Why is there Acqua Alta in Venice?
The Venice Lagoon is located at the northern end of the Adriatic Sea. When a strong southerly wind blows on the Adriatic basin for a long time (the southeasterly sirocco), masses of seawater are transported northwards.
When they find the mouth of the lagoon, they enter through the mouths that open between the sandy cords. The wind must blow impetuously and for a long time for this condition to occur. Even the strong bora wind, coming from the north/northeast, can cause high tides in Venice, more so if it collides with the sirocco wind. Therefore, the wind pushes the masses of seawater into the lagoon.
Another meteorological phenomenon that causes high water in Venice is linked to the great atmospheric disturbances. An area of low pressure in the upper Adriatic and high pressure in the southern Adriatic, causes the water to move to the north of the basin, with an increase in the level of several centimeters. The fluctuations in the level of the Adriatic, known in hydrology as sexes, have a great influence on the high tides in the lagoon.
Turning instead to astronomical factors, it must be said that in the Mediterranean the astronomical tides (linked primarily to the Moon) are much less pronounced than in the ocean, but not for this reason absent.
In certain situations, when the astronomical high tide is added to the meteorological conditions described above, there may be more important high tides. But not always the astronomical high tide coincides with the occurrence of meteorological factors that originate the high tide.
In the occurrence of high water in Venice also enter geological and climatic factors. Venice is subject to strong subsidence, as the lowering of the ground that it’s caused by human intervention, for example with groundwater drainage due to the industrialization of the coastal territory in the last 20 years.
The level zero cm of the Venice High Tide
In order to answer this question, it is first necessary to establish a reference point and a size for each phenomenon of high water. The zero ideographic levels from which to measure the episodes of high water in Venice was conventionally chosen is located at Punta della Salute.
A tide equal to or greater than 140 centimeters above the zero-tide gauge of Punta della Salute is considered exceptional. This happens on average once every three years.
The exceptional tides above 140 cm lead to a flooding of about 57% of the city, according to data from the City of Venice. In this case, the red warning code is displayed in the city. The sirens sound and alert citizens and tourists to the alarm situation.