Why Does Venice Flood?

Understanding Venice's famous Acqua Alta

Wellies, rain boots, galoshes, whatever you call them, bring your puddle-jumping boots to Venice during the Acqua Alta.

Tourists in the Piazza San Marco during the Acqua Alta

What is the flooding?

This seasonal high tide is called the Acqua Alta. It is a high tide phenomenon that happens at various times of the year in Venice. It is most likely to happen in November and December, but is likely anytime between October and March. The high tide is less likely in the summer months, but it can happen.

The Aqua Alta is specifically when the tide exceeds 80cm. "Is that street level?" you may be asking. No, because the streets of Venice are at varying heights. Piazza San Marco is one of the lowest points in the city where flooding from the Acqua Alta is perceptible starting at 82cm. If you're at the Ponte Rialto, your feet won't get wet until the tide rises to 105cm. At the nearby Santa Lucia train station, flooding happens at 135cm tide, which is considered a very high tide.

Why does Venice flood?

The Acqua Alta happens because of a combination of three things: astronomic tide, atmospheric pressure, and wind.

Tide:

Astronomic tides happen because of the forces of the rotation of the earth combined with the forces of attraction by the sun and moon.

Pressure:

When atmospheric pressure lowers, the sea expands and rises.

Wind:

The strength and direction of the Sirocco (pronounced shi-ro-cko) wind which blows from south to east pushing the water of the Adriatic Sea and making the tide higher at its astronomical peak.

(Thank you, this has been, "How Weather Works" with Darcy, not a meteorologist)

There are also human-made reasons for increased flooding in recent years. Among the human intervention is Porto Marghera is an industrial area which has claimed a large part of the lagoon. In addition, deep canals were dug to accommodate large oil tankers and cruise ships. The bridge connecting Venice to mainland Italy, Ponte della Libertà, which includes a road for cars and a railway has also contributed to this issue. Climate change has, of course, affected weather patterns and tides, increasing the frequency and intensity of the high tides.

There are 3 levels of high tide

Sostentuna (high): 80-109cm which floods .1% of public areas like Piazza San Marco and the Basilica di San Marco.

Molto sostenuta (very high): tides above 110cm when 14% of public areas will be flooded.

Eccezionale (exceptional): tides higher than 140cm covering 55% of public spaces in the city.

How long does it last and how often does it happen?

There are 2 high and 2 low tides per day and the Acqua Alta typically lasts about 3-4 hours, however when the tide is very high, around 120cm, it lasts less than half an our on average.

A man trudges through a very high tide in Venice

The first level, Sostentuna, 80-109cm tides happen about 50 times a year. Molto sostenuta tides, or level 2, above 110cm used to be very rare, but now happen about 4 times per year.

Check out our article "When Does Venice Flood?" for resources on how to keep up with flooding in Venice and prepare for your trip.

How is it measured?

While reading this article you may have been asking "80cm high, what does that mean?" or "80cm above sea level?" The reference point for the height of the Acqua Alta is the average sea level in 1897. So 80cm high means 80cm above the average sea level in 1897.

It is also measured in reference to the Punta della Salute (the zero level), so you may see levels written as "80cm zmps" (Zero Mareografico di Punta Salute).

The highest Acqua Alta was recorded on November 4, 1966 at 194cm—that's over 6ft!

The Acqua Alta is a staple of Venice and has been happening for as long as the city has existed. However, strong tides higher than 110cm, which were once a rarity, have been more frequent in the past 50-60 years. So grab some rubber boots and get to splashing because Venice truly is the City of Water.