Environment: Venice Fights Off the Flood Tides

The city of Venice, built on an archipelago in a 212-sq.-mi. lagoon, has long been perched on the edge of disaster. The magnitude of the threat became clear on Nov. 4, 1966, when a storm on the Adriatic Sea inundated St. Mark’s Square in nearly 4 ft. of water and pounded the facade of its revered basilica. But Venetians have come to accept periodic flooding — acqua alta (high water), they call it — as a way of life, while city officials and the Italian government have been slow to realize that Venice’s artistic and architectural treasures are in grave danger.

After decades of false starts, Venice has finally launched its “Moses project,” the building of a giant seawall designed to part the waters and save the city from the sea. Last week, exactly 22 years after the great flood, a 200-ton steel box was towed across the Venice Lagoon and dropped in place at the Porto di Lido, one of the lagoon’s three entrances. If the device works as planned, it will be the first of 60 to 70 sea gates that will eventually stretch 1.2 miles, sealing off the lagoon from the Adriatic Sea.

The action comes none too soon. For the past several hundred years Venice has been sinking — 9 in. this century alone — because of geological shifts in the region and the draining of freshwater wells in and around the city. Although experts believe the sinking has stopped, the city faces an equally threatening development: the slow rise of the Adriatic, largely as a result of a global warming trend that is causing the world’s oceans and seas to expand gradually.

City officials debated various propositions for keeping out the sea, but there was no simple solution. A series of permanent dams was ruled out because that would worsen the city’s already serious water pollution. Without the cleansing flow of tides into and out of the lagoon, the buildup of sewage and agricultural runoff would become intolerable. In 1982 the Italian government turned to a group of more than two dozen design, engineering and construction companies, mostly Italian, known as the New Venice Consortium. After five years of study, the consortium’s engineers came up with a novel design for a flexible seawall that could be raised or lowered at will.

The wall will be made of dozens of individual gates that can be activated separately. Each unit is an empty steel box, nearly 12 ft. thick, 65 ft. wide and from 55 ft. to 88 ft. high, depending on sea depth. When not in use, the boxes will be filled with water and attached by a hinge to a concrete foundation buried in the lagoon bed. If an abnormally high tide threatens the city, the water inside the gates can be pumped out or displaced by compressed air. Suddenly buoyant, the gates swing on their hinges like the jaws of a crocodile, rising to a 45 degrees angle, with the top about 3 ft. above the surface of the waves. After the storm has passed, water is pumped back into the gates, allowing them to sink back out of sight.

When the gates are up, they will oscillate with the sea. Thus even as they hold back the water, they will allow the sea’s wave motion to pass to the lagoon. This will help prevent the gates from making the lagoon more stagnant and polluted. “The technology isn’t new, but the combination is,” says Franco De Siervo, technical director for the consortium. “There is nothing like it in the world.”

Many details remain to be worked out. During the next eight months, the consortium will be testing different types of hinges to determine how well each holds up in the briny lagoon and how easily detachable they are. The engineers are hoping they will not have to dispatch divers to unhook a gate every time it has to be towed into port for cleaning. The consortium is also evaluating systems for scooping out the sediment that could clog the concrete foundations.

If all goes well, the last gates should be swung into place by 1995, forming a chain that will span the three shipping channels connecting the Venice Lagoon with the sea. The total cost is expected to be $5 billion, a small price to pay to save a treasure like Venice.