Venice suffers its fourth-worst flood since 1872

Venice, Italy, suffered ocean flooding 1.56 meters (5.1 feet) above mean sea level on Monday, December 1, its fourth-worst flood since modern record keeping began in 1872. The flooding was triggered by a strong low pressure system that passed through Europe, dumping rains of 1-2 inches that helped fill the salt-water lagoon surrounding Venice with high levels of river run-off. The counter-clockwise flow of air around the low brought warm, southerly winds up the length of the Adriatic Sea, which piled up high levels of ocean water into the lagoon. Sustained winds reached 30 mph on the morning of December 1 in Venice. High tides submerged the city again yesterday, but only to a depth of 1.02 meters. As the low pulls away from Italy and river run-off gradually subsides, no further flooding of Venice should occur this week.


Figure 1.Flooding in Venice from the December 1, 2008 flood. Image credit: Venice Water Authority.

Sinking land, rising seas
When Venice was founded sixteen hundred years ago, sea level was about six feet lower than today. Sea level has risen over the centuries as a natural response to our continued emergence from the last Ice Age, thanks to melting of glaciers and ice caps. Global sea level rose eight inches during the 20th century (2 mm/year). The sea level rose by only about 4-5 inches in the Adriatic Sea near Venice during that time, though. However, the rate of global sea level rise has increased by 50% to 3 mm/year in recent decades and was 5 mm/year near Venice during the period 1993-2008. As the seas have risen, Venice has sunk. Since 1897, natural compression of the sediments Venice sit on has resulted in the city sinking about 1.5 inches. Additional sinking of three inches due to pumping of fresh water from an aquifer beneath the lagoon between the 1920s and early 1970s has left the city 4.5 inches lower than at the beginning of the century. In 1900, famed St. Marks Square flooded at high tide six times per year, on average. By 1999, that number had increased to 99 times per year, thanks to the combined effects of higher sea levels and the sinking of the city.


Figure 2. Satellite image of Venice. The city is on an island in a lagoon that opens to the ocean in three places. Image credit: NASA Earth Observatory.

Jetties
Jetties built at the three entrances to Venice's lagoon help to hold back the sea, but have also acted as barriers to the natural inflow of fresh sand to replenish beaches within the lagoon. This sand now accumulates along the jetties. The jetties also act as barriers that disrupt ocean currents and contribute to coastal erosion.

Barrier island erosion
Venice's lagoon is protected by a number of barrier islands. These islands are steadily eroding, due to construction projects, summer beach crowds, and beach-going vehicles. Sea walls were built along many of these barrier islands in the 14th century, but high tides now regularly overtop these sea walls.

Salt marsh erosion
Additional flooding in Venice occurs because the protective salt marshes in the lagoon surrounding the city are steadily eroding. This is largely due to the fact that 600 years ago Venice diverted the courses of the four major rivers that flowed into the lagoon, forcing them to empty into the ocean directly. This was done so that sediments would not build up and allow attackers to approach the city via land. Without new sediments to replenish them, the marshes in the lagoon have been steadily eroding away. Water pollution has also contributed to erosion problems. The pollution kills eelgrass, the chief building block of the salt marshes.

Protecting Venice
The "official" word on climate, the 2007 report of the U.N.'s Intergovernmental Panel on Climate Change (IPCC), predicts a 0.6 - 1.9 foot sea level rise by 2100. However, they cautioned in their report that due to the lack of knowledge about how melting glaciers behave, the actual sea level rise might be higher. A paper published by Pfeffer et al. in Science this September concluded that when considering these unknown glacier melting processes, the "most likely" range of sea level rise by 2100 is 2.6 - 6.6 feet (0.8 - 2.0 meters). These sea level rise possibilities make it imperative that Venice construct barriers to keep the sea out. In 2003, construction began on the MOSE Project, a $5.5 billion system of inflatable gates that will rise up out of the ocean bed to block incoming storm surges from the Adriatic Sea during storm situations. The barriers will block all three entrances to Venice's lagoon, but allow ship traffic to pass through using a lock system. The gates are scheduled to be completed in 2012. In theory, the gates will protect Venice up to a maximum sea level rise of two meters (6.6 feet), which should be adequate for the remainder of the century. However, some scientists have argued that the gates will only protect Venice up to a sea level rise of one foot, and will allow high levels of toxic pollution to build up in the lagoon without the cleansing action of the tides to disperse the pollutants. A 2005 paper warned that if sea level rises 0.5 meters (1.6 feet), the gates would still allow considerable flooding of Venice, and block or delay 2/3 of all shipping traffic trying to pass through the gates.


Figure 3. Schematic drawing of the MOSE gate in the inflated (closed) position. Normally, the gate lies flat on the ocean bottom. When a storm surge threatens, the hollow top of the gate will be filled with air, and the hinged gate will rise to the surface, blocking any incoming storm surges. Image credit: Venice Water Authority.

Resources
The Venice Water Authority has complete info on the MOSE construction project.
NOVA aired a 1-hour show in 2002 on the sinking of Venice, and has some nice interactive web features to look at.

Jeff Masters