Conventionally, in Venice, a sea level higher than 80 cm above the local datum of Punta Salute, is called "acqua alta": at this level height a lot of problems about transport and pedestrian use of roads in lowest sides of the town (St. Mark's Square) arise. When tide gets over 100 cm, the phenomenon involves a bigger part of pedestrian routes. 
The observed tide in Venice can be thougth as the addition of two distinct components: the astronomical tide, correlated to the heavenly bodies motion, mostly the Moon and the Sun, and the meteorological surge due to atmospheric conditions. 
Phenomenona as the eustasy (fluctuation of sea levels, caused by global climate change) and the subsidence of the Venetian Lagoon (slow, but progressive shift downwards of a sea basin or a continental area) gave their contribution to the enhancement of the high water event frequency.