Acqua alta, literally high water, is the typically understated way in which Venetians refer to the periodic flooding of the city during periods of so-called spring tides. It’s caused primarily due to the natural geography of the Venetian lagoon. The lagoon closed off from the open sea by a series of long islands with gaps in between. At certain times of the year, higher than usual tides are augmented by winds (known as the bora and scirocco) which blow north up the Adriatic sea, forcing sea water into the mouths of the lagoon and preventing lagoon water from leaving.
As this is a natural occurrence, it has always happened. The first recorded instance of acqua alta at Venice was on October 17 589, and it’s a regular feature of chronicles. Particularly high flooding occurred in 1283, 1686, and 1966 when, on November 4 the water was 194cm above sea level, causing a huge amount of damage to the city.
Scientific records of the phenomenon only began in 1872, so we have no way of knowing the frequency or average levels of acqua alta before that. It’s possible that chroniclers only mentioned it when it was particularly high. (We know from their descriptions that the 1283 and 1686 events were comparable to 1966.) However, it’s believed that it has occurred more frequently in the last two hundred years due to several building project that have take place, notably the building of the Ponte della Libertà in 1841, (the rail and road bridge linking Venice to the mainland), the development of the industrial port on the mainland at Marghera, and so on. We can add to this increased subsidence of the city and rising sea levels due to global warming.
Nowadays, the tides are monitored from a special station near the Punta della Dogana, across the Canal Grande from the Piazza San Marco. If particularly high levels of water are expected an alarm will sound around the city, a bit like an air raid siren, about three hours before the high tide will happen.
The lowest parts of the city, in and around the Piazza San Marco are 80cm above sea level, so this is the minimum needed for some flooding to occur. High tide, which occurs twice a day is usually between 50 and 60cm. The Riva dei Schiavoni, which is where the Piazzetta and Piazza San Marco meet the open lagoon is 110cm above sea level so when tides of more than 111cm occur, there is significant flooding in the Piazza. Most of the rest of the city is safe as long as the tide is lower than 120cm.
When the square begins to flood, water bubbles up through the circular, white marble manhole covers in the square, like water coming up the plughole. It then fills up from the middle, it’s lowest point until the water leaves the square under the clock tower and enters the street known as the marzaria. By this point, wellingtons are needed to walk in the square. It’s very eery, because although the seagulls arrive and start swimming in the water, the square’s population of pigeons swoop around the square, not knowing where to land.
To cope with the flooding, the city council places trestle tables, known as passarelle in the streets making walkways. Locals all don their wellington boots, while tourists buy brightly coloured, plastic disposable versions, and stand in the square taking the ultimate Venetian selfie. A few hardy folk will take their shoes and socks off and go paddling in what Napoleon, had he seen it flooded, might have called ‘the finest paddling pool in Europe’. However, I wouldn’t advise that as I’m not sure that the water is all that clean.
The photographs in this essay record the acqua alta which occurred at 12.15 on November 7, 2017.