| |
1. Sea level
The rise in global sea levels over the past century has resulted in
an increase in the mean sea level at Venice of over three inches
since 1897. Coupled over the same period with a lowering of the land
beneath Venice of six inches, large portions of the city are left
awash on far too many days of the year.
2. Square
In 1900, water at extreme high tide spread across Venice's treasured
St. Mark's Square seven times—and this was typical for the
time. By 1996, the city flooded 99 times. The dousing threatens
famous edifices like St. Mark's Basilica and the Doges' Palace and
makes it difficult to deliver goods and simply get around, even on
the raised wooden walkways erected in times of flood.
3. Tidal gauge
In 1897, Venetians established a tidal gauge near the church of
Santa Maria della Salute, with the zero mark indicating mean sea
level. At the time, average tides in the northern Adriatic (without
meteorological interference) oscillated between roughly a foot above
and a foot below this mean level. They fluctuate similarly today;
the trouble is that the mean sea level in and around Venice is now
over nine inches higher than it was in 1897.
4. Ground level
Since 1897, natural compression of sediments beneath the city has
resulted in a lowering of the ground level by about an inch and a
half. More significantly, subsidence brought about by the pumping of
freshwater from an aquifer beneath the lagoon between the 1920s and
early 1970s left Venice resting a good four and a half inches lower
in the lagoon. Subsidence in the historic center is most pronounced
in Castello, the city's easternmost district.
5. Canal
The city's vulnerability is most evident along urban canals.
Motorboat wakes batter buildings, damaging their footings and
foundations and exacerbating the deleterious effects of
exceptionally high water, or acqua alta. The highest tides
disrupt commerce and transportation by making it impossible for
boats to pass under bridges.
6. Building
In the 1600s, normal tides seldom rose over the stone footings at
the base of buildings; today, they do so regularly. The outcome is
risalta salina, or salt rise, as seawater creeps into
permeable stucco and brick above foundations of impervious Istrian
marble. Maintenance is challenging and costly, but if it's
neglected, crumbling and even collapse can result.
Satellite view of lagoon
|
|
|
| |
1. Sea wall
In the 14th century, Venetians built defenses of wooden piles and
rocks to protect the shoreline against the Adriatic. Four centuries
later, as the sea began to threaten the city itself, they
constructed elaborate seawall defenses, or murazzi. Today,
tides regularly breach these walls, making them largely irrelevant.
2. Jetty
The jetties built at the three entrances to the lagoon have held
back more than the sea. They have acted as barriers to the natural
inflow of fresh sand to replenish beaches within the lagoon; that
sand now accumulates abnormally around the jetties. The stone
barriers have also disrupted currents, increasing coastal erosion.
3. Island
The Lido and other barrier islands serve as naturally built
defensive battlements, yet they are suffering from erosion both
natural and human-made. Waves and currents eat away at the
littorals, a process furthered on these long, narrow islands by
houses, campsites, summer beach crowds, and beachgoing vehicles.
4. Salt Marsh
Salt marshes capture sediment, filter pollutants, and increase the
amount of organic material in the soil. Eelgrass, the chief building
block of salt marshes, also retards erosion, because its long roots
consolidate the lagoon bed. Today, with water pollution killing
eelgrass, salt marshes in the lagoon are suffering.
5. Fishing boat
Certain fishing techniques have a devastating effect on lagoon beds,
leaving them barren. A suction device used illegally by some
fishermen, for example, vacuums up between 16 and 20 inches of bed
in a search for clams. All other organisms unfortunate enough to be
in the way get sucked up as well.
6. Tanker
Tankers have ferried more than 12 million tons of chemicals and
crude oil products through the lagoon each year over the past
decade. In 1995, five tons of light crude oil spilled into the
lagoon. While the impact from this accident was minor, experts have
estimated that one-third of a large oil tanker's load could provoke
an ecological disaster in the lagoon, severely damaging fish and
mollusk farms, clam gathering, and wildlife.
Satellite view of region
|
|
|
| |
1. Dump
Authorities have identified no fewer than 17 abandoned dumps in the
lagoon. They contain about five million cubic yards of waste
material. Rain and tides help distribute garbage and pollutants from
these dumps around the lagoon, threatening water quality and
wildlife.
2. Factory
Between 1950 and 1970 when the industrial zone of Marghera was
developed, refineries, chemical plants, and heavy-metal factories
drained their liquid waste partly into the lagoon and partly into
dumps. Today, 80 percent of Marghera's industrial waste is treated,
but the effects of the previous dumping remain, with currents and
erosion continuing to disperse pollutants.
3. Sea
Meteorological conditions can accentuate high tides, worsening the
impact of higher sea levels. Winds like the bora, a cold northerly
in the Adriatic Sea, and the sirocco, a hot, dust-laden wind from
the Libyan desert, can trigger dangerously high tides, particularly
during periods of low atmospheric pressure.
4. River
Centuries ago, in order to halt the silting up of the lagoon, the
Republic of Venice diverted four rivers that originally flowed into
the lagoon so that they emptied directly into the sea. The loss of
the silt and sediment that normally replenished the lagoon has been
slowly transforming the delta environment into a marine one.
5. Farm
While Venetians were able to divert rivers around the lagoon, they
were and continue to be unable to stop runoff from agricultural and
livestock farming in the lagoon's roughly 700,000-square-mile
drainage basin. Today, fully 53 percent of the phosphates and other
pollutants that enter the lagoon come from these sources.
6. Town
Venice's drainage basin is heavily developed, with more than 100
municipalities. Nearly half of all pollution that washes into the
lagoon comes from industrial, civil, and urban sources in the
region. About 1,400,000 people live in the basin, but when one
considers the quantity of nutrients and organic matter generated,
environmentalists say, it is as if 4,000,000 people lived there.
|
|
|
