Acqua Alta: high tide
Venice can be flooded by high tides (acqua alta) during winter most likely in November and December.
Venice has taken and takes her reason of being, her greatness, her character of a unique city in the world from water; that is why being Venetian has always meant being part of this "waterhood", experiencing its positive sides and facing its specific problems without dramatising them.
Among the problems typical of Venice the most evident derive from the water transport and traffic, and the discomforts caused by acqua alta. Acqua alta is nothing but a rather hefty high tide, such as often to flood, in some periods of the year(during winter, most likely in November and December), the lower points of many fondamente (streets bordering canals), calli (streets), and campi (squares). Only very seldom are wide parts of the city flooded.
These floods are generally limited to a few centimetres of water, so that, usually, a pair of common Wellington boots is enough to overcome the problem. This contrasts with a misunderstanding common among those who do not personally know the matter and are lead to think, also by TV information, that acqua alta means water reaching to one's navel.
It isn't obviously so: for instance, when "+100 cm acqua alta is mentioned, that metre only represents the increase in comparison with the average sea level (the average sea level conventionally accepted is the 1897 one measured at the Punta della Salute). At this level, only a very small number of city points get flooded. Acqua alta is due to a number of causes connected first of all with the astronomical tides and the geographic position of the Adriatic Sea, and emphasised by particular meteorological conditions. In addition to these there are local effects peculiar to the Lagoon, caused by winds, rains and, especially in the past, the contribution of continental fresh waters.
- under 80 cm: the tide is regular
- 100 cm : about 4% of the city is under shallow water
- 110 cm : about 12% of the city is under shallow water. Sirens wailing over the city three-four hours before the peak - low alert.
- 120 cm : about 35% of the city is under shallow water. Sirens wailing over the city three-four hours before the peak - medium alert.
- 130 cm : about 70% of the city is under shallow water. Sirens wailing over the city three-four hours before the peak - high alert.
- 140 cm : about 90% of the city is under shallow water. Sirens wailing over the city three-four hours before the peak - emergency!
Floods of the catastrophic 1966 level (194 cm!) are considered unlikely more than once in 180 years. The high tide (acqua alta) most serious in the recent years was on November 16, 2002 (147 cm); it was the fifth in the history of Venice.
Don't panic: the floods rarely stay more than few hours and there is no danger to human life.
During last century the Lagoon and its functionality were deeply modified by the action of man, who has, indirectly, contributed to magnifying the problem. Tide height is moreover subject to variations in relation with several meteorological issues. High tides get particularly huge when the barometric pressure undergoes a substantial decrease and/or there is a strong bora or scirocco (south-east) wind. The widest tide excursions usually occur at new and full moon periods (syzigies), in the first and last quarter of the moon (quadratures) the abundant high tide phenomenon is less likely to happen.
More information and forecast:
Centro Previsioni e Segnalazioni Maree
Palazzo Cavalli, S. Marco 4090
tel. +39.041.2748787, fax +39.041.5210378
Aut. answering service +39.041.241.1996