EDITORIAL

Sinking Cities and Rising Sea Level

Many of the world's great cities, including Lon­ pollution, raw sewage and urban trash. don, , , Houston, and Mexico Sinking has resulted in fractures, tilting and City, are sinking*. With the exception of Mexico irregular surfaces throughout Venice. The columns City, these cities are located on coasts where they of St. Mark's Basilica lean noticeably and the floor are also vulnerable to a long-termworld-wide rise in of the church has irregularities up to 30 cm. Evi­ sealevel. In low coastalareas, such as New Orleans, dence offlooding and chemical decay at the base of sinking increases the flood risks from the Mis­ the buildings where the lagoon waters are on con­ sissippi River andfrom and storm surges from tact is everywhere; however the decay most readily the Gulf of Mexico. London faces similar problems visible is the chemical and physical degradation of when storm surges are driven up the Thames River the stone buildings and sculptures. Factories in the from the English Channel. area discharge tons of sulphur dioxide into the Venice is the world's best example of a city al­ atmosphere each year so the city is being dissolved ready partially submerged by a combination of slowly from the waters of the polluted lagoon and sinking and rising sea level. Venice is located on from an acid atmosphere. Experts estimate that 30 several low marshy islands in a lagoon that is con­ percent of the city's ornate fabric has already nected with the Adriatic Sea. The city has been been lost. builtonpiles driveninto softsedimentofthe bay, so There are three courses of action open to the it's essentially standing in the lagoon waters where Venetians: they can raise the city, they can reduce it is exposed to tides and storms and is particularly the high-water levels through flood control measures, vulnerable toany rise in sealevel. Sixty-two percent or they can erect barriers against storm surges. of Venice is now flooded when tides and storm Raising the city is impossible so Venice is trying to surge reach 1.2 m. This water level has occurred 14 control the flood tides by reducing the size of the times inthe last25 years. Floodlevels whichwere of openings with flood gates between the lagoon and little concern 50 years ago, now cover the Piazzo of theAdriaticflood gates. However, these canonly be SanMarco 20 to 30 days a year between Octoberto considered temporary measures. April, the season ofthe strong southeasterly winds, In Venice, as in London, New Orleans, Tokyo, the Siroccos. and Long Beach, all great coastal cities One result of the sinking is that chemical pollu­ that are sinking, dikes and levees are needed to tion in the Venetian lagoon has become a much keep out the sea, massive pumping systems are more serious problem to the buildings of Venice. used to remove flood waters, and innovative (and The lagoon is at the end of a drainage basin which expensive) movable or expandable dams have been supports a population of 1.2 million, in addition to constructed to obstruct and prevent large port and industrial development. Consequen­ flooding. For example, the Thames barrier in Lon­ tly, thelagoon waters thatmove in andoutofVenice don cost over $1 billion. Landfills are used to raise with every are highly polluted. In fact, Venice the elevations ofparts ofthe cities, and engineering itself does not have a sewage system so it depends works are needed to prop up sagging and leaning on the flushing action ofthe daily tide to rid itselfof buildings. waste. The lagoon is overloaded with industrial Although almost all of the coastal cities that we have mentioned have bothinitiated and aggravated *We discussed the problems of sinking cities, with particular their problems by the withdrawal of ground water emphasis on ground water removal from the aquifers beneath them, in a recently published article inAmerican Scientist (Vol 74, from beneaththem, the continuedrise in sealevel is pp.38-47) really what challenges engineers in their efforts to

-v- Editorial continued keep out the sea. If the predicted increase in sea appearto be toraise the elevation oftheland by fill­ level rise occurs due to a global warming, the en­ ing behind the dikes or actually relocating some of gineeringworks which presently keep the ocean out the cities. It will be the largest effort, the greatest will be severely tested and if the highest sea level challenge, the most expensive undertaking yet faced predictions that have been projected in recent by man. years are correct, then the results would be disas­ trous for most of the world's great coastal cities. Robert Dolan and Grant Goodell Living and working behind high levees like the Department of Environmental Sciences Dutch will probably be the only solution for the University of Virginia short term. For the longer term, the choice would Charlottesville, Virginia

i!!l1i~

-vi-