"Acqua Alta" in Venice - Floodings phenomenon

How many times have you seen images of Venice on your tv, which showed it covered by many inches of water? This is the so called "Acqua Alta", a phenomenon connected with the high tide of the sea (and the Venice Lagoon) that, depending on its intensity, could cause floods in some different areas of Venice itself. This frequently happens in autumn or in spring, and be aware from what you heard about: it is not dangerous and it only lasts a couple of hours, it mainly takes place in lowest areas (as for example San Marco and Rialto) and usually tourists love it, as it’s both strange and funny (only for tourists, not for workers!) to see St. Mark Square completely flodded and similar to a salt lake, even if 1 feet deep.

"Acqua Alta" in Venice - Forecasting and warnings

The high tide depends on moon and sun but also on winds and precipitacion, so since all those things are connected with tricky phisycal laws, it’s possible to estimate water tide just a few days in advance through complex calculations and survey instruments, usually from the tide station of Punta della Salute in Venice. In Venice, the government developed a warning system which uses a new alarm, located on Giudecca Island, with two different tones: the first one is to warn Venice people, the second one is to warn about the tide intensity.
High water peaks in Venice history were:
- November the 4th 1966: + 194 cm
- December the 22nd 1979: + 166 cm
- February the 1st 1986: + 158 cm
- December the 1st 2008: + 156 cm (non official stats say + 170 cm)

A Question: what does +156 cm mean???

A level of + 156 cm means that water level is 156 cm higher than the average level of the sea, which is stated as level 0. This does not mean you will see more than one meter and half of water covering Venice!!! Take in mind that when we have + 90, lowest areas start being flodded, but many parts survive up to + 140. After this level the situation becomes complicated and the city is more or less flodded, even if it goes back to the normality quicky, due to tide cycles (for six hours it grows and for next six hours it decreases), as it happens on December the 1st 2008, when once it touched its peak, thanks also to warm wind, it quickly went down.