Water level in Venice changes constantly according to the tide that pushes water into the lagoon from the Adriatic Sea and draws it out again, twice a day, regulated by the moon. Meteorological factors such as wind and atmospheric pressure further influence water level both in the Adriatic region and within the Venice lagoon.
With some time lag, the water level in the lagoon affects the level in the canals of Venice. The morphology of the lagoon has an influence on this process, and, in turn, the behaviour of water in the inner canals of Venice critically affects the condition of the urban fabric.
Venice has experienced occasional flooding since its origins, but the issue now is the damage caused to the urban fabric by capillary rise of water up the walls and into the floors as water levels routinely rise above the impermeable base of most buildings, and through the crumbling drainage and sewage networks. A new film describes the delicate state of the urban fabric very well (an English language version will soon be available): http://www.venicebackstage.org/it/
Problems begin when brackish lagoon water rises above the stone bases of the buildings and comes into contact with the more permeable parts, the plaster and brickwork. When the water reaches 110cm above the zero reference level set in 1892, flooding of the city also starts to be extensive (14% of pavements are inundated).
It happened 18 times in 2010, and is the level at which the mobile barriers (Mose) are planned to be closed to defend Venice from flooding. These barriers are scheduled to become operational in 2014. An exceptional high tide, involving water levels of 140cm or more, occurs roughly five times per decade and floods more than 55% of the pavements and squares, as well as a large number of buildings.
The mean water level (calculated on the basis of the daily maxima and minima over twelve months) is now at least 25cm above the zero reference level. This means that it is frequently at or above what is officially considered a very high tide (i.e. 80cm above reference zero), the point at which chronic damage to the building fabric is exacerbated. 2010 was the worst year since records began for high tides: looking at figures for every year since 2000 the average number of times the water level went above 80cm is 92 times a year, which is 13% of all tide peaks and about 232 hours per year?. The variability is noteworthy: in 2007, this occurred only 59 times.
Experts are also now beginning to arrive at estimates of what future sea level rise caused by climate change will mean for Venice (qv; The Ecological Implications of Climate Change on the Venice Lagoon Unesco, Venice, 26-27 May 2011) and the most conservative figure is a rise in the mean water level relative to the city of 50cm by 2100—but it may well be considerably higher. At 50cm, it would be at the level of what is considered today a very high tide. The barriers would have to close almost daily should everything else remain unchanged, and buildings would crumble at an even faster rate.
A new film describes the delicate state of the urban fabric very well (an English language version will soon be available): http://www.venicebackstage.org/it/