Dossier: Acqua Alta in Venice, 29 October 2018: What Happened, And How Does The Lagoon Work

The first days of November are a thoughtful time in Venice every year, and even more so in 2018. Less than a week ago, an excessive high tide hit the Lagoon, and even now, acqua alta occurs on a daily basis. We received so many messages from our readers, also on  social media, and would like to thank you all for getting in touch!!! So we promised to prepare a blog post with background information, trying to answer your questions WHAT EXACTLY HAPPENED ON 29 OCTOBER 2018, and why this acqua alta turned out so severe.

To do so, I need to go back some time in history, to the Tenets of the Healthy Lagoon, laid down in the promissio ducale of Doge Pietro Lando in 1545This article was published originally in our Venice Heritage Course, as monthly update for November 2018.

The short-term effects of acqua alta are called “nuisance”, as I’ve seen repeatedly on social media during the past week: What you do right NOW is clean up. You swipe and disinfect the floors, and the walls!, and you open the doors and windows hoping for ventilation and the sun (?! not really shining) to help dry up wet patches in your home. You even switch on the heating to accelerate the dry-up process, even though you know you are wasting money. This layer of filth and mud that acqua alta leaves behind in our homes IS a nuisance RIGHT NOW.

But it’s only a tiny part of the issue. 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 just due to severe acqua alta like the one we just saw, when the water level climbed to approx. 156 cm, but also minor ones, “routine” episodes.

Acqua alta has serious effects on the duration of life of Venice IN THE LONG RUN (= time span up to 10 years), as salt water attacks the facades, climbing and making them porous and crumble. In my thesis, Ecologia e Urbanizzazione della Laguna di Venezia, I covered longevity issues of Venice, and the precautions the Republic of Venice took to ensure the existence of their city. Yes, in addition to architectonic heritage, Venice has accumulated an immense environmental heritage, and knowledge, dedicated to “managing a Lagoon over time! In my opinion, this forgotten voice should act as basis when it comes to taking decisions on shaping and altering the Lagoonscape in the first place.

Has there always been acqua alta in the Lagoon?

Yes. High tides (called acqua alta until 140 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 intact in the past?

I distilled “Five Tenants of Lagoon Maintenance” from the myriad of documents the Repubblica Serenissima di Venetia left us. By the year 1545 (Promissione ducale of Doge Pietro Lando), Venice was in imminent danger of becoming a uninhabitable swamp. Imagine – the Republic of Venice had to fight the RIVERS, and not the sea!

Those rivers discharging their sediments into the Lagoon: Brenta, Sile, and Piave in particular. The engineers of Venice succeeded in turning the situation around in 1610, when the last of the five dams (Taglio del Novissimo), built to keep out the sediments of the rivers, was put in place and working. But the Savi alle Acque, the Authority entrusted with managing the Lagoonscape, were well aware that the focus of maintenance had to be shifted. Their measures had saved Venice, but the fate of the Lagoon was turned into the other extreme. As the rivers couldn’t depose their sediments any longer, the Lagoon was fast becoming an arm of the sea.

Five tenets to ensure a balanced, and healthy, Lagoon, minimizing the risk of excessive high tides:

  • Laguna sana e intatta: Only an intact Lagoon guarantees the survival of Venice and her island communities (Le Venetiae). Striking the right balance between tidal processes and river sediments take priority in maintaining the Lagoonscape: Monitoring, planning, deducting a strategy, implementing measures, and assessing results are the pre-requisites to steering the life of the Lagoon by means of dams and fortifications. It’s about managing the natural processes of interramento (river sediments turning the Lagoon into a swamp) and trasformazione in mare aperto (floods eroding the Lagoon islands, thus turning it into an arm of the open sea).
  • Spartiacque: By 1610, the Lagoon was divided into two areas, Laguna viva and Laguna morta (NB: morta doesn’t mean “dead” in this context, but less exposed to salt water): There’s a watershed running between salt water and prevalently fresh water, just north of Venice. Thus, there are two parts of the Lagoon to be surveilled and kept alive with proper measures.
  • Laguna Viva: South of Venice, the Laguna Viva is located, regularly nourished by the currents coming in from the open sea, via the bocche di porto (Lagoon entrances). This part of the Lagoon must be prevented from turning into an arm of the sea, as sediments accumulate sideways of the river mouths, and thus need different preventive actions than Laguna Morta.
  • Laguna Morta. North of Venice, Laguna Morta is located, whose salinity levels are considerably lower, or even zero in front of the river mouths. This part of the Lagoon must be prevented from turning into a swamp, as river sediments accumulate in front of the river mouths. Thus, it requires other precautionary measures than Laguna Viva does. ==> So now you see – not one master plan to protect the Lagoon, but at least two are required!!
  • Make sure the tide cycles can do their work appropriately. Tidal currents remove and accumulate sediments, and their destructive work is sometimes enhanced by the forces of sismi (quakes) and subsidenza (natural sinking of islands), which man cannot control. Sediments need at least several decades to build up, but invariably reduce the ideal depth of the Lagoon (around 100 cm). But then, tide cycles are steered by giving the tides SPACE to do their work: Emergency areas capable of soaking up excessive waters are needed, when water masses enter the Lagoon, to save the islands from being flooded. We realize that space may be scarce. For this reason, keep the barene islands alive, which act as sponge soaking up excessive water. A minimum of 50 per cent of Lagoon surface should consist of barene. (NB: now, this figure is approx. 18 per cent).
  • There will always be emergencies (emergency = the combined effects of adverse meteorological conditions and lunar phases like full and new moon). Still, putting these five tenets to work was how the Republic succeeded in limiting damage, actively shaping the Lagoonscape to help Venice survive.
This image shows barene – muddy, flat islands, flooded during normal high tides.

400 – 1610: Measures taken by the Republic of Venice to safeguard the Lagoon and Venice from turning into a swamp due to river sediments: I Tagli 

  • Assetto stabile (Lagoon Master Plan): Separating currents, creating an artificial watershed between salt and fresh water currents by means of tagli (dams) cutting through the Lagoon. The first dam (argine) was built in 1324, stretching from Campalto to Resta d’Aglio, corresponding to today’s Canale dei Petroli. These dams had side effects, though, as they caused floodings on the mainland. For this reason, the Republic built only minor settlements on the shores of the Lagoon, classified as emergency flooding zones.
  • Ponte de l’Lovo: In 1509, Venetian engineers had the land tongue Ponte de l’Lovo removed, a long island formed from river sediments which almost reached Venice.
  • Escavazioni: From 1530, ALL canals in Venice were thoroughly cleansed and excavated once every decade.
  • Tagli: The dams Taglio del Re and Taglio di Cavazuccherina were finished in 1543, and the northern Lagoon was saved from turning into a swamp by shifting the course of the rivers.
  • Deviazione dei fiumi: Shifting the mouth of the Piave river out of the Lagoon, towards the east, was a long story, but it worked. The waters of the Sile were channeled into the old river bed of the Piave. This is how the engineers saved  Torcello and the northern Lagoon islands from turning into a swamp. (NB: Before, the Sile river discharged its waters in front of Mazzorbo and Torcello, which is why Torcello was given up in the first place).

1430 – 1797: Measures taken by the Republic of Venice to safeguard the Lagoon and Venice from turning into an arm of the sea, due to excessive high tides: Fortificando le bocche di porto e le isole 

  • Bocche di porto: Two of the five bocche di porto (entrances separating the Lagoon from the open sea) were closed. Over-sized cogs of the Venetian military and merchant boats had to anchor OUTSIDE the Lagoon, along Scanno della Piscotta, a sandbank off the Lido, acting as successor of the port facility on Torcello. Trabaccolo boats were allowed to enter and anchor on Riva degli Schiavoni.
  • Fortificazioni in città: Doge Andrea Gritti (1523 – 1538) focused on overhauling all fondamente, and the Savi alle Acque had the old wooden and brick stone fortifications removed, and replaced with Istrian stone. This was a decade dedicated to overhauling Venice, as raising ocean levels were felt in the Lagoon for the first time.
  • Canale di Santo Spirito: In 1727, creating the Canale di Santo Spirito allowed direct access for smaller cogs to the Bacino di San Marco (the canals from Punta Sabbioni to San Marco were winding and long!!). To lower the risk of severe flooding, the bocca di porto di Pellestrina was narrowed.
  • I Murazzi: When it became clear to the Venetian engineers that the general ocean level was rising between little ice ages in the early 18th century, dams were built from huge chunks of Istrian stone, fortifying and raising the low-lying islands Lido and Pellestrina. These cyclopean dams allowed salt water to enter the Lagoon, balancing out salt and fresh water levels, and nourishing flora and fauna. While inaugurating one part of the dams in 1753, Doge Francesco Loredan wished the city well, expressing his conviction that these Murazzi would succeed in preventing the worst fate of the city for at least 500 years. And that’s what the Murazzi did: they saved Venice on 4 November 1966, despite being damaged, and last week, on 29 October 2018, from waves >6 meters tall.

Since 1797: The Lagoon is in danger of turning into an arm of the sea

  • Interramenti: One third of the canals of the city were filled in the 19th century. Prominent examples are Strada Nova and Via Garibaldi ==> the space for high tides to dilute in town was reduced.
  • Ponte della Ferrovia: Large areas were excavated to build the Railway Bridge (ponte della Ferrovia) under Austrian occupation. Artificial islands (casse di colmata) were built from debris, further reducing the surface of the Lagoon.
  • Porto Marghera e zone industriali: In 1918, Marghera port was built on the former emergency areas, and large portions of the Lagoon were filled to extend the industrial zones. Rivers swelling during storms had little space to disperse waters, discharging their floods into the Lagoon.
  • Canale dei Petroli: The excavation of this canal along the former tagli (dams, see above), caused huge surplus debris. By 1962, Lagoon surface, and the barene islands, were reduced by 40 per cent. High waters cannot disperse easily in the Lagoon, leading to the devastating effects of the acqua granda on 4 November 1966.
  • Scavo di canali in Laguna: Deep water canals allow cruise ships and oil tankers pass by San Marco, but also cause floods to reach San Marco in less than 30 minutes.,

Why didn’t the Mose barriers work on 29 October 2018?

The Mose barrier system is still inactive. These huge barriers are expected to work from 2022. As a result, the Murazzi, built as precautionary measure by a prescient Republic of Venice between 1714 and 1782, still keep the city alive.

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8 Comments

  1. Thank you for this. Knowing Venice well and loving it so much, I’ve been shocked and saddened to see the photos of such a badly flooded city over the last week. I do hope very much it survives a very long term.

    Posted 11.4.18 Reply
    • Iris wrote:

      Thank you!! Hopefully Venice will survive, she’s been very resilient so far ..

      Posted 11.4.18 Reply
      • I must come back soon. Haven’t been since 2015.

        Posted 11.4.18
      • Iris wrote:

        Hope you can return soon!

        Posted 11.4.18
  2. Love your blog. Thank you.

    Posted 11.4.18 Reply
    • Iris wrote:

      Thank you so much Sue!!

      Posted 11.5.18 Reply

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