World’s big cities have that sinking feeling

Press, 6 December 1985, Page 12

World’s big cities have that sinking feeling

By

JOY ASCHENBACH,

National

Geographic News Service

London is sinking. So are Tokyo, Bangkok, Shanghai, Houston, New Orleans, and even landlocked Mexico City and Las Vegas. More than a dozen of the world’s great cities are slowly sinking into the lands on which they were built, some at worrisome rates, dropping as much as a foot a year. Most are sagging, not under their own weight, but because so much groundwater has been pumped out from under them. Most are located in coastal areas, sitting on soft sand, silt, or clay, the kind of soil that easily compacts when large volumes of water are withdrawn.

Compounding the sinking situation is the global rise in sea levels, up to a foot or more a century, which raises the risk of flooding for some cities.

Although cities have been sinking for centuries, the problem has become worse in the past 40 years, according to two geologists, Robert Dolan and Grant Goodell, of the University of Virginia’s environmental sciences department. The unprecedented demand for water for people and industry has tapped the cheapest source: the ground beneath our feet.

When cities sink, buildings crack or drop unevenly, and streets and sidewalks buckle. “People used to live with it. As structures sank, they’d build on top of them or move away,” Dr Dolan says. “There’s an ancient church in Italy’s Po Valley that has three sets of columns. The first set is now in the basement under water. “But modern life complicates matters. New construction is expensive, land values are high. Subways, sewers, and often utility lines are underground, so you can’t afford sudden variations in land levels. Also, over the centuries, many structures have become historical, treasures and cannot be moved from their threatened positions.”

Venice, a classic case of a city already partially submerged by the sea, increased its rate of sinking tenfold between 1950 and 1970 by withdrawing groundwater at a time of sea-level rise. As a result, the chemical pollution in the Venetian lagoon has become a serious

threat, decaying the base of buildings. London’s renowned Big Ben tower, now under repair, was cracked and leaning about 18 inches because the city has been sinking along the Thames, about a foot a century. The $1 billion Thames Barrier, completed last year, is designed to control storm surge from the English Channel and prevent London’s vital subway system from flooding. Because of the continued sinking of the city, the barrier, 10 movable

gates built side-by-side across the river, is estimated to be effective for about 100 years. Step off the curb in Mexico City, and you may step down more than 0.3 of a metre in some places. The world’s largest megalopolis, built 2239 metres up on a lake bed, is a rare example of an inland sinking city. It drops as much as 10 inches a year. “The big problem is that the land does not all go down together,” Dolan says. “This differential subsidence has sunk some buildings a few feet and some from

5.4 metres to 8.5 metres.” The famous El Angel monument, erected in 1910 for Mexico’s centennial of independence, appears to be ascending into the sky as the buildings and streets around it sink. Every few years, a new stair step has to be added to its pedestal. El Angel itself, like other structures in the city, rests on pilings set deep in solid ground. The entire Thai capital of Bangkok is in danger of falling below sea level by the year 2000. The water table that keeps the city afloat has dropped severely because of some 11,000 wells needed for drinking water. Bangkok has sunk nearly a metre in the past 30 years, and recently that rate has

accelerated in some sections of the city. A number of United States cities, from Savannah, Georgia to Long Beach, California, are in various states of depression. Some are slight, such as Las Vegas, which is being pulled down because the enormous weight of the water trapped in Lake Mead, by Hoover Dam, is depressing the earth’s crust. Two of the worst cases are Houston and New Orleans. “Houston is a textbook case of man’s impact on the environment,” Dr Dolan says: “Low coastal area, soft sediments (clay and sand), rapid industrialisation, explosive population and urbanisation, and the dredging of a big ship channel.” As a result of its Texas-size thirst for groundwater, the nation’s fourth-largest city has sunk 1.2 metres in the past 80 years, and its western section is expected to sink another 1.2 metres in the next 20 years.To the east, whole neighbour hoods in the Baytown area along the Houston Ship Channel have been abandoned to Galveston Bay. “Sinking is an almost inevitable consequence of industrial expansion,” Dr Goodell says. Ironically, development occurs where groundwater is available and cheap. New Orleans is so soggy that millions of gallons of seep water must be drained from it each day. About 45 per cent of the city is at or below sea level. The Superdome is supported by 2266 concrete piles driven down more than 45.7 metres. Unlike Houston, New Cretans is sinking from long-term natural subsidence. “No-one sat around a conference table some 260 years ago and discussed the variables in developing the site,” Dr Dolan says. “Now we have to defend it and save it.”

Almost all sinking cities have taken some steps to control groundwater withdrawals, and a number also have constructed enormous engineering works to do everything from lessen the threat of flooding to prop up sagging and leaning buildings. But even in cities where sinking has been slowed or stopped, there has been almost no recovery in ground elevation,