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    15 Nov 2016

    The Tide Barriers of Venice

    Flooding has been a part of Venice since ancient times. Between autumn and spring, the city is affected by bouts of periodic flooding known as aqua alta, or “high water”, caused by a combination of astronomical tides, seasonal rain and strong winds, that pushes flood tides into the Venetian lagoon from the Adriatic sea submerging the lowest parts of the city for three to four hours. During these periods, wooden benches are placed end to end to create temporary raised walkways, and business block their doorways until the waters subsides. Although the city has adapted to flooding remarkably well, their instances have become more frequent and more severe in the last few decades. Partly at blame are rising sea levels, but the city itself is sinking.
    Venice has been sinking for centuries. The very foundation underneath her —the salt marsh, into which wooden pilings were driven to prop-up a maze of heavy marble palazzi and churches— is still compacting and settling. Tectonic forces are are also pushing the ground over which the city stands. The sinking was exacerbated in the 20th century when many artesian wells were sunk into the periphery of the lagoon to draw fresh water from underground aquifers. Over the last hundred years, Venice has sunk by 9 inches, and it continues to sink at a rate of 1-2mm per year.

    In November 1966, after an exceptionally large flood submerged the city in six feet of water, the city authorities decided that the only way to safeguard the city at high tides was to build a tide barrier that could separate the city from the lagoon. As it happens, Venice already has a string of barrier islands as natural protection against flooding. But this barrier of sandbars is breached by three inlets, which are essential for water flow and shipping, but still leave the lagoon vulnerable to high tides.

    In 2003, Venice started building a series of inflatable flood gates at the three lagoon inlets. When not in use, the gates remain horizontally on the seabed allowing ships to pass over, but when weather forecast predicts tides higher than the normal level, an automated control system pumps compressed air into the inflatable gates until they emerge out of the water. The rise of the flap gates temporarily isolates the lagoon from the sea, protecting the lagoon from high tides.



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