As the prefectural governor has determined that a dam is necessary for regional flood control, the prefectural government must work out an effective plan for its construction.
Kumamoto Gov. Ikuo Kabashima has asked the central government to build a dam on the Kawabe River as a flood control measure for the Kuma River. The Kawabe River is a branch of the Kuma River, which flooded following torrential rain in the Kyushu region in July.
Sixty-five people died in Kumamoto Prefecture in the wake of the heavy rains that hit the region. It can be said that the magnitude of the damage has forced a shift from the conventional policy of opposition to dam construction.
The Kawabegawa dam construction project was announced in 1966 following a series of floods. In 2008, Gov. Kabashima expressed his intention to scrap the plan as there had been strong opposition among local residents due to concerns over the possible negative impact on the environment. The administration of the then Democratic Party of Japan halted the construction in the following year.
The central and prefectural governments and municipalities in the river basin had been discussing flood control measures other than a dam for more than 10 years. The alternative measures included enlarging the river channel to increase water-flow capacity by dredging the bottom of the river, and raising the height of the embankment. However, none of the ideas were realized due to huge costs and long construction periods.
It is not surprising that residents in an area hit hard by heavy rains have called for the construction of the dam. "People want to protect both the lives of residents and the environment," Kabashima said. By the end of this fiscal year, the central and prefectural governments and other entities plan to draw up flood control measures that will include the construction of a dam.
Kabashima is calling for a "flood retention dam" that stores water only in the event of heavy rain. Discharge outlets at the bottom of such dams allow water to continue flowing in normal conditions, thereby making it possible to reduce the burden on the environment. Land, Infrastructure, Transport and Tourism Minister Kazuyoshi Akaba has also indicated that he is considering the idea.
The land ministry has estimated that a dam on the Kawabe River could have reduced flooding by 60% during the Kyushu rains this year in areas in and around Hitoyoshi, a municipality in the Kuma River basin. However, this is based on the assumption of a reservoir-style dam that stores water during normal conditions.
It has been pointed out that it may not be possible to adjust the flow volume of water from a flood retention dam if its discharge outlets become blocked with earth, sand or driftwood during heavy rain. It is expected to take more than 10 years to complete a flood retention dam. It is necessary to thoroughly examine whether the construction of such a dam on the Kawabe River will be really effective.
A dam alone will not prevent damage from occurring even if it is constructed.
The ministry has been promoting "river basin flood control," focusing on multiple countermeasures in the entire basin. In addition to dams and embankments, the ministry intends to facilitate the construction of anti-flood ponds to draw water into farmland and other structures, and strengthen other approaches such as encouraging early evacuations.
The participation of local residents is indispensable for flood control in the river basin. The collective strength of communities will be the key to protecting people's lives and livelihoods.
Many residents are still opposed to the construction of a dam. The central and local governments must make efforts to thoroughly disclose information and provide detailed explanations to gain the understanding of local residents.
-- The original Japanese article appeared in The Yomiuri Shimbun on Nov. 25, 2020.
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