Science, Tech & Environment

Cities and Rising Waters

The flooding New York City experienced from Sandy could become a more common occurrence as climate change causes sea levels to rise. The World's Rhitu Chatterjee explores how coastal cities in other countries are protecting themselves from inundation.

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The severe flooding caused by Sandy didn't come as a complete surprise to the City of New York.

Officials there have for some time been looking at ways to make the city more resilient to storm surges. Six years ago, they approached a Dutch scientist for advice.

The Netherlands is a low-lying country that is well known for its heroic efforts at holding back the sea.

The Dutch scientist, Jeroen Aerts of VU University in Amsterdam, says New York officials wanted to know what they could do to make the city more flood-proof. But, at the time, they weren't interested in big engineering solutions.

"They were a little bit reluctant in terms of considering large scale protection measures, like surge barriers," he says.

Surge barriers, or flood barriers, are carefully engineered concrete structures, often built in tidal inlets, to prevent flooding in coastal areas.

These and other large structures, like levees, are expensive, and New York wanted inexpensive solutions, Aerts says.

But that changed last year, after Hurricane Irene caused huge economic damage. "Then they also saw this sense of urgency that maybe (we should) also look at other options," he says.

Aerts is currently collaborating with New York to develop plans for levees and storm surge barriers, and on ways to make new and old buildings more flood proof.

Aerts says New York could adopt some solutions from his country — for example, the network of surge barriers that protects the city of Rotterdam and its port, or the idea of raising beaches with fresh sand every few years.

"Each time the sea rises by a little bit, a few inches, we simply put some more sand on our beaches," he says.

Aerts's colleague Pier Vellinga says New York should also consider some tips from Venice.

"Over the last twenty years, the frequency of flooding has increased in Venice from about four times a year to like 20-40 times a year," says Vellinga.

Vellinga has been helping Venetians put in a range of measures to help protect themselves from these floods.

Some of them are simple; for instance, raised walkways that help pedestrians keep their feet dry.

Then there are more complex and expensive engineering solutions, like a set of mobile underwater barriers.

"It's a set of doors that are hinged to the sea floor," says Vellinga. "They lift up when the water is very high, and then they block the entrance" from the Adriatic Sea to the city.

Whatever action New York takes, experts say the city has to be prepared to redesign its plan depending on what the future brings.

Tim Reeder is the regional climate change program manager at the environmental agency in the U.K. He's involved a program called the Thames Estuary 2100.

It's a plan to protect London and surrounding areas from floods and other climate-related risks, and it can be tweaked based on the degree to which the climate changes over the coming decades.

"Given the fact that we don't know how much of sea level rise exactly we're going to get as a result of climate change," he says, "the plan can be adapted so that we can continue to get flood protection for the next hundred years in London."

One component of the plan is a surge barrier called the Thames Barrier.

"The Thames Barrier is part of a large flood protection system including 330 km. of sea walls to protect London," says Reeder.

Since its completion in 1983, this barrier has protected London from floods over a hundred times. But if the sea level rises by another foot, this barrier may no longer offer enough protection, in which case it will have to be modified to hold back more water.

Reeder says cities like New York, Rotterdam, Venice, and London face an uncertain future, and they should work together to prepare for a common threat.