Floods - acque alte
- have been an element of the Venetian winter for hundreds of years, but since the middle of the twentieth century there's been a relentless increase in the frequency with which the city's streets become immersed. It's now very rare indeed, between October and late February, for a week to pass without flooding, and it's not at all uncommon for flooding of some extent to occur on every day of the week - indeed, in the notoriously soggy winter of 2000 there was an acqua alta
on thirty consecutive days. An acqua alta
begins with water seeping up through the pavement of the Piazza and other low-lying areas, such as Campo San Polo, forming puddles that quickly merge into a shallow little lake. Soon after, you'll notice that wavelets are spilling over the quayside in front of the Palazzo Ducale. Sometimes it doesn't progress much further than this, but often it gets much worse. If you hear sirens wailing over the city it means that there's about four hours to go before the peak of a serious acqua alta
, which is defined as a flood that rises in excess of 90cm above the mean lagoon level at the Salute. (Instruments on the side of the Campanile di San Marco display a continuous measurement of the water level and a prediction of the day's high tide - if the red light is on, a big flood is coming.)
Having lived with acque alte for so long, the city is well geared to dealing with the nuisance. Shopkeepers in the most badly effected areas insert steel shutters into their doorways to hold the water at bay, while teams of council workers lay jetties of duck-boards along the major thoroughfares and between the chief vaporetto stops and dry land. In extreme instances even these measures are not sufficient, and the duckboards get washed away from the Piazza, but usually the city keeps functioning through the inundation, and even on severe days there are some sectors that remain above the waves - maps at most ACTV stops show the routes of these walkways and where the high ground lies. However, Venice's pavements don't drain very efficiently, so you will find yourself splashing through water many hours after high tide. On a serious acqua alta day almost every Venetian is kitted out with rubber boots, and you'd be well advised to follow suit - there are plenty of shops selling them cheaply. And one other tip: if the water's high and duckboards are in place, use them - if you try to improvise a route down the back-alleys, the odds are that sooner or later you'll end up beating an ignominious retreat in the face of an unruly canal.