Venice and water, it’s a story of love and hate. The canals and the lagoon are the main characteristic of Venice and give the city its relaxed atmosphere. Rowing and regatas are part of the daily life of every Venetian. On the other hand, the destructive power of the water is also the biggest enemy of the infrastructure of the city.
In this post, I will give you some insights on what it entails to build a city on water, on the impact of the tides (acqua alta and acqua bassa) and on the large ‘Mose’ project that should protect the city.
Construction of the city and the buildings
The city of Venice as we know it now is not a natural island (or group of 118 islands), but an artificial city built on the water. In the 8th and 9th century, Venice was only an archipelago of small islands separated by one large and many narrow canals. These islands became inhabited from the 9th century onwards. When the population grew and houses became increasingly larger, they shored up the ground and created additional land. The Fondamenta Nove for instance was built in the late 16th century, whereas the Riva degli Schiavoni and the Zattere were added in the 19th century.
The whole city is supported by millions of wooden piles, each about 14 cm thick and up to 3 metre long. To give you an idea, 12,000 tree trunks were used to support the weight of the Rialto bridge, 100,000 for the campanille on the San Marco square and more than a million for the Santa Maria della Salute church.
DID YOU KNOW? Not only the woods of Venice’s Italian hinterland were almost completely cleared for the construction of Venice, but also the coasts of Istria and Dalmatia, on the opposite side of the Adriatic. The barren landscapes of the eastern Adriatic coastline still bear witness to this deforestation.
These piles are driven deep into the soil, until they reach hard clay which could hold the weight of the buildings. The wooden piles have been (more or less) intact for more than 500 years and kept the city above the water, thanks to a combination of reasons. First of all, the type of wood used (oak or larch) is very water resistant. Second, wood only rots when both air and water are present. Hence, the wooden piles were protected due to the lack of oxygen in the water underneath the buildings. Third, the extremely large amount of silt and soil blasted the wood, which turned it into stone at an accelerated pace. Finally, the piles were placed as closely together as the soil of the ground would permit and stone and rock were thrown in between to keep the silt from rising up.
The Science Channel created a short movie on the foundations of Venice, with nice visuals about the piles. It gives you a very good image of what is hidden below your feet when you walk in the calle in Venice.
Also the houses need to be constructed in a special way, as they have to stay upright on an unstable soil and resist the forces of the water. You might for instance have noticed that most buildings in Venice have marble floors. The rationale behind this is not only related to aesthetics, but mainly due to the fact that marble is impermeable to water. Other examples of specific Venetian construction requirements are the outer walls which are wider than inner walls and which have larger and deeper foundations or the ‘paratie’ that you find in front of the doors to avoid water entering the house at acqua alta.
I won’t go into all the technical details, but if you are interested, you could watch the movie ‘Venice: Backstage’ from Insula spa, the operative arm of the City for urban maintenance. It explains the different construction techniques used in Venice to protect the houses from the water. The book ‘Elements of Venice’ also describes in detail the different architectural elements (façade, stair, corridor, floor, …) of buildings in Venice.
Acqua alta and acqua bassa
The phenomenon of acqua alta (high water) is very well known, even to people who have never visited Venice. Acqua alta generally takes place in winter time, when a combination of astronomical tides, strong south wind (scirocco) and sea waves cause a larger inflow of water into the Venetian lagoon. The consequences are that part of the city is flooded, paratie have to be put in front of the doors and the heightened walkways have to be set up. Most of the times, the flooding is limited to a rather small area around the San Marco square and it only lasts a couple of hours, until the tide changes again. Exceptional high tides (140 cm above sea level), which correspond with a flooding of approximately 50% of the city, have happened only 8 times since 2000.
DID YOU KNOW? The median level of the sea in Venice is not the same as in the rest of Italy. Venice uses the ‘tide-level-zero’ at the Punta della Salute (measured in 1897), where a monitoring station is located. In the rest of Italy, the median sea level (IGM) refers to the level in Genoa in 1942, which is about 23 cm higher.
The highest acqua alta occurred in 1966 when a tide of 194 cm completely flooded Venice, as well as Chioggia and other islands and villages in the lagoon. Follow this link to read my post ‘Acqua granda: The story of an eventful day in Venice’ which describes in detail the terrible events of that day. At that time, UNESCO launched an international request for funding to repair the damages to the city. Several international committees were created, of which many still exist to preserve the history of Venice. You can read more on this topic in my post ‘Discover how Venice finances its cultural heritage’.
Even though acqua alta is something ‘common’ for Venetians, it always gets a lot of attention from the world media, who seem to worry much more about it than the residents. Venice is often referred to as the sinking city, and unfortunately there is some element of truth in it. The city is now 25 cm lower compared to the sea level than at the beginning of the 20th century. One of the underlying reasons is the use of the ground water from the subsoil under the city for the daily life. As a result, and as you can see on the graph from the ‘Centro Previsioni e Segnalazioni Maree’, the frequency of reaching a water level of 110 cm is increasing over time. The current problem of the climate change and the rising sea level adds another level of complexity to it. Jay Griffin, an architecture student from the Technology University of Sydney, looked into this topic and compared Venice to other regions in the world with the same problem. You can read more about it by following this link.
There is also the opposite phenomenon, i.e. acqua bassa. When this happens, the water in some canals is so low (from -50 cm, even down to -70 cm) that boats get stuck and the waterways cannot be used. The destructive impact from the water on the houses is also visible at that time below the waterline. Acqua bassa happens less often than acqua alta, but it has been occurring more frequently in the last years due to the fact that many canals haven’t been dredged and cleaned for a long time. Due to changes in the law (Legge Speciale), all financing went primarily to the Mose project instead. As with acqua alta, it’s a matter of patience (of a couple of hours) until the water flows again normally.
The Mose project
To safeguard Venice from the flooding at high tide, the Mose project (referring to the biblical person who split the sea) was launched in 2003. Even though the goals of the project (protect the islands in the Venetian lagoon from the water, while maintaining and restoring the delicate ecological equilibrium) make a lot of sense, the Mose project is a very sensitive and delicate topic in Venice. The project was originally due to be finalized in 2011. However, legislative and financial problems and scandals have led to a much prolonged timeframe. The project is now under the control of Consorzio Venezia Nuova, appointed by the Italian Anti-Corruption Authority in December 2014, and should be finished in 2018. The total cost of the project is estimated at approx. 5.5 billion euros.
DID YOU KNOW? The Venetian lagoon covers a total area of 550 km². It is separated from the Adriatic Sea by a littoral strip of 60 km, which is interrupted by 3 inlets at Lido, Malamocco and Chioggia. The depth of the lagoon, besides the waterways, is generally between 1 m and 3 m.
The Mose project is not only focusing on the problem of the increase in high tides and floods (as discussed above in the paragraph on acqua alta), but it is an integrated project that intends to restore and protect the ecosystem in the lagoon as well. The erosion of the littorals resulted in the gradual disappearance of beaches, while the environmental deterioration led to a loss of typical habitats such as salt marshes and shallows.
The Mose project is therefore much more than an installation of mobile barriers as it also implements solutions for these environmental issues. The following actions have already been taken and/or are ongoing.
- Recreation and protection of salt marshes to safeguard the natural habitats and to guarantee the essential ecological biodiversity in the lagoon.
- Reconstruction of (new) beaches and coastal dunes and construction of outer breakwaters to defend from sea storms and stop the erosion.
- Raising of quaysides and public paved zones in the lowest parts to defend urban centres from floods.
- Improving the water and sediment quality by isolating polluted land from the canal shores in Porto Marghera and from dumps and by creating special wetlands between the mainland and the lagoon to filter the water.
- Installation of mobile barriers at lagoon inlets to protect against floods.
The installation of the mobile barriers is a technically challenging project. There will be 4 barriers of approx. 200 metres wide at the inlets of Lido (2 barriers), Malamocco and Chioggia. Each barrier consists of approx. 20 gates, which can be manipulated individually. The barriers should protect Venice and the other lagoon islands up to a sea level of 300 cm, way above the historical level of 194 cm. The control and management of the system takes place from the control room at Arsenale Nord. So far, the barrier in the Lido-Treporti inlet is already finalized, while the foundations of the other 3 are in place and they will be fully installed in the coming two years. The system will only be used once all 4 barriers are complete.
The barriers will be raised to separate the lagoon from the sea when a high tide of 110 cm or more is forecasted (although this could change in the future depending on the requirements). In such an event, the barriers are filled with compressed air to rise up out of the sea. Once the danger is gone (usually a 4 hour period), the barriers are filled with water to lower them below the sea level. Depending on the tide, they might be used simultaneously, separately or even partially with only a limited number of gates. To avoid interference with the normal port activities, locks have been installed next to the barriers of Malamocco (merchant ships), Chioggia and Lido (fishing boats, emergency vessels and pleasure craft) to ensure that ships can enter or exit the lagoon when the barriers are up. The only ones who will be stuck are the large cruise ships.
An info center (Puntomose) has been opened at the Arsenale Nord to inform citizens and tourists about the need for and the added value of the project. You can visit it as an individual but they also organize info sessions for schools and groups. If you want to understand the project and the complexity, it is certainly worth a visit. It is open on weekdays (Monday till Friday) from 9.30-17.30. Information on the Mose project and the organization is also available on this website.
Overall, Venice and water is mainly a love relationship. As soon as the winter is gone, the lagoon is filled with boats of all types (more info in ‘Don’t miss the daily boat parades on the Venice canals’) and the Venetian residents take part in regatas or in a ‘fresco’ (a stroll on the canals with food and music).
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