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Mardi, 27 Décembre 2011 20:17

How to Save Venice: Make It Float

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How to Save Venice: Make It Float

By Scott K. Johnson, Ars Technica

Everyone knows that on a sinking ship, you want to pump water out. But what do you do with a sinking city? In this case, the plan might be to pump water in.

The city of Venice has long been valued for its unique character. Built in a lagoon along the coast of Italy, the scenic city is crisscrossed with canals. Its waterlogged nature draws a steady stream of visitors, but also makes it vulnerable to costly flooding. The region sometimes experiences unusually high tides, locally referred to as “acqua alta.” The phenomenon is caused by winds that drive water to “pile up” on the north end of the long and narrow Adriatic Sea. When that coincides with a high tide, the City of Water gets even wetter, and the water level can rise by 1-2 meters.

Two factors are exacerbating the flooding risk to the city: global sea level rise and subsidence. In short, the sea is rising and the city is sinking. Like other cities built on river deltas, the sediment beneath the city is compacting over time. In a natural setting, this compaction would be offset by the deposition of fresh sediment at the surface, but the rivers feeding the lagoon were diverted in the 1500s. As a result, the land surface is sinking, and the salt marshes are suffering for it.

The pumping of shallow groundwater in the mid-1900s also contributed to the problem. Water in the pores between grains of sediment provides pressure that bears some of the load. When pore pressure decreases, or water is removed completely, grains can be packed together more tightly by collapsing the pore spaces. As sediment is compacted, the land surface drops. While the effect was small (less than 15cm), Venice doesn’t have much wiggle room.

A remarkable system of inflatable gates that could close off the lagoon during dangerously high tides, dubbed the MOSE Project, has been in the works for a while now. Funding issues and environmental concerns have plagued the initiative, but it continues to move forward.

Recently, another idea has been discussed. Just as withdrawing groundwater can cause subsidence, injecting water can reverse it. It’s not entirely a two-way street—much of the pore space lost during compaction can’t be recovered—but increased pore pressure can begin to unpack the sediment. Injection was used successfully in Long Beach, California in the late 1950s to halt subsidence caused by oil and gas extraction as well as groundwater usage. After the land surface dropped nearly 30 feet, injection stabilized the subsidence and a slight rebound in land surface elevation (a little over 30cm) was even seen in some spots. Early research indicated that a similar amount of uplift could be achieved in Venice, which could make a big difference for a city on the edge. The precision of those predictions was limited, however, by the lack of detailed knowledge about the layers of sediment beneath the city.

A new paper, published in Water Resources Research, adds that information and uses it to show that the idea really could work in Venice. Without boreholes around the city to provide observations of the stratigraphy, researchers have relied on data gathered by seismic surveys. Like the familiar sonar systems used by submarines, seismic surveys require a (much more powerful) signal to be generated so its return can be analyzed as it bounces off sediment in the subsurface. That’s been difficult to pull off around Venice, though, as the lagoon is too shallow for large boats to be used. And, attempts to use potent air and water guns as seismic signal sources caused problems by kicking up large amounts of sediment.

Back in the 1980s, though, oil and gas companies hadn’t yet been banned from using explosives in settings like this. The Italian National Research Council acquired a large amount of old, raw seismic data from an Italian oil company, and the researchers were able to use it to construct a high-quality, three-dimensional model of the stratigraphy below Venice. This allowed them to confirm the presence of a continuous layer of impermeable clay below which injected water could increase pore pressure, rather than simply bubble up to the surface. It also allowed them to determine the thickness and extent of the various layers proposed to be used for the injection.

The group simulated the effects of 12 injection wells in a ring around the city. The results showed that, after 10 years of continuous seawater injection (a total of almost 150 million cubic meters of water), the city could be lifted 25-30 centimeters. That would greatly cut down on the frequency with which the MOSE floodgate system would have to be activated each year. That, in turn, decreases operational and maintenance costs, and reduces the ecological impact of the system. In addition, the uplift around the city would benefit the slowly-drowning salt marshes in the lagoon.

The study also shows that by varying the pumping rates at each of the 12 wells, a very uniform uplift can be maintained across the city. If some areas of the city rise faster than others, buildings could be damaged—a result that would be counterproductive to the entire enterprise. With careful management, the researchers say that the difference in uplift between two points 100 meters apart would be less than 1 millimeter.

While it may initially sound far-fetched, this could become part of Venice’s plan to mitigate flooding issues, which will only worsen in coming decades. Battling “acqua alta” would be much easier if the city had the high ground.

Image: Daveybot/Flickr

Source: Ars Technica

Citation: “A new hydrogeologic model to predict anthropogenic uplift of Venice.” By P. Teatini, N. Castelletto, M. Ferronato, G. Gambolati and L. Tosi. Water Resources Research, Vol. 47, W12507, Pg. 17, Dec. 7, 2011. DOI:10.1029/2011WR010900


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