Venice and the three outlets between the Lagoon and the Adriatic

The city of Venice flourished for centuries in the centre of a vast, natural Lagoon. In recent decades, however, the Lagoon has lost its natural balance: this is the key to problems such as the "high waters" that regularly flood low-lying areas such as St. Mark's Square.

The deep shipping channels dredged through the Lagoon are a central factor. In the early 1800s, the Lagoon's outlets to the Adriatic Sea were at most 4.5 metres (about 14 feet) deep. The Lagoon itself was shallow - under 1 metre (3 feet) deep on average - with a network of natural channels radiating from the three outlets. These depths preserved a natural balance and were fine for sailing ships and fishing boats - not for a modern port. The construction of long jetties at each outlet in the late 1800s blocked currents laden with sediment from entering the Lagoon. As a result, the channels through the outlets grew deeper (the three outlets are visible to the right of and below Venice in the satellite photo). The major damage came, however, with the dredging of deep shipping channels in the 20th century: the oil tanker channel, dug in the 1960s, now cuts across the Lagoon like a wound, 14.5 metres (about 45 feet) deep.

The result? High tides - once partially blocked by the outlets - now enter the Lagoon quickly. On top of sea-level rise and other problems, this means that Venice floods more quickly and more often.

The deep channels also create strong currents that erode natural features such as salt marshes in the Lagoon, one of the Mediterranean's most largest wetlands and an important way-station for migrating birds.

A further problem is that large areas of the Lagoon have been filled in. The industrial zone at Porto Marghera was built on a huge landfill. Other areas have been closed to tides, in particular the once-public (now privately run) "fishing valleys". As a result, the Lagoon has 15 square kilometres (6 square miles) less area for high tides to expand, and flooding in Venice is worse.

For decades, politicians and planners strived to "modernise" Venice without paying attention to its environment. To reduce flooding in Venice, the Lagoon's environmental balance should be addressed first.