Venice – city on a lagoon
Venice is one of the world’s most unusual cities. It is located off the north-eastern coast of Italy (see Heinemann Atlas 3rd edn, page 147). Founded in the 6th century, Venice was the world’s dominant seaport during the late medieval period. It remains well known for its beautiful architecture, marbled buildings, paintings and frescoes. In 1987 Venice was recognised by the United Nations Educational, Scientific and Cultural Organization (UNESCO) as a World Heritage site.
Venice has developed on a series of 118 small islands that are threaded by 160 canals. The city lies in the centre of a lagoon that is separated from the Adriatic Sea by a thin strip of sand. The lagoon is approximately 50 kilometres long and 15 kilometres wide, and its average depth is just 1 to 2 metres. The city is built on millions of wooden poles that are hammered into the swampy ground of the lagoon. For transport around Venice, a city without roads or cars, people either walk or use the boats that traverse the canals.
Acqua alta
Given its location, Venice has always been affected by the regular rise and fall of the tides. But a unique combination of geographical factors has meant that there are times when a higher than usual tide is responsible for flooding parts of Venice. Venetians, the inhabitants of Venice, have termed this event the acqua alta (or rising waters).
Concerns about higher tides
While the acqua alta has always been a part of life in Venice, there are concerns that the tides are now becoming higher and more threatening. The world community became aware of how disastrous the acqua alta could be when a massive flood engulfed Venice on 4 November 1966. The tide on that day rose to a record height of 1.94 metres above normal. More than 5000 Venetians lost their homes. Numerous paintings and other works of art were lost or damaged irreparably by the rising waters. Given the unprecedented magnitude of the flood, research began on whether Venice was in fact ‘sinking’. Scientific research on why Venice experiences regular flooding also began.
Research: tides, ‘sessa’ and ‘bubble’
Tides
The first factor to be studied was the tide itself. The sea level rises and falls simultaneously every six hours at the entrances to the three main canals of Venice.
Sessa
With the Adriatic Sea being almost ‘enclosed’ deep within the Mediterranean region (see Heinemann Atlas 3rd edn, page 147), scientists discovered that its waters are subject to a ‘sessa’. In other words, the waters within the basin of the Adriatic ‘rock’ from side to side like the water in a bathtub.
Bubble
Additionally, a moisture-laden southerly wind brings much precipitation to the Venetian region causing a ‘bubble’ in the sea level which is pushed towards the north of Venice.
Sea levels in the Venice region are highest in the autumn and winter months when the southerly is the prevailing wind. When the sessa and the winds coincide Venice experiences its acqua alta.
Plate movement and global warming
Scientists have also determined that Venice lies near the boundary of the African and Eurasian plates. You can see this in the map of plate tectonics in the Heinemann Atlas 3rd edn, page 184. Subduction of the African plate beneath the Eurasian plate means that Venice has been sinking at the rate of about 2.5 centimetres each century. This finding, combined with rising sea levels due to global warming, explained why Venice’s regular acqua alta has been reaching record levels in recent years.
In November 2000, a flood event in Venice affected more than 93 per cent of the city. A similar flood just a few months later in January 2001 (during which the water rose 111 centimetres above normal levels) was estimated to have cost the city more than 11 billion Italian Lire (approximately $11 million
AUD) in lost working hours when Venetians were either trapped in their homes by the rising waters or forced to evacuate.
A watery future?
Given these findings, there is much concern among the world community that rising sea levels could result in the city of Venice being lost in a watery grave forever.
Archeo Project
The recent Archeo Project, run by the European Union, endeavoured to determine the extent of the effect of climate change and rising sea levels on the Venetian acqua alta.
The research undertaken in this project has used artwork by Canaletto, a Venetian painter (1697–1768). Canaletto used a camera obscura to help him create his artworks. The camera obscura was an 18th-century device that projected an image onto a sketchpad or canvas. Using this method, Canaletto was able to produce strikingly realistic paintings that even included details such as the green scum left on the side of buildings by the regular high tide.
Using this information, researchers determined that the level of the high tide in Venice had risen 80 centimetres since Canaletto’s time. Clearly, something needs to be done in order to save the city of Venice from becoming a modern-day Atlantis.
Saving the city
Since a massive flood in November 1966, many ideas have been put forward about how Venice could be saved. One of the most controversial of these has been an undertaking to install sea defences or floodgates around the city. Systems of floodgates are used with much success in other parts of the world to control tides. Examples include those used at the mouth of the Thames in the United Kingdom and at IJsselmeer in the Netherlands (see the Heinemann Atlas 3rd edn, page 141).
Moses Project
The Moses Project is an ambitious plan to install seventy-nine 300-tonne floodgates around Venice that are hinged to the floor of the lagoon. These movable barriers could be raised and lowered as required.
However, in the face of rising sea levels and the continued subsidence of Venice under the Eurasian plate, the Moses Project has been criticised as a short-term solution. Opponents claim that within 50 years the floodgates would need to be ‘up’ almost permanently, effectively sealing off the city from the open waters of the lagoon that have been its lifeline in terms of trade, transport and communication. The subsequent transformation of the Venetian canals into a stagnant pond and the effects of this on marine life and health standards could be devastating.
'Safeguarding
Venice’ campaign
Perhaps a more holistic view of the situation confronting Venice has been taken up by UNESCO. Its 'Safeguarding Venice' campaign has been in operation for more than 30 years. UNESCO has determined that there are three keys to saving Venice from the rising waters of the lagoon in which it lies. These are:
- combating the subsidence of the city
- limiting the erosion of the contours of the lagoon
- installing defences against the sea and pollution.
Your work in this Atlas Update will be to determine the effectiveness of the efforts of UNESCO and others to save one of the world’s most culturally significant cities.
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