Coastal Processes, Hazards, and Society

Case Study 4: MOSE flood barrier in Venice, Italy


Case Study 4: MOSE flood barrier in Venice, Italy

Although the threat is different from that in the case of the Thames Barrier, like London, the gated flood defenses of the MOSE project in Italy are designed to protect Venice from high tides and surges from the Adriatic Sea, thereby reducing flood frequency. The project takes advantage of the three main tidal inlets connecting the Venice lagoon to the sea, where a series of hollow gates on hinges initially resting on the bed will rise and close the inlets during periods of high tides. The MOSE barrier uses a completely different method than the approach used in the Thames Barrier. The idea, however, is similar in that for much of the time, when there is no imminent threat of flooding, the gates are filled with water and resting on the seabed; when floodwaters threaten Venice, the gates are closed in response to incoming high water. The smart building elements in this approach are in the design, the operation, and the utilization of processes and water properties to minimize operating costs, energy, and maintenance. The gates simply close under the influence of gravity by slowly filling up with seawater, and once fully open, the added weight of the steel when added to the weight of water keeps them submerged. To close the gates, the opposite of gravity, buoyancy, is used. Pumped air forces water out of the gates, and since the air density is more than a thousand times less than seawater, the gates are lifted into the closed position, rising above the water surface. The lack of mechanical infrastructure and arms to perform these steps keeps operating cost low, although, as we learned in earlier modules, the upfront capital costs for such projects can be in the billions of dollars.

Recommended Video: Venice - Lagoon: The MOSE System for the Defence Against High Waters (6:28)

Credit: Ministry of Infrastructure and Transport, Venice Water Authority. Film edited by Consorzio Venezia Nuova (concessionary for implementing the measures for the protection of Venice and the environmental restoration of the lagoon) September 2008. Film making: Konstantin Skatchinski. Editing: Stefania Alberti, Paolo Cottignola. Narration: John Taylor.
Click for a transcript of the Venice - Lagoon: The MOSE System for the Defence Against High Waters video
JOHN TAYLOR: This is the Venetian Lagoon with the city at the center. And these are the lagoon inlets, the openings which connect the lagoon to the sea. This is where the MOSE system will be realized, designed to solve the problem of high water, which is becoming increasingly frequent and intense. Imagine that this system is already in place. Let us draw nearer to the Malamocco inlet, framed by two long jetties. A breakwater outside of the inlet constitutes the outpost of a system of defense from the sea. The breakwater, at about 1,300 meters long, has a dual function. It reduces the intensity of the tidal currents and protects the inlet and the new navigation lock from wave motion. But the heart of the system of defense is underwater, where the mobile gates that protect the area from all high waters, including extreme events, are installed. The mobile devices are made up of a row of gates, which is set in a trench in the bed of the inlet that reaches from one end of the inlet to the other. The row is made up of metal gates, with one side attached to their base by means of two hinges. Special paint is applied to this exterior, which protects them from the aggression of the marine environment. Now let's see how they're made inside. Inside, we see the modular metallic structure which assures maximum resistance of the gates to the pounding of the wave motion. But how do they function, exactly? When a tide higher than 1 meter and 10 centimeters is forecast, compressed air is pumped into the gates to empty the water that keeps them on the seabed. As the water is expelled, the gates, rotating around the axis of the hinges, rise until they emerge and isolate the lagoon from the sea. When the high water ebbs, and the water in the lagoon and the sea reaches the same level, the gates are filled, once again, with water until they return to their original position. The fixed structures at the inlet, such as the outer breakwaters, and the raising of the pavement, and lagoon banks in the inhabited areas of the lagoon, contribute to the reduction of the number of the most frequent floods. This optimizes the management of the mobile barriers and limits their use to three to five times a year, given the current level of the sea. The entire system of defense will be extremely efficient, functional, and flexible. Opening the gates, which we see here, is carried out according to precise procedures, with which the possible increase in the water level of the lagoon resulting from the flow of the rivers from rain, from local increases due to the wind, and water passing between one gate and another is also taken into account. All together, there are four rows of gates-- one at the inlet of Malamocco with 19 gates, one at the inlet of Chioggia with 18 gates, two at the inlet of the Lido, which is twice the length of the other inlets, each with 21 and 20 gates, respectively. The gates have varying dimensions depending on the differing depths of the inlet canals. The lengths range from between 18 and 28 meters. The thickness varies from 3.6 to 5 meters. And the width, the same for all, is of 20 meters. At the end of the raising phase, the upper part of the gates emerges until it forms a continuous line which closes the inlet, preventing the tide from entering and maintaining the water in the lagoon at a safe level. The system is designed to sustain a difference in level between the sea and the lagoon of up to 2 meters. It's therefore capable of protecting the Lagoon of Venice, its inhabitants, its extraordinary areas, it's invaluable historic, artistic, and environmental heritage, even in the case of a relevant increase in the sea level during the next 100 years. When the barriers are in operation, port activities can continue without interruption thanks to the navigation lock, which allows for the passage of ships and is situated along the Malamocco inlet. At the Lido and Chioggia openings, refuge havens with small locks allow for the passage of fishing boats, emergency vessels, and leisure boats. Along the inlets, there are also plants for the functioning and control of the barriers. This development of new activities and new qualified work also implies the safeguarding of Venice.