By: Colin Kolbus
In the northern part of Venice sits an unassuming bookstore aptly named Libreria Acqua Alta. Walking inside reveals isles crammed with books piled high, resembling the tight alleyways that brings customers to the store in mention. But the eclectic collection is not what makes this store unique, it is the “shelves” that hold them up: gondolas, boats, and bathtubs (Figure 1).
The city of Venice is seated in the middle of the Venice lagoon, protected from the Adriatic Sea by narrow strips of land. Due to its location, it is susceptible to flooding. Venetians are no stranger to flooding, but over the past 50 years the events have become exacerbated due to a rising sea level caused by global climate change. The relative sea level in the lagoon has been increasing 2.5 mm per year, measured from 1871 to 2014, at an accelerating rate (Camuffo, Bertolin, & Schenal 2017). During these high-water events, the streets flood and become difficult to navigate. Residents have vacated the first-floor apartments to move to higher ground for the risk of property damage. Businesses invest in large door jams to keep the water from getting in (Figure 2). Libreria Acqua Alta took it one step further and keeps their merchandise out of harms way in watertight vessels. The store’s name translates directly to “High Water Library”.
A protection project titled MOSE has the lofty goal of protecting Venice and its lagoon from the symptoms of a rising sea-level. The MOSE project is composed of several parts: the largest being a set of mobile barriers installed at the mouth of the lagoon and other environmental reclamation projects. Will this project be sufficient in protecting the city from an uncertain future shaped by climate change?
This essay will chronicle the MOSE project and each component function in protecting the lagoon. I will then follow with a critique of the project, specifically how it deals with the problem of a changing climate.
2. The Problem facing Venice
Venice lagoon has a surface are of 550 km^2, of which nearly 400 km^2 is water (Day, Rismondo, Scarton, Are, & Cecconi 1998). Historically, three rivers discharged into the lagoon, but nowadays only the smallest, the Dese River remains. The other two now directly discharge into the Adriatic Sea. The Dese carries freshwater into the increasingly saline lagoon, as well as high concentrations of agricultural runoff (Figure 3). Large jetties built at each inlet of the lagoon significantly reduce the import of marine sediment. This change causes the lagoon to lose 1.1×10^6 m^3 of sediment per year (Bettini et al. 1995), accelerated by the increasing presences of motorboats. This is felt most in the “minor” islands and natural environments surrounding Venice because these areas have natural shorelines that offer little protection from the battering waves (wave pollution). Slowly, the soil that makes up the island is washed out to sea.
The height of the city varies depending on the location and time of day. The one of the most shallow locations is near the Salute Church in the Piazza San Marco, which also happens to be the highest traffic areas in the city (Turismo Venezia 2019). The lagoon oscillates between two low tides and two high tides a day, semidiurnal. The mean sea level has steadily increased over time with an acceleration in the last 50 years. There are two possible measurements of sea-level: relative sea-level is the water’s location on the land, one of the most important to gather because this is the value that determines the frequency of flooding, and the absolute sea level is the level of the lagoon measured off the based zero used by ocean hydrologists everywhere. Relative sea level is composed of the absolute sea level and the vertical land movement, the water with the land height must be measured in tandem to obtain a useful value. Camuffo et al. measured the heights of water steps (Figure 4), a step placed right at the water’s edge to help passengers’ step in and out of boats. The steps are rarely moved and date back to the age of when the building was built. This allowed them to ascertain a value of +0.30 ± 0.04 mm year−1 century−1. This means that the sea level is rising at a faster rate every year (Camuffo et al. 2017).
An intervention needed to happen or the high water events would make Venice uninhabitable. 2011 hosted 18 high tide events, the most on record (Figure 5). The water slowly comes in with the high tide and only lasts 3-4 hours.
It’s the damage from these events that makes them the largest threat to the Venetian way of life. Venice was built on a system of wooden piles driven into the clay below. Due to technology at the time, the piles do not interact with bed-rock and the city remains in place from friction between the sediment and the piles, an indirect foundation. When the foundation of a building meets the water in the canal, a large water-proof brick wall is built to encase the sediment and keep it in place. This technology worked for hundreds of years, but with the wave pollution from motorized boats and a rising sea level, the infrastructure is degrading. Traditional water-proof clay bricks won’t degrade underwater since they are always submerged. However, they will fail in a dry-wet environment that occurs with the rising and falling of the tide. Bricks are made of porous clay that can absorb saltwater with capillary action. Once the brick dries out, such as after a high-water event, the salt recrystallizes and expands within the brick, rendering it useless (Figure 6).
A waterproof Istrian stone was laid within the foundation to prevent water from raising up the wall of the buildings there. Nowadays, these stones lie well below the water line, leaving the water free to make its way up brick walls causing severe structural damage (Insula Spa 2011). As the brick fails, running water can make it’s way into the sediment foundation. Slowly, the sediment below these buildings is washed out with the tide in a positive feedback loop as brick degrades and more sediment becomes exposed. When the sediment transports out of the foundation, the piles don’t have enough friction to support the force of the structure above it and it subsides into the clay to equilibrate the forces. Without intervention, the islands that compose Venice would slowly fail and become too dangerous to inhabit.
That is why the Italian government devised the MOSE project: to decrease the frequency of high-water events in the city of Venice through the development of barriers at the inlets of the lagoon.
MOSE stands for Modulo Sperimentale Elettromeccanico [Experimental Electromechanical Module] or more romantically an allusion to Moses parting the Red Sea to protect the Hebrews from the Egyptians and water. MOSE is a collection of interventions, from local to lagoon wide, to respond to high-water events. Consorzio Venezia Nuova (CVN), on behalf of the Venice Water Authority, is responsible from the implementation of MOSE (Figure 7). The largest part of the project, 60% of the budget, is devoted to implanting four mobile barriers at the three inlets to the lagoon. The project is designed to be a local, diffused, and large-scale engineering intervention. Local projects protect the exposed “minor” islands while diffused interventions involve natural environments around the lagoon. Other components include rebuilding and reinforcing the coast, securing polluted materials, and gathering data about the complex lagoon.
3.1 The Mobile Barriers
Installation of temporary dams at the inlet of the lagoon took years of planning and engineering. The idea was proposed in the 70s following the flood of 1966, the largest one on record where the city was inundated with 166 cm of water. The Italian government required that the defense system could “not significantly modify flushing between the sea and lagoon, create any visual impact or interfere with the landscape and local economic activities” (Consorzio Venezia Nuova 2019). These specifications will be revisited later with critiques on whether these have been achieved. This calls for something that can temporarily be deployed during flooding events and hidden away when it is not needed.
The final design is a series of 78 gates that lie at the bottom of the three inlets of the lagoon (Lewin and Scotti 1990) which began construction in 2003. Located at the Lido, Malamocco, and Chioggia inlets, the barriers would completely block off the lagoon from the ocean in a high-water event. The system is made of two parts the caisson and the gate (Figure 8). The caisson is a large 25,000-ton housing structure for the gates. Installation of the caissons in the seabed required extreme 1 cm precision which is challenging when the structure is around the same size as an apartment building (Enerpac 2016). The caissons get cemented into the bed and then the gates can be lowered into place. The watertight gates size depends on the inlet. Each gate has a pipe for the introduction and expulsion of compressed air, measurement devices, anti-corrosive anodes, and a complex set of hinges. The hinges allow the gate to move out of the caisson when they are filled with air causing it to rise to the surface (Consorzio Venezia Nuova 2019). They were designed to resist forces well in excess of what is capable from the underwater marine environment. To install and maintain the gates, a specially made barge called the “jack-up” is used (Figure 9). It positions itself over the caisson, lowers legs to attach itself to the structure for stability, and the gates are lowered into place.
To deploy the gates, the Control Centre constantly monitors weather and will identify storm surges 5 to 7 days in advance. 36 hours before the event, the decision-making process is launched, and series of deployment protocols is initiated. The gates are filled with compressed air, evacuating them of water causing them to rise to the surface. The barriers act as a dam and keep any water from entering or leaving the bay. Each inlet is equipped with a canal lock system to rise and lower boats into the lagoon during deployment so that economic activity can remain relatively constant. It is projected that the barriers will only be used around 5 times a year when the system is ready, but there is skepticism on whether this will be upheld (Fletcher and Da Mosto 2004).
The regular maintenance of the gates will occur every 5 years with extraordinary maintenance every 15 years. Maintenance tasks include washing, painting, and fixing any damaged structural steelwork (CVN 2019). A large problem with the mobile barrier system is the sediment transport out of the lagoon mentioned earlier. When they are deployed, sediment is going to continue to travel to the inlet but will be stopped and settle at the gates. The act of deploying and retracting the gates also transfers energy into the water that will move more sediment. It is unclear how the barriers will change the amount of sediment lost from the lagoon since they haven’t been fully deployed yet. Lagoons are complex environmental systems and calculating transport in the new configuration of the lagoon will need to be studied during deployment. It is known, however, that particles will collect in the caisson of the gates and so cleaning it out will also be apart of maintenance. How frequently this dredging will need to occur is unclear.
3.2 Protecting the “Minor” Islands
Many cities beside Venice lie within the Venice lagoon. Being smaller and more exposed, they can be more susceptible to flooding than the larger city. MOSE has managed many local interventions in these areas. In many places, the quayside (the pathway located directly on the open water) needed major work because that is the surface the absorbs the energy from the battering waves. The wave pollution in the lagoon is mainly caused by The walkways were raised and reinforced to supplement the large project: the mobile barriers. Piazza San Marco, which has been raised several times already, cannot be raised anymore without affecting the architectural style of the buildings around it. The MOSE project, in 2003, redid all the drainage in the square with the addition of one-way valves and underground rainwater basins (Fletcher and Da Mosto 2004). With the Piazza flooding to this day, local interventions are insufficient at addressing the problem for the time scale needed (Figure 10).
3.3 Rebuilding the Coast
The mobile barrier system relies on thin barrier islands to separate the lagoon from the ocean. These areas, like coasts all around the world, are experiencing high levels of coastal erosion due to a rising sea-level. In the case of Venice Lagoon, sediment transport moves sand on the coast in a northern direction with new sand continuously traveling up from the south. However, when humans build on coastal environments or build jetty, this transport is severed, and the littoral begins to lose sediment without a natural way to replenish it. MOSE coastal protection plan involved installing a breakwater to absorb wave energy before it hits the coast. Groins, long rocky outcrops, were placed along the coastline that serve as a catchment for sand and prevent it from moving further up the beach. Also, 10*10^6 m^3 of sand was dredged and used to replenish the shore (CVN 2019). This intervention is common procedure and has a high level of reliability. In some cases, native beach grasses were planted to increase the probability of sediment retention. However, beach nourishment can be ineffective in some areas due to coastal morphology. In these areas, the money is better off focused somewhere else (Bezzi, Fontolan, Nordstrom, Carrer, and Jackson 2009).
3.4 Restoring Natural Habitat
Consorzio Venezia Nuova has been performing lagoon habitat restoration since the 1990s. This diffusive intervention means that areas over the entire lagoon are receiving improvement. All this work is to combat the erosion altering lagoon morphology. The lagoon has been deepening and slowly turning into a marine bay because of the large volumes of sediment lost daily. The delicate ecosystems of salt marshes, which provide protection from high water, are vanishing. Salt marshes are aquatic environments that lie just above the mean water line and are home to most of the biodiversity in the Venice lagoon. MOSE has stepped in and reconstructed 1.65*10^7 m^2 of salt marshes and mudflats (CVN 2019). To reconstruct these areas, edge reinforcement is used to create a containment area for the new sediment. Sediment is then dredged into the marsh area filling above the mean water level to accommodate for compaction (Day et al. 1998). Pioneer species colonize the artificial salt marsh within the first year, with ‘complete’ integration in 10 years. Sea walls and fences are also installed in channels that experience large amounts of wave motion, to further prevent erosion. This like many other parts of the project are met with criticism from scientists and residents alike.
4. Critiques of MOSE
With large-scale projects like MOSE, decisions are going to be met with opposition form the public. I will discuss several large critiques returning to the specifications made by the Italian government: the system could “not significantly modify flushing between the sea and lagoon, create any visual impact or interfere with the landscape and local economic activities” (CVN 2019). The mobile barrier system cost €6 billion of the total €9 billion MOSE budget. An argument could be made that the money devoted to the mobile barrier system would be better off spent elsewhere. However, in the case of the Venice lagoon, intervention needed to occur at the local, diffused, and large-scale engineering levels so it was a necessary cost. Without it, Venice wouldn’t last long enough to consider next steps of action.
4.1 The Barrier
The mobile barrier system has a few problems that will need to be addressed in the coming years once the system is on line. The first being the large amounts of sediment that is going to settle in the caissons. Dredging the sediment is on the list of maintenance chores, but will this be enough? A majority of sediment transport occurs during extreme flow events, which in most cases is when the barriers will be deployed.
Many scientists are concerned about the frequency and duration of barrier deployment (Figure 11). With the limits currently set by CVN, the wall will only be deployed 5 times a year. In the future, with a 50 cm increase in global sea level predicted by the Intergovernmental Panel on Climate Change, the barriers would be deployed for 187 days a year, occasionally for weeks at a time (Del Bello 2018). Oxygen depletion in the stagnant water, anoxia, could cause an array of negative effects on the lagoon. Venice does not have any municipal wastewater treatment and most waste goes through a primary treatment process at the building before being discharged directly in the canals. While this system ‘works’ with the tide bringing fresh water into the lagoon twice daily, when the gates are closed for weekends on end the waste remains in place. Human effluent is high in nitrogen and phosphorous. Large concentrations of it may lead to eutrophication of the canal, large algae blooms that choke out other aquatic life. There would be a loss of biodiversity, which could nullify the effects of the habitat restoration mentioned earlier. If this scenario is achieved, it would violate the specifications defined at the start of the project: flushing between the sea and lagoon would be significantly altered. On the other hand, studies are underway to use the barriers to induce a current within the lagoon to avoid anoxic summer conditions (CVN 2019).
There is also concern about the effectiveness of the Venice government at running the system. CVN will only realize the project and run it for two years before it is passed off to be used by the municipality. With a history of extortion and money-laundering through MOSE leading to the resignation of Venetian Mayor Giorgio Orsoni, the public remains apprehensive at the governments ability to use the barriers appropriately (Yardley and Pianigiani 2014).
4.2 Coastal Nourishment
Coasts around the world are experiencing the same problem as the barrier islands: the beach is disappearing. Nourishment has the potential to work, but this is reliant on how the beach is managed following intervention. It increases coastal protection but rarely returns the dune morphology to what it was before. To optimize nourishment yield, it may be necessary to follow up with re-vegetation or fence installation. Wrack (beached organic matter) removal and beach raking was witnessed on the Venice coast following the intervention (Bezzi et al. 2009). This is in part due to the large tourist population in the area and resorts tendency to “clean” beach property of organics, a necessary part of a health coastline. Behavior that leaves the beach bare and subject to further erosion.
4.3 Salt Marsh Restoration
The diffused intervention in the marshes are perhaps the most beneficial to the lagoon ecosystem. The salt marshes are the best natural defense against flooding with the ability to retain large amounts of water. Artificial salt marshes, physically and biologically simpler, do remain distinct from natural ones. Many of them are made of steep edges, sometimes reinforced with walls, that prevent the development of mudflats and shallows that offer another form of environment for different species to inhabit (Fletcher and Da Mosto 2004). Different grasses and birds thrive in the new system, while others decline in population.
The larger problem is that the lagoon ecosystem is temporary. With every perturbation, the system reacts to maintain equilibrium and slowly evolves into something different. The natural environment wants to change but the human environment is relatively unchanging. The transport of water and sediment needs to be controlled in order to control the future of Venice (Muraca 1982). MOSE project may be a losing battle against the elements, but a necessary one for the future of the city.
5. The Future
The MOSE project has bought Venice time; and with a life span of 100 years, it is not a final solution. Looking forward, we must ask “Who is the future of Venice for?” Is it for Venetians, nature, or the world? As tourism overtakes the city and the Venetian pollution dwindles, the city feels further from the traditional Venice that captured the world to begin with. The natural environments of the lagoon tend to go unnoticed by the common visitor, but those areas are crucial to the success of Venice. UNESCO gave the title of World Heritage Site to “Venice and its Lagoon”, the future of the city is bound to the future of the lagoon.
All the world has a stake in the future of Venice. “Venice symbolizes the people’s victorious struggle against the elements as they managed to master a hostile nature” (UNESCO 1987). The lagoon has challenged its inhabitants for hundreds of years and yet the city remains. MOSE is just another form of protection in modern times. The harmonious relationship between natural and built environments in the Venice lagoon is an example for the rest of the world. With the threat of global climate change at everyone’s doorstep, our solution must work with nature and not against it.
6. Works Cited
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