Acqua Alta and the Mose Project. The lagoon is sustained by a delicate ecosystem; channels and shallows kept clean by the Adriatic tides sweeping in, whilst the rivers feeding into it have been diverted, in order to avoid silting up.
Until the 20th century the system essentially worked. Today, acqua alta or high water flood tides, now threatens the future of Venice. The MOSE Barrier project nearing completion, will hopefully secure this great historic city for future generations.
Since the 1920’s, the development of commercial harbour and oil refinery at Porto Marghera, deep water channels have been dredged to provide access to the facilities. New man-made islands have replaced a third of the wetlands. This together with the vagaries of global warming and changing weather patterns, have caused an increasing incidence of acqua alta or high water flooding of the city; especially during the winter period. Tapping of the region’s water table also had the effect of causing the sinking of Venice and it is thought that the city is now 25 cm lower compared to the sea level; than at the beginning of the 20th century. However, over the last fifty years this aspect has significantly slowed.
Industrial pollution has caused damage to the buildings and artwork, and a high level of respiratory problems. The high phosphorus and nitrogen content of the Lagoon has stimulated algal growth affecting the water quality and depleting marine life. Speed restrictions on motorised boats have been introduced to limit the damage to underwater piles on which the city is built.
Acqua alta occurs regularly during the winter months. For non-Venetians, it is amazing to see how residents in the city show great adaptability; taking the phenomenon of high tides and exceptional water levels in their stride.
The city has a series of flexible measures in place that limit the extent of the problem. Sirens sound a warning throughout the city when a high tide is forecast; information is provided in real time via the web and mobile telephones.
Elevated temporary platforms are set up in the parts of the city with heavier pedestrian traffic, while some public water transport lines are diverted to all-weather routes. An average high tide lasts about two and a half hours and citizens, commerce and transport are warned in time to plan alternative routes, to put all goods away safely and to wear boots or waders.
Most of the time, the flooding is limited to a rather small area around the San Marco square and it only lasts a couple of hours, until the tide changes again. Exceptional high tides (140 cm above sea level), which correspond with a flooding of approximately 50% of the city; have occurred only 8 times since 2000.
In 1966 disastrous floods affected Venice,which was covered by up to 2 metres of water; as well as Chioggia and other islands and villages in the lagoon. A UNESCO international appeal was launched to save the city and several international committees set up; galvanising the authorities to act to secure the city’s future.
Less well known and publicised is the opposite phenomenon, “acqua bassa” (photo: below).
When this happens, the water in some canals is so low (down 50-70 cm); that boats get stuck and the waterways cannot be used. The destructive impact from the water on the houses below the waterline, is also visible at that time. Acqua bassa happens less often than acqua alta, but it has been occurring more frequently in the last years; due to the fact that many canals have not been dredged and cleaned for a long time. Due to changes in the law, financing went primarily to the Mose Project. As with acqua alta, it’s a matter of patience and a couple of hours; until the water flows again normally.
The Mose Barrier Project.
The goal of the project was to protect the islands in the Venetian lagoon from exceptional high water, while maintaining and restoring the delicate ecological equilibrium. The erosion of the littorals resulted in the gradual disappearance of beaches, while the environmental deterioration led to a loss of typical habitats such as salt marshes and shallows. (Definition of littoral: a coastal region, especially the shore zone between high tide and low tide points).
The whole project was therefore much more than an installation of mobile barriers, as it also implements solutions for these environmental issues and the installation of the mobile barriers was a technically challenging project.
The stated aims of the project were as follows:-
- Installation of mobile barriers at lagoon inlets to protect against floods.
- Reconstruction of existing and new beaches and coastal dunes and construction of outer breakwaters to defend from sea storms and stop the erosion.
- Raising of quaysides and public paved zones in the lowest parts to defend urban centres from floods.
- Recreation and protection of salt marshes to safeguard the natural habitats and to guarantee the essential ecological biodiversity in the lagoon.
- Improving the water and sediment quality by isolating polluted land from the canal shores in Porto Marghera and from dumps and by creating special wetlands between the mainland and the lagoon to filter the water.
Funding and development for the construction of huge mobile steel barriers or gates, across the three lagoon openings to control the tides; was approved in 1994. The project is continuing in the tradition of the Venetian Republic, which intervened in the lagoon to protect against natural threats. Ancient Venetians built sea walls and diverted two major rivers; which to them were huge engineering projects. In fact, the Venetian Republic actually suffered more from flooding than now; as they had difficulty in predicting it and people were regularly killed.
There are 4 barriers of approximately 200 metres wide, at the inlets of Lido (2 barriers), Malamocco and Chioggia. Each barrier consists of approx. 20 gates, which can be manipulated individually.
LEFT: The reconstructed beaches and outer breakwaters at Pellestrina. (© Ministero delle Infrastrutture e Trasporti – Provveditorato Interregionale per le Opere Pubbliche del Triveneto – Consorzio Venezia Nuova)
RIGHT: The Lido-Treporti gates. (© Ministero delle Infrastrutture e Trasporti – Provveditorato Interregionale per le Opere Pubbliche del Triveneto – Consorzio Venezia Nuova)
The working of the gates is somewhat similar to London’s Thames Barrier, thanks to the knowledge and experience shared within the International Network for Storm Surge Barrier Managers; of which London and Venice are both members. A recent cost-benefit analysis has shown that the barriers will more than pay for themselves in 50 years; through the reduction of maintenance and repair costs caused by the annual floods.
The barriers will be raised to separate the lagoon from the sea, when a high tide of 110 cm or more is forecast. In such an event, the barriers are filled with compressed air, to rise up out of the sea. Once the danger is past (usually a 4 hour period), the barriers are filled with water to lower them below the sea level. Depending on the tide, they might be used simultaneously, separately or even partially with only a limited number of gates.
To avoid interference with the normal port activities, locks have been installed next to the barriers of Malamocco (merchant ships), Chioggia and Lido (fishing boats, emergency vessels and pleasure craft); to ensure that ships can enter or exit the lagoon when the barriers are up. The only ones who will be stuck are the large cruise ships.
Delays, Concerns and Corruption. The project was met with delays and initially even concern from some conservationists, that the scheme would damage the sensitive ecosystem and exacerbate the problem of high-water tidal flooding. The project was also slowed due to significant corruption and misappropriation of funding and the final costs for the project approaches Euro 6 Billion. There are new concerns over the level of maintenance these barriers will require. The whole project is a rather sensitive and delicate topic in Venice.
The MOSE Barrier project is finally nearing completion and the “Puntomose” control and information centre is located in a converted chapel in Venice’s North Arsenal. The command office has been simulating the control of the gates since 2011, collecting data that enables the creation of mathematical and statistical models that can be entered into the decision support system. The main factors being considered were wind, water level, waves, pressure and fresh flood from the rivers. The barriers will be able to support a high tide of three metres and hopefully will protect Venice for a century.
Facing the Future. Against this background there is a need to keep the city alive and functioning and for the city to adapt to face the future with confidence. Outwardly Venice is a magical historic city; however, it is also a rather difficult, inconvenient and expensive place to live. Everything has to be brought in; food and transport costs are high. Loss of city centre business and manufacturing, globalisation, lack of reasonable priced rental accommodation together with the previously mentioned problems, has caused a significant exodus particularly amongst the young. Many have moved to the mainland towns of Marghera and Mestre, on the edge of the lagoon.
Now the population is 55,000, down from 160,000 in 1940; a figure put in perspective by the fact that visitor numbers are said to be approximately 20+ million visitors annually. The city has now become reliant on tourism, but this has brought it own set of problems.
In high season some 30,000 people visit the city daily; many are day-trippers often spending little. This has put an enormous strain on the infra-structure and often discourages those with greater spending power. Ideas introduced are increased visitor transport prices and taxes on accommodation.
Venice needs to change and adapt in order to bring a sense of normality in its inhabitants, avoiding becoming some sort of large historic theme park, solely dependent on tourism. Its future probably lies as a Centre of Excellence for culture, academia and business.
For centuries the winged lion, symbol of Venice has watched over the city, keeping it secure. Let us hope that the Mose Barrier project coming to completion and designed to protect against up to 3 metres of Acqua alta ; work as planned.
The future of this historic city must be a concern to the whole world.
The idea of Venice sinking has long captured the imagination and has featured in countless artistic works.
Venice as a metaphor for death and human frailty dates back to the early 19th century. The fall of the Venetian Republic in 1797, created a predisposition to embrace any sign of Venice’s downfall.
“In the fall of Venice think of thine,” wrote poet Lord Byron in 1812.
John Ruskin’s book of 1860, provided details of the city’s stones for the generations he feared would never see it.
When the bell tower in St Mark’s Square collapsed in 1902, it was taken as a sign that Venice was sinking.
In the 20th century, Thomas Mann’s Death in Venice consolidated the association of the city with mortality.
Keates quoted the following: – “The idea that St Mark’s Basilica will one day become a species of Debussian cathedrale engloutie, with coral-encrusted statues and fish swimming across the mosaics and the Pala d’Oro turned into an oyster-bed; is no doubt an alluring one for some”
He continued, “Frankly I don’t share it. Neither am I, or my fellow trustees, in favour of the ‘Let it fall down’ school of thought, which is simply an excuse for idleness, apathy and philistinism. If the thought of Venice sinking captures the imagination, then it should be as a wakeup call to save the city as an essential basis of the civilization we cherish. The world owes huge debts to Venice, including modern systems of democratic government and the printed book.”
Finally, in the theatre director Robert Lepage’s 1990 production of Tectonic Plates, he famously flooded the stage of the National Theatre; to stage an art auction in which punters were bidding on famous paintings, while wading knee deep in water.
——–Acqua Alta and the Mose Barrier——–