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How Megacities Can Survive Rising Tides

"Despite trillions of dollars of assets located in coastal flood-prone areas, investments in protection have often been inadequate."
Lower Manhattan without power in the aftermath of Hurricane Sandy in 2012. Image: Derek Mead

If we've learned anything from the last decade of coastal disasters, it's that floods are devastating and more are coming. And in the future, the damage will be even more costly, as more people are moving to beachfront megacities, while at the same time the oceans are rising and extreme weather is becoming more common.

For a metropolis like New York City, preventing the next Hurricane Sandy isn't an option. The only hope is to mitigate the damage it does when once it blows by. But the devil's in the details: Do you try to build new levees and reshape waterways to slow storm surges, or do you require property owners to retrofit their buildings with increased storm protection? On a macro scale, do massive engineering efforts make more economic sense than simply rebuilding whatever gets knocked down?

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Finding answers means evaluating a complex set of variables: How storm patterns and sea level are expected to change as the climate warms, where people are moving within the city, and how much future damage will cost versus how big a bill mitigation efforts will ring up in the present.

How we can strike that balance is the subject of a fascinating new paper published today in Science, which uses New York City as a model. The authors don't mince words about the current state of affairs for coastal megacities, writing that "despite trillions of dollars of assets located in coastal flood-prone areas, investments in protection have often been inadequate."

First, the authors developed a model for the prevalence and cost of major storm events tailored to NYC, built from 549 storm-surge simulations, which ran the gamut from "extremely low probability events," like Hurricane Sandy, to more frequent storms. By calculating the block-by-block costs of those simulated storms, the team led by Dr. Jeroen C. J. H. Aerts of VU University in Amsterdam was able to develop a map of projected storm costs for NYC.

A map of the damage model used in the study, which includes the number of residential buildings and total buildings in each borough. Image: Aerts et. al supplemental materials

The authors estimated that NYC alone is expected to spend $174 million a year on flood losses if no new management techniques are implemented. The team compared their model to the real-world costs of Sandy, and found it tracked "very close to the actual damages Sandy triggered."

So, with a baseline of how much storm damage costs now, the team built a trio of models to project those costs into the future based on low, medium, or high effects of climate change on sea level rise and storm activity. That allowed the team to develop a benefit-cost analysis of two main mitigation strategies—implementing new building codes to better deal with floods, and large-scale flood management, like building new storm barriers—along with a hybrid of the best parts of both.

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As you might expect, implementing new building codes aimed at decreasing flood damage, such as requiring new buildings to be elevated and adding improved flood protection to new infrastructure, is the most cost-effective move to make.

But, as the authors note, "given the large existing building stock in NYC, this might only have marginal impacts if another hurricane occurs in the next few years," and getting people to comply with a new set of flood regulations is a big challenge. Additionally, wet and dry floodproofing can only go so far to mitigate damage, and may not make much of a difference when it comes to the flood of the century.

The other option is to implement major flood barriers throughout NYC's waterways. The image below left shows one of three different scenarios laid out in the Science study, in which a combination of levees, improved water management, and surge barriers are designed to mitigate storm surge.

While such an effort would be more effective at slowing flooding, it's also extremely costly. The authors peg the maximum investment required by NYC to be between $14.7 and $21.8 billion, with upwards of $100 million a year required for maintenance.

"If I'm telling you to elevate your house, it's something you can do for $20,000 to $30,000," co-author Dr. Erwann Michel-Kerjan of the University of Pennsylvania said in a phone call. "But if you're talking about New York City, it's billions to build barriers. If you look at the maps, these are enormous."

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"It takes five to 10 years to design these things, and another five to ten to build them, which is why they need to be analyzed today if we're going to build them 20 years from now," he continued.

The image to the above right shows the hybrid strategy, which would include cost-effective barriers coupled with the most effective building code changes based around FEMA's flood zone data for once-a-century floods.

The results are pretty stark. "None of the flood protection–barrier strategies is economically attractive under current climate conditions or a low climate change scenario," the authors write, meaning that over the next 100 years, if climate change produces a smaller impact than expected, investing in large-scale flood improvement isn't worth it; it'd be cheaper to simply clean up after a hurricane leaves.

However, under the middle climate change model, the hybrid model is worth it. And if climate change is worse than predicted, with "rapid ice melt and significant increase in storm activity," instituting a hybrid plan of storm management could save some $50 billion.

A table of benefit-cost ratio (BCR) for three barrier models, as well as the team's hybrid model. A BCR over 1 means that implenting changes will save money over the next century.

This is all from a macro viewpoint; stopping floods is essentially impossible, and a strict economic analysis doesn't count the fact that regardless of what's done, people are going to lose their homes and lives. But improvements will make a difference, if we start working on them soon. Put simply, the authors write that "it is economically effective to invest in a storm surge barrier system in 2040 if climate change develops according to the middle scenario; still this strategy needs to be studied now."

Of course, there's the big question (which the authors actually refer to as such) of figuring out who's going to pay for things. Improvements of individual buildings will most likely be covered by property owners, while the most costly infrastructure improvements will likely require a mix of city, state, and federal funds.

"The big question in all of these things is who's going to pay for it?" Michel-Kerjan said. "Who's going to pay for these barriers, or who will pay for the economic consequences and even the human consequences if we fail to build them?"

One interesting proposal from the paper argues that "with more than 50 million tourists visiting the city every year, a simple $10 resilience fee—equivalent to the maximum September 11 airline security fee anyone traveling in the United States is now paying for a roundtrip ticket—could help."

"Enhancing financial protection is critical, too," the authors write. "That 80 percent of households and 95 percent of small businesses in the area inundated by Sandy did not have flood insurance is disturbing, as it is available from the federal government at a subsidized rate for many. Recent legislation passed by the US Congress phases out some of these subsidies over time, which is likely to make investment in risk-reduction measures even more appealing in order to lower the cost of more expensive flood insurance."

In any case, it's positive to see that there are basic economic arguments for improving NYC's flood management—and economic viability is the bottom line, at the end of the day. While the exact model for NYC can't be cut-and-pasted for other cities, the general approach appears to be a rather flexible one. And that's important, because right now, we're still not fully sure how climate change will affect our coasts, but the question is a matter of degree. As the authors conclude, "uncertainty is inherent to such estimations, but it should not be used to justify not doing anything."