ENVIRONMENT / CONTROLLING THE TIDE : Goal in Venice: Clean Water at the Right Level

Water is Venice’s element, but once more this fantasy city is caught in its flux. Too much water; too little. Certainly too dirty. Altogether a dilemma too hard to solve.

For centuries Venice drew sustenance and strength from the sea as capital of an audacious trading republic whose galleys tamed the Adriatic and the Mediterranean. Now, in a diminished Venice that feeds off tourists, the sea is more bane than boon.

Venice monitors the waters of its beleaguered 212-square-mile lagoon the way farmers count sheep. Now, while the water is pungently low, a new round is being fought in a never-ending civic controversy over how to prepare for the next time it is too high.

For Venice, a dry winter has meant empty side canals choked with sludge and garbage that appall visitors and enrage residents.

Because of the winter drought, the prospect for the summer, when most tourists come, is for many bugs, more sludge and a bumper crop of the slimy green algae that has become a regular visitor to Italy’s Adriatic coast. Last year, Venice canal sweepers harvested an average of 500 tons a day of algae.

There are no sewers in Venice. All refuse spills directly into the city’s 176 canals. As a result, according to a recent report, there is three feet of sewer sludge at the bottom of most canals.

All it takes is for the water level to drop about two feet, as it did this winter, for romantic canals to become smelly open sewers. Many projects for cleaning up the canal mess have yielded only sparse results.

By contrast, high water, always a Venetian problem, has been a civic obsession since Nov. 4, 1966, when a disastrous flood swept through the city, damaging buildings and works of art.

A generation later, engineers now say they can conquer the flood threat, but environmentalists insist they are paddling up the wrong canal: The real threat to Venice’s future is pollution, they contend.

On the lagoon bed offshore sits a full-scale working model of a 200-ton stainless-steel sluice gate, hinged and hollow, 66 feet wide, 56 feet high and 11.5 feet thick.

Engineers want to build a barrier of 80 such gates across the three openings that allow Adriatic waters into the lagoon. In normal times the gates would be invisible, anchored to the seabed in concrete housings. When high water threatened, they could be quickly raised by replacing the water in the hinged upper arm with compressed air. In about 30 minutes, Venice could be sealed by a steel fence 10 feet or so above the height of the sea.

Managers of the Venezia Nuova consortium of private engineering and construction companies that designed the gate say the system has the virtue of allowing normal tides to come and go naturally, replenishing and flushing the lagoon.

A year’s on-site test of the prototype sluice gate off the mouth of the Lido has been successful, Venezia Nuova consortium director Giovanni Mazzacurati said in an interview.

“We knew it would work. Now we are fine-tuning it,” Mazzacurati said. “It has been an extremely complex project, but the hardware is done. Now, it is a question of the software--the political decisions to carry the project through.”

As such things do in Venice, the consortium’s mobile dike plan, at a projected cost of $4.5 billion, has run into opposition.

Environmentalists such as Riccardo Rabagliati warn that it is pollution, not high water, that will kill Venice.

“Let’s stop degradation first and then worry about the high water,” said Rabagliati, a physicist who heads the Venice office of Italia Nostra, a national environmental group. “While governments have talked about pollution for years, fish die, the algae spreads and nothing has been done.

“Venice,” he declared, “is not a city meant to cower behind a dike.”

How the sluice diskes work 1. When Venice’s lagoon is at normal or low levels, the hollow, 200-ton steel gates would be filled with water and lie in a concrete housing on the bottom. 2. If seawater threatens the city, water in the gates would be forced out and replaced by compressed air, raising the gates to block incoming seawater.