TOKYO — Flood damage is growing in Japan, yet many cities are still promoting development on low-lying land. Although the five-year average of the total area hit by water-related disasters changed little over the 30 years through 2021, the per-hectare damage cost rose by 3.5 times.

For example, an aerial photo of Satte in Saitama prefecture, north of Tokyo, shows an increased number of homes and commercial facilities on farmland near the Tone River. Many of them stand on land deemed at risk of flooding at least 3 meters deep.

“We are not in a position to limit development, given the need to boost the population amid the declining birth rate and graying society,” said a Satte city official. “We are ready to carry out prompt evacuation in case of a disaster.”

Many other local governments face similar predicaments. “The number of residents and businesses in flood-risk areas has been growing in recent years,” said Yasunori Hada, a professor at the College of Risk Management in Nihon University.

Nikkei analyzed population data in the national census and government lists of areas at risk of inundation from a “once in a century” deluge. The study found that such a torrential rain could have affected 24.59 million people in 2020, up by 760,000 from 1995, or just over 3%.

Many low-lying areas were made accessible during the postwar economic boom by the increased construction of dams and embankments. Relatively low land prices in such areas also attracted potential homeowners and businesses.

The trend gathered steam after the urban planning law was amended in 2000, facilitating projects deemed to meet local needs. Eager to revitalize their economies and boost their populations, municipalities rushed to ease curbs on development in areas at risk of flooding. Given the current difficulty of maintaining local populations, many of them remain reluctant to put the brakes on such projects.

While better flood controls have kept the size of affected areas mostly unchanged, the amount of damage has increased sharply as downpours have become more intense, such as torrential rains caused by bands of dense clouds called linear precipitation zones.

Floods and other water hazards caused an estimated damage of 62.14 million yen ($395,000) per hectare in 2021, up 3.5 times from three decades earlier, according to the Ministry of Land, Infrastructure, Transport and Tourism. “The finding reflects not only the development of low-lying areas but also the increased use of [valuable] digital equipment by factories and residents,” Hada said.

Global warming and other factors have also played a role in causing torrential rains. Heavy rainfall of at least 80 millimeters per hour occurred 237 times over the 10 years through 2023 — 1.7 times more than half a century earlier. In the decade through 2021, only 2% of 1,741 municipalities in Japan escaped damage linked to floods and landslides.

Flood-related losses to business activities have also become serious. Among manufacturers outside mountainous areas, work stoppages and lost sales from floods of over 2 meters would both be about 10 times worse than those from floods of less than 0.5 meter, according to an estimate by the Cabinet Office based on damage from recent rainstorms.

Effective measures to reduce flood risks are indispensable for industrial complexes built on paddy fields and dry riverbeds. But only 50% of companies have business continuity plans that cover floods, compared with 98% that have such plans for earthquakes, according to a Cabinet Office survey released in March.

Although a legal revision that took effect in 2022 bans development projects in high-risk areas, it may not be easy to reverse the flows of population and businesses. Many experts say disaster-prevention measures need to be improved, including updating hazard maps for floods and other natural disasters.

“Conventional flood control measures alone may be inadequate to cover the risk of water hazards, given their increased severity,” said Koji Ikeuchi, a professor emeritus at the University of Tokyo. A flood-prone Japan is facing the tough challenge of achieving both regional development and safety.