Relative Sea Level Changes in the Lagoon of Venice, Italy. Past and Present Evidence.

If no conservation work is carried Out, Subsiding coastal areas are prone to progressive submersion by the sea and to changes in their physiographical features. In Italy, the Lagoon of Venice is a Peculiar example on this situation, where the Relative Sea Level Rise (RSLR) is the result of the sea level rise and the land subsidence both natural and man-induced. In non-urbanised areas in the Lagoon, the mean rate of RSLR over the last 5000 years was about 1 mm/year, assuming the lowest figures in the last 1000 years. In built-up areas, where compaction of superficial marshy ground due to huge load of buildings must be added, the rate is between 1 and 2 mm/year. Over the last century, the RSLR has totalled about 23 cm, equal to a mean rate of nearly to 2 mm/year. In the Lagoon of Venice, both natural and anthropogenic evidence of RSLR is revealed in sediments. Natural evidence goes back to the period prior to human settlement, and that due to mail's activities dates back to historical times. The works in question were mainly carried out ill order to avoid having to abandon settlements due to exceptional flooding. In the archaeological areas of the Lagoon, human settlements act as proxies clarifying evolution. In some places, man has abandoned previous settlements: for example, the island of S. Lorenzo was urbanised in the first few centuries AD, remained inhabited until the XIV century, and was then abandoned, due to a new increase in sea level. At other times, man succeeded in preventing this process by infill works, or by raising paved levels above mean sea level, as oil the island of S. Francesco del Deserto, which has been inhabited since the I century AD. Instead, other non-urbanised areas in the Lagoon, such as salt-marshes or beach cordons, have disappeared with the passing of time, and now lie well below mean sea level. This natural situation is clearly testified by foraminifera - useful proxies of past situations of environmental evolution and its local chronology. In the urbanised areas, and chiefly in the historical centre of Venice, buildings have acted as proxies of the RSLR in historical times. In the last 100 years, technical improvements have allowed ground elevation and sea level to be regularly monitored. At present, inside the Lagoon, some areas lying at approximately sea level, such as high marshes, are undergoing a significant increase in flooding, mostly attributable to the RSLR, and are becoming perennially submerged areas due to the lack of intervention. In view of the considerable importance of the city of Venice, much conservation and restoration work is being undertaken, in a everlasting confrontation with the RSLR.