Torrential rains triggered by typhoons and other factors in 2018 and 2019 forced officials operating dams to release millions of tons of water downriver for fear of exceeding dam capacity.

The decisions had consequences that would lead to a lawsuit filed this past January by plaintiffs who lost their homes or relatives due to disastrous flooding.

Due to the difficulties in accurately assessing the volume of water expected to reach the dams, four dams in the two-year period released more water than eventually filled the reservoir.

Flooding caused by two dams in Ehime Prefecture in July 2018 led to around 3,500 homes being inundated with water and eight fatalities.

Bereaved family members filed a lawsuit in January seeking compensation from the central government and other entities. The plaintiffs claim that the excessive volume of water released caused the flooding. The central government is planning to put up a fight.

Ideally, the volume of water released from a dam is calculated as being the same that enters the dam.

But this is not an exact science due to the number of tributaries that could be sending water to a dam, making it difficult to calculate how much water will fill a dam.

Officials estimate the eventual volume based on the volume of water that entered a dam between the previous 10 to 60 minutes.

Heavy rains that pounded western Japan in July 2018 caused eight dams to release water downriver for fear of exceeding the dam capacity, according to officials of the infrastructure ministry. The following year when Typhoon No. 19 tore a destructive path across a wide area of Japan in October, six dams released water.

The Asahi Shimbun submitted requests for information disclosure to the infrastructure ministry and prefectural governments that manage the dams to get a more precise picture of what happened.

It obtained data about water levels at the dams as well as the volume of water that entered and was released from each dam.

The data showed that water levels at four dams decreased while water was being released, indicating that a greater volume was being released than was filling the dam.

The four dams were the Kanogawa and Nomura dams in Ehime Prefecture, which released excessive volumes of water in 2018, and the Shiobara dam in Tochigi Prefecture and the Takashiba dam in Fukushima Prefecture that released excessive amounts during Typhoon No. 19 in 2019.

The data showed that water levels at the four dams stopped rising between roughly 25 minutes and two hours after water was released. Normally, the volume of water that is later released is set at the same level as the volume filling the dam so as to maintain a constant water level.

But the data showed that the water level at the Shiobara dam dropped by about 1.65 meters, while the water levels at the three other dams also fell by between about 1 meter and 35 centimeters.

Authorities managing the dams said the volume of water released is based on calculations done by computers. Dam officials separately calculate the water levels based on actual measurements at the dam to ensure that the volume of water released is accurately grasped.

But the drop in the water levels indicates that the actual volume of water that entered the dam was not as high as expected based on the most recent amounts measured.

A common occurrence at the four dams is that rainfall decreased greatly in the area around the dams soon after the dam water level reached its peak.

Dam officials in Ehime Prefecture maintained at a meeting of local residents two months after the flooding in 2018 that the dam was not responsible for the intensified damage.

Kenji Watanabe was the official at the infrastructure ministry’s Shikoku Regional Development Bureau in charge of managing the two dams.

In response to questions from The Asahi Shimbun, he explained that what officials meant at the meeting was that records at the dam office showed that the volume of water released did not exceed the volume that reached the dam.

“Under the regulations for operating the dam, the volume of water released is set at the volume that enters the dam,” Watanabe said. “At that point in time, the most important task is to release water based on the volume calculated by the computer. Dam officials cannot make an independent decision to restrain the volume of water released.”

Local residents in Ehime also claimed that they were not promptly informed about the release of water from the two dams.

The infrastructure ministry set up a panel of experts which compiled a report in December 2018. One conclusion was that the reason was unclear as to why an inconsistency arose in the data that showed the water level falling even though calculations showed the volume of water reaching the dam exceeded the volume that was released.

“Data is the starting point for all discussions, so an assessment cannot be made if the data is inaccurate,” said Hirotake Imamoto, professor emeritus of river engineering at Kyoto University. “This is a phenomenon that could occur in the future, so there is a need to immediately review the calculation methods for the various figures used.”