VENICE HIGH FLOODING


 

IMPORTANT: Acqua alta forecast for next days »

 

 

Venice high water (acqua alta):

 

High water (Acqua alta in italian) is the term that indicates the exceptional tide peaks that occur periodically in the Venetian Lagoon, where they cause partial flooding of Venice.
The phenomenon occurs mainly between fall and spring, when the astronomical tides are reinforced by the seasonal winds which hamper the usual reflux.
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The event can be categorized as:

 

  • intense when the measured sea level is between 80 cm. and 109 cm. above the standard sea level (which was defined by average measurements of sea level during the year 1897)
  • very intense when the measured sea level is between 110 cm. and 139 cm. above the standard sea level
  • exceptional high waters when the measured sea level exceeds 140 cm. above the standard.

 

When the phenomenon of High Water happens, flood level isn't uniform throughout the entire city of Venice. This is due to several factors, for example the distance of the area from the canals, the presence of unexpected dams (such as parapets) the different altitude of every zone respect to the sea level, the layout of water drainage network (which is connected to the sea and permits water to rise).
There were several studies about acqua alta and the influence on the different zones of Venice. Most recent results are reported below [26 November 2009] and shows that with a tide level up to 90 cm city isn't affected:

 

Tide level % of city flooded
< 90 cm
none
100 cm
5 %
110 cm
14 %
120 cm
29 %
130 cm
44 %
140 cm
55 %
160 cm
70 %
180 cm
79 %

 

To allow pedestrian circulation during floods, the city installs a network wood planks on iron supports (called passerelle) along the main pedestrian paths. This walkway system is generally set at 120 cm above the standard sea level.

 

venice flooding acqua alta

 

Till December 2007 18 sirens were used to warn population of intense high waters. They were activated about 3 hours in advance when the Center predicted a very high tide (more than 120 cm). Further acoustic warning were given out as soon as it was possible to predict an exceptional tide (up to 140 cm). Finally the sirens were used again when the tides exceed 160 cm. Nowadays sirens sound a first "await instructions" whistle to catch the population's attention, then produce a sequence of whistles whose number increases with the expected tide level.
When an event is forecast, owners of commercial and residential property that is likely to be affected are contacted by phone (a free service provided by the municipality) or SMS.
Everyone can sigh up to the free SMS alert service which send you a text message directly to your mobile phone. More info here.

 

These are the maximal documented tide levels:

 

  • 194 cm on November 4, 1966
  • 166 cm on December 22, 1979
  • 158 cm on February 1, 1986
  • 156 cm on December 1, 2008
  • 151 cm on November 12, 1951
  • 147 cm on April 16, 1936
  • 147 cm on November 16, 2002
  • 145 cm on October 15, 1960

 

High Water forecast for next days »

 

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