Controlling floods an ongoing struggle for municipalities

The heavy rains that hit Kyushu this month led to flooding in multiple rivers in the region, causing incredible damage. Municipalities are now faced with the need to rapidly revise flood control measures to prepare for frequent disasters caused by heavy rain.

“We looked at flood control measures that don’t rely on dams, but haven’t been able to bring projects to completion due to costs and other factors,” said Kumamoto Gov. Ikuo Kabashima on July 5, expressing frustration about flood control measures on the Kuma River, which experienced significant flooding.

The Kuma, which is listed as one of three rivers in Japan with particularly strong currents, is known as the “abare-gawa,” or “rampaging river.”

In 1966, a year after a major flood, the central government announced plans to build the Kawabegawa Dam in an upstream tributary in Kumamoto Prefecture.

Construction failed to progress in part due to local opposition. In 2008, Kabashima announced the project was being scrapped.

In 2009, the Democratic Party of Japan-led administration, which advocated getting rid of dams, decided to halt construction of both the Yamba Dam in Gunma Prefecture and the Kawabegawa Dam.

The central and Kumamoto prefectural governments compiled 10 proposals that combined projects such as river-widening and the raising of residential land, which were presented to flood-prone municipalities last year.

However, they were met with criticism over their cost and duration — from about ¥280 billion to ¥1.2 trillion over a minimum 45-year construction period. This year’s flooding has seemingly occurred amidst a complete lack of direction.

Due to a lack of alternatives, construction on the Yamba Dam resumed some time ago. When Typhoon No. 19 struck the area in October during impoundment testing, it was evaluated that the dam had helped prevent flooding. It is unclear whether damage would have been avoided if the Kawabegawa Dam had been completed.

However, a resident from Hitoyoshi, Kumamoto Prefecture, who was affected by the disaster, said, “There’s no doubt we’ve been stuck with half-baked flood-control measures because of project cancellations.”

There are more than 20,000 rivers nationwide and the government has invested about ¥32 trillion in flood control over the past 32 years alone (based on initial budgets). This has built 335 multipurpose dams for Class 1 water systems and about 9,000 kilometers of levees.

Despite this huge effort, the government has completed only about 70% of the required levees, and projects on rivers mainly managed by local municipalities have faced delays.

However, unprecedented disasters involving heavy rains have occurred on a nearly annual basis.

The reality is that building more dams and levees would not completely prevent damage caused by floods.

The number of rivers exceeding flood risk levels was up fivefold in 2019 from 83 rivers in 2014. The heavy rains that hit western Japan two years ago and Typhoon No. 19 caused severe flood damage.

To prepare for increasingly severe disasters under these circumstances, the government from this fiscal year has decided to start promoting “basin flood control,” which is to entail a combination of hard and soft measures.

In addition to building levees, dams and other infrastructure, the effort is to involve sending overflow water into fields and reservoirs, relocating facilities and raising residential land, while assisting the creation of evacuation timelines for individual residents and improving information provision throughout basin areas.

Furthermore, flooding assumptions will be revised from forecasts based on past data to also include the effects of climate change.

The Yomiuri Shimbun Kuma River Bridge No. 1, which carried the JR Hisatsu Line, was swept away in the flooding, seen in Yatsushiro, Kumamoto Prefecture.