Is Venice sinking or crumbling?
I know the title sounds pretty tragic, but at the end of the beautiful ecological walk of Venice I took on Sunday 6th Feb 2011, organised by Context Travel and guided by Luca Zaggia, that is the way I felt!
Professor Luca Zaggia is a geologist who works for the Science Marine Institute of Venice and who has a great experience on the past, present and future of Venice.
He met us at the vaporetto stop of San Silvestro, on the banks of the Gran Canal, by Palazzo Chiurlotto last Sunday morning.
The day could not have been more perfect. The sun was already shining very early in the morning and the temperature was not that cold, considering it is February, more like a Spring morning rather than a Winter one.
Right from the start Luca put us all at ease and started to explain why the walk was called ecological: because he would explain the relationship between the living organisms and the environment and then the causes, the damages and the possible solutions for the sinking of Venezia.
Luca told us Venice is situated in a lagoon of over 550 km2 (possibly the biggest in the Mediterranean Sea). This lagoon is one of the richest in Italy because that’s where you find the largest variety of birds of Northern Italy. The fauna lives on the numerous sandbars, all almost submerged by sea water. Many birds come here every year for their reproduction. The landscape is changing for the action of the wind and the tide but these are necessary for the renewal of the water in the lagoon.
In the heart of this breathtaking environment lies one of the most beautiful cities of Italy: Venice. But despite its beauty, the ecosystem is complex, it is deteriorating and dragging Venice down with itself.
Called Venetia ( the X Regio) during the Roman Empire, it was made up roughly by the territories we now know as Veneto, Friuli, Trentino and Istria. The southern boundary was represented by the Adriatic Sea: a broad area subject to progressive changes in terrain, with wide-ranging rivers, which combining their action with that of sea waves, gave birth to a wetland-type environment, with numerous lagoons. It was a dynamic ecosystem “, a sort of “middle way” between the environment of the inland, relatively stable, and the sea.
This area, which was part of the land of the Venetians, later assimilated by the Romans, was at that time, inhabited by fishermen, salinari salt workers, all experts in the art of the construction and the operating of vessels suitable for both the lagoon and the waterways environment. The same area, among other things, perhaps because of its peaceful atmosphere was used as a “resort” by the wealthy inhabitants of the surrounding Roman towns (Padua, Altino, Aquileia).
With the gradual disintegration of the Empire and the invasions of Germanic people, especially in the sixth century, the lagoon areas became a shelter to those who saw their land and their possessions at the mercy of the invaders.
To adventure on rivers and canals was not easy for those who did not know the area, and the sandy beaches were excellent protection against a possible attack (by sea). It was in particular, the current lagoon of Venice to see its population grow more. Of course this also meant a profound change in social composition in the lagoon area. Many refugees were well-off merchants or land owners or farmers of the inland cities of Altino and Oderzo.
How is Venice built?
If the interior was in the hands of Germanic tribes, the lagoon remained, however, in the Latin orbit as part of the Eastern Empire, depending directly from Ravenna. From the outset, therefore, Venice established a deep bond with the Byzantine world.
At the end of the seventh century, the inhabitants of the lagoons were no longer governed by the maritime tribunes, the Byzantine military commanders, but they had an independent command under a “dux”, hence the term “doge”. Thus was born the first form of the Venetian state (even if linked to Byzantium): the Dogado.
Towards 810 the government of doge Agnello Particiaco moved from Malamocco and settled in the area of Rivo Alto (Rialto), the centre of the lagoon. This is the History of Venice officially begins.
So these are the origins of this beautiful city, built on water and wood.
This idea developed at a later time. The original inhabitants were fishermen who spent their days in their boats passing from a piece of land to another and living on wooden houses resting on poles equal to those which can still be seen passing through the channel that runs from Pellestrina and Chioggia.
The next step was that to build the houses in wood, resting on poles on the ground near the shores. The soil of Venice, however, is made of very fragile and unstable soil and , when the city started to develop in masonry, the Venetian thought to consolidate the soil by planting heavy duty poles in oak or larch on more solid ground.
This is a layer called caranto and it is present in the mud. The driven stakes underwent a process of mineralization that has made them increasingly resistant instead of rotting them. This has made them extremely stable on the ground on which they are built.
So why is wood which was planted nearly one thousand year ago now rotting away?
The image of the characteristic Briccola, the pole or group of poles planted at the edge of the channel to mark the limit of navigability, gnawed at sea level and seems to be attacked by beavers, is now part of the typical landscape of the lagoon.
Venetians always know the rate at which different types of wood are attacked and eroded when immersed in water, according to the wood base and thickness of the structure. But in reality there is another more important factor that determines the duration of a pile driven into the water … and it’s where it is placed.
If a pile of eight-inch diameter could theoretically last for many years in water, even ten or twenty years, its duration is often much lower, in some cases up to little more than two. To understand the reasons for such a fast decay we need to know the mechanisms of this phenomenon. In fact, contrary to what one might think, the mechanical action produced by the waves, particularly noticeable on the surface, in this case is not the primary cause. Not that its effect does not contribute to the process, but he is not the primary mechanism.
The trigger, a real cause for erosion of the woods in a lagoon and ocean, is a biological fact. It is a question of biodemolition phenomena due to the combined action of moulds and bacteria with the very fast action of the xylophagous invertebrates (ship-worm).
The xylophagous are organisms that feed on wood, digging tunnels, and though in different ways, they have an effect on the wooden structure, weakening the strength of the structure and increasing the attack surface for mould and bacteria.
Bacteria are capable of degrading the cellulose, the substance which gives strength to plant cell walls. Such action may be carried out by different bacteria in both aerobic environment, as in wood exposed to water, and in anaerobic environment, as in substrates embedded in sediment. The bacterial attack is still slow and not very aggressive, especially in anaerobic environment. This is why a portion of the timber immersed in asphyxiated sediments remains essentially unchanged for years, often for centuries.
This is the main reason why poles of the same tree species and the same thickness can have very different duration (up to ten times lower) in fresh water, compared to those located in areas of the open lagoon or related to marine areas . The progressive deepening of inlets to allow the entry of big cruise ships tends to increase the penetration of salt water, effectively increasing the diffusion area of all species in ship-worm lagoon.
Somehow the ancient Venetian knew this and in fact when they built their house on poles they used a specific method to avoid the rotting of the wood and the erosion by salt water of the stones and wood.
The poles would be planted completely in the sediments of the lagoon, then on top they would place a base made of wood and on top again various layers of Istrian Stone. They would work out the maximum level to which the high tide would come during the years and made sure to finish off the foundation with a last layer of Istrian Stone before building the house on top, to stop the humidity climbing up the walls of the new building.
Unfortunately the increase of the level of the high tides (caused by global warming and other environmental factors), the increase amount of days of Acqua Alta and the tidal waves caused by the motor boats, are all causing the water to surpass the original level defined by the last layer of Istrian stone. This can be seen everywhere in Venice.
So Venice is sinking! Well, we all pretty much knew that (even if we might have not known why).
But another terrible thing which is happening to the city is the erosion of the stones by the salt water. The stone are literally crumbling away, just like when you break a cookie in your hands. And not only the stones, but the wood too!
The bases of all the buildings are covered by a white layer: a chemical reaction of the salt water exposed to the sun, which is literally disintegrating the stones and bricks.
This is the real problem of the Acqua Alta. It is not just the everyday disruption of life which it causes, but also the long term damages it causes to all the buildings of the city. Various solutions have been tested, but up to now nothing seems to work.
It must be said that the problem as increased incredibly since the 20th century: the introduction of motor boats, the increase amounts of days of acqua alta, the increase on the overall average level of the water of the lagoon, the lack of maintenance of the small canals, the pumping out of water from the lagoon during the 70s by Porto Marghera. All of these have contributed to make things much worse.
Talking about the maintenance of the canals, did you know that in Venice there is no sewage system? Or better, there is not a conventional sewage system.
So, you are asking yourself, how do they get rid of their number ones and number twos??
Simple, the water does that! OK, let me explain it to you.
We must make a distinction first between public buildings (museums, hotels, restaurants, shops, government buildings etc.) and private houses.
The first ones are obliged to install a septic tank where all of the sewage flows in and which are emptied every so often by specialised companies. But private houses, instead, are still using the original system invented by Venetian 500 years ago.
Venice special sewage system consists of a network of tunnels in the walls called gatoli. This network of collectors, whose origins date back to the sixteenth century, is used to collect soiled waters and rain waters, which then flow out into the waters in the canals of the city. Only in parts of more recent buildings – mainly those located on the edge of the physical city, like St. Helena, Giudecca, Santa Marta, Sacca Fisola, Murano – there are more modern type networks which go as well directly into the lagoon waters.
So, you will ask yourself, what happens to all the dirty water which flows in the canals?
Well, when the high tide comes, twice a day, it flushes out the dirty water away, making possible for the lagoon to clean itself. Obviously the heavy stuff goes to the bottom of the canals increasing the level of the canal and that is why during the Repubblica della Serenissima, the canals used to be completely emptied and cleaned up every 30 years. It was a natural septic tank with a natural depurator.
Back in the 17th century the Venetian system was the best sewage system in existence.
Unfortunately this is not so good now, since we tend to throw down our toilets all sort of detergents which are very armful to the living bacteria of the lagoon, killing the natural ecosystem which kept the sewage working and free from smells and diseases.
For a while the canals were not so well maintained as in the past, but in the last few years, thankfully, the canals are emptied out regularly and the sediments are taken to the Isola Delle Tresse (a damp). In the past the sediments were used to fertilise the agricultural fields of Venice, but now the sediments are toxic and cannot be used so they are disposed of on the little island.
During our walk, Luca showed us all the damages caused by the salt water and by the sinking of the city. He told us many people are studying the possible solutions, but no-one has come up with the definite one which can save the city.
We had a fantastic morning walking through all the little hidden calli, empty of tourists and locals.
Luca is a very knowledgeable person, with a real passion for Venice and its history. Time passed so quickly with him, we did not even noticed it was time for lunch! So after a nice tour of the local osterie until late afternoon, with aching feet and a big smile on our face, we called it a day and headed home, wandering if our grandchildren one day would still be able to do what we did: just walk in the most beautiful city in the world!
So, before the city sinks and crumbles, hurry up and come and visit it. You will not regret it!
The tour was organise by Context Travel and guided by Luca Zaggia.
For information you can visit Context travel website: http://www.contexttravel.com/city/Venice/walking_tour_details/Ecology_of_Venice0