By: Colin Kolbus
University of Washington Department of Civil and Environmental Engineering
September 18th, 2019
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).
Due to its location, the city of Venice is susceptible to flooding. It is seated in the middle of the Venice lagoon, protected from the Adriatic Sea by narrow strips of land. Venetians are no stranger to flooding, but high water 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 at an accelerating rate, measured from 1871 to 2014 (Camuffo, Bertolin, & Schenal 2017). During 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 prevent 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 with the largest being a set of mobile barriers installed at the mouth of the lagoon and other environmental reclamation projects. But, 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
The interesting hydro morphology or shape of the Venice lagoon is integral in the problem of high water. The entire 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 or groins 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. The loss of sediments is felt most in the “minor” islands and natural environments surrounding Venice because their natural shorelines that offer little protection from the battering waves (wave pollution). Slowly, the soil that makes-up the islands is washed out to sea.
The height of the city out of the water varies depending on the location and time of day. The one of the most shallow locations in all of Venice is near the Salute Church in the Piazza San Marco, which happens to be the highest trafficked area 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 which is the water’s location on the land (very important because this is the value that determines the frequency of flooding) and the absolute sea level which 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. To measure the accelerating increase of the relative sea level, Camuffo et al. measured the heights of water steps which are built into the foundation of buildings to help passengers’ step in and out of boats on the canal (Figure 4). This method allowed them to ascertain a value of +0.30 ± 0.04 mm year−1 century−1. In other words, it means that the sea level is rising at a faster rate every year (Camuffo et al. 2017).
It’s the structural damage from the increasing frequent high water events (Figure 5) 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 below 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. With the wave pollution from the advent of motorized boats and a rising sea level, the foundation is degrading. Traditional water-proof clay bricks won’t breakdown 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. 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 through the walls via capillary action. Nowadays, these stones lie well below the water line, leaving the seawater free to move up the walls causing severe structural damage (Insula Spa 2011). As the submerged 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 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 overall 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 when the city was inundated with 166 cm of water, the largest on record. The Italian government required that the defense system would “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). The barrier began construction in 2003 and is located at the Lido, Malamocco, and Chioggia inlets (Figure 3). 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 are cemented into the bed and then the gates are lowered into place. The watertight gate’s specific 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). The boat positions itself over the caisson, lowers legs to attach itself to the structure for stability, and the gates are lowered into place.
A protocol has been developed to deploy the gates. The Control Centre constantly monitors weather and identify storm surges 5 to 7 days in advance of use. On the day of, the gates are filled with compressed air which evacuates them of water and causes them to rise to the surface. The barriers act as a dam and prevent water from entering and 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 structure is ready, but there is skepticism on whether this will be upheld (Fletcher and Da Mosto 2004).
Once online, the barriers will require regular maintenance every 5 years and extraordinary maintenance every 15 years. Maintenance tasks include washing, painting, and fixing any damaged structural steel work (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.
3.2 Protecting the “Minor” Islands
Being smaller and more exposed, the minor islands in the Venice Lagoon 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 walkways were raised and reinforced to supplement the large project: the mobile barriers.
The 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 a jetty, the shore begins to lose sediment due to impeded transport. The MOSE coastal protection plan involved installing a breakwater to absorb wave energy before it hits the coast. Groins or jettie, 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 delicate ecosystems of salt marshes, which provide protection from high water, are vanishing. Salt marshes are aquatic environments that lie just above the relative sea level 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 relative sea 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.
4. Critiques of MOSE
Large-scale projects like MOSE are often met with opposition from 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 functioning. The first problem 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 this will only assuage the problem.
The second problem is 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 projected by the Intergovernmental Panel on Climate Change, the barriers would hypothetically require deployment 187 days a year, occasionally for weeks at a time (Del Bello 2018). Oxygen depletion in the stagnant lagoon water, anoxia, could cause a cascade of negative effects. Venice does not have municipal wastewater treatment with most waste settled in a primary treatment process before being discharged directly in the canals. While this system ‘works’ when the tide bringing fresh water into the lagoon twice daily, the waste remains in place during week long deployments. Large concentrations of wastewater may lead to eutrophication of the canals, large algae blooms that choke out other aquatic life. This would cause 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 barrier 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 that the barriers will be used appropriately (Yardley and Pianigiani 2014).
4.2 Coastal Nourishment
Coastal nourishment has the potential to restore eroding beaches; but its impact is reliant on how the beach is managed following intervention. To optimize nourishment yield, it may be necessary to follow 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). Resorts have a tendency to “clean” the beach of organics, but these materials are a necessary part of a health coastline. This leaves the beach bare and subject to further erosion. Behavior by property owners on the shore needs to change in order for the intervention to make any long term impact. Eventually, the sand will be washed away and require further replenishment.
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. One minor critic with the artificial salt marshes is that the remain distinct from the natural ones. Many of them are made of steep edges and are biologically simpler. The harsh edges of the new marshes prevents the development of mudflats and shallows. Different species thrive in the new system, while others decline in population (Fletcher and Da Mosto 2004). An intervention that is unable to return the ecosystem fully to its former stasis.
5. The Future
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. It is not a final solution, but it has bought the city some time. With the growing threat of global climate change, future solutions must continue to work with nature and not against it.
6. Works Cited
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