Dear readers,In Venice, everyone could tell you about the flood of 4 November 1966, when the water level reached 194 cm on Piazza San Marco. My generation knows this event only from hearsay and until two weeks ago, it seemed unlikely that we would ever witness a similar one. After all, they used to happen only every 150-200 years. read more
Previsions had announced acqua alta, a serious flooding made worse by the southwesterly libeccio wind. Yet, in the early evening of 12 November, NOBODY expected a disastrous flood happening every 200-300 years, brought about by a combination of adverse meteorological factors and moon phase (new moon or full moon, plus one week before and after).read more
Click here to read the Italian version Following the cruise ship accident of 2 June 2019, discussions were sparked on the future of Venice and a possible referendum on the new location of the port of Venice.
Six terms explain what works and what doesn’t for the Lagoon and Venice. There is no Venice without the Lagoon: Thus, any proposal regarding the port of Venice has to take six terms into account, used by Venetian engineers since the 11th century.read more
The two weeks before and after November 1 are always thoughtful here in Venice: It’s the time when severe flooding occurs, more often than in other months. A severe episode of excessive flooding (acqua granda) happened on 28-30 October 2018. In this article, we explore why this acqua alta turned out so severe, and what it had in common with the devastating floods of 4 November 1966.
Did you know? The engineers of the Republic of Venice set up the Tenets of the Healthy Lagoon, laid down in the promissio ducale of Doge Pietro Lando in 1545.
The short-term effects of any acqua alta are called “nuisance”. Cleaning up as best as you can comes first: You swipe and disinfect the floors and the walls! and you open the doors and windows hoping for ventilation and the sun to dry up wet patches in your home. You even switch on the heating to accelerate the dry-up process, even though you know you’re wasting money. This layer of filth and mud that acqua alta leaves behind in our homes IS ALWAYS a nuisance, reminding us of what happened even months and years afterwards, as it leaves stains and rims on the walls and floor.
Acqua alta leaves its marks and crumbles the facades of Venice! We must bear in mind how acqua alta affects the chances of Venice surviving in the FUTURE, in the mid-term (<10 years) and long-term (>10 years). Not only severe acqua alta like in October 2018 when the water level climbed to approx. 156 cm, but also minor “routine” episodes weaken the buildings.
Acqua alta determines whether Venice can physically survive, as salt water attacks the buildings, making them porous and crumble. In my thesis, Ecologia e Urbanizzazione della Laguna di Venezia, I covered longevity issues and the measures the engineers of the Republic took: They have accumulated an immense wealth of know-how on how to steward the Lagoon. In my opinion, this forgotten voice should be heard when decisions are taken today.
Was the Lagoon flooded regularly in the past?
Yes. High tides (acqua alta < 150 cm, acqua granda > 150 cm) occurred from the beginning, and the causes are the same as today. Once every 50 – 100 years, the tides simply stop working and the Lagoon remains flooded for several days, looking like an immense lake! It happened on 4 November 1966, and it happened to a lesser degree on 29 October 2018.
How did Venetians keep the Lagoon alive in the past?
I distilled Five Tenants of Lagoon Stewardship from documents published by the engineers during the times of the Republic (421-1797). By 1545, the Lagoon was well on its way of turning into an uninhabitable swamp: The engineers of the past had to fight the RIVERS, not the sea!
Those rivers discharging their waters and sediments into the Lagoon: Brenta, Sile, and Piave in particular. The engineers succeeded in turning the situation around in 1610, when the last of the five dams (Taglio del Novissimo), built to keep out the sediments, was doing its work.
The Savi alle Acque, the Authority entrusted with managing the Lagoon, were well aware that the focus of Lagoon stewardship had to be shifted: They had saved Venice, but without the sediments building natural barriers, the Lagoon was fast becoming an arm of the sea.
This is why the Savi alle Acque observed five tenets to ensure a balanced and healthy Lagoon, minimizing the risk of excessive high tides:
- Laguna sana e intatta: Only a healthy Lagoon guarantees the survival of Venice and her island communities (Le Venetiae). Managing the tides and river sediments prudently is done by building dams and safeguarding the barene (marshlands).
- Spartiacque: By 1610, the Lagoon was divided into two areas, Laguna viva and Laguna morta (morta means less exposed to salt water): Just north of Venice, a watershed divides the salt water areas from the fresh water zones in the north: There are two Lagoons to be managed properly!
- Laguna Viva: South of Venice, Laguna Viva is located, nourished by the currents arriving from the sea through the bocche di porto (inlets). Laguna viva must be prevented from turning into an arm of the sea, as sediments accumulate sideways of the river mouths.
- Laguna Morta. North of Venice, Laguna Morta is located, whose salinity levels are low and even zero at the river mouths. Laguna morta must be prevented from turning into a swamp, as river sediments accumulate in front of the river mouths.
- Managing the tide cycles. The currents nourished by the tides remove and accumulate sediments. Sediments need at least several decades to build up, but invariably reduce the ideal depth of the Lagoon (around 100 cm). Tide cycles need space to do their work: Emergency areas capable of soaking up excessive tides are needed. For this reason, the marshlands in the Lagoon called barene are so important as they act as sponge soaking up excessive water. At least half the Lagoon surface should consist of barene. Currently, they make up 18 percent.
- Emergencies are defined as adverse meteorological conditions (rain, scirocco winds) happening during full or new moon.
Just a short overview on how Lagoon stewardship was successful until 1797 (end of the Republic), and what happened afterwards.
400 – 1610: Measures taken to prevent the Lagoon from turning into a swamp: I Tagli
- Assetto stabile (Lagoon Master Plan): Separating sea and fresh water currents and defining the watershed by means of tagli (dams). The first dam was built in 1324, stretching from Campalto to Resta d’Aglio, corresponding to today’s Canale dei Petroli. These dams caused floods on the shores and they were classified as flooding zones. Only military fortifications and small villages were located there.
- Ponte de l’Lovo: In 1509, Ponte de l’Lovo was removed, a long island formed from river sediments which almost reached Venice.
- Escavazioni: From 1530, canals in Venice were thoroughly cleaned once every ten years.
- Tagli: The dams Taglio del Re and Taglio di Cavazuccherina were finished in 1543, shifting the rivers out of the Lagoon. This is how Torcello and the northern Lagoon islands were saved from turning into a swamp by the sediments of the Sile river.
1430 – 1797: Measures taken to prevent the Lagoon from turning into an arm of the sea: Fortificando le bocche di porto e le isole
- Bocche di porto: Two of the five bocche di porto (inlets) were closed. Over-sized merchant cogs and the Venetian armada had to anchor OUTSIDE the Lagoon, along Scanno della Piscotta, a sandbank off the Lido: This “floating port” was created when the former port at Torcello turned into a swamp. The smaller trabaccolo boats were allowed into the Lagoon and anchor along Riva degli Schiavoni.
- Fondamente fortificate in città: Doge Andrea Gritti (1523 – 1538) focused on overhauling the quays and used Istrian stone to fortify them.
- Canale di Santo Spirito: In 1727, the newly dredged Canale di Santo Spirito allowed smaller cogs to enter into Bacino di San Marco. To counter-balance this dredging, the inlet at Pellestrina was narrowed.
- I Murazzi: When it became clear that the ocean level was rising after the end of the little ice age in the 17th century, the Murazzi dams were built of Istrian stone, raising the low-lying islands Lido and Pellestrina. In a speech Doge Francesco Loredan made in 1753, he expressed his conviction that the Murazzi would be fit to protect the city for another 500 years. He was right: Despite being damaged, the Murazzi saved Venice on 4 November 1966 from being completely flooded. And they saved Venice on 29 October 2018 from drowning under waves more than six meters tall.
Since 1797: The Lagoon is turning into an arm of the sea
- Interramenti: One third of the canals of Venice were filled in the 19th century, which considerably reduced the area for high tides to expand.
- Ponte della Ferrovia: The Lagoon was excavated to build the Railway Bridge (ponte della Ferrovia) under Austrian occupation, which damaged the watershed severely.
- Porto Marghera e zone industriali: In 1918, Marghera port was built in the former emergency areas. One third of the Lagoon was urbanized to extend the industrial area. Rivers swelling during heavy rainfall now discharge their excess waters into the Lagoon.
- Canale dei Petroli: Dredging this canal along the former tagli (dams) caused the destruction of 40 percent of barene in the southern Lagoon.
- Scavo di canali profondi: Deep water canals allow cruise ships and oil tankers to enter but also cause floods to reach Venice in less than 30 minutes.
Why didn’t the Mose barriers work on 29 October 2018?
The Mose system is still inactive, and when it will be fully operational isn’t sure as of now. The task of keeping Venice alive is done by the Murazzi, built by the engineers of the Republic of Venice.
Imagine just another mild November morning in Venice, starting slightly misty and clad in silvery clouds. The skies are leaden like in the picture above. In the late morning, light rain sets in and the wind feels warm in your face while the rain is getting more persistent and so is the southerly breeze. In the evening, the water level rises to 1,20 meters, but then a peculiar thing happens.
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 ..)