What is Venice acqua alta? Acqua alta facts explained

Everybody knows that from time to time beautiful Venice has to deal with this phenomenon. But how many of us actually know what is Venice acqua alta? In this post I’m going to answer to the most common questions about it.

What does acqua alta mean?

Nothing but high water. The correct pronunciation is AH-kwah AHL-tah (AH not OH).

What is acqua alta?

Acqua alta is mainly a natural phenomenon. It is due to the tides and it occurs especially when the high tide combines with winds and long waves. Because the winds prevent the water outflow. It’s in these occasions that the water reaches some areas of the city. Venice is at the centre of the Lagoon. The connection with the sea happens through three harbour entrances, in Italian “bocche di porto” (literally port mouths): Lido, Malamocco and Chioggia.

When does acqua alta happen?

Acqua alta mostly happens during autumn and winter. There are hydrographic and tides control stations scattered around the city, the lagoon and at open sea. The reference point (the zero level), functioning since 1906, is at Punta della Salute, near the Basilica. The tide beyond 80 cm zmps (Zero Mareografico di Punta Salute) is considered acqua alta. In this case, the lower parts of the city are already affected (like piazza San Marco).

Are there also non natural reasons?

Yes. The industrial area of Porto Marghera is part of it. Its building reclaimed some parts of the lagoon that once served as additional water basins. Moreover, to allow the passage of oil tankers, a deep canal was dug. This canal has increased the flow rate in the lagoon. Also other works had a part of this situation. Among them the construction of the Railway bridge and the Ponte della Libertà. In the last 137 years the city of Venice lost about 28 cm of altitude, because of these operations. And of course also the climate changes and the glaciers melting contribute to this situation. Consequently the acqua alta effect has increased in the last period.

How long does acqua alta last?

Acqua alta normally lasts a few hours (three or four).

How high does the water get in Venice?

A normal tide is under 80 cm and does not affect the city.
It is sostenuta between 80 and 109 cm. The level above 80 is reached about 50 times per year and it only floods the 0,1% of the public itineraries (piazza San Marco is among them, cause it’s a very low area of Venice, at about +70. The Basilica entrance is the lowest part of the whole city: +63 cm).
Above +110 the tide was by tradition defined eccezionale. It once happened rarely. Unfortunately, it now happens about 4 times per year, flooding the 14% of the public itineraries. Nowadays It is called molto sostenuta.
While the tide is eccezionale if it reaches or exceedes +140, flooding the 55% of the paths.

Further info about the image above at archive.comune.venezia.it

How can people manage with acqua alta?

Forecast are spread through newspapers, signs and internet. In case of exceptional events people living or working at low levels are warned thanks to a phone call or sms service, so that they can be prepared and install the water bulkheads. If the level is expected to exceed 110 m, the warning sirens are activated three hours in advance. When the phenomenon is more frequent some walkways are located in the lowest areas of the city. They allow people to pass without the need to wear boots. But only if the water levels is less than 120 cm. Over 120 cm, boots are needed. You can buy them in Venice. But some hotels have spare boots for their guests. Otherwise you can also find some disposable shoe-covers. You can consult the forecast service online at www.comune.venezia.it

Venice acqua alta Facts

Already in 1500 Venetian dogi started to worry about the situation. They ordered the deviation of some rivers which flowed into the Lagoon. In the following centuries the sea level has increased though.
The maximum height was recorded on November 4 1966: 194 cm. It was then nicknamed acqua granda (great water). It was due to the combination of different phenomenons: acqua alta, heavy rains and strong winds. This extraordinary event left many Venetians without home and caused severe damages to many buildings and to Venice’s artistic heritage. 75% of shops, workshops and commercial activities were damaged or destroyed. The city remained isolated for two entire days. Moreover the water was so high that the rescue boats could not pass under the bridges and reach the houses. You can watch this video
The walkways are long 4km overall.
Due to the increased sea levels, some vaporetti change their itinerary or temporarily stop their route.
Many are the damages to the economic activities. It’s not only a problem of “mere flooding”. You have to consider also: lack of supplies, lack of profit for closure, hours of work to safeguard the goods.
People do not live on ground floors any more. Only shops and warehouses are now on ground level. The ways to protect the goods are water barriers or sawhorses. Except the Acqua Alta Bookshop of course! 😉
The 75% of the acqua alta happens in November, December and October (in order of frequency).
Acqua alta also hits Chioggia and sometimes Grado and Trieste.
The famous MOSE is a system of mobile gates created to close the lagoon from the sea in case of high tides. But that’s another story for another time. If you want to read more about it, go to my post: When will venice flood barrier be finished?