Venice -ACQUA ALTA (high water)
With this term the Venetians indicates the invading tide , at certain times of the year and for a short time ,in some areas of the low city .
As is known, the natural phenomenon which consists in rhythmic swing level
the sea, in consequence of the mutual gravitational attractions that are
between the heavenly bodies (mainly the moon and the sun .) The amplitude and the period of the astronomical tide is different from sea to sea . Generally in the Mediterranean
the oscillation is small ( a few decimetres ) while in the Adriatic it is wider ,and in
value is officially took on the Tidal station situated in “Punta della salute”(the peak of the health ) which records the data of the tide gauge located in the channel Giudeca( old tide gauge ) and the new tide gauge located just a few meters away from it on the other side of the channel .Are considered ” normal” the tide that sways in the limits of 80 and – 50 cm ; supported that with the values between 80 and 109 cm; and exceptional what is equal or exceeds 110 cm.
The first historical reference to ” High water in Venice” does Paul Deacon in” Historia Langobardum “in the year 569.Other vague quotes about it back in the years
782,840,885 . References of the second millennium have more precise dates :
-February 1240 – ” invaded the streets more than the height of a man ”
-25 February 1341 – ” two feet even more ”
-1386 – “Eight feet most of the usual”
-10 August 1410 – “large water growing in all the earth that was never view”
-10 November 1442 – ” four steps above the ordinariness ”
-3 October 1535 – ” so much water that the wells were dammaged ”
-2 November 1559 – ” high waters in the streets ”
12th October 1574 – ” Lido di Chioggia is broken in five places”
– November 1686 – ” the fundamentals (the banks) almost all submerged and the gondolas were going into haberdashery ”
-21 December 1717 – ” come to the steps of the high altar of San Antonio church”
It should be noted that in the past , many floods were due to the overflow of the rivers
in the lagoon.Then these rivers are diverted from the lagoon.
But , thanks to the institution of civil engineering of Venice (1848) , the quotes are more precise :
-1848 Water reached +140 cm
-1867 … +153 Cm
In the new century, it should be noted
-On 4 November 1966 it was achieved the level of +194 cm at 18:00 o clock
the highest tide ever recorded in the history of observations.
-2002- a record year of tides.
-16 November : +147 cm,
-14 November – December 8 : “32- tides; more than +80 centimeters of which 10 are above of +110 and 5 above of +120 cm”
The main reason is attributed to the phenomenon of subsidence , both of natural and
anthropogenic origin due to outcoming “emungimenti” of water and gas from underground. Has been esteemed a lowering of the land -4 cm , due to the first phenomen and -10 for the second ,and +2 are here recovered thanks to the elasticity of land after the arrest of “emungimenti” in seventies.
Then there is another element contribution and the natural “eutatismo “- raising the average level of the sea attributed to the melting of polar ice caps caused by the
Earth’s average temperature . This increase was calculated to be +11 cm.
The phenomenon is especially common in autumn and spring , when combined
with the” sirocco “winds , which , exhaling from the channel along the entire length of seabasin , impede the smooth flow of water ; or” bora” , hindering instead locally lagoons and rivers flowing from the Venetian coast .
The damage produced by high waters are of various types , being attributable to at least two categories:
– damage to physical structures – degradation or deterioration in the walls , through the
phenomenon of so-called capillary rise of water brackish .(the wetting and subsequent
the evaporation and crystallization produces salt and with time the disintegration of the mortar ,
– Socio-economic damage – are the result of the interruption of production activities ,
the loss of well-being of both residents and tourist
Prospects : the defense of the tides – the temporary separation of the lagoon from the sea with barriers at the inlets of the harbors -MOSE -modullo sperimentale eletromagnetico,(electromechanical experimental module) http://dipoco.altervista.org/venezia-m-o-s-e/
By http://dipoco.altervista.org/ Cercodiamanti
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