Acqua Alta

Acqua Alta is a phenomenon that regularly occurs in the city of Venice, Italy, most commonly during high and spring tide. It involves the flooding of the most low-lying areas of Venice, or, in more severe cases, up to 96% of the City. By official definition, acqua alta occurs when water is higher than 90mm (3.54 inches) above normal tide.

One of the lowest points of Venice is St. Mark's Square (or the "Piazza San Marco") and this point is flooded most regularly (see pictures).

The municipality of Venice has implemented a number of measures to ensure life continues as normally as possible. For example, boarding for pedestrians to walk on is erected during particularly severe times of flood (although it is advisable to wear rubber boots). However, some homes and business premises have to be evacuated. The Coses research centre in Venice estimated that the floods cost the city 11 billion Italian lire a year in lost working hours.

The frequency and severity of these floods have been increasing over the last century. The frequency of acqua alta has increased from less than 10 times a year to over 60 times a year. as regards severity, the worst case of acqua alta was that of the 1966 flood, where waters registered over 1m higher than normal tide-level (180 cm) and 96% of the city was flooded.On 31 October 2004 the waters reached 135cm and 80% of the city was flooded. On Monday 8 January 2001, more than one-eighth of the city's surface was flooded, as the waters rose to 111 centimetres above sea level.

Effects

Acqua Alta affects different amounts of the city depending on severity. Summarised below is the percentage of the city that is covered under different tidal heights:

Causes

The causes are diverse in terms of human and physical influence, but there are two main factors, these are earth subsidence and eustatic sea level change.

The city is very vulnerable to changes in sea level, as it is built on low islands in the Venetian Lagoon which have existed since the end of the last Ice Age. The buildings are supported by wooden pilings. Earth subsidence was at one point the predominant contributing factor to the flooding, with the city sinking by approximately 100mm in the 20th century. However, the subsidence was mainly due to large-scale groundwater extraction by industry in the 1950s and early 60's. The sinking largely stopped when caps were placed on mainland artesian wells. Today, subsidence is estimated at between 0.5mm and 1mm a year, mostly due to the compression of land beneath the city's millions of wooden pilings and geological factors. It is necessary to note that Venice usually sinks between 2-4cm each century.

Eustatic sea level change is the other significant contributor to the rising frequency of Acqua Alta. The Intergovernmental Panel on Climate Change (IPCC) has estimated that the global average sea level has risen by between 10cm and 25 cm in the last century. This change is caused by a number of different contributing factors such as retreating glaciers (for example in the Alps), melting continental ice-shelves and thermal expansion of water, all considered to be a result of Global warming [http://www.smithsonianmag.com/specialsections/ecocenter/venice.html] .

Certain conditions are known to enhance the effects of acqua alta, and these are:
* A very high tide (usually during full moon).
* Low atmospheric pressure.
* A sirocco wind blowing up the shallow Adriatic, forcing water into the Venetian lagoon.

Prevention

The MOSE Project (an acronym for "Modulo Sperimentale Elettromeccanico"; in English, "Experimental Electromechanical Module"), or "Project Moses" as it is better known, is a plan to install 79 separate 300-ton flaps hinged on the seabed to separate the Venetian Lagoon from the Adriatic. The scientific consensus is that this will prevent all but the most minor cases of Acqua Alta, but the Italian Green Party has suggested that the flood gates would prevent the Lagoon from being flushed by the waters of the Adriatic and would be severely detrimental to life within the area.

References

For a detailed look at this issue and the history behind it, read "Venice Against the Sea: A City Besieged" by John Keahey (Thomas Dunne Books, St. Martin's Press, March 2002)

External links

* [http://www.salve.it/uk/sezioni/voltabarozzo/voltabarozzo.htm Venice Water Authority]