Acqua alta (high water) is the term used to describe a tide that is 80 cm above sea level as measured in 1897. That was the year in which the first gauge for measuring tidal activity was put in place at the Punta della Dogana. Zero on the gauge marks the sea level in 1897. Nowadays, the sea level is at least 23 cm above zero.
Acque alte are officially categorised as:
Marea sostenuta (80-109 cm)
Marea molto sostenuta (110-139 cm)
Alta marea eccezionale (140+ cm)
An acqua alta of only 80 cm is enough to flood the lowest parts of the city, which also happen to be the busiest, namely the Piazza San Marco and, in particular, the Basilica di San Marco.
In recent years the frequency of acque alte has increased considerably. In 2014 there were 189 compared with only 35 in 1983.