Acqua Alta

Acqua alta, Piazza San Marco, Venice
Over the last few days, we have been 'enjoying' a spate of acquae alte, a sure sign that autumn has arrived. 

Acqua alta is the term used in Venice to describe a tide that is 80cms above sea level, as measured in 1897. That was the year in which the first gauge for measuring tidal activity was put in place when a hydrographic station was built at the Punta della Dogana. Zero on the gauge marked sea level in 1897. Since then, on account of subsidence, sea level is at least twenty-three centimetres above zero.
Hydrographic station, Punta della Dogana, Venice
Acquae alte are officially categorised as:

Marea sostenuta (80-109cms)
Marea molto sostenuta (110-139)
Alta marea eccezionale (140+)

An acqua alta of only 80 cms is enough to flood the lowest parts of the city, which also happen to be the busiest, namely the Piazza San Marco and the Basilica. In recent years the frequency of acquae alte has increased considerably. In 2014 there were 189 compared with only 35 in 1983. 


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