When we talk about "acqua alta" (high water) it means that part of
the city (how much exactly can vary) is covered by the high tide.
It is a phenomena that occurs almost only during the winter months and is well-known
by both Venetians and tourists who have had the luck to witness it. However,
it is often exaggerated by the media and misunderstood by those who have never
visited the city.
It has often been reported that there is high water at least forty days a year
but as the inhabitants know, there is "high water" and "high
water".
Indeed, there is a lot of confusion regarding the centimetres of the tide.
 Below
80 cm. one cannot even talk of high water.
Once the level reaches 100 cm, many people believe that you can only get around
the city if you swim. The truth, however, is that less than 4% of the surface
of the historic city centre is submerged by a couple of centimetres of water
and it does not cause too much inconvenience.
The sirens that forecast high water 3-4 hours in advance only go off if the
level is expected to be above 110 cm.(12% of the surface under water).
The wooden walk ways that can be seen around the city ensure movement around
the city in the main areas up to120 cm. (35% of the surface under water).
 Only
once the level reaches130 cm. (70% of the surface under water) the effects really
begin to make themselves felt and at 140 cm. (90% of the surface under water)
one starts to speak of an emergency. However, such extreme tides are rare: approximately
two every three years.
You mustn't forget that the tides rise and fall regularly and the peak only
lasts a couple of hours. You only need a bit of patience before you see the
city freed from the grips of the water.
The causes of the tides are the following:
- astronomic: the attraction of the sun and the moon cause the regular
rise and fall of the water: "6 hours rise and 6 hours fall". You therefore have
two maximums and two minimums a day.
- meteorological: a strong south-east wind ("scirocco") may
cause the tide to increase by as much as 1 metre.
- geographical: the seiche is a sort of long wave that runs through the
whole of the Adriatic Sea with a period of approximately 22 hours.
The frequency of the floods over the last decades has, however, mainly been
caused by man - the numerous fillings of earth to create more space for the
industries of Porto Marghera and the digging of deep canals for oil tankers
has disrupted the delicate equilibrium of the hydrodynamics of the lagoon.
The increase of the tide levels and the decrease of the ground level due to
the extraction of water and gas have also played a role, leading to an additional
increase in tide level (approximately 20 centimetres).
Further information on the phenomena of high water can be found in Observatory
pointsulla Valutazione dell'Impatto Ambientale of the Ministry of the Environment
and the Comune di Venezia
More information and forecasts can be found at
Centro Previsioni e Segnalazioni Maree
Palazzo Cavalli, S. Marco 4090
30124 Venezia
tel. +39-041-2748787
fax +39-041-5210378
Aut. answering service +39-041-5206344/5207722
translated by C.Cawthra
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