Acqua alta


Acqua alta is the term used in Veneto for the exceptional tide peaks that occur periodically in the northern Adriatic Sea.
The peaks reach their maximum in the Venetian Lagoon, where they cause partial flooding of Venice and Chioggia; flooding also occurs elsewhere around the northern Adriatic, for instance at Grado and Trieste, but much less often and to a lesser degree.
The phenomenon occurs mainly between autumn and spring, when the astronomical tides are reinforced by the prevailing seasonal winds that hamper the usual reflux.
The main winds involved are the sirocco, which blows northbound along the Adriatic Sea, and the bora, which has a specific local effect due to the shape and location of the Venetian lagoon.

Causes

Precise scientific parameters define the phenomenon called acqua alta, the most significant of which is measured by the hydrographic station located nearby the Basilica di Santa Maria della Salute.
Supernormal tidal events can be categorized as:
Generally speaking, tide levels largely depend on three contributing factors:
Two further contributing natural factors are the subsidence, i.e. the natural sinking of the soil level, to which the lagoon is subject, and eustasy, i.e. the progressive rise of sea levels.
While these phenomena would occur independently of human activity, their effects have increased because of inhabitation: the use of lagoonal water by the industries in Porto Marghera sped up subsidence, while global warming has been linked to increased eustasy.
Venice's "Tide Monitoring and Forecast Center" evaluates that the city has lost 23 cm in its elevation since 1897, the year of reference, 12 of which are attributable to natural causes, 13 are due to the additional subsidence caused by human activity, while the "elastic recovery" of the soil has allowed the city to "gain back" 2 cm.

Geophysical determinants linked to the Adriatic Sea

The long and narrow rectangular shape of the Adriatic Sea is the source of an oscillating water motion along the basin's minor axis.
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.
Because the timeframe of both oscillations is comparable to naturally occurring 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.
Should meteorological conditions hamper the natural outflow of excess tidal water, high waters of greater magnitude can be expected in Venice.
: the fondamenta Venier flooded on December 1, 2008.

Specific characteristics of the Venetian lagoon

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 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.
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 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 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:

Affected portions of the city

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, the presence of full parapets along the proximate channel, and the layout of the sewer and water drainage network.
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:
Sea LevelArea of Venice submerged
+90 cm1.84%
+100 cm5.17%
+110 cm14.04%
+120 cm28.75%
+130 cm43.15%
+140 cm54.39%
+150 cm62.98%
+160 cm69.43%
+170 cm74.20%
+180 cm78.11%
+190 cm82.39%
+200 cm86.4%

To assist pedestrian circulation during floods, the city installs a network of gangways 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.

Monitoring, alerting and control

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. The centre's unique expertise on the phenomenon enables it to produce forecasts of remarkable accuracy, usually for the following 48 hours, 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.
When an acqua alta event is forecast, owners of commercial and residential property that is likely to be affected are contacted by phone 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 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.
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.

Countermeasures

The MOSE project 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" 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.

Statistics

Regular scientific record-keeping of lagoonal water levels is considered to have begun in 1872, although some researchers suggest pushing this date to 1867, when an exceptional event was measured. However, because the first modern marigraph for regular tide monitoring was installed in Venice only in 1871, most documentation on the subject adopts the following year as the golden standard.
The Venetian Institute for Science, Literature and Arts was appointed to the task by the newly formed Italian Kingdom, thus replacing the Magistrato alle Acque in 1866 upon annexing the city. The Institute ceased to exercise its monitoring and record-keeping functions in 1908, when the task, along with records and instruments, was passed to the Hydrographic Office of Venice.
After the unprecedented acqua alta of 1966, the city set up a dedicated service to analyse data, monitor fluctuations, and forecast high tides, which is also charged with continuously keeping the population informed. Renamed Tide Monitoring and Forecast Center in 1980, the service has absorbed the record-keeping functions of the Hydrographic Office.

Historical records

Early records

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 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.

Middle Ages

The first documented description 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, completely destroyed Metamauco, Venice's political centre before the Doge's residence was moved to Rialto.
Local chroniclers report that in 1240 "the water flooded the streets higher than a man". 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".
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".

Modern era

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".
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... 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.
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 and 1867.

Exceptional high waters since 1923

According to the records of the Tide Monitoring and Forecast Centre of Venice, these are the maximal documented levels :
In Kozue Amano's utopian science fantasy manga series Aria and its anime adaptation, acqua alta is a phenomenon that happens in the lands on Mars referred to as Neo Venezia.
Donna Leon refers to acqua alta in multiple books in her Commissario Guido Brunetti Mystery Series, set in and around Venice. For example: