Educational // But how high is high-water?

In Venice (and only in Venice) the median level of the sea as measured in 1897 is used as the reference level. The reason is so that the situation of embankments and buildings in relation to the tides may always be monitored with respect to the same reference point, known as the ‘tide-level-zero’ at the Punta della Salute, where a monitoring station is located. In the rest of Italy, the median sea level (IGM) refers to the level in Genoa in 1942, which is about 23 centimeters higher.

The phenomenon of “acqua alta” is determined by exceptional conditions when high tide, low-pressure atmospheric systems, wind and in some cases persistent rain coincide. A high tide is considered “normal” when it reaches a level of +80 centimeters. This occurs around 50 times a year and floods only 0,1% of public circulation routes.
The flooding includes Piazza San Marco which is very low (in some points it barely reaches +70 cm). A high tide is “exceptional” when it reaches or exceeds the level of +110 centimeters, flooding approximately 12% of public circulation routes (on an average about 3-4 times a year). In this case pedestrian circulation is guaranteed by 4 kilometers of raised boardwalks installed by the city administration in the lowest sections of the main circulation routes. The projects to raise the level of public paving aim for a level of +120 centimeters; the tide comes in at a level above this marker once or twice a year. The dramatic tide on November 4 1966 reached a level of +194 centimeters.

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