A Dynamic Environment
The Venetian Lagoon is not a static body of water; it is a dynamic system in constant motion. Its morphology—the shape of its channels, mudflats, and marshes—is continuously reworked by tides and human activity. The lagoon floor consists of navigable channels, shallow tidal flats called velme, and salt marshes known as barene. These features are not fixed. Analysis of historical maps and sediment cores reveals that tidal channels naturally migrate across the lagoon floor. This horizontal channel migration occurs at rates of 10 to 20 meters per century. Over the 6,000-year history of the lagoon, this constant shifting has extensively reworked the subsurface sediment layers.
This natural evolution is driven by the twice-daily tidal pulse from the Adriatic Sea. The flow of water carves out channels and deposits sediment, building up the salt marshes. The marshes, in turn, are dissected by a network of smaller, meandering channels. Studies of these smaller channels, based on aerial photographs from 1938 to the present, show migration rates between 6 and 25 centimeters per year. This delicate balance between erosion and deposition has shaped the lagoon for millennia, creating a unique intertidal environment. Human interventions over the last two centuries have drastically altered this equilibrium.
A System Losing Ground
Since the 19th century, large-scale engineering projects have fundamentally changed the lagoon's sediment budget. To keep the three inlets to the Adriatic Sea (Lido, Malamocco, and Chioggia) from silting up, long jetties were constructed. These structures interrupt the natural longshore transport of sand, trapping sediment that would otherwise nourish the lagoon. This has resulted in a severe sediment deficit. Between 1927 and 2002, the lagoon experienced a net loss of approximately 110 million cubic meters of sediment.
The consequences are significant. Without a sufficient supply of new sediment, the lagoon is deepening and its internal structures are eroding. Comparison of bathymetric surveys shows that the average depth in the central part of the lagoon increased from 50 centimeters in 1811 to 1.5 meters today. The salt marshes, which provide a habitat and natural defense against flooding, are disappearing at an alarming rate. Since 1901, the total area of salt marshes has shrunk from around 170 square kilometers to just 43 square kilometers. Some studies report a loss of 72% of the lagoon's salt marshes in the last 100 years. This erosion is accelerated by wind-driven waves, which have a greater effect as the lagoon deepens. The MOSE flood barrier system, operational since 2020, further complicates the situation. While it protects the city from high tides, its closures can alter sediment transport, potentially trapping sediment within channels and preventing it from reaching the marshes.