VENICE, Italy--If you've heard that this famous city, built on islands, is sinking, you've only heard part of the problem. Over the last 100 years, it has indeed lost 23 centimeters of land, but today, there are bigger problems facing Venice and seriously threatening its future.
Venice and other nearby towns are inside the 550-square-kilometer 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 fall and autumn. At the same time, erosion of the littorals--the areas close to shore--mean that local beaches, which are considered crucial defenses for developed areas against storms, are increasingly disappearing. And finally, a local petrochemical center 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.
In order to address these problems, the Venice Water Authority--created in the 16th century--along with the Italian Ministry of Infrastructure and Transport, and a consortium of Venetian engineering firms, has embarked on a series of major projects, including a massive $6.7 billion infrastructure project called the Mose system, as well as a beach reconstruction project and efforts to counteract the degradation of the lagoon environment and its natural habitats.
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.
The three inletsThis 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.
Mose project at the inletsThis 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 totaling 41 gates at the Lido inlet, 19 at the Malamocco inlet, and 18 at the Chioggia inlet.
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.
Gate in actionIn 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.
Gate functionThis 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 harbors 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."
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.
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.