What happened in Venice on November 12th. The post by Piero Martin, professor of physics for chemical and materials engineering at the University of Padua, taken from his Facebook
profile high water, especially for those observing from outside the city. I hope some information is useful anyway.
The tide in the Venice lagoon is mainly due to three effects:
1) The astronomical component
2) The meteorological component
3) The oscillation of sex
The astronomical component does no damage: it is an oscillation with a period of twelve hours due to the gravitational interaction between the earth and the moon, and can be calculated with precision and well in advance. It is essentially what is found in the various calendars that are produced every year.
The problems come from the other two.
The meteorological component of the tide is caused by processes that force the normal astronomical trend. To understand them better, let’s remember how the Adriatic is made: a sort of narrow sleeve that goes from south east to north west, closed at the top and with shallow waters towards the north.
The first forcing is the wind, in particular the wind that blows from the south / south east, typically the sirocco. In very simple words this wind can ‘push’ the sea towards the upper Adriatic and slow down, if not prevent, the flow of water.
A second forcing and a minimum of pressure near Venice (typically when there is bad weather): the pressure difference between the southern and northern Adriatic and another forcing that pushes the sea towards the north.
The sex is instead the large-scale oscillation observed in the Adriatic following a localized perturbation. Put simply: take an elongated basin full of water, raise it for a moment at one end and then lower it: a wave is created that goes back and forth. When the basin is the Adriatic and the tidal “wave” is the meteorological component mentioned above, you have the sex. Its period is 22 hours.
The tide forecasting system of the municipality of Venice uses meteorological data from various stations along the Adriatic coasts from south to north, both in Italy and in the former Yugoslavia, precisely to try to refine the forecast as much as possible (here you will find technical details on the models used by the Tide Reporting and Forecasting Service).
In practice, the exceptional tides are due to the concomitant action of the three contributions, that is when a strong meteorological component is added to a normal astronomical maximum.
This is what happened on November 12 in Venice: around 10 pm the measurement station on the CNR platform located off the Lido measured gusts up to 30 m / s (about 100 km / h) and minimum pressure around 986 hPa. These heavy contributions added up to the astronomical maximum, predicted for 22:55 around 60 cm and produced the dramatic and exceptional level of 187 cm.
The municipality’s forecasting system is equipped with various tide measurement stations, in the lagoon and in the open sea.
In the case of events like this one, I personally kept under observation the measurement station located on the CNR platform off the Lido.
Even with the risk of oversimplification, tide measurements at sea usually, although not always, anticipate by about an hour (again more or less, I apologize but I’m not an expert at that point …) those in the center historical. In fact, it is the time that the tidal wave takes to “make the way” from the open sea to the city passing through the harbor mouths.
You can visually understand this by looking at the graph below, which I produced (except for unintentional errors) using data from the Municipality website. You can see how the celestial curve (measurements at sea) anticipates the orange one by about an hour (measurements in the historic center, Punta della Salute).
