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, with more than a hundred floods a year – though most of these are minor. 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 – most boat stops have maps of where those walkways run. The usual high-tide season is September to April, with the worst flooding between November and February.
A grand plan is being implemented to protect the city, involving building a tidal barrier across the three entrances to the lagoon. Nicknamed Moisè (Moses), the barrier aroused considerable opposition, both to its cost and to its potential environmental impact. However, mounting concern about global warming gave the matter more urgency and has led to widespread acceptance. More than twenty years after the first plan was submitted, work finally began on the barrier in 2003 and is due to be completed in 2014.