Wednesday, January 5, 2011

Cities Beneath The Sea: Venice

It was when we noticed that everyone else at Christmas Midnight Mass was wearing waterproof boots that we realized we were in trouble. The sirens for acqua alta hadn’t sounded yet. But obviously, word had gone forth from Venice’s weather service that this Christmas Eve, high tide was also bringing high water. And now the water was coming. --James G. Wiles, "Christmas, As The Water Rises, " The Bulletin, December 24, 2010

Acqua Alta is the term Venetians use to describe the high water that floods parts of the city under certain conditions. During the high tides in autumn and winter, the Piazza San Marco, the lowest area of the island, becomes totally flooded with water. Debate continues about whether Venice is sinking into the Adriatic Sea or not, but without a doubt the flooding occurs and locals continue looking for ways to deal with it.

The acqua alta has always been a part of life in Venice. Winds over the Adriatic affect the normal rise and fall of the tide in the area, usually in the spring and autumn. Other cities in the region, such as Trieste, also experience the effect to a lesser degree than Venice.

The weight of the city itself, built on pilings driven into the lagoon beginning in the fifth century AD has caused Venice is sink up to seven centimeters per century. In the meantime, Venetians dredged out the surrounding lagoon, reducing horizontal support for the city and exposing it more directly to high tides. A November 1686 acqua alta may have reached 254 centimeters above normal sea level. All this has caused Venice to sink up to seven centimeters per century.

In more recent times, digging deep wells for industry and construction of infrastructure may have contributed to the sinking of Venice. The number of high acqua altas has also increased in recent decades, leading some to believe rising sea levels account for a large part of the apparent sinking. Venice’s Tide Monitoring and Forecast Center estimates about half the 24 centimeters of sinking in the past century is caused by human activity.

During an acqua alta, tourists at the Doge’s Palace stand in line at the Doge’s Palace on wooden platforms set end-to-end across the plaza (pictured above). Meanwhile, civil engineers work on the MOSE project, a series of gates that can be raised during exceptionally high acqua alta. Many have balked at the cost – more than $3 billion dollars – but the only alternative may be to abandon Venice in as little as 100 years.

For James Wiles's complete article in The (Philadephia) Bulletin, click here.

For a look at how the MOSE system will work, click here.

See my post on Venice's history as a maritime power here.

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