What's "acqua alta" (high water)?
"Acqua alta" (high water) is a phenomenon which generally takes place in Venice in winter time, when a combination of astronomical tide, strong south wind (scirocco) and seiche (see glossary below) can cause a larger inflow of water into the Venetian Lagoon.
Detailed explanations: The tide
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 (see glossary below) and the subsidence of the Venetian Lagoon, which, together, have caused an altimetric loss of about 26 centimetres in the last century.
Detailed explanations: Exceptional high tides
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.
Detailed explanations: The tide permanence
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.
Detailed explanations: Graphics and statistics - Monthly distribution
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.
Detailed explanations: The flooding percentage
Does Venice completely go under water when "acqua alta" occurs?
No, only exceptional high waters affect the qute whole town and even on those occasions the water-line is really remarkable only in the lowest areas. These are the flooding rates of the town in relation to high water levels (measured on the mareographic zero): +100 cm: 5%; +110 cm: 14%; +120 cm: 29%; +130 cm: 43%; over +140 cm 54% of the town is covered by water.
Detailed explanations: The flooding percentage
How many times in a year can a high water of + 110 cm occur?
From 1966 to 2009 high waters of + 110 cm on the mareographic zero occurred only 4 times a year, covering 14% of the town.
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.
How does water rise and fall in Venice?
High water is not brought to town by a swollen river, therefore it must not be confused with a flood, since it is just a natural phenomenon related to the alternation of high and low tides. In fact, water invades the town very slowly from the canals and the rising tide lasts only for some hours. Once the peak is reached (which again lasts a few hours), the water level starts decreasing again until it leaves the streets empty and wet, like after a rainfall.
What to do in Venice when there is "acqua alta"?
"Acqua alta" is not a dangerous phenomenon: it is important to understand that most of the time high tides cause very limited inconvenience to Venetians and tourists. In these cases, the only thing to do is to be patient and wait a few hours for the following ebb. Otherwise, a suggestion for the most curious people is to buy a pair of boots and visit the town in a quite unusual way.
seiche: the periodic movement of sea waters, a sort of long wave which washes all Adriatic coasts
mareographic zero: the conventional reference level for measuring water-lines
scirocco: warm south-east wind pushing Adriatic waters towards the Venetian Gulf
subsidence: slow, but progressive shift downwards of a sea basin or a continental area
eustasy: fluctuation of sea levels, caused by global climate change.