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Natural hazards
Cities at risk from below

As urban centers expand, people build more and more underground spaces that remain unmapped. Their interconnections during natural hazards such as floods are a potential threat, according to researchers, who sounded a clarion call on this “hidden vulnerability” of large cities at a U.N. conference held in Tokyo last January.

“Relative to geological time scales, urban development in the past century has taken place extremely fast,” said Srikantha Herath, an engineering professor at the United Nations University, in a press release on Jan. 12. Herath’s colleague Janos Bogardi, director of the university’s Institute for Environment and Human Security, noted that most urban centers grew up on delta or river flood plains and other natural-hazard-prone places, such as hillslopes, because these spaces have rich agricultural land or are otherwise desirable for human habitation. Many regions may not have experienced “extreme events” that would have put their urban construction to the test.

As space becomes limited, Tokyo, New York and other “mega-cities” have expanded, by building underground garages or storage spaces, in addition to underground subways and infrastructure for water, electricity and sewage, which has increased risks, the researchers say. Japan had 17 “incidents” from 1999 to 2001, including some fatalities, during events where flooding may have spread far from the initial source through underground connected spaces. The U.N. team is calling for better mapping and integrating the data to determine how these underground zones might relate to one another.

Most modern construction in the United States is digitally mapped, says Keith Clarke, a geography professor at the University of California, Santa Barbara, but not in formats that would be easily incorporated into a larger database for GIS purposes. Also, he says, older buildings generally are not mapped and are often a hazard in places such as California, where seismicity is high, but standards for retrofitting and construction are relatively new. In the Midwest, Clarke says, “there are huge ongoing engineering problems with even the best-planned holes in the ground,” for example, many storm and bomb shelters.

Meanwhile, the U.S. National Geospatial Intelligence Agency is working to map 133 cities, to “integrate digital construction plans, critical infrastructure information and imagery,” Clarke says, something the Homeland Security Department should spearhead. “There is little difference between needs for natural versus human-induced disaster from a data standpoint.”

Naomi Lubick

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