The Lagoon of Venice has lost its natural balance.
The deep shipping channels dredged through the Lagoon are a key problem. 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. These depths preserved a natural balanced 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 sediment and created deeper passages (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 ship 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 - for example, to create the industrial zone at Porto Marghera. 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 need to addressed first.
Look also at:
Italia Nostra VE: We Oppose the Dam System
Italia Nostra VE: Valid Alternatives Exist
The MoSE: a presentation in powerpoint
A dangerous Monster in the Lagoon
VIA: Why MoSE is not good (in Italian)