Sinking City of Venice

PBS Airdate: November 19, 2002
Go to the companion Web site

NARRATOR: Ah, Venice. To Italians she's known as La Serenissima, the Most Serene Republic. For twelve hundred years, the city has performed a magical balancing act.

Not quite land, not quite sea, Venice seems to float in its own world. A world that at first glance appears immune to the passing of time. The nine-hundred-year-old St. Mark's Basilica presides over a giant public square that's changed little since the days of Casanova and Vivaldi. But in recent years, the illusion that is Venice has begun to crack, and this idyllic city has been showing a darker side.

The trouble began on November 4, 1966, when an extremely high tide swept into Venice and refused to leave. For 15 hours, Venice was inundated by the sea. In historic Saint Mark's Square the water was four feet deep. Luckily, no one was killed. But the place was a disaster zone.

In a single day, the city and the world were forced to face a harsh reality: Venice was sinking into the sea.

Today flooding has become a fact of life. Instead of floating above the water, the 15th and 16th century buildings are often filled with it, and the ancient bricks are gradually dissolving away.

CHIANG C. MEI (Massachusetts Institute of Technology): Venice is at a critical point. The problem of flooding must be solved right away.

NARRATOR: But how can Venetians stop the flooding? Not only is the city sinking, but sea level is rising here and all over the world.

EDMUND PENNING-ROWSELL (Flood Hazard Research Center): It's happening. It's happening now. Venice is a trigger. Venice is the first major city in the world to face sea level rise, because it's built right at sea level.

CHARLES E. MCCLENNEN (Colgate University): The Venice you know today cannot be preserved as it is today. It was built on a salt marsh at sea level, in a sinking area, and unfortunately, sea level is rising.

NARRATOR: Is Venice's long and happy marriage to the sea destined for disaster? Or can the City of Canals somehow be saved?

Sinking City of Venice up next on NOVA.

Major funding for NOVA is provided by the Park Foundation, dedicated to education and quality television.

This program is funded in part by the Northwestern Mutual Foundation. Some people already know Northwestern Mutual can help plan for children's education. Are you there yet? Northwestern Mutual Financial Network.

Science: it's given us the framework to make wireless communications clear. Sprint is proud to support NOVA.

And by the Corporation for Public Broadcasting, and by contributions to your PBS station from viewers like you. Thank you.

NARRATOR: Several times a year, sirens sound along the canals of Venice. They can mean only one thing: the water is coming.

Between October and January, the lowest parts of Venice flood almost every day, and much of the city is inundated half a dozen times. During this wet season, Venetians rarely venture far without their rubber boots. And sometimes even that's not enough. They call it "acqua alta," high water.

ALBERT AMMERMAN (Colgate University): It's this sort of slimy, murky water, which isn't very pleasant. But it also contains quite a bit of pollution. So you really don't want to stick your hand in this water or have this water in your entry to your house, in your hallway. Schools can't run. Boats have troubles getting under the bridges. There's a whole series of dysfunctional things which happen with acqua alta.

NARRATOR: It's not just inconvenient. The tide is insidious, creeping into every building without regard for its historical value. Salt water eats away at floors and walls, no matter what century they were built in or what genius architect designed them. The front of St. Mark's Basilica, perhaps the most famous Venetian landmark, is adorned with stones from around the Mediterranean. All of it is being corroded by floods, almost on a daily basis.

Venice is famous as the city of romance, the city of Casanova, where lovers forget their worldly woes. It was once an extremely rich and powerful city, built by merchants and bankers who controlled a shipping empire throughout the Mediterranean. Her wealth and beauty flowed directly from the sea.

So how is it that the water is now threatening to undo all that it made? In the case of Venice, it comes down to "location, location, location." If you were planning to build a city, you could hardly pick a less practical spot.

At the northern tip of the Adriatic Sea, historic Venice sits on what is actually several dozen islands, within a 200-square-mile shallow lagoon. A long chain of barrier islands guards the lagoon from the sea, but three openings in the chain allow ships, and the tide, access to the city. Flooding occurs when exceptionally high tides break through these inlets.

Normally, the tides are controlled by the moon, but the sun also plays a role. These astronomical tides are easily predicted, and they aren't very extreme. The difference between high and low tide in Venice can be as little as an inch or two, or it can be more than three feet, during a full or new moon. But you can't blame the moon for Venice's flooding problems.

CHARLES E. MCCLENNEN: The acqua alta, those extreme high tides, are caused primarily by the weather, strong winds out of the south driving Adriatic waters to the north and forcing it into the lagoon and causing flooding. So predicting acqua alta is really based on predicting storms. And it's not that easy.

NARRATOR: Bad weather was the cause of the 1966 deluge. Wind and low pressure created a giant storm surge that pushed water into the lagoon and wouldn't let it out. The flood was a complete surprise.

Since then, Venice has paid much more attention to the weather and its effect on the tides. Out on the Adriatic, twelve miles from shore, this platform collects data 24 hours a day. Temperature and humidity, wind speed, atmospheric pressure, anything that contributes to a storm surge. All this information is relayed to a team within the city. If flooding is predicted then alarms go off.

Given enough warning, Venetians can minimize the damage. Doorway barriers keep some water out of ground floor shops and apartments, but for most of the city, there's no stopping the sea.

These events are becoming more frequent. A century ago, St. Mark's Square, the lowest point in Venice, flooded about nine times a year. Nowadays, it happens about 100 times. Buildings in Venice were constructed to withstand some contact with the sea, but these floods are pushing them beyond their limits.

To understand why, just look at a typical Venetian foundation. Here, a canal has been pumped dry for repairs, revealing a few of the secrets of the city's success and some of its major weaknesses. Strip off the outer layers of the foundation and you'll find a forest of wooden pylons. These logs were pounded through the soft mud, 10 or 15 feet to the bedrock below. Surrounding the pylons are several layers of water-resistant stone. As long as the sea washes against this lower level, the structure above is well protected.

But now, the flooding is rising beyond the stone foundation to the building itself, which, in most of Venice, is made of brick. The bricks, sometimes coated in stucco, are soft and porous and much more vulnerable to corrosion. Chemists at the University of Venice are studying how this works.

GUIDO BISCONTIN (University of Venice): It's very interesting to see the effect of the water on the sample, which is no different from what happens to the walls in Venice. I'm immersing the brick in a few millimeters of water that contains a small quantity of salt.

NARRATOR: The salt travels up through the brick. As the water dries, the salt crystallizes. With every new flood, the salt dissolves once again and bores a little more into the brick. Eventually, the brick will crack and crumble away. This is what's happening inside all the walls of historic Venice.

Almost everywhere, ground floors are damp and moldy. Many residents have moved upstairs or out of town. Since the 1950s, Venice has lost over half its population. Today, fewer than 70,000 people live here.

Residents are trying to escape the flooding, but they're also running away from the tourists. About 15 million tourists pour into tiny Venice every year, and most of them stay less than a day.

They drive prices up on everything from food to rubber boots. So every day, Venice becomes less of a living city, and more like a museum, a museum which is often very, very wet.

But does it have to be this way?

GIANFRANCO VIANELLO (Restaurant owner): Just look at the Netherlands, the whole country's below sea level and not a drop gets in. I don't see why we, who are nearly three feet above sea level in St Mark's Square, are not able to stop this water.

NARRATOR: The Netherlands used to be one of the most waterlogged nations on earth, with more than half the country below sea level. Since the 1600s, legions of windmills have powered water wheels that lift water up into canals. These channels flowed back to the sea, which was held at bay by earthen dikes.

But this system was not foolproof. In January 1953, hurricane-force winds and an unusually high tide blasted the Dutch coast, collapsing the protective banks. Eighteen hundred thirty five people were drowned and 70,000 were left homeless. A third of the Netherlands was under water.

Over the next 50 years, the country spent billions walling itself off from the North Sea. But for Rotterdam, the biggest port in the world, a permanent barrier would have spelled economic disaster.

And so the Dutch came up with this: two mobile gates that can swing out to block the river whenever the North Sea gets out of hand. To withstand the full force of the sea, the structures must be gigantic. Each gate has a steel barrier, seven stories tall, which holds back the water. The arms are as long as the Eiffel Tower is high.

The gates are far from subtle, but Pier Vellinga, an environmental scientist, is still a fan.

PIER VELLINGA (Vrije University, Amsterdam): When the North Sea is rough, we are very vulnerable. And we had the choice between raising all the dikes three, four, five meters, and rebuilding part of the old cities or a one-time mobile barrier. And the population was much in favor of this mobile barrier because it is safer and it has less effect overall on the landscape.

NARRATOR: When the sea's calm, the barriers rest in canals on the riverbank. If a major storm is predicted, the huge arms will rotate toward the center of the river until they almost meet in the middle. Then the barriers fill with water and sink into the riverbed.

Finished in 1997, the gates haven't been tested against a real storm yet, but it's predicted they will be needed, on average, once every 10 years. The Dutch system of dams and gates has been hailed as an engineering marvel, but could it work for Venice?

PIER VELLINGA: What I know of Venice, the people and the visitors do not like to see such a massive structure. And the design which is now discussed for Venice is a more elegant solution.

NARRATOR: The Venetian flood plan calls for several sets of mobile gates placed at the three large entrances to Venice's lagoon. Most of the time, the steel gates lie flat in a special housing on the bottom of the inlet. When extremely high tides threaten the city, compressed air is pumped into the hollow gates, causing them to tilt upwards to the surface.

Each gate measures about 65 feet wide, about 12 feet thick, and 65 to 100 feet tall. They're designed to hold back a high tide more than six feet above normal. When in use, they are supported by the water on both sides. After the tide recedes, water flows into the gates, sinking them back into place.

If the hollow gates look flimsy, take a peek inside. Each gate is supported by an internal steel framework weighing between 300 and 400 tons.

CHIANG C. MEI: The design of the mobile gate system is very innovative. When it's in use, the elements of the barrier are allowed to swing back and forth with the waves. In this way, much of the wave force is transmitted back to the water on both sides, and very little force is transmitted to the foundation and to the supporting structure. So in this aspect, I think this is very, very clever.

NARRATOR: Many engineers believe these mobile gates represent the salvation of Venice. There's just one problem: they don't exist, except in computer simulations like this.

Since the flood of 1966, Italians have been talking about how to protect Venice. Talking. And talking. But somehow, they've been unable to make a decision. Instead, the gates have become a political hot potato, tossed from one administration to the next. And that's meant a lot of tossing. Since 1966, there have been more than 35 different governments in Italy.

ALBERT AMMERMAN: The politicians who are sitting in Rome, who essentially have control of this, who are coming and going, almost on a yearly or two year basis, do not have a clear focus on this. They will not be in office when this is done, because it will take ten years. And this helps to explain why you have this sort of inertia.

NARRATOR: It's not that nothing has been done. Every now and then, there's a flurry of activity—like in the late 1980s, when engineers rolled out a full-size prototype of a single gate, which was attached to a large derrick. The whole contraption was ceremoniously hauled along the coastline to gain public support.

WOMAN: On television they talked about this thing. It's the first time I've seen it.

NARRATOR: After years of inaction, Venetians were understandably skeptical.

SAILOR: Well, now, look. As far as efforts to save Venice go, up to now they haven't tried very hard. They've gobbled up a lot of the money meant for saving Venice, but they've done very little. We hope that now things go well with these mobile gates.

VINICIO GROSSI (Gondolier): Okay, the politicians they make project, blah, blah, blah, blah. Make all the Venetian people now wait.

NARRATOR: Today, 14 years later, the Venetians are still waiting. Construction of the gates has not even started. This is particularly frustrating to engineers like Andrea Rinaldo.

ANDREA RINALDO (University of Padua): Especially in this country, science and technology take a back seat to politics. It's never intentional. It happens everywhere. But in a humanistic country like this, what an MIT professor says and what a gondolier says is basically the same thing. And if I say this, it's because I've been burned on this before.

NARRATOR: Initially, the gate plan required years of testing. No existing mobile gates operate in this way, popping out from the sea floor by means of compressed air. To make sure the concept would work, engineers spent four years fine-tuning the technical design of the single gate module.

Then, to test the whole system, they set up several scale models of the lagoon inlets complete with mini-mobile gates. Tiny ripples simulate the effects of a mighty sea storm. And the response of the barriers is carefully noted.

During the testing phase, engineers did find a potential problem. When test waves hit the gates in certain patterns, adjoining gates rocked in opposite directions, creating holes through which floodwaters could flow.

CHIANG C. MEI: This, I believe, was uncomfortable to the designers. But there have been improvements of design by changing the dimension. So this problem is solvable.

NARRATOR: By making a few minor changes in the size and angle of the gates, the engineers are confident they'll avoid this problem, and the gates will do their job. That is, if the gates ever get built.

Paolo Pirazzoli was born in Venice and has been fighting the gate plan for years.

PAOLO PIRAZZOLI (National Center of Scientific Research): In theory, the mobile gates, with optimal weather conditions, could put an end to high tides. But only in theory, because it has a lot of problems. It's an extremely complicated project. It's fragile and expensive in terms of its construction and maintenance. And it would have a negative impact on the environment.

NARRATOR: Environmentalists make up the biggest bloc against the gates. They worry that the gates will be closed too often and will damage the lagoon ecosystem. Some insist that Venice and the lagoon would be better off left to Mother Nature. But the gate engineers don't buy that argument.

ANDREA RINALDO: In recent years, there's been a growing sense among people that every intervention in the lagoon and in the city is considered "tampering." All these interventions are generally called "the disaster of doing." It's hard for me to agree with this, because it's clear that everything that we see today in the Venetian lagoon, it's the result of an artificial system. It's not the product of natural evolution, but of a massive interference from humans.

NARRATOR: It's true. Humans have been tampering with Venice and its lagoon for hundreds of years. Today, Venice is a collection of built up islands connected to the mainland by an artificial causeway. But sixteen centuries ago, it was a very different place, a bunch of marshes surrounded by a shallow mix of fresh and salt water.

The earliest settlers were probably running away from Attila the Hun and other barbarians. They picked the spot precisely because it was so inconvenient. Foreign invaders gave up trying to get there, figuring it wasn't worth the trouble. Only the locals knew the location of channels deep enough for seafaring vessels.

Safe from invasion, the ambitious Venetians developed a monopoly on trade and shipping in the Eastern Mediterranean. This is what made Venice rich and powerful. But by the 14th century, nature threatened to undo the city's cozy place in the lagoon.

ENNIO CONCINA (University of Venice): The big turning point happens in the middle of the 14th century. It seems that reeds and swamps are invading the lagoon and menacing the city. The swamps corrupt the water and bring disease and death. And in addition, they make the city vulnerable to attack.

NARRATOR: Several rivers which ran into the lagoon were depositing large amounts of silt. Only a foot or two deep, the lagoon was becoming even shallower and more swamp-like. Mosquitoes and malaria were rampant. Many feared the lagoon would fill in completely, and Venice would lose its best system of defense. So the Venetians decided to take control of their destiny.

ENNIO CONCINA: Essentially, there is a decision to proceed systematically, to adjust everything that surrounds the lagoon, and to heal the relationship between water and land through constant control.

NARRATOR: After much debate, the Venetians launched an enormous public works project. Over the course of two centuries, they built a series of large canals, diverting the major rivers around the lagoon so the silt would be deposited elsewhere. The plan worked. The lagoon stopped silting up.

But the Venetians were left with another problem, one they could not solve so easily: their city was sinking. In fact, Venice had been sinking since the very beginning.

The Alps are partly to blame. The weight of this mountain range is bearing down on all of northern Italy, slowly driving it into the sea. But even more damaging to Venice is what lies below.

CHARLES E. MCCLENNEN: Underneath Venice is a salt marsh. Venice is sitting on several layers. The uppermost and thinnest are the lagoon sediments, fairly muddy and, in part, salt marshes, and that's what Venice was built on. Below that is nearly a mile thick of river sediments. Those sediments are slowly compacting.

NARRATOR: In effect, Venice is sitting on a giant sponge filled with water. As the weight of Venice pushes down on the sediments, it squeezes some of the water out, and the sponge gets thinner.

To make matters worse, the seas have been slowly rising for centuries. This is what the people of the lagoon have had to contend with since they first arrived on these marsh islands, nearly 2,000 years ago.

Remnants of their long struggle can be found on the island of Torcello, northeast of Venice. The land here has sunk so low, only a handful of residents remain. Beneath the foundations of this 900-year-old cathedral, archeologist Albert Ammerman has uncovered evidence of how the old lagoon dwellers dealt with flooding.

ALBERT AMMERMAN: What we can always see in the archeological record is the gradual, progressive buildup of the land surface. We can see five or six floors, with just one after the other, six inches, a foot, gradually being built up. Flooding would always be a problem, so their way to deal with it was essentially to come in and continually be adjusting the ground level upward, layer after layer after layer.

NARRATOR: Without the constant drone of water pumps, the entire pit would be a giant swimming pool. But everything here was once high and dry, safe above the tides.

ALBERT AMMERMAN: Well, the earliest evidence that we have is a walkway made of Roman tiles, and that goes back to about 200 A.D.—we're in Roman times—and this is found at five feet below sea level today.

NARRATOR: Anything built on these marshes would eventually sink into the sea. So when a building was too-often flooded, the Venetians would either raise up the floor or they might tear the whole thing down and build a new structure on the old foundation.

But at some point in the past, the Venetians abandoned this strategy.

ALBERT AMMERMAN: One of the fascinating things is that, since essentially 1800, the time of Napoleon, the fall of the Venetian republic, they have stopped doing that. Venice has become, in some sense, a museum. It's become fossilized. The life of the city, also the notion of preservation heritage, stops people from doing what was the thing that Venetians always did, that is, build up the ground level.

NARRATOR: It's been a long time since the ground in Venice was raised on a regular basis. When these homes were built, their front steps would have stood well above the highest tide. Now, coated with green algae, they're visible only at low tide.

Some believe that if Venetians want to fight off flooding, they should follow the example of their ancestors.

PAOLO PIRAZZOLI: The ground level has been increased many times in the past, with demolition and reconstruction. Today, of course, it's not possible to demolish, but you can still make some increases in ground level that would be compatible with architecture and landscape.

NARRATOR: Raising the sidewalks and walls of the canals, perhaps by a foot or less, could go a long way to protect the city from moderate high tides, the most common cause of flooding. When it's done well, as here in one section of Venice, the change is hardly noticeable.

Some people say that this should be the priority for Venice, not the mobile gates.

EDMUND PENNING-ROWSELL: That will solve an enormous proportion of the flooding problems. It won't solve all the flooding problems, but it will probably solve half or three-quarters of them. And you could do that over the next 10 years. It will cost money, but it will solve the problem very seriously in that immediate time scale.

NARRATOR: Plans are in the works to slightly raise the ground in the lowest parts of Venice, including St. Mark's Square where the labyrinth of elevated walkways is an all too familiar sight during the winter months.

Most critics of the gates support this approach, and some have suggested an additional strategy: narrowing the three inlets to the lagoon. The smaller the opening, they argue, the less water can enter the lagoon, and the lower the tide. Depending on how drastic the change, it could lower the tide by eight to 12 inches.

PAOLO PIRAZZOLI: With narrowing of the harbor's mouths, the acqua alta would have to reach almost four and a half feet to cause flooding, and this level is reached only once every three years. This means that we would have the same frequency of acqua alta as we had a century ago.

NARRATOR: But narrowing the inlets would not be a completely reliable defense against acqua alta. It could only work for tides that flow in and out of the lagoon fairly quickly, on a cycle of just a few hours.

But there are some extremely high tides that linger, like the one in 1966. The high tide flowed into the lagoon and stayed for a day. In a situation like this, whether the inlet is wide or narrow, the floodwaters will eventually get in.

PIER VELLINGA: We have studied that in detail, and we came to the conclusion that it only works with low tides, but with exceptionally high tides that last for, say, 24 hours, even making the mouth more narrow does not work at all. It would be a worthless solution.

NARRATOR: Narrowing the inlets is considered a soft solution for Venice, and has been endorsed by some environmentalists. But its effect could be less than kind because it would reduce the amount of clean seawater flowing into the lagoon with each tide.

Today, the lagoon is the largest saltwater marshland in the Mediterranean and is cherished by Venetians as a place of natural beauty. It's known throughout Europe as a bird-watching mecca.

Tens of thousands of birds stop here during their annual migrations. They feed on the plentiful fish and shrimp that thrive in the shallow, salty lagoon waters. If the flow of new seawater into the lagoon is reduced, these creatures will be deprived of oxygen and nutrients. In addition, without the cleansing action of the tides, one of the lagoon's problems may rapidly get much worse: pollution.

ALBERT AMMERMAN: Well, the water in Venice has quite substantial levels of pollution. There's raw sewage. I have a boat in Venice and I take people around in the boat. You can actually see soap suds coming out of people's washing machines. You can see worse things coming straight from their houses.

NARRATOR: Venice has no municipal sewage treatment, so most homes pipe their sewage into the canals. On top of this, there are the factories at the nearby industrial complex of Port Marghera, which only recently stopped dumping chemicals into the water. And there's also agricultural pollution, fertilizers that flow down the Po Valley and into the lagoon. All this pollution is swept out of the lagoon and into the sea, every day, twice a day, by the tides.

If narrowing the inlets would be harmful, then what kind of damage would be done by the mobile gates, which would close off the lagoon completely? How long would the gates actually be closed when high seas threatened?

Recently, nature provided the engineers with another opportunity to test their gates' performance, at least virtually. On November 6, 2000, tidal charts in Venice called for a modest high tide, just a few inches above the mean. Later that day, meteorologists forecast that a storm would push the tide two and a half feet higher, which would cause some flooding. But the storm took a sudden turn for the worse, and by 9 p.m., the tide had risen to four feet, enough to flood 93 percent of the city.

Afterwards, researchers created a computer simulation of the storm to see how effective the mobile gates might have been.

MARIA TERESA BROTTO (Consortium for a New Venice): The gates would have been shut for nine hours, and the water level in the lagoon would have reached not more than two feet, three inches.

NARRATOR: The engineers believe that once the gates were opened again, tidal flushing would soon clean out any built up pollution. But what about repeated closures over the course of an entire season? What effect would these have on water quality?

According to Pier Vellinga, who was on a government panel which evaluated the gates, not much.

PIER VELLINGA: Well, I'm a professor in environmental sciences myself, and we have taken the concerns of the Green people very serious. But when we studied it, we came to the conclusion that the closure of the barrier, say 10, 20 times a year, has about the same effect as when you have lower tides in summer. The claim that these mobile gates would really be bad for the environment, we could not find any ground for.

NARRATOR: If the gates were in place today, they would probably be closed, on average, about seven times a year. Even most environmentalists agree this is not a problem. But what will happen in the future?

Remember, Venice is sinking and the sea is rising. Won't the gates have to be closed more often?

That depends on how much the city sinks and the sea rises in the next 100 years, the expected life of the mobile gates. Figuring that out is tougher than it might seem, because in the middle of last century, Venice started to sink at an unusually high speed.

The problem was soon traced to Port Marghera, where factories were pumping large amounts of groundwater out from deep under the city.

CHARLES E. MCCLENNEN: The groundwater wells were active from the `30s through the early 1970s, I believe, when they were stopped. And they were stopped because it was realized that it was actually drawing the city down at a faster rate than it would be sinking naturally.

NARRATOR: The pumping was a catastrophe for Venice. In less than a century, the city lost about nine inches against the sea. But when the pumping stopped, the sinking slowed down drastically. In the last couple of decades, Venice sank very little.

No one knew how fast Venice would sink in the future, so the gate planners chose several possible scenarios. One scenario was very conservative, predicting that Venice would sink less than two inches in the next hundred years. Before long, this choice was coming under attack, and from an unlikely source.

ALBERT AMMERMAN: We found that the scenarios, the projections that they were using in these impact studies were completely wrong. There were fundamental flaws.

NARRATOR: Albert Ammerman is an archeologist not an engineer. But together with marine geologist Charles McClennen, he had uncovered controversial new evidence about the changing sea level of Venice.

CHARLES E. MCCLENNEN: There was no motivation on our part to study or evaluate the mobile gates. Our initial purpose was really to look at the archeology. Inadvertently, we learned about sea level in the lagoon of Venice.

NARRATOR: By carefully measuring and dating the different levels of ancient buildings, the team tried to determine how fast the seas in Venice have been rising throughout her long history.

ALBERT AMMERMAN: The result of this was we could see the average rate of change was five inches per century.

NARRATOR: The team believed the gate planners had been too optimistic about Venice's rising sea level. And they began to question some of the planners' other conclusions, particularly their predictions about global warming. Global warming is a contentious issue, especially when it comes to its effect on sea level.

Hollywood films have had a field day with the doomsday scenarios resulting from melted polar ice caps. Waterworld, starring Kevin Costner, depicts a future where dry earth is a hot commodity.


NARRATOR: And Stephen Spielberg's A.I. transforms Manhattan into a waterlogged ghost town. If New York is going to look like this, then Venice is a goner, too.

But we don't have to melt all the ice on Earth to have problems. Even with a little bit of global warming, the oceans will expand, that's what water does when it's heated. And if the oceans get bigger, there's nowhere to go but up.

But predicting how far up and how fast is extremely tricky.

PIER VELLINGA: We sometimes think that the oceans are like one tub of water, and when it gets hotter, the water expands and rises. But if the climate changes, then also the wind changes and the storm surges change. So the water will be pushed up differently in different parts of the world.

NARRATOR: Official estimates for global sea level rise in the next hundred years are all over the map—from four inches to three feet. Exactly what's going to happen in Venice is anybody's guess. The gate planners estimated a probable rise of about eight inches, meaning that the barriers would be closed just a few times a year.

CHIANG C. MEI: For this kind of sea level rise, it is probably necessary to close the gate approximately 12 times a year or 45 hours per year. The length of closure is very, very small compared to the time when tidal flushing can be effective.

NARRATOR: But gate critics think that these figures are too optimistic.

ALBERT AMMERMAN: If one accepts notions about global warming, what reasonable scientists around the world think will happen is that over the next 100 years, the midpoint range of what they think will happen is that there will be an 18 inch rise. If that happens, then you're going to have acqua alta all the time and then you're going to have an emergency situation.

CHARLES E. MCCLENNEN: The problem that I see with the mobile gates in the lagoon of Venice is that the closing may have to be too frequent and for too long duration each time.

ALBERT AMMERMAN: What we're talking about is, in a bad year, out in the later years, maybe after about 2060, 2070, you could have, day after day, week after week, some days even two closings.

NARRATOR: Ammerman and McClennen fear that with so many closings pollution inside the lagoon could build to dangerous levels.

But what will pollution levels be like sixty years from now? Aerial surveys, along with sampling, have shown that water quality in the lagoon has improved somewhat over the last decade. And it would be nice to think that at some point in the next 60 years, Venice will build a sewage treatment facility.

But if the worst-case scenarios for global warming come true, it won't matter how clean or dirty the water is. There will simply be too much of it. For some, this is reason enough not to build the gates.

EDMUND PENNING-ROWSELL: Well, in the next 100 years, Venice has undoubtedly got a serious problem, because all the projections for sea level rise indicate a rise of around 50 centimeters to 100 centimeters. And that is going to mean that the mobile gate solution, which has been put forward, will be impossible to implement.

NARRATOR: But when others consider the rising seas, their conclusion about the gates is...

PIER VELLINGA: Well, exactly the opposite. Because we know so little, and we think the lagoon is precious, and Venice is precious, the mobile gates is like a precautionary measure. It will be very useful for at least hundred years, and if sea level comes quicker than you think, at least you have a way to save Venice.

NARRATOR: But are these mobile gates the best way to save Venice? Ammerman can't help wishing there were more choices.

ALBERT AMMERMAN: What you're going at with the gates is one big final thing. It's like trying to throw a long touchdown pass or hit a home run. And what's going to happen if you miss, and that doesn't work? You have nothing in place as a building block for the next generation, to take you out to 2100 or 2200.

NARRATOR: He supports more studies and openness to new ideas. The engineers say, with or without more research, it's time to build.

ANDREA RINALDO: As a general rule, I suspect that "wait and see" when you deal with a phenomenon that change...on a global change scales, would be rather unsuitable procedure, especially if you have to protect treasures of a human kind like the city of Venice, its art, its heritage, its architecture.

PIER VELLINGA: I think you first have to act and learn simultaneously. If you only want to study and the sea level rises quickly, you lose Venice and you lose the lagoon. You lose both. So we think this solution is very good for the short term. And if you postpone and study, and study, and study, we may study until the sea has risen 50 centimeters. Then you're not safe.

NARRATOR: Today, the Italian government seems intent on building the gates, which could take eight to ten years. If the gates are constructed, there is little doubt that they will stop the worst floods, at least for a while.

But at some point, the mobile gates may not be enough to hold back the rising sea. Having invested so much, will the government be able to launch an even bigger project if the gates become obsolete?

EDMUND PENNING-ROWSELL: In the long term, that temporary solution, costing perhaps three billion U.S. dollars will have to be repeated by something more permanent. Venice and its lagoon will have to be sealed off from the sea.

NARRATOR: What would it take to create a permanent solution for Venice? There seem to be two main possibilities: wall off the city from the lagoon, or wall off the lagoon from the Adriatic with permanent dikes, transforming it into a fresh water lake. For a city so intimately linked to the sea, both proposals seem unthinkable.

But the water is rising. Eventually, something will have to give.

CHARLES E. MCCLENNEN: The Venice you know today cannot be preserved as it is today. Because all the data we have on global sea level and local sea level is to our disadvantage when it comes to the city of Venice. It was built on a salt marsh, at sea level, in a sinking area, and unfortunately sea level is rising.

NARRATOR: For now, those who love Venice hold their breath, hoping that the city that has defied time for so many centuries will somehow manage to hold on.

Rising sea levels, shipping channels, the weight of the city itself—on NOVA's Web site use a clickable map to see all the ways that Venice is under siege, at or America Online, keyword PBS.

To order this show or any other NOVA program for $19.95 plus shipping and handling, call WGBH Video at 1-800-255-9424.

Next time on NOVA: He has a madness no medicine can cure. "I think about plants more than anything else." And it drives him to the ends of the earth. "I need to go to a place where no botanist has ever been...I've got it, I've got it, I found it I found it." Orchid Hunter.

NOVA is a production of WGBH Boston.

Major funding for NOVA is provided by the Park Foundation, dedicated to education and quality television.

Science: it's given us the framework to make wireless communications clear. Sprint is proud to support NOVA.

This program is funded in part by the Northwestern Mutual Foundation. Some people already know Northwestern Mutual can help plan for your children's education. Are you there yet? Northwestern Mutual Financial Network.

And by the Corporation For Public Broadcasting, and by contributions to your PBS station from viewers like you. Thank you.


Sinking City of Venice

Produced by
Marco Visalberghi & Julia Cort

Narration Written by
Julia Cort

Directed by
Gianfranco Pannone
Marco Visalberghi

Produced for Doclab by
Paolo Vidali

Edited by
Doug Quade

Additional Editing by
Paolo Vanghetti

Associate Producer
Jennifer Lorenz

Narrated by
Tony Kahn

Stefano Pancaldi
Brian Dowley

Sound Recordists
Matteo Orsini
Chris Grosso

Assistant Camera
Salvatore Bacciu

Daniele Cestana
Giovanni Sollima
APM Music Library

Franco Valenziano, Playup
Sputnik Interactive Design and Animation

Additional Graphics
David Eells
WGBH Design

Assistant Editors
Rob Todd
Dan Van Roekel

Online Editor
Ed Ham

Mark Kueper

Audio Mix
Richard Bock

Archival Material
ABC News
Consorzio Venezia Nuova
Getty Images
Journeyman Productions, UK
NBC News
RAI Trade, Italy
Royal Netherlands Embassy

Special Thanks
Consorzio Venezia Nuova
Archivio di Stato di Venezia
Panasonic DVCPRO High Definition
Dr. Rafael Bras, MIT
Dr. Georg Umgiesser, CNR/Venice
John Keahey

NOVA Series Graphics
National Ministry of Design

NOVA Theme
Mason Daring
Martin Brody
Michael Whalen

Post Production Online Editor
Mark Steele

Closed Captioning
The Caption Center

Production Secretary
Queene Coyne

Jonathan Renes
Diane Buxton

Senior Researcher
Ethan Herberman

Production Coordinator
Linda Callahan

Unit Managers
Holly Archibald
Denise Drago

Nancy Marshall

Legal Counsel
Susan Rosen Shishko

Post Production Assistant
Patrick Carey

Associate Producer, Post Production
Nathan Gunner

Post Production Supervisor
Regina O'Toole

Post Production Editor
Rebecca Nieto

Coordinating Producer
Laurie Cahalane

Supervising Producer
Lisa D'Angelo

Senior Science Editor
Evan Hadingham

Senior Series Producer
Melanie Wallace

Managing Director
Alan Ritsko

Executive Producer
Paula S. Apsell

A Doclab production for NOVA/WGBH, Mediaset, Discovery Networks Europe, and CTB for La 5eme.

© 2002 WGBH Educational Foundation

All rights reserved

Sinking City of Venice

See the Gates in Action

See the Gates in Action
Watch a video about the proposed barriers designed to hold back the sea.

Venice Under Siege

Venice Under Siege
Use satellite images to zoom in on hazards imperiling the city.

Weighing the Solutions

Weighing the Solutions
How Venetians are dealing with sinking.

What Causes the Tides?

What Causes the Tides?
An animated explanation of tidal ebbs and flows.


About NOVA | NOVA Homepage | Support NOVA

© | Created September 2006