Acqua Alta (high water) is a term used in Venice for the high tide in the Adriatic Sea. The water levels reach the air maximum in the Venetian Lagoon, which in turn causes flooding in Venice. This usually happens outside of the Venetian tourist season, between September and April when the tides are strengthened by seasonal winds called Sirocco and Bora that blow along the Adriatic coast.
Venetian Lagoon is a large elongated body of water around 35 miles long and 6 miles wide, which is separated from Adriatic Sea by a sand bar cut by three passages: Lido, Malamocco, and Chioggia. During high tides the water from the sea comes into the Lagoon via these three passages, raising the Lagoon level, and afterwards it goes back out to the sea. These daily tides clean up Venetian canals allowing the water to circulate, and the city to survive without special sewer systems.
There is a special scale of acqua alta levels developed just for measuring tides in Venice. At Punta della Dogana, Venice’s entrance to the Lagoon there is a hydrographic station, which regularly takes measurements. The zero point was defined in 1897 when measurements started. Add 31 to 43 inches to that and you get Venetian high tide. 44 to 55 inches above 0 is very high tide. And above 5 inches is extremely high tide.
The worst acqua alta was recorded in November of 1966, with an increase of 76 inches, and was devastating for the city and its inhabitants. While it was a relatively infrequent phenomenon in the nineteenth century, with about 10 instances a year, the frequency and severity of it increased last century to reach about 60 times a year. Some people think that is because Venice is quickly sinking, but this is not accurate. The pilings on which Venice stands sunk 10cm lower in the twentieth century due to the rise of industrial activity in the Lagoon and pumping out of the groundwater, which caused rapid compression of the layers of land on the bottom of the Lagoon, which holds the pilings. Once the government realized how this was affecting the city, groundwater pumping was stopped and now Venice sinks by less than a millimeter a year mostly due to natural geological reasons.