeters in two days, the river would breach its embankment and flood an extensive part of the district to a depth of 5 meters or more. After the total rainfall reached 246 millimeters over 48 hours by midnight last Friday, the district was flooded roughly to the extent anticipated in the map, with many of the houses inundated up to their second floors. Still, many residents — including elderly people and small children — remained in their homes and drowned.

Evacuation orders were issued by the city to residents in some areas of the district just minutes before the breach of the embankment took place. Officials said the river rose much faster than anticipated. And even though the city had urged the residents to evacuate quickly in the event there was a chance of flooding, many victims ended up being trapped in their submerged neighborhoods.

The disaster raises the question of whether the flood risk information was widely disseminated among residents and whether it had convinced people to take action to ensure their own safety. In the 2015 downpours that hit northern Kanto and parts of the Tohoku region, about a third of the city of Joso, Ibaraki Prefecture, was inundated by flooding from the Kinugawa River. But a subsequent survey showed that many of the residents in the submerged area said they had not seen the local hazard map, which, as in the case of the Mabicho district of Kurashiki, forecast roughly the same extent of immersion that actually happened. It should be reviewed whether the residents of Mabicho had been aware of the district’s flood risk.

To learn lessons for future disasters, the review of the response to the heavy rains should scrutinize whether the evacuation orders were given early enough to allow residents — particularly the elderly — sufficient time to safety evacuate before the floods or landslides took place; whether sufficient efforts had been made in advance to share the disaster-risk information — by the local authorities, at the community level and by the residents themselves — and whether evacuation advisories and other emergency information were properly communicated to all residents.

Extreme weather of unprecedented scale, like the heavy rains that caused the extensive damage in western Japan, is expected to occur with increasing frequency. We need to scrutinize our defenses against such disasters, identify the weak points and fix them.

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