Subsidence and sea-level fluctuations in the territory of Venice: Fid-Bau Portal

Subsidence and sea-level fluctuations in the territory of Venice

Ghetti, A.

AbstractAfter a brief account of the present situation of the Lagoon of Venice, of its origin and presumed evolution in history, subsidence effects are examined in connection with those of eustatism, both with reference to the Venice area. Evidence is given that the level oscillations of the Adriatic sea, which are almost unmodified in the lagoon, have recently reached values as high as 1 m. Subsidence due to geological factors, as supported by many archaeological remains, appears to have been nearly constant at a rate of about 1.3 mm per year. Venice has been flooded in past centuries by unusual increases in tide levels caused by peculiar meteorological factors in the Adriatic sea.An abnormal increase in subsidence has occured in the last 50 years owing to considerable water extractions from deep artesian wells, demanded by the new industrial plants of Porto Marghera at the border of the lagoon. This produced a soil lowering of about 10 cm—a value that has to be added to the geological subsidence and to the rising of the mean sea level. It has, however, caused an overall altimetrical reduction in the level of Venice soil with respect to sea level of over 22 cm between 1908 and now. Recently, the wells have been shut and the abnormal lowering has stopped. Now Venice is flooded by a tidal excursion of only 0.80 m; the city was completely flooded in November 1966 by an exceptionally high tide which reached 1.94 m. Protection works have been planned, consisting of a temporary closure of the three mouths through which the lagoon communicates with the sea, but it will be very difficult to ensure their effectiveness for the future.