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Monday 27th February 2017

Venice Sinking

Rialto Bridge in Venice

Venice - Rialto Bridge on Grand Canal

Why is Venice's miraculous beauty so fragile?

Venice is sinking. Why?

A clarification to make is a distinction that is often missing from reports on Venice.

There are two different phenomena that cause concern in the relationship between Venice and water.

One is summed up by the often-used phrase "Venice is sinking": it is the phenomenon of land subsidence, which has accompanied Venice all her life.

The other is what is called "acqua alta" (high water): this is a product of the regular movement of high and low tides, and occurs those times, more generally in winter, when a combination of tide, sirocco (a strong, stormy wind from North Africa) and seiche (the periodic oscillation in the water levels, like a long wave all over the Adriatic coasts) can generate a greater inflow of water into the Venetian Lagoon, flooding squares or streets of the city and, on rarer occasions, the front steps or ground floors of ancient palazzi.

To improve the acqua alta situation, the MOSE project (Modulo Sperimentale Elettromeccanico) is in process, to be completed in 2016.

These two phenomena are obviously distinct but related, in that the lowering of Venice in relation to the sea level makes the acqua alta events worse.


Causes of sea level changes and of subsidence

The causes of Venice's subsidence (sinking) are both natural and human-induced.

There are many different factors influencing a change in the relative sea level of a particular place. They include changes in ocean currents, atmospheric pressure, winds; changes in the mass of ocean water generated by loss or gain of global ice; changes in the volume of ocean water provoked by temperature fluctuations; earth movements such as subsidence (earth surface going down) or uplift (earth surface rising).

In the case of Venice a major factor is subsidence, dropping in elevation.

Venice has been sinking since its very beginning.

One of the causes of Venice subsidence is entirely natural, related to plate tectonics, and has been affecting the whole area for a long time: the tectonic movement leading to the thrusting of the Apennine Mountains from the south and of the Alps mountain range from the north over the Po River Plain. The weight of the immense Alps is overwhelming all of northern Italy, slowly driving it into the sea. At the same time the Adriatic plate, the ground on which Venice ultimately sits, is being pushed down, under the Apennines.

The Alps (30-40 million years old) and the Apennines are geologically young mountain ranges, which explains why their peaks, that have not had enough time to be eroded and rounded yet, are high, pyramidal and rugged. This means that they are not well stabilised in tectonic terms. Italy itself is a geologically young land.

Another cause of subsidence is the compaction of the sediments beneath Venice under loading.

The weight of Venice centuries-old buildings is basically pushing down on the sediments of the Venetian Lagoon which is a very frail, always changing and dynamic environment. This loading squeezes water out, and the "sponge" on which Venice sits becomes thinner. Under the uppermost layers of the Lagoon on which Venice rests is a 1-to-2-kilometre-thick layer of sediments brought by the many rivers flowing from the mountains and discharging them into the Lagoon. These deposits are compacting, getting denser and decreasing in volume. Furhermore, these deeper layers under Venice:

"erode the pre-lagoonal floodplain that existed before the arrival of the rising Adriatic Sea to the Veneto region, thus increasing the water at the expense of the solid matter."


Groundwater extraction

For many years in the last century much of Venice's subsidence was caused by dewatering.

During the fast-paced industrial development of the early 20th century, an industrial complex was built near Marghera on the edge of the Lagoon, with refineries, chemical establishments and metalworking factories. The Porto Marghera and neighbouring Mestre area on Venice mainland turned Italy's "industrial triangle" Milan-Venice-Genoa (the last a port) into an "industrial quadrangle" with the addition of Marghera. The increased requirements of water for industrial use and drinking water for a growing local population led to the constant extraction of huge amounts of water from deep aquifers below ground and beneath the Lagoon.

Sea level rise and land subsidence

Groundwater removal between 1930 and 1970 had the effect of greatly rising the rate of Venice subsidence in that period.

This problem affects not only Venice but also other coastal cities in the world. Researchers have found that what causes coastal big cities to sink is groundwater extraction.

In 2014, a team of Dutch scientists of the Deltares Research Institute in Utrecht, speaking at the European Geosciences Union General Assembly, revealed the results of their study of coastal cities, according to which the ground in some areas of the world is going down 10 times faster than the sea level is rising, due to groundwater extraction as a source of drinking water.

Subsiding land is a bigger immediate problem for the world's coastal cities than sea level rise, they said, and is wrongly underestimated by governments and policymakers while all the attention is focused on global-warming-induced sea level rise. Dr Gilles Erkens said:

"The most rigorous solution and the best one is to stop pumping groundwater for drinking water, but then of course you need a new source of drinking water for these cities. But Tokyo did that and subsidence more or less stopped, and in Venice, too, they have done that."

Tokyo had sunk by two metres before halting that practice.

Land subsidence caused by groundwater extraction is also a contributing factor to longer and larger floods. Subsidence also tends to occur naturally in coastal cities, because the soil there is weaker.

After the Italian government stopped dewatering in 1970, studies in the 2000s indicated that the major sinking has been arrested and the rate of subsidence has decreased, thus showing that this was an important factor in the descent of Venice. The latter didn't stop entirely, because there are still other causes at work, such as the restoration of buildings which are in that way made heavier.

Pietro Teatini, a researcher with the University of Padova in Italy, explains: "When some people restore their buildings, for example, they load them, and they can go down significantly by up to 5mm in a year." How far they go down, he adds, depends on the type and compaction of soils under them.

The results of the 2014 Dutch study confirm those of two previous papers published in Geophysical Research Letters.

The first paper examined the historically-changing contribution of groundwater removal for human water needs, like drinking, irrigation and others, to global sea level. It found that human removal of water from deep aquifers, water that eventually ends in the sea, has contributed to observed sea level rise from 1900 to 2000.

This non-climate component of sea level rise is both significant and rapidly increasing, at present comprising 15-25% of the current rate of sea level rise. This rate had been falling, but there has been a constant increase in the rate of groundwater removal, which has caused a slight acceleration in the rate of sea level rise over the past 50 years.

The study found that, once this non-climate factor is eliminated, there is no evidence left for a climate-related acceleration. Dewatering was adding a significant, growing, but often overlooked, input of water to the global oceans and was responsible for a non-negligible amount of sea level rise. Without direct human contribution not associated to any climate change, ie dewatering and impounding, the sea level would not even show a clear rate of increase. Therefore, the paper concludes, the apparent acceleration in the rate of sea level rise has been caused solely by dewatering (which is still on the increase) and not by climate change.

The second paper highlights that many geophysical phenomena, including sea levels, undergo a 60-year-period cycle of fluctuations; therefore care should be taken, when talking about "sea level acceleration", that the GSLR (Global Sea Level Rise) is not simply part of this cycle: the 20-year-period of rise considered by the United Nations' Intergovernmental Panel on Climate Change (IPCC) to be evidence of acceleration is too short to reach that conclusion:

"The bottom line is this: the more people look for the anticipated acceleration in the rate of sea level rise, the less evidence they seem to find in support of it. All the while, we eat into the 21st century with a rate of sea level rise not much different from that experienced during the 20th century—and one which was hardly catastrophic, readily proven by a simple look around."



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