For a long time, Venice has been threatened and destroyed by sea tides and floods. Water has always been a concern by the Venetian. It requires care, substantial investments to avoid swamping as much as flooding. At the same time, however, water attracts and fascinates and, without it, Venice would lose its meaning.
A 2014 research conducted in collaboration with UNESCO in Venice identified the main wear and tear phenomena that compromise the stability of the system equilibrium and the survival of the urban fabric: the problem of high water, the wave motion from the wind and water traffic, pollution, deterioration of the building heritage and flooring, changes in intended use due to the progressive loss of resident population, changes in the structure of local trade, in addition to the growing anthropogenic pressure caused by tourist flows.
The conformation and the terrain on which Venice stands required the solution to various problems in the construction of buildings and in urban planning of the city. Geographically speaking, Venice is located in a place that is not suitable for the establishment of a city, which has caused the continuous worry of being submerged. However, for thousands of years, people have never given up their efforts to save the city from its fate of destruction. Through the diversion of the nearby river, the large-scale hydrological environment transformation of the lagoon, and the moisture-proof system created by modern science, this historical city can continue to exist.
The delicate balance of the lagoon, which is affected by the contribution of sediments and fresh water from rivers, by the invasion of sea water based on the tides and the wind, the Venetian carefully control the water regime over the centuries. Venice has been a master of this in the past by modeling the lagoon with hydraulic and environmental management interventions and finding a balance between the lagoon and the city. This equilibrium was broken in the course of the twentieth century due to human intervention leading to the aggravation of the phenomenon of high water.
In May 2003, the MOSE Project (Modulo Sperimentale Elettromeccanico) inaugurated, an experimental model for evaluating the performance of hollow floatable gates; the idea is to fix a series of 78 hollow pontoons to the sea bed across the three entrances to the lagoon. When tides are predicted to rise above 110 cm, the pontoons will be filled with air, causing them to float and block the incoming water from the Adriatic Sea. This engineering work was due to be completed by 2018.
Studies have shown that water waves caused by high-speed ships can also damage the foundations of Venetian buildings. Therefore, the Venice Grand Canal restricts the entry of private boats and sets an upper limit on the speed of navigation to reduce the damage caused by wave slaps. However, some people believe that such restrictions have never been strictly enforced under the huge tourist flow.
Geography and hydrological environment
Venice sits atop alluvial silt washed into the sea by the rivers flowing eastward from the alps across the Veneto plain, with the silt being stretched into long banks, or lidi, by the action of the current flowing around the head of the Adriatic Sea from east to west.
The main canals of the city are the Grand Canal and the Giudecca Canal, the second, disposing to the south of the city, separates the historic center proper from the Giudecca island and is heavily trafficked by large cargo and passenger ships that go to dock at the Maritime Station. In ancient times the most used streets were precisely those of water, which provided the main vision of the city.
In the beginning, the buildings in Venice were difficult to sustain because lacked a solid rock layer to support them. They were built directly on the sand and silt. The huge load-bearing and groundwater activities would cause the buildings to sink.
The early residents of Venice found a solution to keep their refuge on the sandy islands, they learned to build by driving closely spaced piles consisting of the trunks of alder trees, a wood noted for its water resistance, into the mud and sand, until they reached a much harder layer of compressed clay. Building foundations rested on plates of Istrian limestone placed on top of the piles.
This genius approach has a scientific explanation. In an anaerobic environment, the wooden piles driven into the clay layer are calcified by the surrounding environment, and gradually form strong piles. Fixed by a large number of wooden piles, the clay layer will also reduce loss due to reduced flow, forming a relatively stable foundation. In the underground of Venice, there are millions of such tree trunks supporting the entire city.
However, this approach can only delay but not prevent the downward trend of the Venetian land surface. Together with rising sea levels have caused frequent floods and tides that threaten Venice. The gradual lowering of the surface of Venice, has contributed—along with other factors—to the seasonal Acqua alta (“high water”) when much of the city’s surface is occasionally covered at high tide.
Between autumn and early spring, the city is often threatened by flood tides pushing in from the Adriatic. Six hundred years ago, Venetians protected themselves from land-based attacks by diverting all the major rivers flowing into the lagoon and thus preventing sediment from filling the area around the city. This created an ever-deeper lagoon environment.
During the 20th century, when many artesian wells were sunk into the periphery of the lagoon to draw water for local industry, Venice began to subside. It was realized that extraction of water from the aquifer was the cause. The sinking has slowed markedly since artesian wells were banned in the 1960s. Studies indicate that the city continues sinking at a relatively slow rate of 1–2 mm per annum; therefore, the state of alert has not been revoked.
The acqua alta, which indicates the phenomenon of particularly pronounced tidal peaks that occur periodically in the northern Adriatic and with particular intensity in the Venice lagoon such as to cause flooding in the urban areas of Venice and Chioggia and, much more rarely, of Grado and Trieste.
The phenomenon occurs mainly between autumn and spring, when the astronomical tides are reinforced by the prevailing seasonal winds that hamper the usual reflux. The main winds involved are the sirocco, which blows northbound along the Adriatic Sea, and the bora, which has a specific local effect due to the shape and location of the Venetian lagoon.
The particular shape of the Venetian lagoon, the subsidence which has been affecting the soil in the coastal area, and the peculiar urban configuration all magnify the impact of the high waters on city dwellers and on the buildings.
Furthermore, the northbound winds called bora and sirocco often blow directly towards the harbors that connect the lagoon to the Adriatic Sea, significantly slowing down (and, at times, completing blocking) the outflow of water from the lagoon toward the sea. When this occurs, the ebb is prevented inside the lagoon, so that the following high tide overlaps with the previous one, in a perverse self-supporting cycle.
The creation of the industrial area of Porto Marghera, which lies immediately behind Venice, amplified the effects of high waters for two reasons: first, the land upon which the area is built was created by filling large parts of the lagoon where smaller islands just above sea level previously lay. These islands, called barene, acted as natural sponges (or “expansion tanks”) when high tides occurred, absorbing a significant portion of the excess water.
Second, a navigable channel was carved through the lagoon to allow oil tankers to reach the piers. This “Oil Channel” physically linked the sea to the coastal line, running through the harbor in Malamocco and crossing the lagoon for its entire width. This direct connection to the sea, which was obviously non-existent at the time of Venice’s foundation, has subjected the city to more severe high tides.
The MOSE project
MOSE (Experimental Electromechanical Module) is a project intended to protect the city of Venice, Italy, and the Venetian Lagoon from flooding. The project is an integrated system consisting of rows of mobile gates installed at the Lido, Malamocco, and Chioggia inlets that are able to isolate the Venetian Lagoon temporarily from the Adriatic Sea during acqua alta high tides. Together with other measures, such as coastal reinforcement, the raising of quaysides, and the paving and improvement of the lagoon.
MOSE is designed to protect Venice and the lagoon from tides of up to 3 metres. On 3 October 2020, the MOSE was activated for the first time in the occurrence of a high tide event, preventing some of the low-lying parts of the city (in particular piazza San Marco) from being flooded.
The aim of the MOSE project is to protect the lagoon, its cities, inhabitants and the priceless historical, artistic and environmental heritage from all high waters. High waters have become increasingly frequent and intense due to the combined effect of subsidence and eustatism, due to natural and anthropogenic phenomena.
Furthermore, there is always the risk for the entire lagoon area of an extreme and catastrophic event such as that of November 4, 1966, when an exceptional tide of 194 cm submerged Venice, Chioggia and the other inhabited centers. In the future, the phenomenon of high waters could worsen due to the expected rise in sea level as an effect of climate change. In this context, the MOSE, together with the reinforcement of the coastal strip, was designed to protect against tides up to three meters, and will therefore be able to ensure effective protection of the lagoon even if the most pessimistic hypotheses were to be verified.
MOSE consists of rows of mobile gates at the three inlets, which temporarily separate the lagoon from the sea in the event of a high tide. There will be 78 gates divided into four barriers. The gates consist of metal box-type structures 20 metres wide for all rows, with a length varying between 18.5 and 29 metres and from 3.6 to 5 metres thick, connected to the concrete housing structures with hinges, the technological heart of the system, which constrain the gates to the housing structures and allow them to move.
Under normal tidal conditions, the gates are full of water and rest in their housing structures. When a high tide is forecast, compressed air is introduced into the gates to empty them of water, causing them to rotate around the axis of the hinges and rise up until they emerge above the water to stop the tide from entering the lagoon. When the tide drops, the gates are filled with water again and return to their housing.
Operating procedure dictates that the gates will be raised for tides of more than 110 centimetres high. The authorities have established this as the optimum height with respect to current sea levels, but the gates can be operated for any level of tide. The MOSE system is also flexible and depending on the winds, atmospheric pressure and level of tide, it can oppose the high water in different ways with simultaneous closure of all three inlets in the case of exceptional tides, by closing just one inlet at a time, or by partially closing each inlet for medium-high tides.
To guarantee navigation and avoid interruption of activities in the Port of Venice, when the mobile barriers are in operation, a main lock is under construction at the Malamocco inlet to allow the transit of large ships, while at the Lido and Chioggia inlets there will be smaller locks to allow emergency vessels, fishing boats and pleasure craft to shelter and transit.