After decades of floods and heated debates over how to deal with them, Italy on Saturday launched the first stage of a $5 billion project to shelter the Venetian lagoon from the high tides of the Adriatic Sea.

Since its founding 1,100 years ago, Venice has been plagued by frequent floods, water pollution and a steady sinking. The city has dropped 9 inches this century.After flood waters in 1966 inundated the famous St. Mark's Square and damaged most of the old city, the government began studying how to solve the problem.

But the studies became bogged down in political infighting and budgetary problems until three years ago, when the government came up with a plan that includes construction of a line of mobile dikes across the lagoon's three entrances.

The barriers can best be described as giant jaws on the seabed that will open when necessary to slow high tides rushing toward the city. The rising "jaw" will drop back to the seabed as the tides weaken and will remain invisible at normal times.

Deputy Premier Gianni De Miche-lis inaugurated the first prototype in a ceremony at the industrial section of Marghera. The steel dike weighs 1,100 tons and has been dubbed "Moby Duck."

After the experiment is completed, 70 to 80 dikes similar to the prototype, varying in length from 17 to 27 yards and up to 180 feet high, will be built. The system is scheduled to be fully installed in 1995.

Other parts of the plan involve efforts to preserve the 150 canals, shore up the islands and the seawalls around the cresent-shaped, 340-square-mile lagoon, and control pollution.

Members of the consortium supervising the project visited the Thames river estuary and Holland's Schelda river, where similar projects have been built.

They also studied Chicago's work in cleaning up Lake Michigan and looked to the San Francisco and Chesapeake bays to imitate their organization in creating an overall authority to supervise the project.