"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.”