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Science & technology | Saving Venice

Why Venetians are pondering raising their entire city

A EUR5.5bn flood barrier has bought only a temporary reprieve

Visitors stand on on a trestle bridges as they admire St. Mark's
Basilica during an expected high water, in Venice, northern Italy,
Saturday, Oct. 3, 2020. Controversial and long-delayed underwater
barriers passed their first emergency with flying colors on Saturday,
protecting the Italian lagoon city of Venice from a tide that peaked
at 130 centimeters (51-inches), a level that would normally inundate
about half of the city. (Claudio Furlan/LaPresse via AP)
May 24th 2023 | Venice
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The relief in the City of Canals is palpable. For centuries, regular
high tides--acqua alta to the locals--have flowed through Venice,
submerging walkways, flooding buildings, and stopping boats from
passing under its many bridges.

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For most of the city, at least, that is no longer the case. In
operation since last year, after nearly two decades under
construction, a giant piece of hydraulic engineering called the
Experimental Electromechanical Module--known by its Italian acronym 
MOSE--now protects Venice and its lagoon. In a city where waders were
as easy to buy as postcards and ice cream, most people can now go
about their business without consulting tide charts. Property prices
are up, especially for flats and shops on the ground floor.

Or at least, they are for now. Although MOSE is up and running, there
are questions about how long the barrier will last. The flood-defence
scheme was designed to serve for a century. But Hermes Redi, the
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 useful lifespan might be just half as long.

MOSE is made up of 78 hinged steel floodgates that run for 1.6km
along the sea floor beneath the three inlets to the Venetian lagoon.
When a high tide begins, machines that consume enough electricity to
power "a small town", as a technician puts it, compress air that is
blasted into each floodgate. As seawater is forced out, the
floodgates rise into nearly vertical positions. The resulting barrier
holds back the Adriatic until the tide retreats.

That sinking feeling

For all its technological whizzyness, the system has downsides. Cost
is one. Mr Redi reckons each raising of the barrier costs about
EUR150,000 (other estimates are higher). Maintenance costs add up, too.
Sand must be cleared from the machinery. Each floodgate is designed
to be removed every five years for defouling. Last year, when MOSE
was used 36 times, the operating cost was more than EUR70m ($76m).

The barrier also disrupts maritime traffic, causing protests from
both fishermen and the big container ships that call at Marghera, a
busy port in the lagoon. Antonio Revedin of the North Adriatic Sea
Port Authority says a delay can cost an individual cargo ship EUR80,000
a day--though a system of locks, due to come online later this year,
should help.

There are environmental issues, too. Most of Venice's sewage flows
into its canals. As Luigi Tosi, a geologist with Italy's National
Research Council (CNR), puts it, a lagoon that is sealed off too
often would become "first a bathtub, then a sewer". All that means
that mose is only used when tides exceed 110cm. That means that some
low-lying parts of the city, including St Mark's square, still flood.

Rising sea levels will make those downsides 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 110cm in the
Venetian lagoon by 2100, depending on how sharply the world cuts its
carbon emissions.

What is needed, then, is a plan to extend the system's life. Dr Tosi
is among those who think that seawater could be the answer as well as
the problem. They propose to pump seawater underground, and in doing
so to raise the land. That may seem outlandish but, in principle, it
is merely the reversal of something that has already happened.
Between the 1940s and 1970s the extraction of groundwater for use by
industry caused Venice to sink by about 15cm. Pietro Teatini, a
hydraulic engineer at Padua University, points out that there is
precedent from the oil-and-gas industry, which has shown that storing
gas in underground reservoirs can lift the land above.

The area's geology is promising. Its sandy subsoils should be
relatively expandable. Those sandy layers are capped with watertight
clay which would prevent injected seawater seeping upwards to
contaminate freshwater aquifers.

Giuseppe Gambolati, a semi-retired hydraulic engineer at Padua
University, thinks it should be possible to achieve a rise of 25cm
across the entire city within a decade. His proposal calls for
drilling three test wells. If those reveal no show-stopping problems,
then the full job would involve a dozen wells 600 to 800 metres deep
around the city. Dr Gambolati reckons the city could be raised for
something like 2% of MOSE's EUR5.5bn construction cost. Maintaining the
uplift, by continual pumping of water, might cost 5% as much as the
flood barrier's operating expenses.

For now seawater injection remains just an idea. But if something is
not done, then the rising waters may eventually force more drastic
changes. Dario Camuffo, who studies both the environment and Venice's
cultural heritage at cnr, says one option would be simply to abandon
the city's ground floors. Raised pavements could allow people to
enter buildings on the storey above. Another, he says, is that prized
structures might be taken apart for reassembly elsewhere. Mr Redi
worries that Venice's lagoon may need to be permanently cordoned off
from the sea with a dyke. For a city proud of its maritime heritage,
that might be seen more as a humiliation than an adaptation. #

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This article appeared in the Science & technology section of the
print edition under the headline "Water works"

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