Acqua alta is the term used in Veneto for the extraordinary tidal peaks that regularly occur in the northern Adriatic. The peaks reach their maximum in the Venetian lagoon, where they cause a partial flood of Chioggia and Venice. Floods also occur elsewhere in the northern Adriatic, in Trieste and Grado and, but to a lesser extent and much less frequently.
When does this occur?
The phenomenon occurs mainly between spring and autumn, when the astronomical tides are intensified by the prevailing seasonal winds that make it difficult to return regularly. The main winds are known as the sirocco, which blows north just along the Adriatic Sea, and the bora, which has a specific effect due to the location and shape of the Venetian lagoon.
These supernormal tidal events can be classified as follows:
- intense, when the measured sea level is between 109 cm and 80 cm above normal sea level
- very intense when the measured sea level is between 139 cm and 110 cm above the standard
- When the sea level reaches or exceeds 140 cm above the standard.
Tide levels depends upon
The tides depend to a great extent on three factors:
- An astronomical component resulting from the movement and orientation of the celestial bodies, mainly the moon, secondarily the sun and a few other planets (with a decreasing effect according to their distance from the earth); this component depends on the laws of astronomical mechanics and can be accurately calculated and predicted in the long term.
- A geophysical component that depends mainly on the geometric shape of the basin, which strengthens or reduces the astronomical component and, since it depends on the laws of physical mechanics, it can also be calculated and predicted with long-term precision.
- A meteorological component that is related to a variety of variables, such as strength and wind direction, precipitation, position of atmospheric pressure fields and their gradients, etc.
These are forecasted at very short notice and hence, Venetians are not prepared for such emergencies.