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e-Journal

 

Venice and the Environmental Hazards of Coastal Cities
(Released December 2006)

 
  by Carolyn Scearce  

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Subsidence, sea level rise, and storm surge

Contents

Every year Venice loses a little land. Over the last century, through a combination of natural geological phenomena and anthropogenic influences, Venice lost approximately 25 cm in height relative to sea level (Spencer et al., 2005 and Frassetto, 2005). The geological factors that contribute to the loss of land include subsidence and sea level change. Additionally, human activities and alterations to the lagoon environment have increased rates of subsidence and erosion.

Venice lagoon, and the surrounding land, rests on the Adriatic plate in a foreland basin between the Alps and the Apennine mountains. Each year, tectonic processes lead to a loss of about 1.0mm height relative to sea level on the Adriatic plate (Carminati et al., 2005). For millions of years, the Adriatic plate has been subducting under the Apennines, causing the subsidence of the land within and around Venice. The city loses an additional 0.3mm per year due to the gradual rise in sea level associated with the process of deglaciation that has been occurring since the end of the last ice age (Carminati et al., 2005). The combined loss of land due to these natural phenomena averages to about 1.3mm per year.

man wading in flooded city
Flooding in the streets of San Marco
During 1930-1970, humans substantially accelerated the process of subsidence due to the extraction of deep-well water (Butterfield, 2005). Industries located in the district of Marghera extracted much of the water during this period. Groundwater pumping stopped, once the impact of this activity came to be properly understood, and the ground level rebounded by approximately 20mm. Still the overall impact lead to the loss of 120mm of height, nearly doubling the rate of subsidence for the twentieth century.

A number of alterations over the centuries have decreased the levels of sediments entering the lagoon. The process started with the diversion of rivers around the fifteenth and sixteenth centuries. Further, the jetties built at the lagoon's three inlets from 1850-1920 increased the ratio of sediments lost during ebb tide to those returned during flood tides (Di Silvio, 2005). Other factors, including the loss of salt marsh vegetation and the building of deep navigation channels within the lagoon have lead to increased rates of erosion, subjecting the lagoon more and more to the influence of the marine environment.

Loss of land has made Venice more vulnerable to flooding events. When low atmospheric pressure systems develop around northern Italy, it sets up a pressure gradient that allows storm surges. Since Venice is located at the 'dead end' of the Adriatic Sea, during these storm events water piles up in the lagoon (Tomasin, 2005). The past 50 years have seen an unprecedented increase in the frequency and intensity of flooding (Lionello, 2005). The confining shape of the Adriatic also can cause a phenomenon known as seiche to occur. If the seiche within the Adriatic becomes synchronized with the tidal cycle, flooding can continue for days after the storm event has passed (Tomasin, 2005).

Another factor that may affect subsidence rates within the lagoon is the materials on which the city is built. Only one consolidated strata layer exists within the lagoon, a desiccated clay layer known as 'caranto'. The thickness of this naturally occurring geological feature ranges from 10 cm to 10 meters. Scientists speculate that the apparently uneven rates at which the Venetian palaces are sinking may be due to lack of support by this hard clay to hold up the heavy structures (Frassetto, 2005).

When anticipating the future impacts of subsidence and erosion, scientists are concerned as to how climate change may affect the lagoon. A great deal of uncertainty exists about the potential impact of increasing temperatures on glacial melting processes. When calculating the rate of future sea level rise, scientists must not only forecast sea temperature changes in polar regions, but also determine the response time of glacial melting increases associated with such changes. Consequently, extremely wide ranging estimates are offered concerning the rise in Mediterranean waters over the next century. Should deglaciation accelerate over the following decades, the waters around Venice may rise at a faster rate than expected.

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