Since the beginning of the 1900s, exceptionally high tides have become ever more frequent as the levels of the land has dropped and the sea has risen. The floodings cause inconvenience to inhabitants, economic damage due to the suspension of commercial activities, and serious damage to Venice’s architecture and historical buildings.
In the future, flooding could be further aggravated by the predicted rise in sea level resulting from climate change. To address this risk, the Mose barriers have been constructed in the City of Venice; these are designed to cope with a rise in the sea level of up to 60 cm, and to be managed flexibly enough to cope with an increase in the frequency of high tides in various ways, depending on the characteristics and scale of the tidal event.
What is it?
The Mose system was conceived to safeguard Venice from high tides/sea level rise and consequent flooding. It consists of mobile barriers able to temporarily separate the lagoon from the sea and protect Venice from both exceptional destructive events as well as more frequent events. It is being constructed at the lagoon inlets of Lido, Malamocco and Chioggia, the three openings in the barrier island through which tides enter the lagoon.
Complementary works such as new cliffs outside the inlets are designed to mitigate more frequent tide levels. The integration of these operations creates a highly functional defense system.
The system has been designed to respond to different objectives:
- To stop the water in case of exceptional high tide and let the free circulation of water in normal times;
- To maintain the operability of the port;
- To disappear when not in use to clear the impact on the landscape.
The total cost of the Mose system is €5,493 million euros, all of which has been financed by Italian State.
How does it work?
The mobile barriers consist of rows of gates. In normal tidal conditions, the gates (formed by 78 floating pontoons) rest in housing structures at the three lagoon inlets, completely invisible and without modifying exchanges between the sea and lagoon. They are raised only when necessary to block the incoming tide and avoid flooding the lagoon and built areas. Currently, the tide level planned for the operation of the gates is 110 cm (related to the sea level meter placed in Venice, Punta della Salute). This level is not, therefore, a functional limit of the defense system and may be revised and lowered if necessary. Mose can protect the lagoon and its cities and towns from tides of up to 3 meters.
Depending on the situation, the defense strategies can involve simultaneous closure of all three inlets, closure of one inlet at a time, or partial closure of each inlet, as the gates are all independent. The construction of the Mose barriers was preceded by a work programme with a systemic approach, combining physical protection with restoration of the morphological balance of the entire lagoon ecosystem, implemented by the Italian Ministry of Infrastructure and Transport – Venice Water Authority through the Consorzio Venezia Nuova.
The defense of the city from high waters also depends on the integration of other interventions (such as tidal forecasts, early warning, urban maintenance, raising of the city, de-pollution of the lagoon) established by the National Special Law for the Safeguard of Venice (1973) and implemented by all the local governments levels (city, region, etc.).
The Mose barriers are expected to be completed and in operation by 2016. Upon completion, Venice will be more resilient to the exceptional high tides; it will also be necessary to further assess, from a scientific point of view, how the lagoon and the activities that take place inside the lagoon (the ecologic system, the socio-economic activities) will react to the unprecedented impacts of climate change. The challenge will be to learn how to use this complex and powerful system most effectively.
It is always hard to assess the damages and the opportunity costs of not making a city more resilient, especially a delta city. However, climate change related incidents of the past few years such as the flooding in Rome, London and Thailand, as well as Hurricane Sandy in New York show that water-related catastrophes can cost billions. With the Mose barriers, the city of Venice became more resilient to exceptional events, so that tragedies like the one that occurred during the flood on 4th November 1966 do not happen again.
Magistrato alle Acque di Venezia
Palazzo dei X Savi
San Polo, 19
30125 Venice - Italy
 The tide is measured on the mareographic zero at "Punta della Salute" and 97% of the town’s pavements is at +100 centimetres; for example these are some flooding rates of the pavements in relation to high water levels - measured on the mareographic zero-: +120 cm: 29%; over +140 cm 54% of the town is covered by water.