Why is there high water in Venice?
The causes of high water in Venice can be divided into two broad categories of reasons: meteorological and astronomical. The Venice Lagoon is located at the northern end of the Adriatic Sea. When a strong southerly wind (the south-eastern sirocco) blows for a long time in the Adriatic basin, the masses of sea water are transported northwards.
When they find the mouth of the lagoon, they enter through the mouths that open between the sand bars. The wind must blow strongly and for a long time for this condition to occur. The strong bora wind, coming from the north/northeast, can also cause high tides in Venice, especially if it collides with the sirocco wind. The wind then pushes the masses of sea water into the lagoon.
Another meteorological phenomenon that causes high water in Venice is linked to large atmospheric disturbances. An area of low pressure in the upper Adriatic and high pressure in the southern Adriatic causes the water to move north of the basin, with an increase in the level of a few centimeters. Fluctuations in the level of the Adriatic have a great influence on the high tides of the lagoon.
Moving on to astronomical factors, it must be said that in the Mediterranean the astronomical tides (mainly linked to the Moon) are much less pronounced than in the ocean, but not therefore absent. In some situations, when astronomical high tides are added to the meteorological conditions described above, there may be more important high tides. But the astronomical high tide does not always coincide with the occurrence of meteorological factors that give rise to Acqua Alta.
The causes of high water in Venice also include geological and climatic factors. Venice is subject to severe subsidence, such as the subsidence of the land which is caused by human intervention, for example with the drainage of groundwater due to the industrialization of the coastal territory in the last 20 years.
The zero cm level of High Water in Venice
To answer this question, it is first necessary to establish a reference point and dimension for each high water phenomenon.
The zero ideographic level from which to measure episodes of high water in Venice has been conventionally chosen and is located at Punta della Salute. A tide equal to or greater than 140 centimeters above the ideographic zero of Punta della Salute is considered exceptional.
This happens on average once every three years. Exceptional tides above 140 cm lead to flooding of around 57% of the city, according to data from the Municipality of Venice. In this case, the red alert code appears in the city. The sirens sound and warn citizens and tourists of the alarm situation.
