Acqua Alta occurs when winds from the south, south-east blow for a long time, with a force of more than 25 kilometers per hour (or 13.4989 knots). These winds push seawater into the lagoon. The water enters through the open mouths in the long sandbar that, from Chioggia to Punta Sabbioni, acts as a barrier between the Adriatic and the lagoon basin. Since the basin is very shallow, it is forced to take in much more water than it can hold: the level, therefore, rises until it submerges Venice’s “calli” and “Piazze.”
The Acqua Alta anomaly is aggravated by the influence of the Moon, which causes the tides to raise the level of the Adriatic (the maximum of Acqua Alta occurs on high tide), and two nearby rivers and the large quantity of water that they discharge which is also pushed towards Venice by the wind.
How to Manage Acqua Alta
The phenomenon of high water is one of the most famous peculiarities of the city of Venice, which usually arouses a lot of curiosity among tourists but also a lot of discomfort for Venetians.
On a day of Acqua Alta, you might come across streets and flooded “Campo,” you might see the water infiltrating the shops and houses on the ground floor, and sometimes variations to the routes of Vaporetto and/or other public transport.
Acqua Alta normally occurs in Fall and Winter, most likely between November and December, only sporadically during the spring and summer months. It follows the tidal cycle, which increases for 6 hours and decreases for 6 hours.
So you know that if you are caught in the water, don’t panic, just wait for 6 hours before the water goes away!
How High Can High Water Be?
There are different tide levels; but note that the tide level should not be calculated from the walking pavement of—for example—Piazza San Marco, but from the increase compared to the average sea level.
High tide: when the tide level is between +80 cm and +109 cm (31—42 circa inches. Again this level of high tide is not getting to your knees if you are strolling in Piazza San Marco, and most likely the rest of the city alleys are still pretty dry.)
Very high tide: when the value is between +110 cm and +139 cm (in general, up to +100 cm the discomfort remains very slight since only less than 4% of the surface of the historic center is invaded by a few centimeters of water)
Exceptional tide: when the value reaches or exceeds +140 cm (corresponds to flooding of about 59% of the city’s viability, being the streets of Venice are at a variable height).
The sirens announce the arrival of high water 3-4 hours in advance only if the expected level is higher than +110 cm. The walkways around the city are designed to ensure transit up to +120 cm. Only over +130 cm do you begin to feel the effects of high tide consistently.
However, on the night of November 12, 2019, Venice was submerged by 187 cm of water (73.622 inches). A real disaster, on that day the sirens never stopped sending their alarm message.
The Catwalks or Walkways
Walkways are set up where the Acqua Alta occurs more frequently, a good remedy to overcome the inconvenience caused by high tide, especially if you are without boots. They are usually an original experience for tourists.
Usually, when the tide exceeds +110 cm, to move in certain low-lying areas of Venice without risking to find yourself with your feet in the water, it is best to be equipped with knee-high rubber boots. In case you don’t have a pair, you can make up for it with a pair of “pocket boots”, similar to plastic bags but with the sole and easily foldable and transportable in your bag.
Boots can be found in many shops and stalls around the city, for example in Piazzale Roma (but note that prices will rise as fast as the tide rises…). In the event of a tide greater than +140 cm, be aware that even higher boots are needed.
The App For High Water
You can also download an application on your mobile that will allow you to stay up to date with just one click. Hi! Tide Venezia, available for both iPhone and Android devices. And Venice Tides, a cute app that tells you how the tide will rise on a place near you (or your hotel, based on the altitude of such location).
It is recommend to get a little familiarity with the altitude of the various areas of the city when in Venice in the Fall. And to avoid the areas most exposed to Acque Alta in order not to be imprisoned by the waters; In such an unfortunate case just wait until the water level has decreased, six hours, as I said above.
Acqua Alta lasts only a few hours, after which Venice always returns to its normalcy.