Called the acqua alta, the winter flooding of Venice is caused by a combination of seasonal tides, fluctuations in atmospheric pressure in the Adriatic and persistent southeasterly winds, and has always been a feature of Venetian life. In recent years, however, it has been getting worse: between 2000 and 2013 there were eight highest-category floods, which is more than in the preceding fifty years. If the siren sounds, you can expect a serious flood in three to four hours’ time. A system of plank walkways is immediately set up in the low-lying parts of the city. The usual high-tide season is October to March, with the worst flooding between November and February. Most floods, though, are minor, and cause no disruption at all.
A grand plan is being implemented to protect the city, involving building a tidal barrier across the three entrances to the lagoon. The barrier has aroused considerable opposition, both to its cost and to its potential environmental impact. However, mounting concern about global warming gave the matter some urgency and work finally began on the barrier in 2003. Delays and corruption have dogged the project, which is now scheduled for completion in 2017.