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.