Rain too much for dams
The rains caused Hiji River in Ozu City, Ehime to burst its banks on July 7th, killing 4 residents and flooding 4,600 homes.
The district east of the river was the hardest-hit. Water levels there are believed to have reached up to 4.6 meters.
The Nomura and Kanogawa dams are situated upriver on the Hiji. During the rain, authorities took the unusual step of releasing the same amount of water from the dams as was being amassed to stop them from breaking.
Records show that at one point, up to 6 times the normal amount of water was released from the Kanogawa, potentially leading to river surges downstream.
600 tons of water, within the limit deemed safe, were being released from the dam every second until 6 AM on July 7th.
But at around 7:30 AM, river water further upstream began overflowing into the Kanogawa. At that point, worried about the dam breaking, authorities started releasing the same amount of water downstream. This meant they no longer controlled the flow of the river.
By 9 AM, the Kanogawa was pumping out 6 times the limit deemed safe. All this water headed toward the city, causing the massive flooding.
Residents question warning system
A river management official said when massive rain falls for long periods, the dam needs to release as much water as is flowing in to avoid reaching capacity.
Ozu city officials told NHK they warned residents that the river was swelling through a loudspeaker disaster alert system, following a communication from river management officials.
But residents question the handling of the information.
Several people who live along the river say they saw water levels rise rapidly. But one man whose house was destroyed says no one told them about the dam releasing water. He says he understands no one can beat Mother Nature and he accepts the flooding, but he says he would've responded more quickly had he known about the release.
Tomohito Yamada, associate professor at Hokkaido University Graduate School, says when water is being released from a dam, state officials, municipalities, and residents alike must share the information.
He says it must be relayed in an organized manner. He also says there should be drills to help people recognize danger more quickly and evacuate their homes in a timely manner.