Most dams cannot allay flood risks

Not even half of Japanese dams can release water to cut flood risks when faced with the prospect of torrential rains, a recent survey by Kyodo News showed.

According to the survey, 246 out of 559 dams operated by 57 agencies throughout the country are able to release water when there are forecasts of heavy rainfall. That finding indicates that only 40 percent of the country's dams can release their reservoirs' water before floods come.

The survey, conducted in December, was published to mark one and a half years since the torrential rains of July 2018. Those rains triggered massive flooding and landslides, leaving 155 people dead.

Some of the deaths were due to an emergency discharge from a dam in Ehime Prefecture when water levels reached their limit, the survey said, adding that there were also emergency water releases from dams when Typhoon Hagibis struck the country in October.

According to Japan's Ministry of Land, Infrastructure, Transport and Tourism, more than 47 rivers burst their banks when Hagibis swept across the country. Satellite imagery and videos at the time showed houses surrounded by murky brown water and overflowing water submerging nearby farmland.

The Japanese government has been calling for dam operators to prepare for strategic releases and to actively manage water levels.

Forty-five dam operators said they have difficulties complying due to the structures of existing dams, which sometimes lack discharge gates or have discharge conduits with weak drainage capacity.

One of the survey respondents said that if water is pre-released but then there is no rain, the resulting water shortage would entail risks for farmers and water utility companies and negatively affect society at large.

A country that began to build dams for irrigation purposes more than 1,400 years ago, Japan now has many dams more than 100 meters high. However, their reservoir capacities are smaller than those of other dams around the world due to Japan's geographical features characterized by narrow islands and steep terrain.

Kyodo News contributed to this story.