|The twentieth century has definitely jeopardized Venice's capacity to "float" in the lagoon. The city has started to "take in" water more and more frequently and more and more intensely. The average level of the land has been lowered by 23 cm compared to the sea level. The tidal level has grown by some 8 cm because of the morphological modifications which have occured in the lagoon basin. In order to achieve the objective of the defence against high water the Venice Water Authority - Consorzio Venezia Nuova has proposed and developed an integrated system of interventions: on the one hand, local defence in order to protect the lowest urban centres and on the other hand works at lagoon inlets, which come into operation in the case of tides higher than 110 centimetres.|
|The measures of sea level in Venice are in reference to the marigraphic zero chart datum at Punta della Salute, which is an altimetric reference point integrated with the ground level of the historic centre that corresponds to the mean sea level in Venice in 1897. The marigram at Punta della Salute today refers to a level of -23 cm in respect to the current mean sea level. This altimetric loss is due to the effects of eustasy and subsidence.|
The historic centre of Venice and the entire lagoon have become more
and more exposed to flooding. Today, with a tidal level of + 60 cm on
the marigram of Punta della Salute, water begins to invade Piazza San
Marco, one of the lowest areas of the city. With a tide of + 110 cm
the 12% of the city is blocked with water and raised wooden walkways
must be placed along established pedestrian routes.
|At the beginning of the 1900, the average flooding events (based on 100 years) in the lowest parts of the historic centre were 90 events. Today they are 3900.|
|See the film of December 1st, 2008|
During the years, the occurrences of tides equal to or greater than 80 cm has increased. Since the beginning of the century until today, the average yearly occurrence of high waters that are equal to or greater than 80 cm has gone from 10 events in the first half of the century, to 40 events in the second half, reaching almost 60 events in the last five years.
The risk of extreme events. The most devastating flood that Venetians can remember was that of November 4, 1966. The tide reached a level of 194 cm on the marigram of Punta della Salute. On that day, the sea, pushed by a strong scirocco wind surmonted the littoral at Pellestrina and San Pietro in Volta and poured into the lagoon. Venice, like the other lagoon centres, was completely submerged in one metre of water. The damages were incalculable. The occurrence was a great event, in that it forced the awareness that the city would cease to exist without intervention.
|At the beginning of 1900, an event like that of 1966 had the possibility of occurring every 1,000 years (that is, it was pratically impossible). The same event, today, could occur every 140 years, and with a rise in sea level of 20 cm, every 40 years. Thus, extreme events are no longer remote possibilities, but mathematical certainties. The only indefinite variable is how they will present themselves. In fact, Venice is no longer above the intertidal belt, as it was when it was first built.|
The occurrence of the phenomena of high water is mainly connected to two factors which affect, on different time scales, sea level:
High water occurs for the most part during winter months (about 80% of the occurrences are between October and February), that is when the ciclonic perturbation and low pressure are more frequent, with an average duration of about 3 hours.
Astronomical tide and meteorological factors. The "astronomic tide" is the periodic rising and falling of the level of the sea water, due to the combined gravitational pulls of the sun and the moon. The level of the tide, determined by astronomical cycles, can also be influenced by particular meteorological events, such as variations in the intensity of the wind and atmospheric pressure. Dependant in part on these events in the phenomenon of the "sessa", a periodic oscillating in the Adriatic sea. In some cases, these phenomena provoke sudden changes that affect even the average sea levels.
Eustasy and subsidence. Because of the subsidence, the historic
centre of Venice has sunk since the beginning of the century, resulting
in an althimetric loss of 12 cm, while eustasy in the same period has
caused a rise in sea level of approximatly 11 cm.
|The subsidence, that is the lowering of the land level, is caused by two factors: one natural and one induced by man. The natural process is not constant in time and space: the average rate of natural lowering in the Venice area and its hinterland at the beginning of the century was about 0.4 mm per year. Overall, in this century natural subsidence has been 4 cm. The man-induced factor above all is caused by the exploiting of underground liquid resources for industrial uses, beginning in the 30's.|
The tapping of the underground water supply caused a reduction in pressure in the subsoil and, therefore, a contraction of the ground itself, with consequent lowering.
Eustasy, or the variation in sea level, is tied to changes in the world's climate. During cold periods, precipitation is withheld on the continents in the form of ice and consequently the level of the sea lowers. The opposite happens in hot periods.
The 10 highest tides recorded in Venice since 1920
In order to assess the future evolution of subsidence and eustasy the following three scenarios can be considered:
|It should be noted that the effects of the increased frequency of high water on architectural structures and buildings as well as on the lagoon environment are mostly long term. This means that the current damages were caused by the situation previous to the intensification of flooding (a situation that is anything but "normal"). The exact consequences of the high waters of the last decades will manifest themselves in all their seriousness only in the near future.|
|During high water many areas of the city are no longer reachable on foot (the most common way to get around in Venice) or they can be reached only with great difficulty, and, furthermore, the ground floors of many dwellings are flooded. This is not the only a problem that people living in the lagoon area must bear, but it is certainly one of the reasons why the city is abandoned and also one of the reasons why even the most normal maintenance measures must be carried out with greater frequency.|
|The waste water discharges for inhabited centres, planned and constructed centuries ago, once remained under the water of the lagoon only in stretch closest to the canals. Today they are totally underwater. The effect is twofold: on the one hand, it favours the dispersion of waste water in the foundation soils of the city, subjecting the piling to a new kind of aggression and, on the other hand, it accentuates the phenomena of fine sediment loss from foundation soils thereby creating the possibility of the ground giving way within the city. This situation is destined to get seriously worse even if one of the causes which contributed to the acceleration of the lowering of ground level has disappeared: the tapping of the deep underground water supply. The risk which is present today is the imposing eustastic rise due to the green house effect.|