Acqua Alta (high water) is a meteorological phenomenon which usually takes place in Venice in autumn and winter time. How does it happen? A larger inflow of water into the Venetian Lagoon can be caused by a combination of astronomical tide, strong south wind which is called ‘scirocco’, and ‘seiche’ – the periodic movement of sea waters, a sort of long wave which washes all Adriatic coasts. No matter how magical Venice normally looks like – watching it doubled in a mirror of water on the ground adds to a visual experience which is even hard to describe.

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When does an exceptional tide occur?

Exceptional tides (when the water-line is equal to or more than 140 centimetres on the mareographic zero of “Punta della Salute”, located near the Salute Church, in front of St. Mark’s Square) statistically occur once every 4 years. They are caused by a combination of various factors, such as the astronomical tide, low pressure on the Tyrrhenian Sea, strong south wind (scirocco) and the Adriatic seiche. Further two larger phenomena also contribute to increase the water level: eustasy and the subsidence of the Venetian Lagoon, which, together, have caused an altimetric loss of about 26 centimetres in the last century.

How long does an aqua alta last?

High water depends on the tide cycle (the alternation of high and low tides happens every 6 hours): when there is “acqua alta” on the streets this lasts only a few hours during the peak of the high tide (usually 3 to 4 hours). Once water goes down again, things go back to normality.

How often does “acqua alta” occur?

High waters may occur in autumn or winter seasons and are most likely to happen in November and December. But even in these months, high waters usually affect only the lowest parts of the town, such as St. Mark’s Square, whereas exceptional high tides (>= 140 cm) statistically occur only once every 4 years.

But how high can high water be?

High water levels are measured on the mareographic zero at “Punta della Salute” and 97% of the town is at +100 centimetres. This means that the actual water-line is always much less than the high water forecast. For example, an exceptional high tide of +140 centimetres means that 54% of the town is covered by water, but even in the lowest parts the water-line won’t reach more than 60 centimetres on the streets.

What happens in Venice when there’s a high tide?

Venice and Venetians have always been used to coping with “acqua alta”. These are the City Administration’s measures in case of high tide: if there’s a sea level forecast of +110 cm on the mareographic zero, the population is alerted by acoustic signals and with text messages (for those registered at the free high tide information service of the City Tide Centre – Centro Maree Comunale). At the same time, elevated platforms are set along the main streets to allow passage. Public waterbuses keep on working, although some lines may be subject to changes. In any case access to most of the town is guaranteed. Only when exceptional high waters occur (higher than 120 cm on the mareographic zero) the famous “acqua alta boots” are really needed, but even on these occasions the inconvenience last just as long as it takes for the water to go down again, which usually happens in a few hours.

You can check when the next high water is expected here

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