SHORTLY after arriving in Venice on an assignment for the New Yorker magazine, American humorist Robert Benchley sent a now-famous telegram to his editor. “Streets flooded. Please advise.” That was back in the 1930s, but had Benchley visited Venice more recently he might have found the city’s intimate relationship with water less of a cause for amusement. Chances are that its pavements, cafes and houses would have been flooded, too.
Venice is, of course, watery by nature. The city was built on 118 small islands within a lagoon, linked up by more than 400 bridges. Floods known as acque alte (“high waters”) are a fact of life, striking whenever spring tides coincide with storm surges in the Adriatic Sea.
The trouble is, the frequency and severity of acque alte have increased dramatically over the past 100 years. Back in the early 20th century, St Mark’s Square – one of the city’s lowest points – flooded fewer than 10 times a year. By the 1980s it was flooding 40 times each winter. Nowadays the square spends much of the winter doing a fair imitation of a paddling pool, flooding around 60 times.
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Venice’s problem is largely one of subsidence, both natural and man-made. From the 1930s to the 1970s, fresh water was pumped out of underground reservoirs beneath the city to supply surrounding factories. As the water was pumped out of these aquifers – which are rather like rocky sponges – their water-filled pores compressed and the ground sank. Combined with sea-level changes, this has produced an effective rise in sea level of 23 centimetres over the past 100 years. Water is no longer being extracted but natural subsidence, probably caused by plate tectonics, is continuing to drag the city down by 0.5 millimetres a year. Add to that the problem of rising sea level and it is clear that Venice’s predicament is getting steadily worse.
Numerous plans have been proposed to prevent Venice succumbing to the floodwaters, many of them controversial. But if the latest idea gets the go-ahead it will raise more than a few eyebrows. Rather than trying to control the rising water level by keeping the sea out, engineers at the nearby University of Padua want to lift the entire city out of harm’s way by raising the ground upon which it sits.
In the simplest version of the plan, the engineers would pump 18 million cubic metres of seawater a year into an underground aquifer beneath the city. The Padua team say this would reverse the historical subsidence, protecting the city from all but the worst floods. The plan has many critics, not least engineers involved in an ongoing flood-barrier scheme. But it also has some powerful allies who are pressing for a pilot project to test its feasibility.
What both sides can agree on is that Venice needs a rescue plan. The wake-up call came on 4 November 1966, when a catastrophic flood hit the city. First a high tide sent sea levels up to 1.2 metres above normal, inundating most of the city. Then the wind blew in from the south and stopped the floodwater receding. When the next high tide hit, Venice was in real trouble. The city centre was submerged under nearly 2 metres of water, power failed and thousands of drowned rats were flushed into the canals. The surviving rats scurried up buildings and became the unwanted house guests of unfortunate Venetians, many of whom were trapped for days.
Although the frequent floods have not dampened the enthusiasm of visitors, they are threatening the structural integrity of Venice’s treasured buildings and causing the city to lose its allure as a place to live and work. It is the human residents who have been abandoning this particular sinking ship. Medieval Venice was a thriving community of 250,000 people; today’s population of 60,000 is half what it was a just few decades ago.
Come hell or high water
The 1966 disaster initiated a lengthy search for solutions that culminated in 2001, when the Italian government approved an ambitious flood-barrier project called MOSE. Designed to protect the city for the next century, MOSE will consist of a series of 79 flood barriers that collectively span the three inlets to the Venetian lagoon. Normally, the gates will lie on the bed of the lagoon. But whenever sea level threatens to rise by more than 1.1 metres, compressed air will be pumped into them so they float up and block the inlets.
Construction of MOSE began in May 2003. But the ¬3.5 billion project remains controversial, with critics continuing to voice fears about environmental damage to the lagoon and even whether the gates will be enough to protect the city once they are complete in 2011.
Enter the Padua team, led by applied mathematician Giuseppe Gambolati. He and his colleagues are not proposing to scrap MOSE, but they believe that their idea of raising the city could give it a helping hand. “I think MOSE is the real solution to the problem of protecting Venice from acque alte,” Gambolati says. “But Venice uplift would really represent an important help in terms of both the duration of its useful life and the impact on the Venice ecosystem.”
Predictions for MOSE’s useful life are a matter of some dispute. Supporters say that it will protect the city for up to 100 years, while opponents contend that rising sea levels could render it obsolete in 50. If the Padua plan succeeds in raising the city, it could effectively mitigate decades of rising sea level, which is expected to increase globally by 20 to 90 centimetres over the next 40 years.
One of the most controversial aspects of MOSE is its potential impact on the lagoon ecosystem. Astonishingly, much of Venice still has no proper sewerage system, so anything that slows down the exchange of lagoon and seawater could quickly lead to a dangerous build-up of pollution. Scientists are divided over whether MOSE will contribute to the problem, although both camps agree that installing proper sewerage should be a priority. Gambolati argues that raising the city will mean that the gates have to be used less frequently, minimising any risk of the lagoon turning into a giant cesspool.
Raising Venice may sound like a crackpot idea at first, but in the light of the subsidence that occurred when water was pumped from beneath the lagoon, pumping fluid back underneath the sinking city doesn’t seem quite such a daft idea.
Such is its appeal, in fact, that this isn’t the first time the concept has been mooted. Back in 1969, the city’s chief engineer, Eugenio Miozzi, proposed injecting water into an aquifer about 200 metres down. The idea was never pursued, however, because any uplift caused by injections at such a shallow level would make the surface rise unevenly, potentially destroying the city it was designed to save. “The previous proposal was actually like a chat in the bar, with no calculations made of the possible uplift,” says Gambolati. “No investigation of any kind was made.”
So what is different this time? For a start, the Padua team’s proposal targets a different aquifer. This one lies some 600 to 800 metres down and extends slightly beyond the perimeter of the lagoon. “It is a virgin brackish aquifer never used before,” says Gambolati. The Padua team know it is there because of seismic measurements made around the lagoon, along with measurements taken from a borehole 950 metres deep that was drilled below Venice itself by the Italian National Research Council back in 1971. The aquifer sits on a layer of relatively impermeable clay, and is capped by another 25 metres of clay, which should stop any injected fluid from seeping back up towards the surface.
Crucially, because this aquifer is so much deeper, the ground above should in theory rise more evenly. “The deeper you go, the smoother the effect is on the land surface,” says Gambolati. This is because, at such a depth, the weight of the overlying rock forces deformations to spread horizontally, damping down any vertical differences.
The Padua team have run detailed mathematical simulations of the uplift project using real data. This comes from years of work in the Northern Adriatic basin, measuring the way undersea gas fields contract as the gas is extracted and then rebound once the oil companies have finished with them. This happens because water from the surrounding rocks tends to flow into the evacuated areas and the pressure inside starts to recover.
To monitor any rock movement, the Padua team uses weakly radioactive capsules as underground markers. They deposit the capsules down a disused well shaft and then fire them horizontally into the surrounding rock. By periodically dropping a monitor down the shaft to measure how the signals have moved about, the researchers can check how the rock is shifting. In this way they discovered that gas fields don’t just contract when the gas is removed – they re-expand as water flows back in. This makes the team confident that injecting water will cause the aquifer to expand and so elevate the ground above it.
“Before our studies we only had information on rock behaviour in compaction,” says Gambolati. “Data on rock expansion was totally missing.” The characteristics of the target aquifer aren’t known for sure, but Gambolati assumed in his model that they are comparable to the nearby gas field at Chioggia Mare, south of the Venice lagoon, which his team has studied extensively.
The team modelled three scenarios. The first involved injecting compressed carbon dioxide into the aquifer from a circular arrangement of 12 vertical wells. This option would have the added benefit of helping Italy meet its climate-change obligations by locking some 23 million tonnes of greenhouse gas out of harm’s way each year. The model, however, produced only a modest rise of 12 centimetres. Next they tested the more expensive option of pumping CO2 into eight horizontal wells, each 3 kilometres long and converging towards a central point. This produced a more acceptable rise of 24 centimetres. Best of all, however, was simply injecting seawater from vertical wells, which led to a 30-centimetre lift (see Graphic).
As well as giving the greatest lift, injection of seawater would be the cheapest and simplest solution, even though it would do nothing to mitigate Italy’s CO2 emissions. “Tackling two issues at a time would probably be too much,” concedes Gambolati.
The biggest danger would be fracturing of the aquifer rocks, but Gambolati believes this is a remote possibility. At a depth of 600 to 800 metres the rock is in compression from the tremendous weight of the land overhead. This compression counteracts the increase in pore pressure of 1800 kilopascals, so the rock is never in tension and there should be no danger of fractures developing. If the numbers are wrong, however, a fracture could ruin the whole project. “Fractures could propagate to the overlying formations and be potentially dangerous for the uniformity of the uplift,” says Gambolati.
When the Padua team published the results of their simulations (Eos, vol 84, p 546), they were greeted with scepticism by some, in particular the leading supporters of MOSE. Rafael Bras, a hydrologist at the Massachusetts Institute of Technology who has been involved with MOSE since 1995, dismisses the uplift scheme as “nothing more than speculation”. “To argue that you could engineer a controlled lifting of a whole city, built on over 100 islands, is really stretching credibility,” he says. “The dangers of such an operation would be enormous to the structural integrity of the city. As an engineer, I certainly would not want to take that responsibility.”
Bras maintains that MOSE will be up to the job. “The mobile gates are the only system that can deal with floods,” he says. “The truth is that this well-studied plan is an engineering marvel.” Even so, Bras admits that the underlying principle of the uplift idea holds some appeal. “If a scheme could be found to recover subsidence in an effective, safe way then it would be helpful and complementary. I just do not think this one is realistic.”
So does the plan get the same reaction from the anti-MOSE lobby? Paolo Pirazzoli, a geomorphologist with the CNRS, the French national research organisation in Meudon, is less dismissive but says that the scheme does not go far enough to address flooding in the rest of the lagoon. “The Gambolati proposal, if feasible, would be useful in Venice but not in other lagoon centres or on the islands delimiting the lagoon. These would remain exposed to attacks by the sea,” he says.
But Pirazzoli reserves his most scathing attack for MOSE itself: “MOSE is a bad project over the short term, the mid term and the long term.” He believes the real answer lies in lower-budget, lower-impact projects, such as manually raising the ground level in the lowest parts of Venice. “This is already being done,” he says, adding that within 30 years, 97.5 per cent of the city will be more than 110 centimetres above the baseline sea level.
Gambolati and his colleagues have powerful supporters, however. CORILA, a consortium of universities set up to coordinate research into the lagoon’s problems, is seeking funding to enable them to carry out a three-year pilot uplift project at the margin of the lagoon. The full-scale uplift scheme will only get the green light if the Padua team can demonstrate that its computer models match reality, by which time the construction of MOSE should be well under way.
In the days before Venice became the cultural treasure house it is today, the city’s residents took a rather more fatalistic view of the sinking city. Whenever the water threatened to overwhelm a building, they simply demolished it and rebuilt at a higher level. These days, this approach would be unthinkable. But even if fans of MOSE are proved right and the gates hold back the sea for more than 50 years, what then? At least the uplift scheme could buy the city some extra time to find the next solution.