Manifestations

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.

High tides upset the lives of inhabitants as well as causing considerable damage to the architectural heritage.

Frequency of floods for tidal levels higher than 80 cm (average calculated over a ten-year period)

In the following chart, the occurrences of tides equal to or grater than 80 cm 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 cases in the first half of the century, to 40 cases in the second half, reaching almost 60 cases in the last five years.

The occurrence of the phenomena of high water is mainly connected to two factors which affect, on different time scales, sea level:

• the temporary rise in sea level because of the combination of the astronomical tide and the meteorological factors;
• the lowering of ground level with respect to mean sea level, due to the two long term factors of eustatism and subsidence.

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.
Therefore, since the beginning of the century, Venice has lost 23 cm with respect to sea level, causing a greater intensity and frequency of high water. This has produced the result that today there are high water events that during the first decades of the century would not have flooded the city.

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 aboveall 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.
The most critical phase of this process began in the 50's and worsened up until 1970, with an average lowering in ground level of 12 cm at Marghera and 8 cm at Venice.
From that point on the exploitation of water resources from artesian wells was prohibited and the result was a rapid increase in the pressure of the underground water supply with a consequent small but important altimetric recovery of the ground level.

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.
After the last glacier period (about 15,000 years ago), the warming of the atmosphere caused a slight increase in sea level. During the last century, the eustastic rise for the city of Venice, independent of its subsidence, was on the average 1.27 mm per year.

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:

• Scenario A. The first scenario, which could be defined as optimistic, is based on the evolution in the last 20 years, in which the eustatic rise has been dormant. If this should be the case, future expectations equal to only natural subsidence would be expected: about 4 cm in the next century;
• Scenario B. In the second scenario, which could be defined as realistic, the tendencies recorded in this century are transferred to the next century, which without the subsidence caused by the tapping of water, would show an overall increase between 16-20 cm;
• Scenario C. The third scenario, which could be defined as pessimistic, is based on the forecasts of the IPCC (Intergovernmental Panel for Climate Change) whose most recent estimates show the potential increase in the rise in sea level at the end of the next century at about 50 cm. The consequences of such a scenario would be dramatic not only for Venice (that without the mobile flood barriers would practically suffer daily submersion) but for the coastal areas of the entire world. Way before such a scenario occurs, appropriate and timely decisions must be made by political authorities.

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 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.