The temporary flooding that submerges parts of Venice, known locally as “Acqua Alta” or “high water,” results from a convergence of multiple environmental and human factors. This phenomenon occurs when the tide rises significantly above the normal level. Understanding why Venice floods requires examining the interplay between global climate patterns, the city’s unique geography, and the consequences of past human engineering projects. The flooding typically affects the lowest-lying areas, such as St. Mark’s Square, and has grown more frequent and severe, threatening the city’s historical and structural integrity.
Meteorological and Global Drivers
The immediate causes of an Acqua Alta event are meteorological, centering on specific wind patterns and atmospheric pressure anomalies within the Adriatic Sea. The most significant factor is the Sirocco, a warm, moist wind blowing from the southeast that travels up the long, narrow basin of the Adriatic, effectively pushing a wall of water toward the northern end where the Venetian Lagoon is situated. This wind-driven surge is amplified by the enclosed shape of the sea, which can create a standing wave, or seiche, further raising the water level.
Concurrently, a drop in barometric pressure over the area contributes to the water rise through a process called the inverse barometer effect. Lower atmospheric weight pressing down on the sea surface allows the water to bulge upward, adding several centimeters to the tide height. These localized conditions often combine with the regular astronomical high tides that occur during the new and full moons, setting the stage for an exceptional water event.
Over the long term, global sea level rise acts as a constant background driver, making all high-water events more dangerous. As the average sea level steadily increases due to climate change, the base water mark from which all tides begin is incrementally higher. This means that even a moderate storm surge today can easily exceed the historical flood thresholds that once defined an extreme Acqua Alta.
The Sinking City and Lagoon Geography
The city of Venice itself is physically sinking, compounding the effect of rising sea levels. Over the last century, a combination of natural geological compaction and historical extraction of groundwater for industrial use caused the city to subside by approximately 23 centimeters relative to the mean sea level. Although groundwater pumping has largely ceased, natural compaction continues, with the city and surrounding lagoon areas currently subsiding at rates of one to two millimeters per year.
The geography of the Venetian Lagoon also funnels high tides directly into the city’s canals. The lagoon is connected to the Adriatic Sea by three large inlets: Lido, Malamocco, and Chioggia. This configuration acts like a funnel, allowing storm surges and high tides to rush quickly into the shallow lagoon basin.
Once inside the lagoon, the water encounters the city, which is built on wooden piles and low-lying islands. Piazza San Marco, one of the city’s lowest points, is often the first to flood, even during relatively minor high-water events.
Anthropogenic Factors Exacerbating Flood Risk
Human interventions in the lagoon environment, beyond the historical groundwater extraction, have significantly altered the natural hydrodynamics and increased flood risk. The dredging of deep-water shipping channels allowed large commercial and cruise ships access to the port of Marghera. The creation of channels like the Canale dei Petroli deepened the lagoon floor and widened the inlets connecting the lagoon to the sea.
These deeper channels act as high-speed conduits, allowing a much larger volume of water to flow into and out of the lagoon faster than historically occurred. This change in flow regime reduces the natural damping effect of the shallow lagoon. Where it once took 90 to 120 minutes for a tide to fully enter, the process is now quicker, intensifying high-water peaks. The altered flow also contributes to the erosion of the lagoon’s fragile seabed and salt marshes, which naturally dissipate tidal energy.
The MOSE Project and Flood Control
To combat the escalating threat of Acqua Alta, Italy constructed the MOSE project. This large-scale engineering initiative consists of a system of 78 mobile flood barriers installed at the three main inlets connecting the lagoon to the Adriatic Sea. The barriers are designed to be raised only when a dangerously high tide is forecast.
The mechanism uses buoyancy, with the hollow steel gates resting on the seabed, filled with water, during normal conditions. When a high tide, typically forecast to exceed 110 centimeters, threatens the city, compressed air is pumped into the gates, forcing the water out and causing the gates to pivot upward to form a continuous line of defense. This seals the lagoon off from the sea, preventing the surge.
Since becoming operational, the MOSE system has successfully protected the city from several major high-water events. The project is designed to withstand a sea level rise of up to 60 centimeters, offering protection from tides up to three meters high. While it provides a functional defense against immediate flooding, the long-term challenge remains managing the frequent closures required as global sea levels continue to rise.