Flooding: Reasons and Frequency
Since its foundation, Venice has seen an increase in acqua alta occurrences, which has caused both physical and socio-economic damage to the city. This increase in flooding is the result of many natural forces including: more intense winter storms, rising sea levels, and subsiding land, as well as man-made changes to the natural defenses of the lagoon. Although a portion of the flooding is caused by the tides, the main contributing factors are the low-pressure systems and prevailing winds. The first of which is the Bora, a local northeast wind responsible for driving high water surges, up to 20cm, through the northern and southern areas of the lagoon. Secondly, the Sirocco, a southeast wind from the Sahara, pushes water surges from the Adriatic Sea up the coast and through the ‘funnel.’ The combined forces of these two winds and the daily tides caused the historic flood of 1966 when the waters rose to record heights, nearly two meters higher than any previous flood.
Accounts of fifty-four incidents of acqua alta in Venice were recorded between the years 1866 and 1966. Forty-eight of these fifty floods occurred in the last thirty-five years of the hundred-year period. Thirty of these forty-eight floods happened in the final ten years. These numbers do not include the ‘normal’ flooding, which is defined as only covering the pavement. There are 295 instances of ‘normal’ flooding in the period from 1972 to 1974, a marked increase of eighty percent over the period between 1962 to 1964.
 Fletcher, Caroline and Jane Da Mosto. The Science of Saving Venice. Turin: Umberto Allemandi & Co., 2004. p. 32.
 Fay, Stephen, and Phillip Knightley. The Death of Venice. New York: Praeger Publishers, 1976. p. 41.