Table of Contents
The goal of this page is to discover the depth of the Venice system, by realizing how one of the most rich ecosystems in the world is now tightly interwoven with over a thousand years of mankind’s engineering. Venice is a World Heritage Site, it is a phenomenal feat of mankind and one of the hottest tourist destinations in the world where romantics and families alike contribute to the some 20 million that visit each year. However, Venice has a much deeper meaning to the environmentalists, politicians, and engineers who are invested in the well-being and longevity of the region. The city is subjected to damage from daily interaction with the tides, and because of the earths changing climate Venice will become increasingly more vulnerable to a devastating storm. The lesser known interest in Venice is the plethora of unique biodiversity that makes up the largest wetland in the Mediterranean(SOURCE: MOSE website). Human intervention in the lagoon has caused the ecosystem to be forever transformed, and the survival of the city comes at the cost of damaging the lagoon. Thus, the fate of the historic city and the ecosystem have been intertwined, and to preserve both against the changing environment of the world is a challenge that will be faced until the day Venice dies.
II. Background Information
The Allure of Venice
To thoroughly understand the dilemmas of today, one must first understand the origins of the region. Venice began when peasants sought safety from barbarians, and the lagoon provided a defense that enabled them to survive. The Venetians were able to utilize the lagoon to fish, transport food, and rid of waste all while being protected. The benefits of the lagoon allowed the area to thrive, and eventually became an incredible naval and commercial force. As can be seen with the map below, the unique setting with access to rivers inland and the Adriatic Sea contributed to the successful economic and valuable history of Venice.
Like many powerful historic cities, Venice went through ups and downs due to change in leadership and war. These variations in prosperity have contributed to the incredible cultural history found in Venice. Music, art, and interior design are some of the aspects that make up the culture, and this is what draws people to Venice today. However, the most important factor that attracts people to Venice today is the engineering and architecture that define the city. The style of architecture is called Venetian Gothic, and the way the beautiful architecture lines the canals is what makes Venice so irresistible, as can be seen from figure 1.
|Figure 1: View from Bell Tower in St. Mark’s Square.|
The battle between the terrestrial and aquatic systems began the moment that the Venetians began settling in the lagoon, and repeated adaptation has been required in order to continue human’s home in the lagoon. The first impact that man had on the lagoon was the wooden piles that were driven into the subsoil of the lagoon. These piles are the foundation for the city, and the reason why buildings can exist in a lagoon. Millions of wooden piles are used to support the city(Fletcher, 2004), all of which of course needed to be transported from the mainland by water since there were not trees to make pilings out of in the lagoon. The next modification to the Lagoon took place in the 15th and 16th century, when seven rivers were diverted to the north and south of the lagoon. These rivers were transporting too much sediment into the lagoon, which was pushing the lagoon to become apart of the land. The watery environment is what defined and enabled the strength of Venice, so in order to continue they chose to divert these major rivers. For the next few centuries they continuously altered the diversions as sediment gathered in different regions. In 1791 it was decided that there was need to define the boundary of the lagoon, in order to safely control and successfully maintain the region. They piled stones to create the wall, which is constantly under restoration to maintain itself. In the mid 19th century and early 20th three jetties were implemented to “accentuate the tidal current” (Fletcher, 2004). With the development of industry, some major changes to the lagoon bed to deepen channels were also made to allow tankers and cruise ships to enter the lagoon. Along side the engineering changes to the lagoon, the governing bodies have implemented policies to regulate the balance of the ecosystem. An engineer by the name of Bernardo Trevisan contributed to the adopting of these policies. In 1715 he published a book titled Della Laguna di Venezia, Trattato, this book outlined concerns with the lagoon, including the excessive sedimentation from the canals. Figure 2 is a depiction of the opposition between aquatic and terrestrial elements of the lagoon from Trevisan’s book.
|Figure 2: Della Laguna di Venezia, Trattato, 1715|
III. Understanding the Lagoon
The lagoon can be broken down into four distinctive habitats, all of which hold value to the environment and the city. Visually the lagoon changes based on time of day and time of year because of the different levels of the water and what is below or above the water level. The following breakdown of the lagoon will give an insight to the role of each of the habitats.
The easiest ecosystem to point out is clearly the areas that are always underwater. The average depth of the lagoon is approximately 1-3 meter(Fletcher, 2004), but it varies across the navigation channels, open water, and the man made navigation channels. The natural channels are what the locals and small boats use as passageway, and are marked by the wooden pilings throughout the lagoon. The man made navigation channels can be 20 meters deep(Fletcher, 2004) with the purpose of enabling industrial ships and cruise ships.
The bed of the lagoon is made up mostly of seagrass, which is different from the more commonly known sea weed. Sea grass is pertinent to supporting many different organisms by providing food and shelter, fish also use the bed as a nursery for reproduction. The sea grass helps stabilize sediment which is vital to a lagoon since the interaction with the sea and rivers causes incredible sediment transformations (Seagrass, 2015). Due to the importance to the ecosystem, the sea grass is mapped through time and the downward trend of the system can be followed. Data is recorded using a Geographical Information system (GIS) system, which is commonly used by engineers to spatially map and analyze geographical data.
The mudflat areas are a great example of the misunderstood importance of aspects of the lagoon. The mudflats are only exposed at low tides, and appear to be muddy and bare with minor channels running through, but are the home to an array of invertebrate life like worms and clams, and migratory birds use the mudflats as both feeding and breeding grounds. Figure 3 shows an aerial view of the mudflats
|Figure 3: Mudflats|
The mudflats also provide a a defense to the inner parts of the lagoon from storm surges, which aids the life of the inner ecosystem of the lagoon. Mudflats even have economical contributions because of the clam farming that takes place there. An organization called Ecology and Society is dedicated to the “science of resilience and sustainability”, and has an article on the sustainability of clam farming in Venice that can be found here. This article is a great example for the depth of people that are invested in the well being of the Venice lagoon and proof of the importance of the mudflats.
The saltmarshes are the regions that are only covered by water at high tides. This habitat supports salt tolerant plants that should allow for a strong and diverse wildlife to exist. They are another region of the lagoon that supports unique wildlife. Figure 4 shows the salt marsh from the view point of a boat in the canal.
|Figure 4: Saltmarsh|
The survival of a saltmarsh is dependent on the difference between the sediment accumulation and the sea level. This balance has been challenged by mans intervention, and therefor has made these saltmarshes fragile. Rising sea level is also challenging the life span of the saltmarshes, if the vertical accretion can not occur at the rate of sea level rise, they will be buried underwater and eventually washed away and irrelevant. High wave energy is another important factor in the existence of the saltmarsh, since stronger waves will cause the saltmarshes to subside more quickly.
The island habitats generally are not submerged at any point during high and low tide. However, the islands submerging is of course becoming more prevalent with the rising sea level and is the topic of major concern for the survival of Venice. The city of Venice is made up of 118 major and minor islands (MOSE Venezia, 2015). The main city of Venice and Murano are the well known and dense tourist locations, but there are many other islands that are the home to locals and a source of agriculture. Vegetable and fruit production on these islands is vital to the local economy, some of their most important exports being artichoke, asparagus, apricot and persimmon(VeniceExport, 2015). The sea walls are also considered islands, these sea walls separate the lagoon from the sea and are the main line of defense against the hydrodynamic forces of the sea.
Relationship with the Sea and Terrestrial Systems
6,000 Years ago, the Lagoon was dry, and a part of the mainland. When the ice age ended the large flow of water and settlement gave birth to the lagoon. The natural path of any lagoon in the world is to eventually become land, or sea. Simply put, the lagoon can exist because of the balance of material inflow from the rivers and material washed away by the sea, the balance is not perfect and one should eventually outdo the other. Mans intervention is a hard fought battle against nature to preserve the lagoon system in the way that everyone wants.
Every day the water levels have a high and low tide because of the gravitational forces from the Sun, Moon, and the rotation of the Earth. You can see the tide forecast times for Venice here. Tidal information must be known for navigating the channel, especially now days with the large industrial and commercial cruise ships that regularly enter the lagoon. Venetians originally utilized the tide as a way to rid of trash because the trash could be washed away during low tide, but even today some waste is pumped in the canals and washed away with the tide.
One of the main reasons that Venice is such a unique ecosystem is because of the mixture of salt and fresh water. Aquatic animals often require one or the other, and there is only a small percentage of the world where there is a mixture. This means there is opportunity for special biodiversity to exist here. Among the special biodiversity are Euryhaline fish, who spend their juvenile stages in the lagoon and return to the sea once they have developed(Musu, 2012). They do this because the lagoon offers a feeding resource that allows them to safely grow up.
IV. The Foundation of Venice
The foundation of Venice is made up of millions of wooden piles that have been driven into the muddy bed of the lagoon. The bed of the Lagoon is made up of a system of sands, silts, and silty clay’s that have been “chaotically accumulated” from the relationship with the rivers and the sea throughout its existence.(Simonini, 2014). The piles are driven down approximately 10 feet, into a more firm clay that lies beneath the surface. Wood is not nearly as strong in compressive strength as steel or concrete, it also rots when it is exposed to water and air because it creates a home for fungus that destroys the wood.The special circumstance that allows wood to be a incredible material choice, is that the wood is fully submerged, and not exposed to oxygen. Where other materials would deteriorate, the wood is able to maintain its strength. Another special circumstance is the presence of salt water in the lagoon, it petrifies the wood making it stronger than normal wood. The video below gives an excellent visual of how they built on top of the lagoon, by using the very popular Rialto bridge as an example:
When they began they didn’t have the engineering knowledge of today, and put as many piles as they could to ensure safety of design. Like most of Roman engineering, it was based on empirical knowledge. Today the normal, shear, moment, and torsional load details are required to successfully design a foundation. A single pile load capacity is governed by the friction along the sides of the pile, and the end bearing capacity. Thankfully the wood pile foundation has proven strong enough to support the loads put on it by the beautiful buildings and inhabitants of the city.
On top of the wooden piles they laid down wooden cross beams. These cross beams dispersed the weight of the buildings, ensuring that the load path didn’t overwhelm certain portions of the foundation. The next material was water resistant marble, this marble completed the water resistant foundation that has lasted years. The problem is that the rest of the building materials used were not designed to be repeatedly soaked, and are deteriorating with the years of increasingly harsh interaction with the water.
Beginning in the 1920’s water was pumped from deep natural underground reservoirs in order to handle the large volume of water required to support the increase in industry. This proved to be a destructive solution to the demands for water because the reservoirs were a buoyant force acting on the land, and supporting the foundation. It wasn’t until 1970 that it was discovered to be contributing to the sinking of Venice. Now that ground water is no longer being extracted, Venice is barely settling compared to the danger imposed by the rising sea level. Today you can see wells all over the city that have been covered up, some are re purposed as a drinking water fountain but from a different source. Figure 5 shows one of those wells.
|Figure 5: Capped off well|
V. The Dangers Facing Venice
Rising Sea Level
In the beginning, the Venetians were able to alter the lagoon and form it to match the needs of the city, but with the increase of time and intervention the impact has become so great that the environment is changing faster than can be kept up with. The number one risk facing Venice is the rising sea level.
The Intergovernmental Panel on Climate Change (IPCC) projects the future sea level rise due to global warming. The warming of the earth is contributing to rise in sea level because when the ocean waters are warmed, the water will expand, just like any other material or substance that expands when heated. Figures 6 and 7 show the history and projected sea level rise from the 1800’s – 2100 by the IPCC.
|Figure 6: Sea level rise in the 100 years|
|Figure 7: Projected sea level rise|
The high water level is damaging materials in the city that aren’t designed for exposure to water. Many of the first buildings used brick on the exterior, because of its affordability and lightweight properties for construction. Now that the brick is exposed to the lagoon waters due to high tide and boat traffic, the salt is settling inside and destroying the brick from within. On top of the damage from the salt, during winter when the tides are highest, the bricks undergo a freezing and thawing routine, which is another highly damaging process to the brick.
An article posted in the Journal of Coastal Research studied the geometry of a saltmarsh, analyzing aspects like widening of tidal creeks, vertical accretion, and short term sediment deposition. They found that the saltmarsh is deteriorating due to to hydrodynamic energy, which is expected. However, they also discovered that with the rise in sea level, there is an increase in wave energy, this means that the deterioration of the saltmarshes will become more and more rapid.
High water has become quite common, the video below shows a typical afternoon where the lagoon is seeping up through the drains in St. Mark’s Square during high tide. St. Mark’s Basilica adapts to this by setting up wood structures to allow visitors to cross over the puddles that are accumulating in the square as can be seen in figure 8.
The Storm of 1966
High water isn’t always so calm; on November 4th, 1966 Italy survived an incredible storm that defined the risks facing Venice. The storm created a cyclone in the western Mediterranean, which led to a devastating storm surge in Venice and a 22 hour flood event. 90% of the city was flooded, with the sea level reaching 1.94 meters above normal. There were no deaths in Venice, however in the surrounding parts of North-Eastern Italy like Florence, there was 118 deaths. The damage done by this event undoubtedly had shortened the life span of Venice, and reminded the world how valuable the history there is. This event has been the core example for why Venice is at risk, and kick started the conversation and the prioritization for a protection system. The United Nations Educational, Scientific, and Cultural Organization (UNESCO) took special interest in the well-being after this event, and by 1987 Venice was added to the list of World Heritage Sites. After much debate, the solution for venice decided upon was to engineer a way to block off the sea during times of high sea level.
VI. Goals of The MOSE Project
After the 1966 storm, there was strong need for a solution. The solution decided upon was the MOSE Project. The title comes the Italian acronym, the Modulo Sperimentale Elettromeccanico, which translates to Experimental Electromechanical Module, and a word play connecting it to the biblical character Moses and the parting of the Red Sea. This project is supposed to be the answer to the problems facing Venice. The main idea is to raise flood gates at time of high tide, to separate the lagoon from the sea and prevent flooding. It is meant to be flexible, enabling the natural processes of the lagoon but being able to control tides when necessary. The following videos from the MOSE Project website do a great job of defining the project and its goals for defending the city as well as the environment.
In resting position, the gates are filled with water, and since the density of the water inside the gate and outside is the same, it rests at the bottom. When the gates need to rise, they pump air into gate from compressed tanks which in turn forces the water out. The difference in density in the gate now causes an upward force that lifts the gate towards the surface. It takes about 30 minutes to raise the gates, and 15 minutes to lower the gates. This relatively short time to open and close allows for some variability on the operations of the system. The Arsenal is the control center for the MOSE system, and is located in the eastern part of the city of Venice re purposing an old naval ship yard. This is where the tide levels are monitored and the decision to activate the system is triggered. The Arsenal is currently housing some specifically designed engineering to support the MOSE project. Because the gates are so massive they need equipment to transfer the gates from land onto the barge that places them into the water. More intricate than the transfer equipment is the barge required to insert and service the gates. The barge is specifically designed so that it can adjust and handle both sizes of gates so that it can be utilized across the whole system. Figure 11 shows the smaller size of the two gates sizes. To place and remove the gates is a difficult process that is limited by the intensity of the sea. The system requires calm waters so that the barge can drop the gate down into place with high precision. Each mobile flood gate must be serviced every 5 years, so even after the it is in place there will be a strong commitment to maintaining the system. Figure 11 gives you a scale of how big one of the gates is.
|Figure 11: Gate awaiting Installation at the Arsenal.|
While the MOSE Project is an incredible engineering feat, it still is posing some very controversial issues. In my opinion the most significant being the short design life of the MOSE Project. Initially, a 100 year design life may seem like a strong system, but when you realize Venice began almost 1,600 years ago it means the project is only increasing the life-span by 6% and another multi-billion dollar project will be needed to replace it then. So this project is essentially a 7 billion dollar large scaled stop-gap project. Economically, the flood gate barriers will slow down the boat traffic to Venice. This will disrupt trade tourism and public services by creating a queue of ships waiting to enter and exit through the system. A report on the economic impact of the MOSE Project by Vergano breaks it into two categories, direct and indirect costs. The direct costs being mooring and chartering costs that are extra to wait to enter and exit. The indirect costs are the potential losses due to the slowed traffic, and Vergano believes that the reduction of traffic will make Venice a less competitive port.
The local perspective is a very important aspect to understand too. There is little left of Venetian culture that hasn’t been taken over by tourism, and the input of the true inhabitant of the lagoon can often be over looked. In my recent adventure to Venice, I met Tommaso who is a Venetian born and raised, and gives tours of the Venetian lagoon via a traditional Venetian boat. He believes that the damage of flooding isn’t as bad as it seems, and that only people on the first level are impacted, and even then “All you have to do is pick up your furniture.. and open a bottle of wine”. On top of the extreme cost of the system, there has been some political controversy, including the arrest of the Mayor of Venice and 30 other public officials. The prosecutors are accusing the mayor of diverting public funds for bribes and illegal donations. Often when the governing authority is challenged, it contributes to a strong doubt in other areas of the project. Generally, the biggest opposition against the engineers developing the MOSE Project are environmentalists. Gates closing will interrupt natural process in the lagoon impacting water quality and the natural processes of the ecosystem. Potentially harmful algae blooms could return and the stagnate water will make the water quality worse that it already is.
VIII. Analysis and Conclusion
There are many different parties interested in well being of the Venice and the lagoon. It is a world heritage site, and is supported by government resources, and public institutions alike. With all of the complicated variables surrounding Venice, there is one component that will forever be the same. That is the undying fight for its survival and health. The engineers search for a dependable protective system that still promotes the rich ecosystem of the lagoon will continue, but to do so requires intensive research, and time. The Lagoon is one of the most studied ecosystems in the world, and serves as an example for similar environments. Discoveries in the Venice system could lead to ground breaking applications in the rest of the world.
Although there is much controversy surrounding the current protection system, anything is better than no protection. The lagoon still requires much research and understanding, the MOSE Project can prolong the life of the system and allow time for a better engineering system to be developed that can handle aspects of the lagoon that aren’t understood right now. This challenge within the boundary of the lagoon system will undoubtedly lead engineers to creative ground breaking work that hopefully can make Venice feel safe again.
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