Acqua Alta (high water) occurs in Venice about one to four times a year – typically in winter – caused by a combination of astronomical tide, scirocco (strong south wind) and seiche (periodic movement of sea waters). It is considered acqua alta when the water level reaches 140cm or more than the Mareographic zero, a reference point located at Punta della Salute; 97% of the town is at 100cm over the zero point. At an exceptional high tide (occurs once about every three years) with 140+ centimeters increase, 59% of the town would be covered in water – even the lowest streets typically will not get a depth greater than 60cm.
The population is alerted of aqcua alta through acoustic signals and text alerts, elevated platforms are placed along the roads and in cases of exceptional tides, rubber boots are worn. Acqua alta only lasts a few hours, just like a typical tide change, and everything returns to normal as soon as it passes.