Venice sinks from when it was born because the sandy soils with time are compacted and are settled. The phenomenon is called subsidence. The subsidence became very fast in the twentieth century when the industrial center of Marghera began to extract rivers of productive waters from the underground aquifers. If the old photos dated back a century ago show a proud Venice, high on the water, today it is a city sitting on the water’s surface.
At the phenomenon of subsidence has been combined with excavation in the lagoon and above all will add up the raising of level of the seas, which will be made more dramatic and fast when the warming of the climate will melt the polar ice.
The lagoon of Venice is a shallow sea of brackish water divided by the sea by a two islands long and narrow, Pellestrina and Lido, and joined to the sea by three natural channels, Lido, Malamocco and Chioggia, called in italian “bocche di porto” (literally mouths of the harbour) mainly oriented towards south east.
Every six hours the tide grows and every six hours it goes down. When the sun and moon add up their astronomical effect with the contribution of rains and the scirocco wind (its prevailing directions being south and south-east) that swells the Adriatic against the lagoon, in Venice the water rises up to flood St. Mark’s Square (70 centimeters, the lowest point of the city).
On November 4, 1966, there was the most devastating of the “perfect storms” in which all the disastrous effects were concentrated, and the water level reached 194 centimeters, with incalculable damages.
Precise scientific parameters define the phenomenon of high water, and exceptional tidal events can be categorized as:
– intense when the measured sea level is between 80 cm and 109 cm above the standard sea level (which was defined by averaging the measurements of sea level during the year 1897)
– very intense when the measured sea level is between 110 cm and 139 cm above the standard
– exceptional high waters when the measured sea level reaches or exceeds 140 cm above the standard.
