Big cities must address risks of underground expansion, UN University says

With growing population pressures on land in big urban centres, city managers find opportunities in using underground space for transportation, shopping and parking but do not always face the possible subterranean risks from natural disasters, according to United Nations University experts.

"The concentration of people and wealth in such underground spaces is expanding and merits careful examination," says the UNU's Srikantha Herath.

In many areas such facilities have not been used long enough to show the effects of various hazards, Dr. Herath says. "Modelling a variety of catastrophic events is essential for building contingencies into underground infrastructure designs, including evacuations and the emergency containment and transport of flood waters, for example."

From 1999 to 2001, Tokyo experienced 17 incidents of underground flooding, mainly during the July to September monsoon and typhoon months and some resulted in deaths, despite extensive precautions, he says.

In other areas not known to be flood plains, rare incidents of torrential rain can cause disasters, he says. In Venezuela in 1999, rainfall not seen in the previous 1,000 years caused mudslides that devastated nine northern provinces and buried resorts in coastal towns near Caracas.

In Asia, people have expanded their settlements into alluvial plains formed by previous floods, he says. In these areas, "the co-existence of humans and nature has always been precarious, a gamble with nature."

Meanwhile, underground space is usually mapped in relation to the buildings above ground, instead of using subsurface maps showing connections along which water could flow a long way from the source, UNU says.

"It is essential to plan and construct underground space based on information such as connectivity and to design carefully all uncovered entrances to underground spaces," says Janos Bogardi, director of the UNU's Institute for Environment and Human Security in Bonn, Germany.

Underground disasters can also threaten the stability of the buildings overhead, collapsing floors and compromising ventilation ducts, he says.

UNU says it is helping to build a computerized system to simulate the impacts of such natural disasters as floods and tsunamis on towns and cities, beginning with the Japanese city of Owase and moving on to other Asian urban centres.