How was Venice flooded?

Question

If I understand this correctly, Venice was not flooded due to sudden rainfall causing rivers to overflow, but due to the high tides. These high tides were caused by an alignment of sun, which was expanding the waters of the oceans disproportionately on one hemisphere and moon, which shifted the center of gravity of the moon-earth system in such a way that water rose on the same hemisphere.

In the news articles that I see, they also mention strong winds. This would make sense. High baseline + waves = a lot of water in the city. But in these photos everything seems to be calm. No waves, no wind, just an abnormally high level of water:

https://www.theatlantic.com/photo/2018/10/venice-under-water/574396/?fbclid=IwAR1DWLs_W39ndpdLWLZ1f6wlDDw8ZJmcBqyVM555O9DL5a_DhuY0mPQDi38

For me this doesn't make any sense. If the winds subsided and the flooding was caused purely by tidal effects, which are astronomical events, shouldn't it have been predictable months, if not years in advance?

Answer 1

If the winds subsided and the flooding was caused purely by tidal effects, which are astronomical events, shouldn't it have been predictable months, if not years in advance?

The tides are only partially caused by astronomical events. There's always some difference between predicted and observed tide levels. The former typically only use astronomical indicators while the latter reflect all influences. Multiple influences resulted in those exceptionally high tides in Venice on 29 October 2018.

The astronomical influences are biased toward higher than average high tides at Venice at this time of year. The reason is that annual and semiannual tidal components at Venice peak in autumn and winter. Late autumn to midwinter is when Venice historically has experienced most of its exceptionally high tide events.

Two even longer term influences are subsidence and sea level rise. These alone have made tides relative to buildings in Venice 23 cm higher than they were in 1897. Venice suffered only one exceptionally high tide event (high tides at the benchmark Punta della Salute recording site that exceed exceed 140 cm) between 1872 to 1950. In contras it has suffered ten such events in the last two decades (including the 29 October 2018 event).

The straws that broke the camel's back were a confluence of a very strong storm and the shape of the Adriatic Sea. Taken individually, neither would have caused the 29 October 2018 event. The storm was associated with strong southerly winds (winds that blow from the south) that pushed waters northward along the eastern coast of Italy. By itself, this would have resulted in a storm surge that would have raised waters in the lagoon around Venice by 10, maybe 20 centimeters. Some flooding would have resulted if the storm surge coincided with astronomical high tide, but not the exceptionally high amount that was observed. Moreover, that would have been a coincidental fluke of an event.

What those winds did instead was to make the entire Adriatic Sea slosh longitudinally (east-to-west and back). This is called a seiche. Seiches are standing waves that occur in enclosed / semi-enclosed bodies of water. In addition to the Adriatic, seiches also occur in the Baltic (which occasionally floods St. Petersburg in a very similar manner to the flooding of Venice), the Great Lakes, and several other bodies of water. The Adriatic Sea seiche has a period of about 22 hours. The storm that preceded the 29 October 2018 flooding event lasted long enough to make the Adriatic seiche have a rather high amplitude and be rather long-lived.

The flooding occurred when the peak of this 22 hour period standing wave at Venice coincided with the peak of the short term (e.g., the 12.421 hour $M_2$ tidal component) astronomical tidal influences. Adding in the long term influences (annual and semiannual astronomical tides, subsidence, and sea level rise) made the 29 October 2018 flooding event not just exceptional but nearly record-breaking.

Answer 2

1. Venice can't really be flooded by rivers, since there are none worth mentioning throughout the city.

2. High tides are predictable, yes.

3. Take a look at how Venice is situated. The historic city is sheltered behind low sandbanks/islands, which form the foremost coastline. Still the laguna is - obviously - connected to the open sea. Meaning: Huge waves, like in the open sea, wont rush through the city's channels, since they are dispersed by these islands. Still, in some images the water is quite choppy - a sign for rather windy conditions. Furthermore, all the houses of the city act like additional "islands", calming the surface of the water by dispersing larger waves into smaller and smaller ones. However, wind does not only create waves, it pushes the water also into the direction it is blowing to. In case of the Scirocco, which blows from northern Africa across the Mediterranean right into the Adriatic Sea, at which northern end Venice is situated. Thus, this wasn't simply a high tide, it was a spring tide, a combination of favourable planetary alignment and wind conditions. And, as already commented, winds/weather are quite hard to predict.

The whole process actually has a local name: "Acqua alta". It is nothing new, simply the intensity was noteworthy.