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''Acqua alta'' (Italian: '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 which 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. == Causes == Precise scientific parameters define the phenomenon called ''acqua alta'',〔(【引用サイトリンク】language=Italian )〕 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: *''intense'' when the measured sea level is between 80 cm. and 109 cm. above the standard sea level (which was defined by averaging the measurements of sea level during the year 1897); *''very intense'' when the measured sea level is between 110 cm. and 139 cm. above the standard; *''exceptional high waters'' when the measured sea level reaches or exceeds 140 cm. above the standard. Generally speaking, tide levels largely depend on three contributing factors: *An astronomical component, which results from the movement and alignment of celestial bodies, principally the Moon, secondarily the Sun, and marginally other planets (with effects decreasing in logarithmic relation to their distance from the Earth); this component is dependent upon the laws of the astronomical mechanics and can be computed and accurately predicted for the long run (even years or decades) *A geophysical component, primarily dependent upon the geometric shape of the basin, which amplifies or reduces the astronomical component and, because it is dependent upon the laws of the physical mechanics, can be also computed and accurately predicted for the long run (even years or decades);〔((it) Venice Municipality, Tide Monitoring and Forecast Center - Astronomical tide )〕 *A meteorological component, linked to a large set of variables, such as the direction and strength of winds, the location of barometric pressure fields and their gradients, precipitation, etc. Because of their complex interrelations and quasi-stochastic behavior, these variables cannot be accurately modeled in statistical terms. Consequently, this component can only be forecast for the very short run and is the principal determinant of ''acqua alta'' emergencies that catch Venetians unprepared.〔(Venice Municipality, Tide Monitoring and Forecast Center - The meteorological contribution )〕 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「acqua alta」の詳細全文を読む スポンサード リンク
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