Why and when does Venice flood: Can acqua alta be avoided?

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.