Acqua Alta

High Tides and Flooding in Venice

by Durant Imboden

Venice flooding photo Acqua alta, or "high water," can make Venice feel like Atlantis. At its worst, in 1966, acqua alta flooded the city with more than a meter of salty lagoon water; more typically, visitors notice water splashing over canal banks or bubbling up through drains in the Piazza San Marco.

The phenomenon is often mistaken for proof that Venice is sinking. Although the city did sink about 10 cm in the 20th Century because of industrial groundwater extraction, the sinking largely stopped when artesian wells on the mainland were capped in the 1960s. Today, subsidence is estimated at 0.5 to 1 mm per year, mostly due to geological factors and compression of the land beneath the city's millions of wooden pilings. A larger problem is the rising sea level, which will become an even bigger threat as global warming melts the arctic ice caps. Already, the frequency of acqua alta has increased from fewer than 10 times a year to more than 60 times a year in the last century.

acqua alta photo

PHOTOS: Intrepid tourists wade in the Piazza San Marco.

Why (and when) acqua alta floods the city:

Acqua alta occurs when certain events coincide, such as:

A very high tide (usually during a full or new moon).
Low atmospheric pressure.
A scirocco wind blowing up the narrow, shallow Adriatic Sea, which forces water into the Venetian Lagoon.
By official definition, acqua alta occurs when the tide is 90 cm (35.4 inches) above normal high tide.

The phenomenon is most likely to take place between late September and April, and especially in the months of November, December, and October, in that order. The Comune di Venezia's Acqua Alta publication explains: "In the remaining months, frequency is very low, while there has never been an event from June to August."

Not all parts of the city are equally susceptible to flooding, as the following chart from the Comune di Venezia indicates. Also, the actual depth of water in the streets is far less than the "level of tide" might suggest. (See the "extreme case" below, where 135 cm of flooding translated into 40 cm of water in the Piazza San Marco.)

Level of tide	Percentage of Venice flooded
Up to 80 cm	Normal tide
100 cm	4%
110 cm	12%
120 cm	35%
130 cm	70%
140 cm	90%

An extreme case:

On the October day when some of the photos in this article were taken, the acqua alta reached 135 cm. The Piazza San Marco was inundated by at least 40 cm or 16 inches of water in what was billed as the worst acqua alta of the decade (and the 10th worst since the record flood of 1966). Several times during early winter in the last few years, acqua alta has reached 150+ cm and flooded most of the city. (The worst deluge in Venice's history was in 1966, when floodwaters topped out at 194 centimeters.)

Possible solutions:

The Comune di Venezia and various international organizations have been working on solutions that range from MOSE Project floodgates at the Lagoon's entrances (due to be completed in 2014) to raising of pavements in low-lying areas of the city. Much construction of the latter has already taken place, but it remains to be seen whether acqua alta can be tamed without closing off the Lagoon from the sea and turning it into a freshwater lake.

Next page: What to expect, how to prepare

In this article:

Acqua Alta - Introduction

What to expect, how to prepare