How the survival of Venice will hinge on MOSE flood defences

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It’s a well-documented fact that Venice is slowly sinking and experts predict sea levels around the historic Italian city will rise dramatically during the next 100 years. Venice has had to undertake inventive protective measures, unique to their setting, which are currently being put in place in a passionate bid to avert potential catastrophe to come.

Taking centre stage in a wider programme of flood defence works is an incredible feat of marine engineering: MOSE.  Short for ‘Modulo Sperimentale Elettromeccanico’, the name is also a play on Moses parting the waters of the Red Sea. Once complete in 2016, MOSE will provide three barriers across the Lido, Malamocco and Chioggia inlets to the Venetian Lagoon that will temporarily isolate its waters from the Adriatic Sea during high tides.

After more than 30 years of controversy and political wrangling about how best to manage the increasingly frequent floods affecting Venice, MOSE finally got underway in 2003. Last October a trial section worked successfully in tests. Since then, the project has been powering ahead.

On course for 2016

About 80% has been completed with three major elements now in place as of January this year, namely two barriers across the Lido inlet (one consisting of a row of 21 gates and the other of 20) linked by a small artificial island, and a giant lock at Malamocco. The super-lock will allow large ships and cruise liners to enter and exit the lagoon when the flood barriers are up. More modest locks for pleasure craft and smaller vessels will be built at the Lido and Chioggia inlets.

Before looking at the simple but inspired way MOSE works, here are some figures to help convey the massive scale of the operation. At present, some 4,000 people are directly or indirectly involved the construction of MOSE. Four barriers totalling 1.6km in length will consist of 78 individually controllable gates.

The gates range between 18.5 x 20 x 3.6 m and 29.5 x 20 x 4.5 m in length, width and thickness. Each will be fitted with two hinges weighing in at 42 tons a piece. When not in operation, the gates will lie flush to the seabed in concrete caissons, concealed from view.

Old physics, new twist

Here’s a synopsis of MOSE’s operating principle, which combines Archimedes’ ancient principle of buoyancy with ultra-sophisticated and flexible modern management technologies.

In normal tidal conditions, the gates are full of water and rest in their caissons. When a high tide is forecast, compressed air is introduced into the gates, emptying them of water and causing them to rotate around the hinges. They rise up until the tops are above water, thus blocking the sea behind from entering the lagoon.

When the tide drops, the gates refill with water and hinge back down to the seabed. Once MOSE is fully operational, the inlets will be closed for an average of four to five hours during tides of over 110 cm. This will include the time it will take to raise the gates – about 30 minutes – and to lower them again – about 15 minutes. Sirens will warn shipping in the area that the barriers are about to go up.

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So far MOSE has already cost the Italian government around $7 billion, which in turn begs the question, “Is the flooding serious enough to justify the expense?”. The short answer is yes. Floods caused by exceptional high tides of over 110 cm are becoming more and more frequent. In 1920 there were just two, while between 2001 and 2010 there were 64. At 194 cm, the worst flood to date occurred in 1966.

Today, towns and villages in the lagoon are on average 23 cm lower than at the start of the 1900s. At the same time, the 4th Intergovernmental Panel on Climate Change report predicts an increase in sea level of between 18 and 59 cm during the next 100 years. Taking the ‘even worse case scenario’ approach, MOSE has been designed to cope with an increase of up to 60 cm. In extreme circumstances, the gates could be further raised above the optimum slope of 45 degrees.

Those behind MOSE

MOSE is being directed by the Consorzio Venezia Nuova on behalf of Italy’s Ministry of Infrastructure and Transport – Venice Water Authority. The Consorzio consists of a group of the country’s leading international construction and marine engineering firms, and local cooperatives and firms with extensive experience of operating in the lagoon.

Marine engineers involved on the four MOSE sections are E Mantovani SpA (Lido inlet, northern side), Grandi Lavori Fincosit SpA and Nuova CO.ED.MAR (Lido inlet, southern side), Grandi Lavori Fincosit SpA and Impresa Pietro Cidonio SpA (Malamocco inlet),  Società Consortile Condotte, Co Ve Co, and Società Cooperativa San Martino (Chioggia inlet).

In addition to MOSE, a number of other flood defence measures are being put in place by the Consorzio. These include the raising of quaysides and paving in low-lying urban areas, the installation of small flood gates in some of the city’s canals, the reinforcement of barrier islands and jetties in the lagoon, and the reconstruction of 1,600 hectares of salt marshes to help protect the lagoon’s delicate eco-system.

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Gail Taylor

Written By admin 
February 03, 2014 17:20 pm
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