Venice’s lagoon often rises to 40 inches above its normal level during 'acqua alta’ or high tides, particularly in autumn and winter.
But anything above 50 inches risks flooding the city and causing chaos for its 60,000 permanent residents and the tens of thousands of tourists who descend on it each day.
The worst flood in modern times was in 1966, when the lagoon rose more than six feet and caused widespread damage.
Experts say the severity and frequency of floods is becoming worse due to silt deposits raising the floor of the lagoon and a rise in sea levels caused by global warming
Flood control barriers under construction are not expected to be operational until 2012. Link -Thanks, Freshome!