That sinking feeling

By Huw Thomas, Editor

Rising sea levels and subsiding land threaten the City of Light’s very existence. Can an ambitious engineering plan provide a solution or is Venice doomed to watery grave?

“Since the start of the last century the land on which Venice is built has subsided by an average of 23cm across the city”

As any visitor to the city can attest, Venice is like nowhere else on earth. It's twisting network of canals, narrow streets and stately bridges have captivated visitors for centuries. Settled on a series of marshy islands in a huge lagoon on the Adriatic coast some 2000 years ago, Venice's position on the Adriatic coast saw it to become a major seafaring city state and powerhouse of international trade.

But now the proximity to water that enabled Venice to punch so high above its economic weight threatens the city's very existence. Venetians are no strangers to flooding. The so-called acqua alta (high water) has been a regular feature throughout the city's history, but its effects are becoming more acute over time. Since the start of the last century the land on which Venice is built has subsided by an average of 23cm, resulting in increasingly regular flooding. This subsidence has been accompanied by rising sea levels, a problem that is only set to get worse in the coming decades as global warming pushes tides even higher. Hard data offers a stark example of the seriousness of the situation; between 1926 and 1935, high water events of 110cm or more occurred seven times. In the decade between 1996 and 2005, that number had grown to 53. It is not only the frequency of flooding that has increased during the 20th century, but the severity too. A 194cm event, the worst ever recorded, caused havoc on 4 November 1966, but swells of 140cm or more have become distressingly commonplace. The most recent on 1 December 2008 saw a water level of 156cm swamp streets and piazzas.

Venice is no longer the economic goliath it once was. In place of huge volumes of international trade, the city now draws its wealth from the 20 million tourists who visit annually. They come to bask in Venice's sights and history and they hugely outnumber the just 60,000 Venetians for whom the city is a year round home. If it is to remain both a holiday destination and a viable place to live for residents, something needs to be done to halt the sea's progress. Historic buildings, their durable stone foundations already swamped, now show telltale smears of green algae in their vulnerable brickwork. Water bubbles up through manhole covers in Piazza San Marco, even on clear and flood free days. This threat of rising waters led to the 2003 authorisation of the MOSE project. Both an acronym for Modulo Sperimentale Elettromeccanico and the Italian name for Biblical Red Sea parter Moses, MOSE is an ambitious plan to build mobile floodgates at the Lido, Malamocco and Chioggia lagoon inlets. It will eventually cost almost €5 billion and hopefully protect Venice from the acqua alta for decades to come.

Stemming the tide

On the face of it, the MOSE system seems fairly simple. When unusually high sea levels threaten the city, floodgates will rise from the lagoon floor to stem the flow. Coupled with better breakwaters and a number of other permanent defences, the system should protect the city from the worst tidal effects. But few things really are that simple. From the beginning MOSE has been a major source of controversy, with numerous objections on environmental, financial and operational grounds. The organisation tasked with overseeing this huge project is the Consorzio Venezia Nuova (New Venice Consortium) an alliance of major Italian construction firms. Giovanni Cecconi is an engineer with the Consortium who has been working on the project since its inception. A construction effort of this size and type has presented plenty of engineering challenges. "For instance the differential settlement, the foundation," says Cecconi. "The technique is one of the most universal, but this has to be done in four different places, historical places, and we have to compact the soil and predict the behavior of these cases. You can find floodgates around the world, but not a row of independent floodgates spread out around so many inlets. This provides great flexibility and they can move each other so the load is transferred to the foundation, but you can have dynamical behavior. So we had to do plenty of investigation in the resonance behavior to avoid that. We modified the size and shape of the gates in order to have a system that cannot resonate through the normal weather, and also during the extreme flooding. We used mathematical modeling to define the domain of risk of instability. Then with the physical model we did the complete modeling of all relevant phenomena that because the mathematical model has certain assumptions. These studies started since the very beginning and we finished them immediately before the start of the project."

A striking feature of the project is its duration. Despite being approved in 2003, the system is not expected to be operational until 2014. According to Cecconi, it is not only technical challenges that have made the process of building MOSE a slow one, but also political considerations. The project has been a football for various factions in Italy's often tumultuous political landscape, only gaining final approval when it was adopted by Silvio Berlusconi's administration. "When you have such a big project the decision is mainly political and it's like a flag," says Cecconi. "The project goes ahead when one party is winning and the other is losing and so on. This takes time to build success, but because the project was very much ahead when Berlusconi went into power it triggered the start-up because the project was ready and reliable. The administration took a chance to promote itself through the project."

 

But Berlusconi's approval hasn't ended opposition. Now, nearly seven years since its approval and still four years from completion some continue to see MOSE as an expensive way for the government to funnel money to its friends in industry that fails to address the real issues facing Venice. Even the city's mayor, Massimo Cacciari, has added his voice to protests from conservation groups and concerned residents.

While there seems to be general consensus that the MOSE gates will at least be effective in protecting against the acqua alta, a key concern is the impact they might have on the lagoon's ecosystem. A natural flow of water through the three inlets is vital to prevent the lagoon's waters becoming stagnant and polluted. This is a particularly big problem for Venice as the city has no municipal sewage system and relies on the tides to flush waste into the Adriatic. However, Cecconi believes that this should not cause too much trouble. "We have decided in the environmental assessment to use the gates to preserve Venice for all the high tide and also the exceptional ones, instead leaving the more frequent one to be treated through local rising of the banks and local protection," he says. "This means that now assuming that we want to elevate a small number of time of closure, only 3-5 times a year for a few hours, 3-5 hours. This means less than one percent of the time during the flooding period with very little impact to the port because we have also navigational lock."

But critics suggest that this is a short-sighted assessment. If global trends continue, we could see sea levels rising considerably in the not too distant future, which could necessitate the gates being in use far more often than currently planned. "The phenomenon of high waters could be further aggravated by the predicted sea level rise due to climate changes," says Luigi D'Alpaos, Professor of Hydrodynamics at Padua University. "If the relative sea level increased by about 50 cm in the next century, in accordance with Intergovernmental Panel on Climate Change previsions for the global rate of sea level rise, the barriers of the MOSE system would enter into operation almost every day in winter, and for longer periods of time. Such an occurrence will bear as a consequence relevant environmental problems related to the strong reduction in the intensity of tidal currents and therefore to their role in governing water exchange between different portions of the lagoon. It is also worthwhile noting that Port activities will suffer the more frequent closure of the inlets in the case of a marked increase in mean seal level. In fact, operational continuity and accessibility to the port to commercial ships, with the mobile gates raised, is ensured only through the navigation lock at the Malamocco inlet, and this represent a limitation to port activities."

Port in a storm

Port activity is a significant factor in the debate on safeguarding the city. In September 2009 the Venice Port Authority released a report on plans to further develop Porto Marghera, just 4km northwest of Venice, into a major shipping hub. To ensure that larger ships could access the port, the Authority has proposed a programme of dredging which includes deepening the Malamocco-Marghera channel to 16 metres. Professor D'Alpaos expresses serious concerns about the long-term effects such a move could have on Venice's ecology. The movement of sediment between the lagoon and the Adriatic is greatly influenced by shipping channels, with the asymmetry between ebb and flood meaning that far more sediment is expelled from the lagoon than brought in. "In the ebb phase, once the current has moved beyond the heads of the jetties, it maintains a jet structure that moves out towards the sea penetrating for quite a long distance into the open sea," he says. "As a consequence, a sediment plume forms which flows out into the sea during the ebb phase, whereas only a small amount of sediments, that were carried out to the sea during the ebb phase, returns to the lagoon during the following flood phase. Such a strongly asymmetric behaviour has led to a very intense erosion of the tidal flats in the Venice lagoon, which is well documented by the comparison of the bathymetries starting from 1901."

These effects are likely to be compounded by the impact more and bigger ships would have on Venice's environment. "Boat-induced wakes and currents, particularly in the case of large boats, further contributes to the erosion processes taking place in the lagoon," continues D'Alpaos. "This clearly emerges if one compares the elevation of tidal flats flanking the Malamocco-Marghera channel characterising the present configuration and the bathymetry surveyed in 1970, immediately after the excavation of the Malamocco-Marghera channel. From all this it follows that increasing the number and size of boats entering the Lagoon, will inevitably enhance the erosion processes occurring on the intertidal areas flanking the navigable channels."

It is here that the MOSE system enters the debate over port expansion. The Port Authority dismisses concerns over the effects of deeper shipping channels by pointing to the mitigating factor of the new flood defences. In the report on its plans for Porto Marghera, the Authority simply states that: "The problem of the hydraulic equilibrium is solved because it will be manageable through judicious use of the MOSE system."

But according to Professor D'Alpaos, the idea that the MOSE project can provide an answer just doesn't stand up. "The erosion process due to wind waves. Boat-induced wakes will not be mitigated by the MOSE system," he states. "The gates, in fact, do not have any effect on the reduction of wind-induced erosion processes which depend on water depth, wind climate, and fetch. All of these quantities do not feel the presence of the gates. As to the boat-induced wakes within the Malamocco-Marghera channel, obviously such a process is not influenced by the presence of the gates. It depends mainly on the shape of boat keels and on boat speed."

Venice Port Authority declined to be interviewed for this article, but it would be fair to say that the Authority's President Paolo Costa is in general disagreement with those who oppose plans for port expansion. In response to a report from charity the Venice in Peril that highlighted the dangers fresh dredging posed, Costa accused the fund of "pseudo scientific profiteering" and insisted that the port expansion would bring both economic and environmental benefits.

That sinking feeling

The growing disparity between a subsiding Venice and a rising sea level remains a major issue, with some suggesting that the rate of subsidence is being underestimated by planners. Albert Ammerman is an archaeologist who has been digging into Venice's past for many years, with some surprising results. Ammerman's excavations have uncovered a walkway of Roman tiles dating back to about 200 AD which now lies about 1.5 metres below current sea levels, indicating that the actual rate of subsidence is around 13 centimetres per century. In the past Venetians would counteract these effects by engaging in a constant process of construction. "What we can always see in the archaeological record is the gradual, progressive buildup of the land surface," said Amerman in an interview with PBS News. "We can see five or six floors, with just one after the other, six inches, a foot, gradually being built up. Flooding would always be a problem, so their way to deal with it was essentially to come in and continually be adjusting the ground level upward, layer after layer after layer."

However, Venice's status as a historical destination has seen such efforts grind to a halt. "One of the fascinating things is that, since essentially 1800, the time of Napoleon, the fall of the Venetian republic, they have stopped doing that'" Ammerman continued. "Venice has become, in some sense, a museum. It's become fossilized. The life of the city, also the notion of preservation heritage, stops people from doing what was the thing that Venetians always did, that is, build up the ground level."

This illustrates the biggest challenge facing the city, that of safeguarding Venice without destroying what makes it worth protecting. If current predictions are in any way accurate, the only way to effectively prevent the city being submerged will be to completely seal off the inlets into the lagoon. Along with the environmental problems of such a plan, there would also be significant economic impact as access to cruise and cargo ships would be severely restricted. This would give rise to a whole new range of engineering challenges if the city were to remain viable.

But there remains one more option. Instead of entering an arms race with rising sea levels, why not just raise the land up instead? It's not as bizarre an idea as it sounds. Venice is built on spongy swampland. Over the years the weight if the city has pushed down, squeezing out water and compressing the earth below. Borrowing techniques used in oil exploration to access hard to reach deposits, a research group at the University of Padua has suggested that salt water could be pumped into a sandy layer 600-800 metres beneath the lagoon. "This plan could lift Venice by up to 30 cm in 10 years, and Venice could be raised safely by using actual technology," says Professor D'Alpaos. "This plan is contested and contrasted by some people, but I think it deserves much more attention, and that analyses and studies on such a direction need be supported. After all, the process of lifting the city with advanced technologies could somehow mimic the actions undertaken by the ancient Venetians, who used to demolish whole islands and rebuild them over the rubble at higher elevations."

If it could work, this solution offers perhaps the best hope for Venice's prolonged future, allowing the city to stay dry while keeping its architectural magnificence trapped in amber. The MOSE plan can certainly stem the tide for now, but if Venice's 2000 year history is going to endure for further millennia it is only the first step in an ongoing engineering challenge.

MOSE in action

When a tide of 110cm or more occurs, the MOSE gates spring into action. Normally laid flat on the lagoon bed, the gates are filled with air and raise up to create a barrier to the incoming seawater. To minimise environmental impact, the gates only remain in use until sea levels return to normal. They then fill with water and sink once more the bottom of the lagoon. While this is an adequate response to intermittent threats of high waters, opponents of the system are concerned about the impact on Venice's ecosystem if rising sea levels require their more frequent use.