A year after torrential rains hit western Japan, newly disclosed data by the Fukuoka Prefectural Government showed that massive inland flooding of the six tributaries flowing into the Chikugo River in the prefecture occurred because the amount of rain that fell is estimated to be about three to 10 times the capacity of the pumping systems.
Experts point to the difficulty of building infrastructure that can completely prevent disasters triggered by climate change, saying that the most effective way to protect one’s life is to evacuate at an early stage.
On July 6 and 7 last year, as the Chikugo River and its tributaries began to swell simultaneously, the tributaries’ floodgates were closed to prevent water from flowing back from the main stream. But the tributaries’ pumping systems failed to discharge the rainwater, resulting in the streams breaching their banks and flowing into farmlands and residential areas in the city of Kurume and surrounding areas.
According to the Meteorological Agency, the amount of rainfall in Kurume in the 48 hours up to 2:20 a.m. on July 7 last year marked a record high of 383.5 millimeters. The prefectural government’s estimates show that the amount of water flow in the six branches far exceeded the capacity of the drainage system: roughly 250 tons per second in the Tachiarai River, 170 tons in the Yamanoi, 140 tons in the Jinya, 100 tons in the Kanamaru, 85 tons in the Shimoyuge and 30 tons in the Egawa. But the pumping systems can process only 27.2 tons of wastewater in the Tachiarai and 23.2 tons in the Yamanoi.
Following the disasters, measures to prevent flooding have been taken for only one out of the six tributaries — the Yamanoi — while local authorities are still deciding what to do with the other five rivers.
“Considering the amount of water flow (at the time of the flooding), it is clear that increasing the number of drainage pumps isn’t enough to solve the problem,” said Toshimitsu Komatsu, professor emeritus of river engineering at Kyushu University. “Since there are so many tributaries and small rivers, it’s hard to take measures to control the amount of flow in all of them.”
Increasing the capacity of the drainage pumps at tributaries could also raise the risk of flooding for the Chikugo River, he added.
According to the prefectural government data, which was obtained through an information disclosure request, last year’s flood waters rose in some areas to a depth of 1.6 meters. Around 2,000 houses were inundated from their foundations up to a depth of 1 meter. The data also show that flood waters inundated more than 300 facilities including office buildings to a depth of more than a meter, which is believed to be a life-threatening level.
At that time the prefecture had no system to notify residents that the water gates would be closed, which could cause inland flooding. And although the flooding did not cause any harm to local residents, the inundation came as a surprise to many of them. The outcome prompted the central and prefectural governments, as well as the Kurume Municipal Government, to decide to alert residents in the future of possible flood risks via the internet and vehicles with loudspeakers.
Komatsu stresses the need for residents to always think about how to evacuate in case of an emergency.
Citing the Mabicho district in Kurashiki, Okayama Prefecture, where floodwaters reached a depth of up to 5 meters following torrential rain last year, Komatsu said: “If the water depth is up to 2 meters, people can survive by going upstairs, but in the case of Mabicho, many people died on the first floor because floating furniture prevented them from reaching the stairs.
“Whether or not you can save your own life depends on your efforts in daily life to look for safe places on higher ground where you can evacuate.”
He warns of risks of inland floods at tributaries with sluice gates or drainage pumps installed at a point where they merge with the main stream.
“There are lowlands with such risks across the country, including the Saga Plain with the Rokkakugawa River,” he said.
This section features topics and issues from the Kyushu region covered by the Nishinippon Shimbun, the largest newspaper in Kyushu. The original article was published on July 8.