Venice Flooding

Venice flooding, locally known as “acqua alta”, or “high water” in English, is a phenomenon that affects the city usually during the winter period, when the astronomical tide, sirocco, seiche waves, or a combination of several of these elements, cause an increased flow of water into the lagoon of Venice.

An exceptional tide, meaning a tide equal to or greater than 140 cm above normal as measured at Punta della Salute, statistically occurs only once every four years.

These exceptional tides are caused by the coincidence of several factors: the astronomical tide, depression on the Tyrrhenian Sea and the resulting strong sirocco winds, and seiche waves in the Adriatic.

Then there is the phenomenon of subsidence of the Venetian lagoon, which, during the last century has resulted in the loss of Venice in altitude of about 26 cm above the sea level.

The acqua alta period follows a tidal cycle (the tide rises for 6 hours and falls the next 6): in the days where there is Venice flooding, this only lasts for the middle part of the rising phase.

So, usually acqua alta in Venice lasts for only about 3-4 hours. Immediately after, once the water levels have lowered, life in the city returns to its normality, as people here have gone through this cycle throughout their lives.

Acqua alta flooding usually occurs in autumn and winter months, with the most likeliest months being November and December. During this period, it is possible that there are only a few days of flooding, often to levels that affects only the lower parts of the city (such as Piazza San Marco).

An exceptional tide (Venice flooding of over 140 cm) happens, statistically, once every 4 years.

The tide predictions and the levels recorded are compared to a standard level at Punta della Salute, which is corresponding to the average sea level of 1897.

Venice’s city streets are not all at the same level, and instead, there is great variability. For example, an exceptional tide of 140 cm corresponds to a flooding of about 59% of the city’s streets, with water levels ranging from a few millimeters to 60 cm in Piazza San Marco, one of the lower areas in the city.

Only exceptionally high water levels affect almost the entire city, and even then, only in the lower areas the water levels are really remarkable.

The following are percentages of city under water compared to the amount of increased water levels:

+100 cm: 5%,
+110 cm: 12%,
+120 cm: 28%,
+130 cm: 46%, and
after 140 cm, the city is flooded for more than 59%.

When the acqua alta exceeds 110 cm, which happens on average about 4 times a year (sample years 1966-2009), in the hours when the tide is at its highest, only about 14% of the city is covered by water.

As a city, Venice has always been accustomed to living with high water levels.

Today, when the tide is expected to exceed 110 cm above the mean sea level, the population is warned by an acoustic signal and text messages are sent to those who subscribe to the appropriate service from the local weather service.

At the same time, wooden paths are set up on platforms within the main streets of the city.

Water buses continue their services, but may change, at times, some routes, but still provide access to almost the entire city.

Only in case of very high tides, typically greater than 120 cm on average above the sea level, to move comfortably in Venice, the locals took out their acqua alta “boots”. However, the discomfort only lasts for a few hours, the time it takes for the water flow back into the sea.

Acqua alta is not a dangerous phenomenon, with the high tide being, in most cases, just a limited discomfort for both Venetians and tourists. If you do come across the flooding during your visit here, remember that normally, you just have to wait only a few hours for the water to be completely drained.