Imagine the following: During a particularly wet rainy season, runoff water flows into tributaries of the Yodogawa River faster than a series of dams, built to avoid such a problem, can handle it.

The roaring floodwaters then are funneled into the Yodogawa, and the pressure builds as a wall of water races past the Umeda district of Osaka, toward Osaka Bay.

As the water races along, the levees and flood walls protecting the area, including underground passageways used daily by tens of thousands of people, burst under the pressure of the deluge. In just over an hour, all of north Osaka is under 3 meters of water and the underground passageways are completely submerged.

A plot for a B-grade movie? No, the above scenario could very likely take place, according to officials at a local branch of the Construction Ministry.

To alert the public, ministry officials recently produced a video showing, with the aid of computer simulation, what could occur. The video was produced last year by the Yodogawa branch of the Construction Ministry’s Kinki bureau.

“Naturally the purpose of the video is to show the kind of damage that could occur in the event of a flood. Hopefully, after viewing this video, people will begin to think more about flood control,” said Katsuhiro Onuma, director of the branch’s investigative office.

The 10-minute simulation shows what would happen if the retaining walls on the river bend near Tenjinbashi 6-chome Station collapsed in a flood. Within 30 minutes, nearly 1 meter of water has washed over the area, filling the station. The water then spreads toward JR Osaka Station and the 180,000-sq.-meter Umeda underground shopping center.

After 50 minutes, the floodwaters reach the Umeda area and start filling up the surrounding subway entrances. After 1 1/2 hours, Osaka Station itself is flooded, and the underground area east of the station is now nearly 1 meter deep and rising.

After three hours, at least 1 meter of water has covered northern Osaka, and most of the underground area in Umeda has flooded. Two hours later, the flood peaks, as the entire Umeda district is under between 1 and 3 meters of water.

The underground water then pours through the city subway tunnels heading south, filling up each station along the way. Some eight hours after the initial break in the flood walls near Tenjinbashi 6-chome, most of Umeda is submerged as is virtually the entire subway system down to the bay.

The video predicts the city’s disaster prevention facilities for the subway system would be rendered useless because they are located deep underground and would be the first areas to be flooded.

On the other hand, there should be plenty of time to issue warnings to those still underground in Umeda, and to evacuate the area before the floods hit. “We estimate that the Yodogawa experiences serious flooding, similar to what we’ve simulated, about once every 200 years. There was fairly severe flooding in 1953, but with all of the construction since then, the next flood could cause worse damage,” Onuma said.

While walls exist to hold back the Yodogawa, Onuma said no studies have been conducted down the length of the entire 9.2-km stretch between Tenjin 6-chome and Osaka Bay to determine if there are weak points that need reinforcement.

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