On the morning of October 3, 2020, St. Mark's Square in Venice stayed dry despite a powerful high tide. At that moment, enormous metal gates were rising from the seabed in the three inlets that connect the Venetian Lagoon to the Adriatic. In little more than an hour, they had formed a barrier against the water. It was the first time MOSE, one of the most complex engineering projects of our time, had proved itself in action.
Why an ordinary dam won't work for Venice
Venice sits in a lagoon separated from the sea by narrow sandbars. Three inlets link it to the Adriatic: Lido, Malamocco and Chioggia. During storms, the sea drives water through them into the city, causing the floods known as acqua alta, or "high water."
Building a dam might seem the obvious answer, but for Venice it is impossible for two reasons. First, the lagoon must constantly exchange water with the sea; without the ebb and flow of the tides, it would turn into a swamp. Second, Venice is a major port, and a permanent wall would block the way for cargo ships and cruise liners. The engineers needed a barrier that would appear only in moments of danger.
How MOSE works
The name MOSE is an acronym for Modulo Sperimentale Elettromeccanico, or "experimental electromechanical module." It echoes the Italian name of Moses, the prophet who, according to the Bible, parted the waters of the sea. The system consists of 78 steel gates grouped into four barriers. Each gate is a hollow box up to 29 meters long.
In normal conditions, the boxes are filled with water and lie in concrete housings on the floor of the inlets, leaving shipping unobstructed. When a flood threatens, compressed air is pumped into them under pressure, forcing the water out. The gates become lighter and, pivoting on hinges, float upward. The entire lift takes about half an hour. Once raised, the gates form a slanted wall that holds back the force of the sea. When the danger has passed, the air is released, the boxes fill with water again and sink back to the bottom under their own weight. In this way, the main work is done by the buoyancy of the water rather than by complicated lifting machinery.
Design and logistics
The most sophisticated components of the system are considered to be the hinges that hold the gates. Rather than keeping the gates rigidly in place, they allow them to sway slightly, dissipating the energy of the waves. Inside the concrete housings on the seabed run service tunnels with electronics and control systems. So that the city is not cut off while the barriers are raised, the system includes locks that let fishing boats and merchant vessels pass through.
The price of salvation
Construction of the system, approved in 2003, was dogged by corruption scandals and soaring costs. In the end, the project cost more than 5.5 billion euros.
Despite its successful performance, the project still has its critics. The metal structures sit in salt water and require constant protection against corrosion. Environmentalists have also voiced concerns: as global sea levels rise, the barriers may have to be raised more and more often, which could disrupt the natural exchange of water in the lagoon. Nevertheless, MOSE is today the only defense of Venice's historic heritage against the destructive power of the sea.
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