Saving the sinking city of Venice

The famous Italian city gets flooded up to five times a year, and a massive infrastructure project is under way to protect it against the rising tides, as well as to save its beaches and marine habitats.

Special gates could keep the Adriatic Sea at bay and protect Venice, Italy, from the increasingly common high tides that threaten the island city with floods and continued erosion.
(Credit: Consorzio Venezia Nuova)

Not far beyond this famous city, just at the border between the Venice Lagoon and the Adriatic Sea, a group of concrete behemoths is under construction. A lot of very smart people think they will be part of the solution that could save the island town from drowning.

The notion that Venice is sinking has been around for a long time, and in fact, it has lost 23 centimetres of land over the last 100 years. But these days, it faces a troika of what may be more insidious challenges: rising tides that each year brings more and more floods; the degradation of beaches and shore areas; and gradual pollution of its marine environment.

To tackle this, the Italian Ministry of Infrastructure and Transport, in conjunction with the Venice Water Authority and the Consorzio Venezia Nuovo — a group of local engineering firms — have embarked on a giant infrastructure project known as Mose that, it is hoped, will keep the rising waters at bay and begin the process of permanently protecting Venice. The group hopes to have the system up and running in 2014.

The heart of the project is a system of 78 disappearing, oscillating, buoyancy flap gates that will be activated when tides rise higher than 110 centimetres inside the lagoon. There are three inlets to the 500 square-kilometre lagoon. Forty-one gates will be installed at the Lido inlet (21 at Lido-Treporti and 20 at Lido-San Nicolo), while 29 will be installed at the Malamocco inlet and 18 at the Chioggia inlet.

"Normally, the barriers [will be] full of water and rest on the sea bed," according to information provided by the Consorzio Venezia Nuova. "When flooding is forecast, compressed air [will be] pumped into the gates, emptying them of water and causing them to rise up and emerge, blocking the tide as it enters the lagoon. When the tide drops, the gates [will be] filled with water again and return to their housing. Thanks to its operational flexibility, the Mose [will be able to] cope with floods in various ways — by simultaneously closing all three inlets during an exceptional event, by closing one inlet at a time, or partially closing each inlet (the gates are independent) during medium to high tides."

The highest tide on record to hit Venice was 194 centimetres, in November 1966, and the Mose gates are expected to be able to handle tides of up to 300 centimetres. Yet, even if a major storm hits the area and the gates are all raised — for as much as five hours, the average expected closure time — normal shipping activities should not need to be interrupted. That's because part of the project includes a large lock at Malamocco that will be able to handle even the largest ships. And smaller locks have been built at the two other inlets for smaller boats.

We got a chance to visit some of the biggest infrastructure construction of the Mose project, and while the gates have yet to be made, they're still very much a part of the future reality of this iconic city.

Just adjacent to the mouth of the Malamocco inlet, for example, a massive artificial pier has been built to handle the fabrication of the caissons — or basements — that will house the gates when everything is ready to go. The caissons themselves may well be the biggest part of the effort, since they are giant concrete and steel boxes that will rest on the sea bed, and on top of each will be three of the 20-metre-wide gates.

Once built, they will be moved on a system of 84 trolleys to what is known as the Syncrolift, a tremendous launching bay where the caissons will be lowered into the water — they have many large hollow sections, so they'll float — and then get towed to their final resting place nearby. Later, the gates will be brought out and connected at special hinge points.

At Lido, which I also visited, the process is different. There, the area of the canal where the gates will be installed was drained, and the caissons were built in place. Next spring, we were told, the area will be re-flooded and prepared for the gates.

It's too early to tell, of course, if the Mose project will have the desired effect. But the consortium of the Italian Government, the local water authority and leading engineering firms have bet a lot of money — up to US$6.7 billion — and a lot of prestige on the idea that these gates can keep Venice's rising tides from impacting the city, its shorelines or its marine environment.

And if it works, and Venice becomes safe even in the biggest storms, the long-held belief that the city is sinking may itself finally be ready to be put out to sea.

(Credit: Consorzio Venezia Nuova)

Venice and other nearby towns are inside the 550-square-kilometre Venice Lagoon, which is connected to the Adriatic Sea. Rising tide levels inside the lagoon are resulting in these towns, including Venice itself, becoming more frequently flooded — three to five times each winter and autumn. At the same time, erosion of the littorals — the areas close to shore — mean that local beaches, which are considered crucial defences for developed areas against storms, are increasingly disappearing. And finally, a local petrochemical centre and deep canal excavation are contributing to reduced water and sediment quality, as well as the deterioration of local habitats, such as shallows and salt marshes.

The massive $6.7 billion infrastructure project called the Mose system is all about the construction of a large series of sophisticated gates that are going to be placed at the entry to the three lagoon inlets. The idea behind the gates is that they can sit idle in the water when unneeded, but during storms, they can be automatically raised to isolate the lagoon from the Adriatic Sea. This should, according to the Consorzio Venezia Nuova, defend the city against floods, as well as protect port activity, the morphology of the lagoon and its water quality.
(Credit: Consorzio Venezia Nuova)

This image shows the Venice Lagoon and the three inlets that allow shipping traffic in and out, but through which storm surges have regularly flooded Venice. The gate systems of the Mose project will be installed at the three inlets.
(Credit: Consorzio Venezia Nuova)

This diagram explains the new infrastructure that is being installed at each of the three inlets of the Venice Lagoon. A total of 78 large gates will be installed in four rows, hopefully by 2014, at the three inlets — two sets totalling 41 gates at the Lido inlet, 19 at the Malamocco inlet and 18 at the Chioggia inlet.
(Credit: Consorzio Venezia Nuova)

This illustration explains the five major elements of each of the gates. When needed, the gates will rise between 40 and 42 degrees from the hinge seen in this image in order to protect the interior of the lagoon from the seas. The gates will be opened only in groups of threes in order to keep too much water from flooding through the other inlets.

In this illustration, the Adriatic Sea is on the right side, and the Venice Lagoon on the left. The gates are intended to maintain a lower water level in the lagoon than in the Adriatic.

The thickness of the gates will vary from inlet to inlet, from 11.8 feet to 16.4 feet. The lengths will also vary depending on the depth of the canals where they are located, from 60.7 feet to 95.8 feet. But all the gates will be 65.6 feet wide.
(Credit: Consorzio Venezia Nuova)

In this illustration, we can see one of the gates as it will look when it is in use. Today, the Consorzio Venezia Nuovo's construction contractors are working on building the many caissons that will house the gates, and the gates themselves will be built prior to the planned 2014 debut of the Mose project. When needed, the gates should be able to be opened in about 45 minutes, and closed in about the same amount of time.
(Credit: Consorzio Venezia Nuova)

This illustration explains the basics of how the gates are intended to work. The gates, which are hollow, will rise up at their hinges from the caissons on which they are installed under water.

According to a document from the Consorzio Venezia Nuova, "When a high tide is forecast, compressed air is pumped into the gates to empty them of water, causing them to rise above the surface and create a continuous barrier dividing the sea and the lagoon for the time necessary. Small craft harbours connected to locks allow vessels to enter the lagoon when the barriers are raised — small locks for pleasure and fishing boats at Lido and Chioggia and at Malamocco, a lock for large ships heading for the port."
(Credit: Kathleen Craig)

While the caissons — otherwise known as basements — that will house most of the gates are being constructed away from where they will be installed, that's not the case at the Lido inlet. There, because there is enough space to build them, they will be constructed in place.

In order to do that, the area around the caissons was drained of water, a stone system placed underneath to protect the bed of the caisson, and then the whole thing reinforced with a system of piles to avoid the collapse of the bed. Then, at Lido, the caissons were built on top of this system.

Here we see one of the Lido caissons under construction. It will hold three gates, each of which will eventually be installed on top of it and connected with special hinges.

The plan is that in the spring of 2012, all the water will be put back into this section, and this caisson and those around it will all be back underwater.
(Credit: Kathleen Craig)

Just alongside the Malamocco inlet, a huge construction site has been set up for the building of 18 of the caissons. They will be made at the site, and then launched into the water via a special system called the Syncrolift. The caissons will then be towed into place nearby.

All 18 of the Malamocco caissons are being constructed at the same time, although according to a very specific plan, and not at the same pace. Here we see a row of caissons that are under construction.
(Credit: Kathleen Craig)

This is at the Lido site, and we see a curving row of installed caissons, each of which will house three gates.
(Credit: Consorzio Venezia Nuova)

This is an aerial view of the caisson construction going on at the Malamocco site. What this picture shows is that the caissons, while constructed of steel and concrete, have large hollow spaces in them that will allow them to be buoyant when launched into the water before being sunk into place at the nearby Malamocco inlet.
(Credit: Consorzio Venezia Nuova)

This is an aerial view of the construction and placement of the caissons at the Lido inlet.
(Credit: Consorzio Venezia Nuova)

This is an aerial view of the Malamocco inlet (in the foreground) and the Malamocco caisson construction site (on the left). The caissons are being built there and will be launched from the Syncrolift and towed into place in the inlet. Later, the gates will be installed atop the caissons.
(Credit: Consorzio Venezia Nuova)

In this archival photograph, we see Venice underneath flood waters. The city is being flooded up to five times each year, and the Mose project is intended to protect the city from the rising tides that threaten the city's well-being.
(Credit: Kathleen Craig)

Each of the caissons will be launched into the water via the Syncrolift. In order to get to the Syncrolift, however, they must first be rolled into place on specially designed tracks. And they will be placed on top of 84 large trolleys. Here we see a long row of the trolleys, each of which has its own hydraulic piston jack that is used to support some of the massive amount of weight of the caisson.
(Credit: Kathleen Craig)

This is a close-up of one of the 84 trolleys that will be used to roll each of the caissons into place at the Syncrolift.
(Credit: Kathleen Craig)

These are the tracks on which the caissons will be rolled into place at the Syncrolift. The tolerances for the construction of the tracks was minuscule — a fraction of a millimetre — because there was almost no room for error in ensuring that the caissons are moved totally straight toward the Syncrolift.
(Credit: Kathleen Craig)

Here, we see a trolley for each of the tracks underneath one of the caissons. There is an extremely delicate plan in place for moving each one of the caissons into position to be rolled onto the Syncrolift. It involves sliding them forward and to the side from their construction places. When moving to the side, each trolley will lower its jack, rotate 90 degrees and then raise its jack, one trolley at a time. This is expected to be a very slow, laborious process.
(Credit: Consorzio Venezia Nuova)

In this archival photograph, we see the world-famous Piazza San Marco under water.
(Credit: Consorzio Venezia Nuova)

This is an aerial view of the Syncrolift. You can see how it is at the end of the artificial pier on which the caissons are being constructed. There is one row of caissons that is lined up directly with the Syncrolift. But two rows are to the side, and the caissons being made there will have to be moved to the side using the process described in the previous caption.
(Credit: Kathleen Craig)

Each caisson is being built with two tunnels inside that will house all kinds of electric and mechanical systems, and which will be waterproofed so that people can operate inside of them.
(Credit: Kathleen Craig)

These are a matching pair of the connectors that will join the gates with the caissons. Three of these connectors will be used for each gate.
(Credit: Kathleen Craig)

Two long rows of special Rolls-Royce winches are installed alongside the Syncrolift in order to lower the caissons onto the lift, from which they will be launched into the sea.
(Credit: Kathleen Craig)

These are the many pillars that are in place for the construction of a future caisson.

Via CNET