Aqua Alta in Venice: What it Is and Why it Happens

You may have seen videos and photos in the news of Venice underwater from time to time, with tourists lugging suitcases through rivers or frolicking in Piazza San Marco wearing rain boots. This flooding is unfortunately common and is simply called "high water" or aqua alta. Venice, as a city built on muddy islands in a shallow lagoon, has always been at risk of flooding. Floods were recorded way back near the founding of the city, over 1000 years ago. The city was built on water and understands how to deal with it. But these days, the flooding has increased exponentially and gotten much more severe. Why?

What is a Flood?

Let's start with the definition of flooding. A flood could be considered when there is an unusually high tide, but the locals usually only call it flooding when water enters the lowest place in the city. Unfortunately, the lowest point of the city is also the most historic: the entryway to the ancient Basilica of San Marco. Even if there is a trace amount of water there, it is considered a flood.The water does not rise in the lagoon and wash over the quay, as you may imagine. Instead, it seeps up between the stones of the streets and through the storm sewers. That means that any point in the city that is low will flood, regardless of how close to a canal it is.I've seen these floods many times. I once saw a bit water burbling up slowly through the sewer covers in Piazza San Marco. By the time I'd crossed the piazza, it was completely covered in water. It's a creepy, unstoppable force, like the blob.When I began my career as a tour guide about 20 years ago, Venice flooded a few times per year. These days, my local guide friends tell me that it can flood up to 180 days per year. And more than just higher frequency, they are getting deeper.

What Causes Aqua Alta?

The high water in Venice is a result of a combination of factors. How many factors occur at the same time will determine the severity of the event.

Tide

The natural tide of the Adriatic Sea means that the water in the lagoon has a high and low tide twice per day, as it always has. Just like any tidal area anywhere in the world, those tides fluctuate based on the movement of the sun and moon, and can be predicted with some accuracy months in advance.This tidal movement has always been a good thing for the city. Sewer lines were never really necessary in the past, people threw everything into the canals, as the change of tide would flush out the water of the lagoon and bring in fresh seawater.Things have changed in the lagoon, though. In the early 20th century, chemical plants and factories were built along the edge of the lagoon on the mainland. Deep canals were dug into the lagoon to accommodate transport ships. And a deep canal was dredged just in front of the island along the Giudecca which now accommodates cruise ships.These deep canals have had a disastrous effect on the health of the lagoon. The volume of water entering and leaving twice a day has drastically increased, eroding the pilings on which the city stands. The sheer volume of sea water entering the lagoon now has led to more floods.

Wind

Venice sits at the top of the thigh of Italy's leg, at the end of the Adriatic. Typically, water should be flowing southwards, as the Alps empty into the sea. There are two winds that can change that. The Scirocco is a strong wind coming from the south, which pushes water up the Adriatic and into the lagoon. The Bora is another strong wind, which can flow towards Venice as well. If either of these winds is blowing, the water will be higher than normal.

Weather

Contrary to what would seem likely, flooding in Venice has nothing to do with rainfall. Rain can make things wetter, but it is a larger force that makes the rain correlate with flooding. It's the pressure. Low pressure weather systems usually bring rain. Low atmospheric pressure allows the sea to rise higher than normal, so aqua alta often occurs in rainy weather, but does't always.

A Sinking City

People often ask me why Venice is sinking, and assume that is why the city floods. It is true, Venice is sinking, but so is your house. Settling is a natural consequence of any building. When you build on muddy islands in the middle of water, it would make sense that there would be settling over 1000 years. There has been, but not as much as you would think.Over the past 500 years, the settling has been only a few centimeters overall. Some scientists have used the detailed paintings of Canaletto from the 1700's to make comparisons. (As an aside, the Queen of England has the largest collection of these famous views of Venice.)For centuries, the people of Venice have raised up their city as it has settled. The main square, Piazza San Marco, used to have a brick pavement. As flooding increased, new paving stones were layered over the top, raising the level of the piazza. Keep your eyes open as you walk around the city, you'll see plenty of buildings with steps buried by a higher sidewalk. I saw the sidewalk raised near Zattere only a few years ago. And so it goes in the city in the water.The typical settling of the city has been consistent for generations, but has become more rapid in recent decades. It makes some sense. The city is built on wooden pilings, trapped under water and slowly petrifying. The introduction of propeller boats in the canals is very recent, and the swirling of the water around the pilings has been eating away and the centuries-old supports. And much worse, the enormous cruise ships sloshing water around the city have cause irreparable damage to the underpinnings of the city.

Rising Waters

The last and most concerning factor is rising sea levels due to global climate change. You can debate me all you like about the reality of climate change, but debates don't help the people of Venice. They live with it. They see it. The water IS higher than it has ever been. Even a tiny increase in global sea levels can be serious for those who live on the water. And as the sea levels continue to rise, the future of Venice becomes murky.

A Dangerous Combination

The highest aqua alta occurs when all of these factors coincide. Wind, natural lunar high tide, and low pressure all together will raise the water to dangerous levels. When that happens, the residents get an alarm several hours in advance, although most track these things days and weeks ahead. There's even an app for tracking the tides.A tide that is more than 140 cm above sea level will cause a serious flood of the majority of the city. 150 cm, which occurred this past week (Oct 2018) flooded 75% of the city. While this sounds apocalyptic, the people of Venice are remarkable in their resilience. Teams of shopkeepers helped each other in the days ahead to raise everything off the floor. Although many shops, museums and restaurants closed, some stayed open and served customers in their high rubber boots.Raised planks are put on the main arterials, and maps are available to show you the routes that typically stay dry. The people cope, even when things are really bad.The aftermath of a severe high water can take days to recover from. Anything that has touched the water needs to be cleaned. The water is not just salty sea water, but it is water that has mixed with sewers and dirty streets. I cringe when there's a high water and I see tourists swimming or playing in it. EEEEWWW. The last time I was caught in a high water I threw my shoes away.

No Solution

I wish I had a nice conclusion to this piece. I wish I could tell you that a savior is on the way. But there isn't. The MOSE project, a series of gates that should block seawater from entering the lagoon, has been a terrible disaster. After decades of work and billions of Euros, it still doesn't work and probably never will. The cruise industry seems to have a death lock on the government, and more ships appear every year, undermining the delicate balance of the lagoon. And the water rises.The good news is the resilience of the Venetian people who still choose to make their lives on the island. They dutifully prepare for the floods and scrub everything clean once the water is gone. They don't panic and stoically deal with everything that comes their way. They keep the island functioning as a city, not just a museum. They have my respect, fighting the good fight. So, if you visit the city, be kind to the locals. Thank them for their efforts to keep the city alive. It's not easy, but their hard work makes it happen so we all have this world treasure to appreciate.