The Acqua Alta: a natural phenomenon in Venice
High water, a phenomenon Venetians have always been used to, is nothing else but a tide peak, usually occurring in autumn and winter, mostly between November and December, when the astronomical tide, the strong Sirocco wind, the “sessa” phenomenon in the Adriatic – or a combination of these elements – determine a higher inflow of water in the Venetian Lagoon.
The height of the tide varies according to different meteorological factors. The tides are higher when the barometric pressure is particularly low or when the Sirocco wind is blowing. The widest tide excursions usually occur at new and full moon periods, in the first and last quarter of the moon the abundant high tide phenomenon is less likely to happen.
High water in Venice follows the tidal cycle, that rises for 6 hours and then lowers in the next 6: in the days with high water, this phenomenon persists only in the central hours of the rising phase. High water in Venice usually lasts about 3-4 hours, and when the level of the water lowers, the city goes back to normal.
But how high can the high water get? When the tide level is measured, the point of reference is the average sea level measured at Punta della Salute. To let you better understand, nearly all of the city (97%) is at about 100 cm over the average sea level. This means that the quantity of water that can flood the streets is always much lower than the maximum forecast tidal level. For instance, an exceptional tide of 140 cm corresponds in reality to about 60 cm of water in the lowest points of the city (Saint Mark’s Square) and floods about 54% of the historic city centre.
sustained tide
the tide level is between +80 cm and +109 cm over the average sea level
very sustained tide
When the value is between +110 cm and +139 cm;
exceptional tide
When the value reaches or exceeds +140 cm.
INFORMATION AND COMMUNICATION SERVICES FOR HIGH WATER
Three times a day the Tide Forecasting Centre of the City of Venice makes a forecast of the tide levels for the following two days and communicates it to the city. About three hours before the arrival of the tidal peak a system of alarm sirens, placed in the strategic points of the city, is triggered to warn the entire population.
WHAT TO DO IN CASE OF HIGH WATER
Venice is used to cohabiting with high water. It is good for tourists to know that when tides above 110 cm over the average sea level are forecast in the city, the population is warned by sirens.
At the same time routes on gangways are prepared in the city’s main streets. These routes are indicated also by notices put up on the landing jetties of Actv, the local public transport company.
The Actv water buses continue their service, sometimes partially changing route, but anyway guaranteeing access to nearly all of the city.
Only in presence of a very sustained tide, higher than 120 cm over the average sea level, you need RUBBER BOOTS to walk in Venice. DO NOT USE plastic trash bags to cover your feet because water leeks inside immediately, it is also very dangerous because you can easily slip and hurt yourself. Anyway we remind you that this phenomenon lasts only a few hours, i.e. the necessary time for the water to flow back out to sea.
When there is high water not all of Venice goes under water. Only exceptional high water affects nearly all of the city, and anyway, even in this case, the water level is really relevant only in the lowest parts, one of these being Saint Mark’s Square.
To have an idea of the extent of the city that is flooded in presence of sustained tides, the following table indicates the flooding percentages according to the tidal levels:
+100 cm: 5%;
+110 cm: 14%;
+120 cm: 29%;
+130 cm: 43%;
above 140 cm more than 54% of the city is flooded.
Written by Margherita Pasotto
