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Sinking City of Venice

Venice Under Siege

Satellite view of city

Venice homepage

Satellite view of Venice (city)

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

Satellite view of Venice 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

Satellite view of Venice 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.

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NOVA Home Find out what's coming up on air Listing of previous NOVA Web sites NOVA's history Subscribe to the NOVA bulletin Lesson plans and more for teachers NOVA RSS feeds Tell us what you think Program transcripts Buy NOVA videos or DVDs Watch NOVA programs online Answers to frequently asked questions