High Tides and Flooding in Venice, Italy
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.
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.
Why (and when) acqua alta floods the city:
Acqua alta occurs when certain events coincide, such as:
The phenomenon is most likely to take place between late September and April, and especially in the months of November, December, and October, in that order. The Comune di Venezia's Acqua Alta publication explains: "In the remaining months, frequency is very low, while there has never been an event from June to August."
Not all parts of the city are equally susceptible to flooding, as the following chart from the Comune di Venezia 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 the October day when some of the photos in this article were taken, the acqua alta reached 135 cm. The Piazza San Marco was inundated by at least 40 cm or 16 inches of water in what was billed as the worst acqua alta of the decade (and the 10th worst since the record flood of 1966). Several times during early winter in the last few years, acqua alta has reached 150+ cm and flooded most of the city. (The worst deluge in Venice's history was in 1966, when floodwaters topped out at 194 centimeters.)
The Comune di Venezia and various international organizations have been working on solutions that range from MOSE Project 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.
What to expect during acqua alta:
Most of the time, acqua alta is only a mild nuisance: You might have to sidestep a pond or two in the Piazza San Marco or avoid water that's splashing onto a fondamenta, or sidewalk, next to a canal.
However, if you hear a siren wailing, you should be prepared for more serious flooding, especially in low-lying areas of the historic center. A siren means that an unusually high tide will peak in three to four hours. Depending on the height of the acqua alta and where you are in the city, you could encounter flooding for several hours before and after the tidal maximum. Such flooding can vary from a few centimeters to a major inundation like the one in the picture above.
(It's worth noting that Venice's Tidal Center claims that its three-hour forecasts are 95% reliable, with an accuracy of plus or minus 10 centimeters.)
The city provides elevated wooden walkways in areas of the city that are prone to flooding; you can see a map of these pedestrian routes at most ACTV vaporetto stops.
How to prepare:
If you plan to spend time in Venice during the fall and winter months, you may want to pack rubber boots--especially for visits near the full moon or new moon, when acqua alta is most likely to occur. Alternatively, consider packing several plastic garbage bags and strings or elastic bands to hold them in place. Garbage bags may not be elegant, but they can help to keep your shoes and legs dry.
Another option is to buy rubber boots in Venice or acquire temporary plastic rain boots from souvenir stands and street vendors. However, if you have large feet, you may have trouble finding boots that fit--and in any case, you'll need to buy protection before the waters rise.
Again, it's unlikely that acqua alta will be more than a mild nuisance during your stay, and it isn't a danger to human life. (No one drowned during the record acqua alta of 1966, when the entire city was flooded.)
Acqua alta Web links
City of Venice: Acqua Alta (High Tide)
Venice Against the Sea: Flood Zone
Acqua alta overshoes
Next page: Acqua alta photos - Dorsoduro
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