MOSE: How does it work, and can it save Venice in the long term?

Whitish clouds move quickly across the night sky while tides are rising gradually. The next morning, Venice awakes to steel blue skies: A warm gust of wind (scirocco) caresses your face when you open the window in the morning, feeling ice-cold in the next minute. Or, the sky looks petrol blue, with the sun shimmering orange behind the clouds.

When days start like this in October and November, Venetians know what to expect, especially with the full and new moon: Severe acqua alta will occur, flooding the city within the next 12 hours.

In the early morning of 3 October 2020, with that orange sun trying to get the better of the petrol blue-colored clouds, Venetians held their breath because this time, the MOSE flood gates deployed at the Lagoon inlets were scheduled to take over.

And it worked: At 8:35 am, the first of the 78 flood gates deployed at the three inlets of the Lagoon began rising. First, the gates at each side of the inlets came up, and then the others followed, separating the Lagoon from the open sea within 1 hour 17 minutes.

This is a premiere in history: Never before has the Lagoon been separated completely from the open sea. For eight hours, sea water was blocked out, the high tide couldn’t reach the city and thus, Venice was spared another severe flood predicted to reach 135 cm – 4.3 feet. Half the city would have been drowned, recalling the disastrous floods of 12-17 November 2019.

As Mose did its job well to protect Venice, on 3 October 2020, we’ve entered completely new territory, raising the question of whether separating the Lagoon from the sea can become a permanent model to save Venice?

MOSE (MOdulo Sperimentale Elettromeccanico, Experimental Electromechanical Module) is a system made of 78 mobile flood gates deployed at the three inlets of the Lagoon: Lido / Treporti, Malamocco, and Chioggia. MOSE also has a long and “tormented” history, but this is not the topic of today’s article.

Whenever high tides above 130 cm (4.26 feet) are predicted, especially in fall and winter, the mobile flood gates will rise and separate the Lagoon from the open sea.

In addition to the main MOSE system, at Chioggia and Malamocco, smaller local flood gates called mini-mose are deployed, rising whenever tides exceed 80 cm (2.7 feet). When the main Mose system works fine, the local ones are not required.

Which effects could Mose have on the Lagoon environment in the mid-term?

Depending on how often the flood gates will need to rise and protect the Lagoon from excessive high tides, nourishing sea water required to cleanse the Lagoon and feed plants and animals will be blocked out for a certain period of time. As a consequence, halophile Lagoon plants and animals may suffer, as the water temperature will tend to rise and the portion of freshwater pockets increase with time.

A drastic change in Lagoon flora and fauna already happened in the year 1610, when Venetian engineers built the dam Taglio del Novissimo to divert the rivers and save the Lagoon from turning into an unhealthy swamp. As a consequence, the portion of salt water lakes in the Lagoon increased steadily since then.

In our case, with MOSE blocking the arrival of sea water more often, the portion of fresh water lakes in the Lagoon could rise again, turning the situation around to some extent and bringing back the Lagoon closer to salinity levels before the year 1610.

Thus, Mose represents an environmental experiment as we cannot say how the current, prevalently halophile flora and fauna in the Lagoon will react. Also, there are fish farming zones in the Lagoon, especially in the south, that would have to be monitored very closely.

At the same time, ships cannot leave the Lagoon while the flood gates are up, and maintenance for this complex system of flood gates may be very costly and sensitive. The MOSE system has a limited life expectancy of approx. 15 years, which is yet another reason why a viable long-term solution to save Venice from flooding is urgent. By long-term solution we refer to the term adaptation of Lagoon morphology.

There are a number of projects worked out by hydraulic experts of the present and past, containing proposals of how to adapt Lagoon morphology to make sure that Venice can survive in a healthy environment: One of these projects even goes back to the 16th century (hydraulic engineer Cristoforo Sabbadino, 1489-1560) and has been mentioned several times since 2019 by experts like Professor Luigi d’Alpaos. Sabbadino’s project even includes the topic “enlargement of port areas“, by means of separating the port areas from the Lagoon by creating a smaller deep-water basin by means of tagli (dams).

Is there a long-term solution to protect Venice and the Lagoon?

Answers come from the excellent engineers taking care of Lagoon stewardship of the past:

In 1538, Doge Pietro Lando even included Five Tenets of Lagoon Stewardship in his promissione ducale. These tenets were observed until 1797, but in our times, they are as important as ever:

Thus, any long-term solution comprises an adaptation of Lagoon morphology: How many deep-water canals do we really need, where do they have to run, and how can we protect and increase the number of barene (marshlands) required to absorb toxins and excessive high tides.