Acqua alta in Venice (meaning “high water” in Italian) is an extreme high tide that occurs in the Venetian lagoon between autumn and spring due to the geographical configuration of the Northern Adriatic Sea, wind patterns in the area, and the force of the moon.
During an acqua alta, the city puts up boardwalks in strategic areas and Venetians wear high rubber boots help people get around in Venice. Often during these periods when the acqua is too alta, walking is the only option because boats, including the vaporetto (waterbuses) lines, cannot navigate under the city’s bridges. For help getting around the city easily during this season, be sure to download the Prontopia app and get in-person help from a friendly local who knows how to navigate the terrain by foot amid high water, or help finding some emergency rubber boots…
“Acqua alta” is an expression originating from the dialect of Venice, and adopted by Italian, indicating dramatic tides in the Adriatic Sea so high that they flood the city of Venice. The main reason for this phenomenon is the astronomical tide that occurs daily during peaks on the days of the new moon and full moon, which can be calculated years in advance with high precision. Weather also strongly effects the regularity of the tides, especially wind and air pressure. The Adriatic Sea is a long narrow body of water, and the Venice lagoon lies at its north-western end is the Venice lagoon. When Sirocco winds blow from the south-east, water accumulates in the northern part of the Adriatic because of the shallowness of the sea in that area. When both Sirocco and Bora (a wind from north-east) winds blow, a collision occurs, the well-known scontratura. High winds can create a more severe acqua alta as the water is slowed from leaving the lagoon. Additionally, when a storm creates a decrease of atmospheric pressure there is an increase in the water level and vice versa. Consequently, a low pressure area over the Northern Adriatic and a high pressure in the South results in a shift of the mass of water to the area of lower pressure, with a variation that can reach up to 20 cm.
Complicated enough? Keep on reading patiently because other factors alter the tides. One has the appealing name of seiche. Due to its shape, the basin of the Adriatic works as a cavity: a series of longitudinal and transverse oscillations occur, called the “seiches”, whose amplitude progressively fades. The effect of the main oscillation, in the longitudinal direction, must be taken into account when forecasting tides. Subsidence and eustasy also play roles in the high tides: subsidience is the sinking of the land due to natural and man-made causes, and eustasy is the sea-level rising, linked to global climatic variations. Subsidience and eustasy both occur naturally over time, yet at the same time, human causes such as development of the lagoon and global warming have made their effects more severe. The lagoon is a fragile environment, and visitors to Venice can help prevent further damage through awareness and sustainable travel choices.
Acqua alta is measured from a hydrographic station at Punta Salute. A sea level higher than 80 cm above the measuring point at Punta Salute is called acqua alta as at this level many problems arise in the lowest areas of Piazza San Marco. When the tide exceeds 100 cm, 5% of public land in Venice’s historical center is flooded; at 110 cm, it’s 12%, but when the level reaches 140 cm, about 59% of the city is flooded. Tides are monitored and forecast by a specific office, the Centro Maree, which publishes a bulletin once a day in the period when tides are more regular (from September to May) and several times a day when floods are more probable. The city uses sirens to warn residents and visitors of an acqua alta. Today, these sirens have been replaced by electronic loudspeakers that, according to the tide level forecast, emit tones of different pitch. Citizens and commuters can subscribe to a service that automatically sends SMS warnings, but anyone in Venice can use the app “hi!tide Venezia.”
Grazie to Prontopia Local Giacomo for this post. Giacomo enjoys sharing his deep knowledge of the city of Venice, and helping travelers get where they need to go in the city. For a better arrival that will have you prepared for what to expect during acqua alta season, download the Prontopia app, and request a local connection for help getting around hassle free from friendly and passionate Venetians like Giacomo!