The sudden inundation of a city north of Tokyo this week when a river swollen by torrential rains broke through flood embankments was a relatively rare occurrence in Japan, a country with one of the world's most elaborate water-control systems. Here is a look at the disaster and its implications:
A section of an embankment along the Kinugawa River, a tributary of one of the major rivers in the Tokyo region, the Tone, gave way Thursday, allowing the water to surge into the surrounding valley. Many residents who had not heeded warnings to flee were stranded atop roofs and vehicles as the waters rose around them. Authorities said 22 people were missing.
WHY DID THE RIVER FLOOD
For one, a lot of rain. Government data show that the amount of water entering the river system due to the unusually heavy rainfall caused four upstream dams to reach capacity in only a few hours, limiting their effectiveness for flood control.
The relatively steep grade and narrow river bed in the worst-hit city of Joso also raised the risk of flooding. A flood hazard map for Joso shows risks of flooding up to three stories high in much of the valley. The levees along the Kinugawa were due to be reinforced, but work hadn't begun yet.
While tens of thousands of people were instructed to evacuate, many more received only a "recommendation" to leave and many stayed put. That's typical. In Japan, many ignore evacuation warnings, other than the elderly and the infirm, who are not able to escape quickly.
The name "Kinugawa" means "river of an angry demon." It has flooded repeatedly over the centuries. In 1938 more than 300 people died in a flood brought on by a typhoon. Japan's rulers ordered a change to the river's course in the 17th century as part of a larger project to divert the Tone River away from Tokyo Bay and further east, directly into the Pacific.