An insider take on what acqua alta in Venice, Italy, really is

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    Acqua Alta, one of the most misunderstood aspects of life in Venice

    If you looked for informations or news about Venice, you might have read about the unfamous “acqua alta”, or “high water”. When the tide rises higher than usual, the city of Venice ends up being flooded in certain spots. When high water is particularly intense, and there is little news to talk about, italian and international newspaper and media indulge in speaking about this “problem”, often giving wrong informations which lead to a widespread misunderstanding about the intensity, frequency and danger of this event.

    We’ll try to break down this topic and answer the most common question about this phenomenon.

    What is Acqua Alta?

    Acqua Alta, or “High Water”, is exactly that: an exceptional tide that, at the peak of its rise, floods parts of the city of Venice.

    What causes acqua alta?

    The “Acqua Alta” comes mainly as a result of three factors: the distance between the Moon and the Earth, the intensity of a specific kind of wind blowing for a certain amount of time, and the atmospheric pressure. When the Moon is closer to the Earth, the tides are higher. When Scirocco (a warm wind coming from the south) or Bora (a strong cold wind coming from north-east) blow, the water in the lagoon is slowed down or even blocked during its fall towards the open sea. When the pressure is lower, the waters rise higher.

    Does Acqua Alta happens all around the city?

    As every other city, Venice is not built exactly all at the same level. The lowest part of the city, San Marco square, is 80 cm above the average sea level; other areas are up to 1 m higher than that, therefore the percentage of the city being flooded depends on how high the acqua alta is.

    With +110cm, some area of Saint Mark’s square are covered by up to 30cm of water, but only 12% of the city is affected by the tide. With +130cm almost haf of the city is affected (46%). With +160cm, three quarters of the city are covered by water: some areas just a few cm, others as Saint Mark’s square by 80cm!

    How often does Acqua Alta happen?

    Hundreds of times per year!
    Well, partially. Some spots in San Marco are extremely low and get flooded even when the tide is not above average.  Not exceptional acque alte can happen 50 times per year and flood only small parts of the city, while more exceptional acque alte can occurr even 10 times in a year, or not at all.

    How quick does an Acqua Alta happen?

    Luckily enough, when the city gets flooded, there are no streams nor sudden increases of the level of the water. Rise and fall of the tides always take 6h and 12 minutes each, therefore the Acqua Alta is simply perceived as a relatively slow increase of the water level all around and inside of Venice.

    How long does it take for the high water to be completely gone?

    Rise and fall of the tides alternate every ~6h, therefore an acqua alta will flood certain areas and then withdraw all within a couple of hours at most.

    Is acqua alta dangerous?

    No, absolutely not.
    It has been dangerous only once, in 1966.

    On the 4th of November 1966, the high water reached a staggering +194cm over the sea level, covering 90% of Venice surface. Saint Mark’s square got drowned by 115cm of water, as if the lagoon had taken over the heart of the city. That day, for the first and last time, the Acqua Alta became a real danger to the life of the inhabitants of the lagoon. The inhabitated islands of Pellestrina and San Piero, which separate the Adriatic Sea from the Venice Lagoon witnessed the sea level rise over their stone dams. Here you can see some pictures of the most terrible acqua alta ever in the Venetian history.

    Does the acqua alta causes any problems to the inhabitants?

    Yes, it does.
    There are people living on the ground floor and the shops are all on the ground floor, therefore an exceptional acqua alta does indeed cause quite some problems to the inhabitants. Everything needs to be lifted (fridges, couches, wardrobes, goods… everything!), and after the (salty!) acqua alta is over, the place has to be immediately aired and cleaned completely. This is very annoying for everyone, but it really becomes a huge problem for older people. At the same time, you should go to work or do your exams, since it is not considered an “emergency” case.

    Can't people do something about it?

    The acqua alta can not be blocked with a bulkhead, because the water will come up from the pavements and the shower for example. The only way to block it is by completely isolating the house with a “vasca” (“tub“): the whole floor of the house/shop is destroyed, a hole is digged at a specific depth and then fully waterproofed, and then the the floor is re-built. The cost of such an operation can easily be over 100.000€.

    Is the phenomenon of the Acqua Alta scientifically studied?

    Since the foundation of the Venice Tides Forecast and Alert Centre in 1980, experts have studied in depth the tides and every aspect of the acqua alta, reaching outstanding levels of understanding of the phenomenon and offering a great alert service to every Venetian. *

    How are Venetians alerted of the next Acqua Alta? How do they know how high it will be?

    Besides the possibility to personally monitor the tides forecast on the city’s website, in case of an exceptional tide forecast the Centro Previsione e Segnalazione Maree di Venezia, now Centro Previsioni e Segnalazione Maree, immediately sends an sms to everyone who requested this (free) service, giving information about the time and the height of the possible upcoming Acqua Alta. As the tide approaches, further sms are sent to confirm the forecast or to notify about any change, and sirens can be heard loud and clear in every part of the city with sounds coded as such:

    1 long note = forecast of 110cm over the average sea level
    2 increasing notes = forecast of 120cm over the average sea level
    3 increasing notes = forecast of 130cm over the average sea level
    4 increasing notes = forecast of 140+ cm over the average sea level

    Are there any projects in Venice to limit the acqua alta?

    Since 2003, Venice is building 3 big underwater dams which will rise before high tides, preventing the water coming from the sea to flow into the lagoon. Supposed to be ready by 2016 (… will it be?), it costed 5.5 billion €.

    *UPDATE JULY 2016: Venice Tides Forecast and Alert Centre dissolved

    The Venice City Council has recently taken the decision to dissolve, by the 1st of September 2016, 4 municipal institutions, and to internally perform their tasks. This decision was taken in order to rationalize the expenses by disbanding the B.O.Ds (board of directors) and to simplify internal bureaucracy. It does sound very reasonable, doesn’t it?

    One of the 4 institutions affected by this decision is the Centro Previsione e Segnalazione Maree di Venezia, the Venice Tides Forecast and Alert Centre.

    The role played by the Venice Tides Forecast and Alert Centre is essential to local life, and as such we sincerely hope that the choice taken by the City Council will not decrease in any way the investments into the research and understanding of the tides, the quality of the forecast and the promptness of the alerts.

    We’re going to have an answer to our hopes and worries in a few months when the season of the Acqua Alta starts again… stay tuned, and hopefully DRY!!!

    UPDATE AUGUST 2016: THE MOSE IS NOT WORKING.

    We’d like to thank the Venetian administrations and lobbies for having, once more, proved to be a joke. A terrible joke. As of now, 13 years later, all we have is 5.5 billion € less and some thousands of tons of steel rotting in our delicate lagoon.

    Acqua Alta, or High Water, on the Strada Nuova in Venice, Italy. A natural phenomenon often misunderstood buy visitors and media

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