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Okayama pref. gov't had no maintenance plans for 20 years for 3 rivers that flooded

Houses are swept away by floodwaters and mud from a local river in the Mabicho district of Kurashiki, Okayama Prefecture, on July 9, 2018. (Mainichi)

OKAYAMA -- The Okayama Prefectural Government failed for 20 years to work out legally required maintenance plans for three rivers under its supervision that flooded in torrential rains in wide areas of western Japan, officials have admitted.

The prefectural government has been entrusted by the national government with the management of these three rivers in the Mabicho district of the prefectural city of Kurashiki, which are tributaries of the Oda River that also flooded.

The prefectural government has admitted that it was at fault. "Such plans are necessary as countermeasures against flooding and should've been worked out earlier," an official said.

Experts have pointed to the possibility that a "backwater phenomenon" occurred in the flooding. Specifically, the flow of the Oda River rose too high and breached its banks because it got blocked by the mainstream of the Takahashi River, which swelled with far more water than usual from heavy rains upstream.

It is not known which river flooded first. However, a member of an investigative committee set up by the Land, Infrastructure, Transport and Tourism Ministry pointed to the possibility that the Takama River, one of the tributaries of the Oda River, breached its bank as a result of a backwater phenomenon, which reduced part of the Oda River's dike, causing it to breach.

Areas along the Suemasa River, another Oda River tributary that also breached its bank, were hit particularly hard by the flooding, claiming the lives of many people, and the prefectural government's failure to work out river improvement plans could be called into question.

According to the infrastructure ministry, the Oda River breached its dike at two locations including at a point about 3.4 kilometers before joining the Takahashi River, for about 50 to 100 meters. A subsequent survey by the prefectural government has found that the Takama River, its tributary the Madani River and the Suemasa River, a tributary of the Oda River, breached their banks at a total of six locations, for sections ranging from 20 to 300 meters.

These three rivers are designated as class A rivers, for which river development plans must be drawn up under the amended River Act that went into effect in 1997. Such plans typically describe the characteristics of the rivers concerned and how to maintain their dikes and how to restore them if they are damaged by natural disasters.

However, the prefectural government had failed to work out such plans for these three rivers for over 20 years since the revised legislation came into force, or to grasp their depths, widths or the heights of their dikes.

The only data concerning the maintenance of these rivers kept by the prefectural governments is a record that officials conducted annual visual checks on the rivers, which is required by law, last year.

The prefectural government's river section declined to comment on whether its failure to draw up river development plans led to the massive floods.

"We take it seriously that we haven't been able to draw up such plans for 20 years. We can't answer any questions about the causal relationship between the lack of plans and the flooding," an official said.

(Japanese original by Toru Tsukui, Osaka City News Department, and Yuki Takahashi, Okayama Bureau)

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