Last updated on 28 October 2018
Acqua alta doesn’t hit Venice as a surprise. There are simple “laws” that can help predict it, and in this post we are going to show them to you. As I’m updating this post (28 October 2018), it seems that Venice is threatened by a particularly heavy bout of acqua alta. So while we are preparing a new post on this topic, we are inserting more essential information in this one. So here are a few essential rules you need to know about acqua alta.
Not all parts of Venice are flooded when acqua alta strikes. If you’re ready to stick to a neighborhood, on a raised area, you may not even notice acqua alta, and may not even need stivali = boots!
I recall one November when I was closed in by acqua alta after breakfast in at Ristorante Gran Canal. The day started out sunny, and I was hoping to drink a cappuccino on their terrace, but … within half an hour, the tide rose from normal to +50 cm above ground level. The terrace was completely flooded, and the door was shut and boarded up. The birds – sparrows and sea gulls – remained perched on the tables outside. The chairs on the terrace were fastened with a chain to keep them from floating off. Life continued, inside and outside, and ten minutes of heavy rain were followed by sunshine and a mild breeze.
I was able to “escape” through a “secret” back door, leading out to another calle on higher ground. Making a little detour I was able to reach home without getting wet at all. It really depends on where you are in Venice when acqua alta hits ..
Times have changed in the sense that acqua alta now hits Venice all year, yet, you must expect the most severe bouts in the cold season. If you know a few facts about the Lagoon, and are familiar with the moon cycle, you can predict when acqua alta occurs, and even, how strong these bouts will be.
From October through February, we experience excessive high tides. These happen because of adverse meteorological conditions in the cold season, such as heavy rain and scirocco wind (southerly wind) causing the water masses to swell in the Lagoon. I’ve experienced acqua alta episodes in the last few years that didn’t retreat for 10 hours on end.
These episodes are less severe in September and in April. From June to August, acqua alta occurs 1-3 times a month, while you must be prepared that it will happen up to 20 times per month in winter.
# 2 The laws of the tides:
Tides on the northern Adriatic shores are more accentuated because many rivers discharge their water masses in the Lagoon, and because water, pressed in by the southerly wind scirocco, enters the Lagoon via the bocche di porto. This is why the Lagoon of Venice experiences higher tides than other shorelines of the Adriatic sea.
And then, the moon comes in: When the force of gravity is strongest, high tides = alta marea = marea sigiziale (marea viva) occurs with full moon = luna piena, and with new moon = luna nuova. With the rising moon = luna crescente (crescent moon) and waning moon = luna calante, tides are more regular, called marea di quadratura (marea morta).
#3 Summer and winter tides
Twice a day in winter, we experience peak episodes of the tides. That is, high tide = alta marea and low tide = bassa marea occur twice a day in winter, but as a rule only ONCE a day in summer !!!
IMPORTANT: During full moon and new moon, water enters the Lagoon faster than it recedes. During the periods of rising and waning moon, water enters more slowly than it recedes.
#4 What is acqua alta
Acqua alta is an exceptional alta marea (high tide). Even if the Lagoon wasn’t altered in its landscape features, there would still be acqua alta once every 2-3 years, and an excessive one once every 500 – 1000 years. That means the Lido would be flooded and water masses critically submerge all of Venice.
That’s why the stone barrier called i murasi (Murazzi) were built in 1714 on the island of Pellestrina, which together with the Lido island, separate the Lagoon from the open sea. This monumental wall made of huge Istrian boulders, built by the Republic of Venice, saved our city almost 200 years after the Republic had ceased to exist, during the exceptional high tide of 1966!!
It takes about half an hour for the high tide to reach Piazza San Marco, and even longer to arrive in the northern Lagoon. For this reason, salinity levels are higher at the bocche di porto than around the island of Torcello located near the river mouths, in the northern Lagoon. So now you know why fresh water-loving fish live in the northern Lagoon, while the sea fish farming grounds called valli da pesca are located in the southern Lagoon, near the open sea.
(to be continued ..)