Acqua alta

The long and narrow rectangular shape of the Adriatic Sea is the source of an oscillating water motion (called seiche) along the basin's minor axis.[5]

The principal oscillation, which has a period of 21 hours and 30 minutes and an amplitude around 0.5 meters at the axis' extremities, supplements the natural tidal cycle, so that the Adriatic Sea has much more extreme tidal events than the rest of the Mediterranean. A secondary oscillation is also present, with an average period of 12 hours and 11 minutes.[6][7]

Because the timeframe of both oscillations is comparable to naturally occurring (yet independent) astronomical tides, the two effects overlap and reinforce each other. The combined effects are more significant at the perigees, which correspond to new moons, full moons and equinoxes.[3]

Should meteorological conditions (such as a strong scirocco wind blowing north along the major axis of the Adriatic basin) hamper the natural outflow of excess tidal water, high waters of greater magnitude can be expected in Venice.

Acqua alta in Venice, fondamenta di Borgo, December 2019

The particular shape of the Venetian lagoon, the subsidence which has been affecting the soil in the coastal area, and the peculiar urban configuration all magnify the impact of the high waters on city dwellers and on the buildings.

Furthermore, the northbound winds called bora and sirocco often blow directly towards the harbors that connect the lagoon to the Adriatic Sea, significantly slowing down (and, at times, completing blocking) the outflow of water from the lagoon toward the sea. When this occurs, the ebb is factually prevented inside the lagoon, so that the following high tide overlaps with the previous one, in a perverse self-supporting cycle.[4]

The creation of the industrial area of Porto Marghera, which lies immediately behind Venice, amplified the effects of high waters for two reasons: first, the land upon which the area is built was created by filling large parts of the lagoon where smaller islands just above sea level previously lay. These islands, called barene, acted as natural sponges (or "expansion tanks") when high tides occurred, absorbing a significant portion of the excess water.

Second, a navigable channel was carved through the lagoon to allow oil tankers to reach the piers. This "Oil Channel" factually linked the sea to the coastal line, running through the harbor in Malamocco and crossing the lagoon for its entire width. This direct connection to the sea, which was obviously non-existent at the time of Venice's foundation, has subjected the city to more severe high tides.

Porto Marghera and its facilities are not the only human-made contributors to higher tides. Rather, the municipality of Venice has published a study[8] that suggests the following initiatives may have had an irreversible and catastrophic impact on the city's capacity to withstand acque alte in the future:

• the building of the Railroad Bridge (1841/1846) connecting Venice to the land, because its supporting pillars modify the natural motion of lagoonal water;
• the diversion of the river Brenta outside the Chioggia basin, which drained the 2,63 hectares of the river's delta that functioned as expansion tanks, absorbing extra lagoonal water during high tides;
• the building of offshore dammed piers (Porto di Malamocco, 1820/72; Porto di S. Nicolò, 884/97; Porto di Chioggia, 1911/33), which obviously restrict the natural movement of water;
• the building of the Ponte della Libertà (1931/33), which connects Venice to the land;
• the building of the Riva dei Sette Martiri (1936/41), an extension to the Riva degli Schiavoni;
• the building of the artificial island Tronchetto used as a car and bus terminal (17 hectares, 1957/61):
• the doubling of the Railroad Bridge (1977).

Fondamenta del Squero flooded in December 2019

The flooding caused by the acqua alta is not uniform throughout the city of Venice because of several factors, such as the varying altitude of each zone above sea level, its distance from a channel, the relative heights of the sidewalks or pavements (fondamenta), the presence of full parapets (which act as dams) along the proximate channel, and the layout of the sewer and water drainage network (which acts as a channel for the flooding, as it is directly connected with the lagoon).

These factors account for the severity and spread of a supernormal tidal phenomenon; as a city-commissioned study showed, a tide up to 90 cm above sea level leaves Venice virtually unaffected, while 50 cm of additional water affects more than one-third of the city. The study provided Venetians with the following reference guide:[9]

To assist pedestrian circulation during floods, the city installs a network of gangways (wide wooden planks on iron supports) on the main urban paths. This gangway system is generally set at 120 cm above the conventional sea level, and can flood as well when higher tides occur.

The reference hydrographic station at Punta della Salute.

An electronic display showing the tide forecast in Piazzale Roma during the acqua alta of December 1, 2008.

The Tide Monitoring and Forecast Centre of the City of Venice is fed information via a network of hydrographic stations, located in both the lagoon and the Adriatic Sea (on a scientific platform belonging to the Italian National Research Committee, CNR). The centre's unique expertise on the phenomenon enables it to produce forecasts of remarkable accuracy, usually for the following 48 hours (longer forecasts are also issued, but tend to be less reliable, as discussed above), by analysing the meteorological and hydrographic data available.

Forecasts are then announced to the population via the centre's website and dedicated phone lines, through local newspapers, on electronic displays, and at some stops of the vaporetti (public transport).

When an acqua alta event is forecast, owners of commercial and residential property that is likely to be affected are contacted by phone (a free service provided by the municipality) or SMS.
"Very intense" events warrant alerting the whole population, which is accomplished by sounding a dedicated system of sirens located throughout the city.

On December 7, 2007, the alert system was modified (in Venice alone) to signal the magnitude of expected "very intense" tidal events to the population: sirens sound a first "await instructions" whistle to catch the population's attention, then produce a sequence of whistles whose number increases with the expected tide level (according to a published equivalence table).[10]

While not radically innovative, the new system communicates in greater detail the extent of the expected flooding to the population. The previous system, still used in the rest of the Venetian lagoon, only provides three levels of warning: the signal is sounded once for a tide above 110 cm., twice for tidal forecasts above 140 cm. and thrice for those above 160 cm. The new system was first used on March 24, 2008, communicating an accurately forecast tide level above 110 cm.

The MOSE project[11] (which stands for Modulo Sperimentale Elettromeccanico, i.e. "Experimental Electromechanical Module") has been under construction since 2003, the long time period partly because of budget constraints, and partly because of the sheer complexity of the undertaking. The project should significantly reduce the effects of "exceptional high waters" (but not those of lesser, yet detrimental, tidal events) by completing the installation of 79 separate 300-ton flaps hinged on the seabed between the lagoon and the Adriatic sea. While normally fully submerged and invisible, the flaps can be raised preemptively to create a temporary barrier, which is expected to protect the city from exceptional acqua alta.

The first record of a large flood in the Venetian lagoon dates back to the so-called Rotta della Cucca, reported by Paul the Deacon[13] as having occurred on October 17, 589. According to Paul, all rivers with mouths in the northern Adriatic, from the Tagliamento to the Po, overflowed at the same time, completely modifying the hydro-geologic equilibrium of the lagoon.

The first documented description[14] of acqua alta in Venice concerns the year 782 and is followed by other documented events in 840, 885, and 1102.

In 1110 the water, following a violent sea storm (or, possibly, a seaquake and its subsequent tsunami), completely destroyed Metamauco (ancient name for Malamocco), Venice's political centre before the Doge's residence was moved to Rialto.

Local chroniclers report that in 1240 "the water (that) flooded the streets (was) higher than a man".[14] Other events are recorded to have occurred in 1268, 1280, 1282, and on December 20, 1283, which was probably an abnormally significant event, since a chronicle reported that Venice was "saved by a miracle".[14]

Chroniclers report that high tides occurred in 1286, 1297, and 1314; on February 15, 1340; on February 25, 1341; on January 18, 1386; and on May 31 and August 10, 1410.

In the 15th century, high tides were recorded in 1419 and 1423, on May 11, 1428, and on October 10, 1430, as well as in 1444 and 1445. On November 10, 1442, the water is reported to have risen "four feet above the usual".[14]

Monument to sea and land soldiers, sculpted by Augusto Benvenuti, to commemorate the help given by the army during the catastrophic 1882 flood (Biennale gardens)

High waters were recorded on May 29, 1511; in 1517; on October 16, 1521; on October 3 and, again, on December 20, 1535. Local chronicles also attest to floods occurring in 1543; on November 21, 1550; on October 12, 1559; and in 1599.

The year 1600 was characterized by a high frequency of events, with floods on December 8 as well as December 18 and 19. The latter event was probably remarkable, since there are also records of very violent sea storms that, having "broken indeed the shores in several places, entered the towns of Lido Maggiore, Tre Porti, Malamocco, Chiozza, et cetera".[14]

Another noteworthy acqua alta took place on November 5, 1686. Several chronicles of the time, among them one written by a scientist, concur in reporting that "the waters reached the outdoor floor of ... [Sansovino's] Lodge", which is the monumental entrance to the Campanile di San Marco. A similar level was reached during the exceptional flood of November 4, 1966, which allowed scholars in the late 1960s to recreate a likely scenario for the 1686 flood. After accounting for the rebuilding of the Lodge after the 1902 fall of the Campanile and for subsidence, estimates concluded that the tide may have been as high as 254 cm above today's standard sea level.[15]

In the 18th century, records became more abundant and precise, reporting acque alte on December 21, 1727; New Year's Eve, 1738; October 7, 1729; November 5 and 28, 1742; October 31, 1746; November 4, 1748; October 31, 1749; October 9, 1750; Christmas Eve, 1792; and on Christmas Day, 1794.

Finally, in the decades before the installation of the marigraphs, high waters are recorded to have occurred on December 5, 1839, as well as in 1848 (140 cm) and 1867 (153 cm).

Water levels engraved on the walls of Ca' Farsetti, Venice's City Hall

Levels reached by high waters painted outside a Venice shop

These are the highest water levels documented by the Tide Monitoring and Forecast Centre of Venice:[2]

• Maximum high tide level: 194 cm, recorded on November 4, 1966
• Minimum ebb tide level: −121 cm, recorded on February 14, 1934
• Maximum difference between a high tide and the following ebb tide: 163 cm, recorded on January 28, 1948 and on December 28, 1970
• Maximum difference between an ebb tide and the following high tide: 146 cm, recorded on February 23–24, 1928 and on January 25, 1966