Venice’s Downfall Won’t Be Tourism or Sea-Level Rise, but Rather the Tree Trunks It Was Built On

Nowadays, Venice is like an amusement park. It boasts some of the most beautiful buildings in Europe and holds a World Heritage Site status. However, the city’s appeal to tourists lies not only in its architectural marvels but also in its iconic canals. Venice relies heavily on tourism, and the COVID-19 pandemic dealt a severe blow to its economy. This dependence on tourism has created a complex relationship for the city, with both positive and negative implications. More significantly, it poses a threat to the city’s survival.

The underlying issue is that Venice is constructed on millions of wooden pilings that have been remarkably preserved for centuries. Human action is now threatening this foundation, endangering the city’s very existence.

Fleeing to the islands. The Venice of today is situated on an archipelago of 118 islands connected by 455 bridges. The urban framework is truly impressive, and taking a stroll through its streets is a truly special experience. Although it may not seem like the most practical location for a city, the initial settlers didn’t have much choice. The city was established in the 5th century in a space that offered protection against attacks from Germanic peoples.

Originally just a lagoon with an outlet to the Adriatic Sea, it was inhabited by fishermen. However, it provided the perfect defense against attacks, and consequently, people fleeing from surrounding cities due to these “barbaric” attacks settled there. In order to accommodate everyone, the city began to expand and, as is customary, started reclaiming land from the sea.

Piles. The foundation is crucial when constructing a building because it provides the necessary support. However, in Venice, the challenge was the absence of solid ground due to the presence of only mud. Removing the mud was impractical, and laying a brick foundation wasn't feasible. As a solution, millions of trees from the surrounding area were cut down to create piles and foundations for the city starting in the 9th century.

These wooden pillars were driven into the unstable clay soil to a depth of several feet, reaching a strong enough stratum to support the piles. The entire pile was underwater, and the buildings were constructed on the shallow end. In Venice, the only visible wood would be that of the poles used for mooring boats.

Wait, did you say wood in water? Doesn’t it rot? When a living tree is partially submerged in water, it can coexist with it. However, when the tree dies and is in contact with water, it begins to rot because organisms called xylophages start “eating” it. If the wood is completely submerged, these organisms don’t.

This is because the organisms that eat the wood need oxygen, and although there’s oxygen in the water, it’s not enough for them, so the wood remains intact. This is more than just a theory—after certain accidents in Venice, it was discovered that the piles were practically intact after several years.

Wood of the finest quality. The city’s foundations were built using wood from nearby oak and holm oak forests. This wood was resistant to wood-eating organisms and had strong properties to support heavy weight. Over time, the saline water in the lagoon hardened the wood, making it appear more like rock than wood.

Venetians recognized the critical role of wood in their survival. They used it to expand the city and construct their naval fleet, which was essential for defense and trade. To ensure sustainability, the city’s authorities implemented a program to regulate logging, promote reforestation, and prohibit the use of quality trees for firewood. Furthermore, they also used wood extensively in the city’s main buildings.

The settlement. When a building is constructed, it undergoes a period of settlement. This occurs in any city and leads to the building gradually sinking into the ground due to its own weight. Settlement affects many areas, regardless of soil quality, and can be a more serious problem in some areas. For instance, in China, major cities are sinking due to the weight they bear, and in Venice, this occurs dramatically.

Because the ground is soft and clayey, settlement occurs despite efforts to drive the building’s piles into a harder stratum. As a result, some buildings experience a displacement of several inches. Interestingly, many buildings haven’t collapsed yet, although accidents have occurred throughout history.

Acqua alta and defenses. The city of Venice is an engineering marvel, but it faces occasional flooding known as acqua alta between autumn and spring. When the water level rises, much of the city, including popular spots like St. Mark’s Square, floods. This is a major attraction for tourists, who like taking pictures and walking on elevated platforms, but it’s a real problem for the city.

The city estimates that these floods cost about $1.6 million a year in hours not worked because stores can’t open. Additionally, it affects the city’s heritage. The MOSE (Experimental Electromechanical Module) system, which consists of huge defenses (dikes) that can be raised, when necessary, was completed in 2020 to address this issue. These defenses, which cost around about $7.6 billion, are designed to prevent flooding from affecting the city as much. According to officials, they’ve been effective since installation.

Climate change and cruises. The city’s settlement causes it to sink between 0.8 and 1.5 inches every century, a process that could accelerate. Climate change, particularly the melting of the poles, poses a significant threat to all coastal cities, but it can be particularly catastrophic in a city with a delicate state like Venice. Additionally, tourism is also damaging its sea floor.

Venice isn’t sinking due to the millions of tourists it receives annually, but because of the activity of cruise ships and other boats. While Venice has always had boats navigating its canals, the current influx of various types of crafts includes police boats, tourist gondolas, private boats, and delivery boats. There are also cruise ships that come very close to the city, causing to severe consequences in the sea floor and the soft clay soil that forms the base of the city. In fact, regulations have been imposed to prevent these ships from approaching too closely to mitigate the damage they cause.

Venice, you’re not special. While the city is indeed special, the use of piles can be found in various other places around the world. There are several towns in certain regions built on piles, but many of these structures are leaning and have deteriorated due to attacks by xylophagous. Other cities that showcase the use of piles include Amsterdam and Mexico City. Additionally, iconic constructions such as the London Bridge, St. Isaac’s Cathedral in St. Petersburg, the Brooklyn Bridge in New York, Notre Dame in Paris, and Toledo near Madrid are also all built on wooden piles.

Image | Science Channel | Paolo da Reggio | Slice Science | Didier Descouens