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.