 Durant
& Cheryl Imboden'sVenice for Visitors veniceforvisitors.com |
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Acqua AltaHigh Tides and Flooding in Venice
ABOVE: Tourists wade in the Piazza San Marco. INSET BELOW: An intrepid visitor tests the waters. Acqua alta, or "high water," can make Venice feel like Atlantis. At its worst, in 1966, acqua alta flooded the city with more than a meter of salty lagoon water; more typically, visitors notice water splashing over canal banks or bubbling up through drains in the Piazza San Marco.
Why (and when) acqua alta floods the city:Acqua alta occurs when certain events coincide:
The phenomenon is most likely to take place between late September and April, and especially in the months of October, November, and December. By official definition, acqua alta occurs when the tide is 90 mm (3.54 inches) above normal high tide. Not all parts of the city are equally susceptible to flooding, as the following chart from the Comune of Venice indicates. Also, the actual depth of water in the streets is far less than the "level of tide" might suggest. (See the "extreme case" below, where 135 cm of flooding translated into 40 cm of water in the Piazza San Marco.)
An extreme case:On October 31, 2004, when some of the photos in this article were taken, the acqua alta reached 135 cm, and 80% of the city was flooded. The Piazza San Marco was indundated by at least 40 cm or 16 inches of water in what was billed as the worst acqua alta of the last 10 years (and the 10th worst since the record flood of 1966). More recently--on December 1, 2008--the acqua alta reached 156 cm and flooded most of the city.This was the most extreme acqua alta in 22 years. (The worst deluge in Venice's history was in 1966, when floodwaters topped out at 194 centimeters.) Possible solutions:The Comune of Venice and various international organizations have been working on solutions that range from floodgates at the Lagoon's entrances to raising of pavements in low-lying areas of the city. Much construction of the latter has already taken place, but it remains to be seen whether acqua alta can be tamed without closing off the Lagoon from the sea and turning it into a freshwater lake. Next page: What to expect, how to prepare
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The phenomenon is often mistaken for proof that Venice is
sinking. Although the city did sink about 10 cm in the 20th Century because of
industrial groundwater extraction, the sinking largely stopped when artesian
wells on the mainland were capped in the 1960s. Today, subsidence is estimated
at 0.5 to 1 mm per year, mostly due to geological factors and compression of the
land beneath the city's millions of wooden pilings. A larger problem is the
rising sea level, which will become an even bigger threat as global warming
melts the arctic ice caps. Already, the frequency of acqua alta has increased
from fewer than 10 times a year to more than 60 times a year in the last
century.