Tourists were confined to using elevated walkways when water flooded St Mark\'s Square in Venice Wednesday, with seasonal high tides rising 82 centimeters above sea level in the lagoon city. Known as \'acqua alta\', or high water, the flooding has hit Venice for the past five days but is expected to subside for the next week, authorities said. Autumn marks the beginning of the high-tide season, when water routinely spills over the city\'s banks, flooding its streets and squares. The causes are both natural and man-made. Decades of pumping groundwater caused significant damage to the delicate foundation before the practice was called off. Weather experts say the high-water threat has been increasing in recent years as heavier rains have hit northern Italy. Other possible explanations for the phenomenon include the sea floor rising as a result of incoming silt and gas extraction in the sea off Venice undermining the islands. According to a recent study, plate tectonics are also to blame as the Adriatic plate is sliding beneath the Apennine Mountains, causing the area to drop in elevation. Scientists have conceived various ways of warding off the waters since a catastrophic flood in 1966 and a system of moveable flood barriers called MOSE is near completion after years of polemics.