How the Venice Tide Barrier Project Works

Before the city became an iconic cultural landmark, its citizens countered rising water by simply filling in the lower levels of buildings, by or razing buildings entirely and then constructing new dwellings on top of older ones. In effect, it was a slow-motion concession to the unstoppable rising seas. But as Venice's architecture became more precious, no one wanted to destroy it.

Now, rising water slaps against permeable foundations, crumbling them. At high tide, transportation canals become impassable because the tunnels are too close to the waterline. Sidewalks, even elevated versions, turn into giant puddles. An entire infrastructure, and all of its inhabitants, are held hostage to the sea at high tide, particularly in winter.

It's a sloppy, tiring lifestyle. Population has sunk from 180,000 to only 60,000 [source: Squires].

Venice's wet wakeup call came decades ago. On Nov. 4, 1966, extreme flooding caused by a potent combination of high tides, lashing winds and sheets of rain lasted for around 22 hours, trapping people in buildings, causing widespread damage and leaving thousands homeless. It was then that Italians realized they must reckon with the sea, or wind up in it.

Italy made a national priority of protecting Venice and surrounding areas from flooding and extreme weather events. The years following the flood resulted in more floods -- floods of engineering and legislative paperwork.

But even after years of researching solutions, the Ministry of Infrastructure was unable to identify any single proposal to sufficiently address all of the engineering, economic and environmental concerns involved in a project of such enormous scale and complexity. The agency reviewed the possibility of raising important buildings and structures to protect them, as well as sealing off the lagoon by filling in the inlets to narrow or even permanently close them.

But it wasn't until decades later, in the early 1990s, that the government finally began settling on a type of mobile barrier system. Then began a long series of design reports, environmental impact studies and epic red tape wrangling that ultimately resulted in the MOSE dam project, which the government said in 2001 could be completed at a cost between $2 billion and $3 billion [source: Keahey].

The project stalled as people balked at the price tag and uncertainty as to whether MOSE would actually work as advertised. In 2003, the project finally launched, with an expected completion date of 2012 and a cost pushing $4 billion. As of mid-2014, some component testing had taken place, but the project was not finished.

So, how does the costly MOSE project plan to go mopping up the wet streets of Venice?

It took a long time to design and approve MOSE. Construction is taking a long time, too. That's because MOSE is currently the most massive public works project in the entire world. Its basic premise -- to stop high water from entering the lagoon -- is more complicated than it might sound.

There are three primary routes for water flowing in and out of the lagoon: the Lido, Malamocco and Chioggia inlets. When complete, MOSE will block waters entering through those inlets, to levels more than 2 meters (6.5 feet) above those on the city side of the dam.

MOSE is a series of mobile steel gates that are raised and lowered on command. When the tide is out and waters low, these hollow gates fill with water and lie flush with the seafloor, folded away in trenches. As waters rise, engineers use air compressors to blow out seawater and fill the gates with air.

Then, natural buoyancy causes the gates to swing upwards on hinges attached to the seafloor. The top of the gates jut out over the waves, plugging the inlets and stopping rising water from entering the lagoon, and in theory, protecting Venice and surrounding areas from flooding. Because the gates swing to and fro on hinges, they allow for some give, or oscillation, during periods of rough waves and powerful storms. They're also angled so that surges of water toward the inlets won't slam them shut and defeat the project's purpose.

To span the full width of all three inlets, there will be a total of nearly 80 gates, with each individual gate spanning up to 66 feet (20.1 meters) across. At 874 yards (800 meters) wide, the Lido -- the northernmost inlet -- is the widest. At wider points like those found in this location, there will be multiple rows of gates to ensure that as much water as possible is stopped.

The projected completion date is scheduled for some time in 2016. Until then, construction workers are working quickly, in the hopes that they can beat the next big weather event.

What goes into the installation of one of these gates?

Construction teams start by securing the hinged barriers to the bottom of the sea. To install the barriers, they must make perfectly flat ditches in the seafloor. Then, those trenches are lined with concrete. Once the gates are constructed, they'll be sunk to the sea bottom, where they'll lie completely within the trench until they are activated during high water.

The concrete bed does more than provide a foundation for the gates. It also houses the mechanical components needed to inflate and deflate the barriers, and provides service tunnels for engineers, too.

Meanwhile, engineers must also strengthen the lagoon's natural barriers between the inlets. They're adding more breakwaters, jetties and seawalls. They're also adding protection to the sea bed and other underwater structures to make them stronger.

At its deepest, the seafloor of the inlets is about 100 feet (30.5 meters) below the waves, which means that some of the gates must be really, really tall. Each gate is also up to 16 feet (4.9 meters) thick. That's a whole lot of metal. So it's probably not surprisingly that the heaviest of these gates weighs in at close to 350 tons (317.5 metric tons). That's as much as a Boeing 747 airliner.

In spite of their enormity, it takes only around 30 minutes to pump the gates full of air and into place. Reversing the process is even faster. The gates can be flooded with water and resting on the seafloor again in only 15 minutes. On average, the closures will last between 4 and 5 hours, or until high tide has passed [source: NOVA]. During closures, ships will still be able to enter and exit the lagoon through a lock system.

Construction is underway at all three inlets, employing approximately 3,000 workers who contribute directly or indirectly to MOSE. As building continues, project managers are careful not to block more than half of an inlet opening so as to avoid interfering with commercial traffic and other port activities.

Once complete, MOSE will need around 150 caretakers to prevent malfunction and deterioration. That's no small challenge, considering that most of system will be submerged dozens of meters below the waterline.

Building and maintaining MOSE is an arduous task. But as you're about to read, just getting the project launched was a feat in and of itself.

Any project as auspicious as MOSE is bound to draw its share of detractors. And when MOSE was proposed, critics took aim from all sides.

Everyday Italians were immediately put off by the fact that MOSE looked like a giant kickback scheme. The project was proposed by a group of 50 companies, Consorzio Venezia Nuova (CVN), which also happened be the same engineering and construction companies that wanted to build, and thus profit most heavily from, such a large and costly project.

In a country with plenty of other infrastructure needs, MOSE seemed to many like a plan born of corruption. Those suspicions weren't alleviated by the fact that some number crunchers found MOSE to be more expensive and complicated than necessary, especially considering there were similar, cheaper systems elsewhere in the world (for example, London and the Netherlands) that had already proved effective.

Environmental groups also take issue with MOSE. They say that impeding water movement will alter sediment flow, with all sorts of unknown consequences, including the possibility that sand buildup could prevent the gates from opening or closing properly.

There's also concern that hindering the natural inflow and outflow of water in the lagoon will cause irreversible damage to the ecosystem. In essence, these groups say that damming the lagoon even temporarily will increase pollution levels in the water, which would then harm plant and animal life. The assertion is that this is especially true of Venice because the city dumps much of its sewage directly into the lagoon, and the natural ebb and flow of tides helps flush the lagoon of these wastes.

MOSE proponents counter that Venice is long overdue for a modern water treatment system. They also say that by systematically closing just one or two of the barriers, the lagoon's flushing effect could actually be intensified.

MOSE opponents point out that a project of this size and expense may not provide a solid return on investment, in large part because no one really knows just how fast sea levels will rise. Should water levels advance faster than expected, MOSE could be obsolete in just a few decades.

There's also the fact that there are no guarantees that MOSE will work exactly as advertised. There may be weaknesses in the system that won't be exposed until an exceptional storm challenges the city's defenses.

Critics have attacked MOSE by appealing to courts to stop the project. So far, however, none of these efforts have done much to slow MOSE's progress. Now that the inertia of construction has taken hold, it's unlikely to stop due to bureaucratic or environmental disagreements.

As of mid-2014, MOSE moves onward. Its backers are confident that the system will work as promised. They also say that even if current scientific estimates are wrong and sea levels rise more quickly than Venetians hope, MOSE still buys Italy at least a century worth of time to find a better and more permanent solution [source: Poggioli].

Once the project is actualized, engineering and construction companies will wait to see how well their design and planning fares against Mother Nature's fury. And the still-dwindling number of native Venetians will wait, nervously, to see if MOSE, their city's lifeguard, can save them from falling prey to a relentless, uncaring sea.