Acqua alta

An acqua alta is an exceptional tide peak that occurs periodically in the northern Adriatic Sea. The term is applied to such tides in the Italian region of Veneto. 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:

intense when the measured sea level is between 80 cm and 109 cm above the standard sea level ;
very intense when the measured sea level is between 110 cm and 139 cm above the standard;
exceptional high waters when the measured sea level reaches or exceeds 140 cm above the standard.

Generally speaking, tide levels largely depend on three contributing factors:

An astronomical component, which results from the movement and alignment of the Moon and the Sun; this component is dependent upon the laws of the astronomical mechanics and can be computed and accurately predicted for the long run
A geophysical component, primarily dependent upon the geometric shape of the basin, which amplifies or reduces the astronomical component and, because it is dependent upon the laws of the physical mechanics, can be also computed and accurately predicted for the long run ;
A meteorological component, linked to a large set of variables, such as the direction and strength of winds, the location of barometric pressure fields and their gradients, precipitation, etc. Because of their complex interrelations and quasi-stochastic behavior, these variables cannot be accurately modeled in statistical terms. Consequently, this component can only be forecast for the very short run and is the principal determinant of acqua alta emergencies that catch Venetians unprepared.

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.

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 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" physically 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:

the building of the Railroad Bridge 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, which obviously restrict the natural movement of water;
the building of the Ponte della Libertà, which connects Venice to the land;
the building of the Riva dei Sette Martiri, an extension to the Riva degli Schiavoni;
the building of the artificial island Tronchetto used as a car and bus terminal :
the doubling of the Railroad Bridge.
''Acqua alta'' in Venice

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 half of the city. The study provided Venetians with the following reference guide:

Sea level	Area of Venice submerged
+90 cm	1.84%
+100 cm	5.17%
+110 cm	14.04%
+120 cm	28.75%
+130 cm	43.15%
+140 cm	54.39%
+150 cm	62.98%
+160 cm	69.43%
+170 cm	74.20%
+180 cm	78.11%
+190 cm	82.39%
+200 cm	86.4%

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.