Photo/Illutration Temporary housing is flooded in Wajima, Ishikawa Prefecture, on Sept. 21. (Mayumi Ueda)

About 20 percent of Japan’s population resides in areas that are at risk of flooding when heavy rain causes rivers to overflow, according to a recent Asahi Shimbun analysis.

The number has increased by approximately 900,000 to 25.94 million people over the 20 years leading up to 2020, prompting experts to call for the relocation of residents to safer areas.

This increase represents a 3 percent rise, despite a 1.5 percent decrease in Japan’s total population over the decade from 2010, when the nation’s population peaked.

Among more than 30,000 rivers nationwide, The Asahi Shimbun selected around 3,000 with the highest flood risks for analysis, utilizing data from the land ministry and the internal affairs ministry.

The number of people living in areas with projected flooding of 3 meters or more has increased by 70,000 to 2.57 million.

Additionally, 260,000 people reside in areas with projected flooding of 5 meters or more.

In Tokyo, 4.15 million residents—nearly 30 percent of the city's population—live in areas that are at risk of flooding.

Following the capital are Saitama Prefecture with 2.77 million residents in flood-risk areas, Kanagawa Prefecture with 1.7 million, Aichi Prefecture with 1.6 million and Hyogo Prefecture with 1.4 million.

Over the two decades, 20 prefectures experienced an increase in the number of people living in areas with a high chance of flooding.

Since the post-war period of rapid economic growth, advancements in flood control have enabled the development of low-lying areas that were once at greater risk of flooding.

Because these areas have relatively lower land prices, they have attracted an influx of residents.

The relaxation of zoning district regulations in 2000 also contributed to population increases in low-lying areas, even though restrictions have been reinstated in some locations.

“Some local governments hesitate to limit development in flood-risk areas because they are worried about population decline,” said Yasunori Hada, a professor at Nihon University specializing in disaster management.

“However, we should focus on land use that takes disaster risks into account, especially given the annual occurrence of flooding,” he said.

Hada suggested relocating people to vacant houses in safer areas as a possible solution.

(This article was written by Takashi Togo and Kanato Ishii.)

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