Venice floods because the city is simultaneously sinking into its foundations and facing rising sea levels, while its position at the head of the Adriatic Sea funnels storm surges directly into a shallow lagoon. Over the past 150 years, Venice has lost about 23 centimeters of elevation relative to the sea, roughly the length of a standard ruler. That may sound small, but for a city built at water level on a collection of mudflats and wooden pilings, it is the difference between dry streets and regular flooding.
How Tides, Wind, and Weather Combine
The periodic flooding Venice experiences has a name: acqua alta, Italian for “high water.” It is not one single cause but a collision of forces that happen to align at the worst possible times. Normal astronomical tides in the Adriatic are relatively modest, but during certain moon phases, gravitational pull from the moon and sun raises the baseline water level. On its own, that rarely causes problems.
The trouble starts when a specific weather pattern forms over the Mediterranean. A low-pressure system settles over the Tyrrhenian Sea (west of Italy), while high pressure sits over central and northern Europe. The pressure difference between these two systems generates strong winds that blow southeast to northwest, straight up the length of the Adriatic and toward Venice. These winds push enormous volumes of water ahead of them in what meteorologists call a storm surge. The Adriatic is long, narrow, and shallow, so it acts like a bathtub being tilted: water piles up at the northern end, right where Venice sits. The lagoon’s narrow inlets then trap that water, preventing it from draining back out quickly.
When a strong storm surge coincides with a high astronomical tide, the water can rise dramatically. Venice’s worst flood on record struck in November 1966, and the city experienced another severe event in November 2019. These catastrophic events grab headlines, but smaller acqua alta episodes happen dozens of times per year during autumn and winter, sending seawater across St. Mark’s Square and into ground-floor shops and homes.
The City Is Sinking
Venice was built on over a hundred small islands in a marshy lagoon, with buildings resting on millions of wooden pilings driven into soft sediment. Some natural settling is inevitable in that kind of geology, amounting to roughly 3 centimeters over the entire 20th century. That pace, while slow, is relentless and cannot be reversed.
But the bigger story is human-caused sinking. Starting around 1930, factories in the nearby industrial zone of Marghera began pumping massive quantities of groundwater from wells beneath the lagoon. As water was extracted from underground layers of sand and clay, those layers compressed like a squeezed sponge, and the land above dropped. Between 1952 and 1969, the peak years of pumping, Venice sank at a rate of about 5 millimeters per year in the historic center and 6.5 millimeters per year in the industrial zone. Over that 17-year stretch alone, the city dropped roughly 9 centimeters, with some spots sinking as much as 10 centimeters. The industrial zone fared even worse, losing up to 14 centimeters.
Authorities eventually restricted groundwater extraction, and the rapid sinking largely stopped. But the damage was done. That 9 centimeters of lost elevation is permanent. Combined with natural geological settling, the land beneath Venice contributed about 12 centimeters of the city’s 23-centimeter relative drop over the last century. Parts of the lagoon’s edges continue to sink at 1 to 5 millimeters per year today.
Rising Seas Are the Other Half
While Venice was sinking from below, the water was climbing from above. Tide gauge records stretching back to 1872 show that sea level in the northern Adriatic has risen at an average rate of about 1.2 millimeters per year after removing the effects of subsidence. That accounts for roughly 11 centimeters of rise over the 20th century. Add the 12 centimeters of sinking, and you get the full 23 centimeters of relative sea level loss.
Put another way, subsidence and sea level rise have each contributed about half of Venice’s flooding problem. The combined rate of relative sea level rise over the observational period is approximately 2.5 millimeters per year. That pace is expected to accelerate as ocean warming and ice sheet melt intensify. Even a few more centimeters of rise dramatically increases how often acqua alta events cross the threshold where they cause real damage to buildings and infrastructure.
How Humans Reshaped the Lagoon
Beyond groundwater pumping, Venice’s lagoon has been physically altered in ways that make flooding worse. Over the past two centuries, about 34 square kilometers of salt marsh have disappeared from the lagoon. These marshes once acted as natural sponges, absorbing tidal energy and slowing the movement of water. Without them, incoming tides reach the city faster and with more force.
Deep shipping channels dredged through the lagoon to connect Venice’s port to the open Adriatic have also changed water flow. Research comparing conditions before and after channel excavation shows that dredging increased the volume of water moving through the lagoon’s inlets with each tidal cycle. Wider, deeper channels let water rush in more easily during floods. The lagoon that once buffered Venice from the sea now conducts storm surges toward it more efficiently.
How the MOSE Barriers Work
Venice’s primary defense against flooding is MOSE, a system of 78 hinged steel gates installed at the three inlets connecting the lagoon to the Adriatic. When a dangerous tide is forecast, compressed air is pumped into the hollow gates, causing them to rise from the seabed and form a temporary wall that blocks incoming water. The system is designed to handle tides up to 3 meters, far beyond any flood in recorded history.
The standard activation threshold is a forecast tide of 110 centimeters above the local reference level, which is the point where significant parts of the city begin to flood. In practice, operators can raise the gates at lower thresholds (as low as 80 or even 60 centimeters) depending on conditions, offering partial protection during more moderate events. MOSE has been activated successfully on multiple occasions since becoming operational, keeping St. Mark’s Square dry during tides that would have flooded it in past decades.
The barriers buy Venice time, but they are not a permanent solution. As sea levels continue to rise, the gates will need to be raised more frequently and for longer durations, which disrupts port traffic and lagoon ecology. At some point, the frequency of closures could become so high that the lagoon essentially becomes a sealed basin for large portions of the year, creating its own environmental problems. MOSE addresses the symptom of flooding without changing the underlying forces of sinking land and rising seas that are slowly redrawing the boundary between Venice and the water it was built on.