Gondolas and a cruise ship in the background, Venice

Venice Flood Barrier: MOSE project keeps the sea at bay

Despite allegations of corruption, delays and overspending, the construction of the Venice Flood Barrier has an end in sight.

The final phase in the construction of the massive barrier system that will protect Venice and its lagoon from high waters was marked, ironically, by flooding.

In the summer of 2013, the first four modules of the MOSE project – named after the biblical leader who parted the Red Sea – were installed at the Lido inlet, one of the three waterways connecting the lagoon with the Adriatic Sea. The system comprises a series of modular gates that, when filled with water, rest on the seafloor; they will be raised by filling them with compressed air.

However, there has been political controversy along the way. The centre-left mayor of Venice was freed from house arrest on 17 July after police detained him as part of a probe into kickbacks connected with the MOSE project.

Giorgio Orsoni, who told a prosecutor he was innocent, agreed to a four-month sentence in a plea bargain that is awaiting court approval. Thirty-five warrants or requests for arrest proceedings were issued and dozens more people were placed under investigation. Orsoni was alleged to have received €560,000 in illicit contributions to his mayoral campaign in 2010 from the consortium behind the project. Despite this conviction, he is now back in his post.

While this seemingly widespread malpractice and criticism from some civil engineering experts have been going on, the MOSE project has reached its final stretch. By 2016, Venice will be serene once again – at least regarding the flooding problem, and assuming that there are no further delays.

Big civil works like the Venetian MOSE barriers are often opposed by environmental groups in an effort to preserve the uncontaminated natural environment. In this case, however, the ecosystem in question is far from natural. Sediments carried by the many inland rivers originated from the lagoon itself in ancient times by sinking and consolidating below sea level.

Those very same sediments would have spelled the end for the picturesque lagoon many centuries ago, if not for massive engineering projects by the Republic of Venice dating back to the 13th century. The outcome today is not the work of nature, but the artificial result of a trial-and-error process.

When we think of hydraulic engineering works, we think about keeping bodies of water at bay and protecting cities, towns and villages and factories. The upper Thames flooding earlier this year is a case in point. Venice, despite its recent history, historically has the exact opposite problem.

The city’s main concern is how to prevent its main body of water from disappearing altogether. The Venetian lagoon offered the city protection and access to the sea. It was a moat – an essential lifeline for the former maritime power and the very reason the city was founded by refugees from the mainland seeking asylum from barbarian raids.

Heritage of the waterways

The problem the government of Venice faced in medieval times was presented mainly by two big rivers that flowed into the lagoon – the Piave in the north and the Brenta right in front of the city, where now a modern bridge gives overland access.

The Brenta, in particular, kept the city magistrates in check for a very long time. The first solution devised was a simple levee bordering the north margin of the lagoon intended to deviate the flow east. As it was built almost perpendicular to the river, the solution was not destined to last long, and at the beginning of the 16th century a 24km-long canal was built that diverted the river (and a couple of other ones as well) south, outside the lagoon and directly into the sea.

The territorial expansion of Venice on the mainland, which coincided with the construction of the canal, enabled the city authorities to complete diversions, as the upper courses of the rivers were now under direct Venetian control.

The Piave was diverted north into the bed of another river and its old bed was used to receive the waters of the River Sile, which itself was diverted from its natural flow. In total, five major waterways were diverted in the 15th century preventing the lagoon from filling up.

These prominent civil works were completed by the dredging of the inlets. Until very recently these inlets were shallow and were consequently a risk to navigation, upon which trade in the old Republic was based. Subsequently, this allowed the building of many miles of artificial canals and walls and the complete reworking of the natural barriers between the sea and the lagoon. In the 19th century, when the Republic was no more, deep canals were excavated to allow big ships to reach the newly developed port and industrial petrochemical complex.

From ally to enemy

It is only recently that water went from being considered an ally and a lifeline to the city to threatening its very existence. Recurrent high waters periodically flood vast tracts of the historic city, which, incidentally, is not sinking at all. While these events are predictable, and even memorable, with the waterways being connected to the natural tidal cycle, they represent a serious danger.

The record flooding of 1966 triggered the need for a definitive response. The city authorities quickly recognised that only the complete separation of the lagoon from the sea at high tide could safeguard the city. Nevertheless, the actual building only began in 2003. In the meantime, both the Thames Barrier and the Rotterdam Surge Barrier were completed and in service.

The MOSE concept is highly innovative in many respects. The four barriers being installed at the three lagoon inlets are made up of 78 independently operated gates of various sizes. Each gate lies horizontally on the seabed when not in use.

When needed, the water inside is pumped out by compressed air and the gates simply rotate into position under their own buoyancy. The visual impact of such a solution is minimal, and the whole barrage can be closed in just 30 minutes. Once operational, the historic city will be > < protected from tides as high as 3m, which is much lower than the average 7m on the Thames tidal section that the London Barrier has to face. It is, however, high enough to protect Venice, whose lowest parts are less than a metre above mean sea level. The record-setting 1966 tide reached 194cm.

Venice barriers

While the barriers themselves are invisible when not in use, the same cannot be said of the complex structures needed for their operation. All three passages were strengthened with new concrete walls and embankments, including the sockets that connect them to the mobile barriers.

At the northernmost Lido inlet – the widest and most complex, since it leads to two distinct canals – a new artificial island was created, allowing two different barriers to be built: one closing the deeper canal used for tankers and big passenger ships and the other - shallower and leading to the northern part of the lagoon.

This new island will host all the service buildings controlling the barriers. Three storm ports will be available for boats and ships locked out of the lagoon when the barriers are closed.

Four locks are also built to allow passage. Three of them are of small dimensions at the north and south inlets. At the middle inlet of Malamocco, a lock is being built big enough to allow sizeable ships into the lagoon. This will allow the passenger port of Venice and the industrial port of Marghera to remain operational even when the MOSE is closed.

The old Arsenal, the former shipyard of the Republic of Venice in the centre of the city, is being restored and already hosts the main control centre. The barriers are not completely automated, unlike the Rotterdam surge barrier where control is delegated entirely to software. The MOSE barriers must be initiated by human decision.

Once activated the 78 single barriers emerge in groups of four, almost closing the links between the lagoon and the sea and leaving gaps a few centimetres wide between the gates – not enough to allow much water through. Not being linked, each gate – about 20 for each of the four channels – is able to move on its hinges and rotate under its own hydrodynamic push.

The nominal working inclination of the gates when closed is 45°, and the active control system keeps them as close as possible to this position by adjusting the quantity of water inside to act as ballast. Opening the barrier again is achieved by filling the gates with water, or ballasting. They return to their at-rest position, which is flush with the seabed.

Huge prefab concrete bases, also modular, were embedded on the seafloor of each channel to integrate the mobile gates, which are coupled to their bases by 42-tonne hinges. While invisible from above the water, the installation of the bases represented a huge modification of the pre-existing floor. It was a necessary part of the project because, for the barriers to operate, the floor must be completely flat.

Some hydraulic engineers, including Luigi d’Alpaos, author of a comprehensive work on the history of the hydraulic works in the lagoon, have expressed concern . D’Alpaos claims that the future erosion effects of such extensive reworking of the inlets has not been sufficiently studied and could lead to surprises, even impeding the use of the barrage.

Critics have also suggested that the money could have been better spent on a series of smaller projects to alleviate the problem, such as the protection of the lower parts of the city by building higher walls at the canals’ banks.

Also suggested is periodically dredging and cleaning the canals themselves. This was once an operation conducted with the utmost care when Venice was a self-governing city but has since been neglected. When filled with sediments and rubbish, obviously the depth is much less and flooding becomes much more probable.

While all these smaller-scale works are important, the general consensus among the engineering community is that having the possibility to separate the lagoon from the sea is the only way to really safeguard the city in the long-term. Let’s not forget that, while its impact is considerable, this is not the first huge hydraulic engineering project the Venetian lagoon has seen.

Big ships, bigger risks

It’s not only nature that is posing a threat to the delicate equilibrium of the lagoon, but also human activities. Oil tankers and huge cruise ships navigate the lagoon daily, causing pollution. Also, in the case of passenger ships, they pass within metres of St Mark’s Square – all the better to provide guests with the Venice panorama but posing an immense risk to the integrity of the stones of Venice themselves.

The barriers allow for this traffic to continue even when closed, with the big lock being built at the Malamocco inlet. The debate is still raging regarding the big cruise ships that seem to be bent on gigantism. The most modern ships are behemoths of well over 140,000 tonnes and the course they follow to enter and exit the port is right in front of the historical centre.

On the one hand of this dilemma, we have the cash influx guaranteed by thousands of cruisers and the use of the port as a departing point for Mediterranean cruises. On the other hand, the city is faced with the risk to the integrity of fragile historical buildings and pollution caused by the ships.

A choice must be made, and soon, on how to reap the economic benefits of tourism without putting the city itself at risk. One thing is certain; if Venice is to survive, it will have to do so as a working city and not as a mere theme park for cruisers and tourists. Without a sound strategic vision, all these massive works will be for nothing because there will be only an empty, albeit beautiful, shell remaining to protect.

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