Exceptional High Tide on St. Mark's Square
6 May 2015 | From Massimo of Italy
"Acqua alta" (high water) is a phenomenon which generally takes place in Venice in winter time, when a combination of astronomical tide, strong south wind (scirocco) and seiche (see glossary below) can cause a larger inflow of water into the Venetian Lagoon.
Exceptional tides (when the water-line is equal to or more than 140 centimetres on the mareographic zero of "Punta della Salute", located near the Salute Church, in front of St. Mark's Square) statistically occur once every 3 years. They are caused by a combination of various factors, such as the astronomical tide, low pressure on the Tyrrhenian Sea, strong south wind (scirocco) and the Adriatic seiche. Further two larger phenomena also contribute to increase the water level: eustasy (see glossary below) and the subsidence of the Venetian Lagoon, which, together, have caused an altimetric loss of about 26 centimetres in the last century.
High waters may occur in autumn or winter seasons and are most likely to happen in November and December. But even in these months, high waters usually affect only the lowest parts of the town, such as St. Mark's Square. High water depends on the tide cycle (the alternation of high and low tides happens every 6 hours): when there is "acqua alta" on the streets this lasts only a few hours during the peak of the high tide (usually 3 to 4 hours). Once water goes down again, things go back to normality.
Source: Comune Venezia