The relief in the City of Canals is palpable. For centuries, regular high tides—high water for locals – flowed through Venice, submerging walkways, flooding buildings and impeding the passage of boats under its many bridges.
For most of the city, at least, that’s no longer the case. In operation since last year, after almost two decades under construction, a gigantic work of hydraulic engineering called the Experimental Electromechanical Module – known by the Italian acronym MOSE– now protects Venice and its lagoon. In a city where shorebirds were as easy to buy as postcards and ice cream, most people can now go about their business without consulting tide tables. Property prices are on the rise, especially for ground floor apartments and shops.
Or at least, they are for now. Although MOSE is working, there are questions about how long the barrier will last. The flood defense scheme was designed to serve for a century. But Hermes Redi, director general of Consorzio Venezia Nuova (CVN), the Venetian engineering consortium that built it, fears that, thanks to a combination of climate change and the gradual sinking of the city itself, its lifespan could be only half as long.
MOSE it is made up of 78 hinged steel gates that run 1.6 km along the sea floor under the three entrances to the Venetian lagoon. When the high tide begins, machines that consume enough electricity to supply “a small city”, as one technician puts it, compress the air that is released in each lock. As seawater is forced out, the gates rise to nearly vertical positions. The resulting barrier holds back the Adriatic until the tide recedes.
That sinking feeling
For all its technological ingenuity, the system has drawbacks. The cost is one. Redi calculates that each barrier lift costs around €150,000 (other estimates are higher). Maintenance costs also increase. The sand must be cleaned from the machine. Each gate is designed to be removed every five years for depollution. Last year, when MOSE was used 36 times, the operating cost was over €70 million (US$76 million).
The barrier also disrupts maritime traffic, prompting protests from both fishermen and large container ships that call at Marghera, the lagoon’s bustling port. Antonio Revedin of the North Adriatic Sea Port Authority says a delay could cost an individual cargo ship €80,000 a day – although a system of locks, due to come into operation later this year, should help.
There are environmental issues as well. Most of Venice’s sewage flows into its canals. As Luigi Tosi, a geologist at the National Research Council of Italy (CNR), an often closed pond would become “first a bathtub, then a sewer”. All this means that moisés it is only used when tides exceed 110cm. This means that some lower parts of the city, including St. Mark’s Square, are still flooded.
Rising sea levels will make these drawbacks more apparent as the barrier is raised more and more frequently. One paper, published in 2021, predicted a rise in water levels of between 32 and 110 cm in the Venetian Lagoon by 2100, depending on how drastically the world reduces its carbon emissions.
What is needed, then, is a plan to extend the life of the system. The Doctor. Tosi is among those who think seawater could be the answer as well as the problem. They propose to pump seawater underground and, in doing so, raise the land. This may seem strange, but in principle it is just the reversal of something that has already happened. Between the 1940s and 1970s, the extraction of groundwater for industrial use caused Venice to sink by about 15 centimeters. Pietro Teatini, a hydraulic engineer at the University of Padua, points out that there is precedent from the oil and gas industry, which has shown that storing gas in underground reservoirs can lift the earth above.
The geology of the area is promising. Its sandy subsoils must be relatively expandable. These sandy layers are covered with waterproof clay, which would prevent injected seawater from seeping in and contaminating freshwater aquifers.
Giuseppe Gambolati, a semi-retired hydraulic engineer at the University of Padua, believes it should be possible to achieve a city-wide 25 cm elevation in a decade. His proposal provides for the drilling of three test wells. If they don’t reveal any downtime problems, the full job would involve a dozen wells 600 to 800 meters deep around the city. Dr. Gambolati reckons the city could be raised to something like 2% of MOSEconstruction cost of €5.5 billion. Maintaining the elevation, by continuous pumping of water, can cost 5% more than the operating expenses of the flood barrier.
For now, seawater injection remains just an idea. But if something isn’t done, rising waters could eventually force more drastic changes. Dario Camuffo, who studies Venice’s environment and cultural heritage in cnr, says that one option would be to simply abandon the ground floors of the city. Raised floors can allow people to enter buildings upstairs. Another, he says, is that valuable structures can be dismantled for reassembly elsewhere. Redi fears that the Venetian lagoon will need to be permanently sealed off from the sea with a dyke. For a city proud of its maritime heritage, this could be seen more as a humiliation than an adaptation. ■