Acqua Alta
From Wikipedia, the free encyclopedia
Acqua Alta is a phenomenon that regularly occurs in the city of Venice, Italy, most commonly during high and spring tides. 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.
Its causes are both diverse in terms of human and physical influence but also in severity.
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 picture). The municipality of Venice has implemented a number of measures to ensure life continues as normally as possible, but the high waters still cause distruption. Boarding for visitors to walk on are erected during particularly severe times of flood.
The frequency and severity of these floods have been increasing over the last 50 years. On October 31st, 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. The most severe 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.
Over the last century, the frequency of acqua alta has increased from less than 10 times a year to over 60 times a year. The Coses research centre in Venice has estimated that the floods cost the city 11 billion lire a year in lost working hours.
[edit] 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:
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.
[edit] Causes
The flooding in Venice is caused by two main factors, these are earth subsidence and eustatic sea level change. Earth subsidence was at one point the predominantly contributing factor, with the city sinking by approximately 100mm in the 20th Century. However, this was mainly due to large scale groundwater extraction by industry in the 50's 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 do to the compression of land beneath the city's millions of wooden pilings and geological factors.
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 (e.g. Rocky's, Alp's), melting continetnal ice shelfs and thermal expansion of water, all considered to be a result of Global Warming.
[edit] Prevention
The MOSE Project (an acronym for Modulo Sperimentale Elettromeccanico, or in English, Experimental Electromechanical Module ), or Project Moses as its better known, is a plan to install 79 separate 300-ton flaps hinged on the seabed to separate the Venetian Lagoon from the Adriatic. Scientific consensus is that this will prevent all but the most minor cases of Acqua Alta, whereas Green Party campaigners have 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.
