Acqua alta

Acqua alta (pronounced [ˈakkwa ˈalta]; Italian for "high water") is the term used in Veneto for the exceptional tide peaks that occur periodically in the northern Adriatic Sea. The peaks reach their maximum in the Venetian Lagoon, where they cause partial flooding of Venice and Chioggia; flooding also occurs elsewhere around the northern Adriatic, for instance at Grado and Trieste, but much less often and to a lesser degree. The phenomenon occurs mainly between autumn and spring, when the astronomical tides are reinforced by the prevailing seasonal winds that hamper the usual reflux. The main winds involved are the sirocco, which blows northbound along the Adriatic Sea, and the bora, which has a specific local effect due to the shape and location of the Venetian lagoon.

Precise scientific parameters define the phenomenon called acqua alta, the most significant of which (i.e., the deviation in amplitude from a base measurement of "standard" tides) is measured by the hydrographic station located nearby the Basilica di Santa Maria della Salute. Supernormal tidal events can be categorized as:

Generally speaking, tide levels largely depend on three contributing factors:

Two further contributing natural factors are the subsidence, i.e. the natural sinking of the soil level, to which the lagoon is subject, and eustasy, i.e. the progressive rise of sea levels. While these phenomena would occur independently of human activity, their effects have increased because of inhabitation: the use of lagoonal water by the industries in Porto Marghera (now ceased) speeded up subsidence, while global warming has been linked to increased eustasy. Venice's "Tide Monitoring and Forecast Center" evaluates that the city has lost 23 cm. in its elevation since 1897, the year of reference, 12 of which are attributable to natural causes (9 because of eustasy, 3 because of subsidence), 13 are due to the additional subsidence caused by human activity, while the "elastic recovery" of the soil has allowed the city to "gain back" 2 cm.

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