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Research Identifies Changing Coastal Flood Risk

By Kevin Wilcox

Two separate studies find a compounding effect between climate change factors that are often researched separately.

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Researchers have determined that high levels of storm surge accompanied by periods of extreme precipitation are far more likely to occur now than they were in decades past. Wikimedia Commons/George Miziuk Gamweb

September 29, 2015—Flooding along portions of the populous East and Gulf coasts in the United States has become more frequent and more severe in the last 50 years and is likely to become significantly worse in coming decades, according to two separate research reports published recently in the journal Nature Climate Change.

An international team of researchers at the University of South Florida (USF), University of Maine, and University of Siegen (Germany) examined the linkage between storm surge and heavy rains and reported their findings in the paper "Increasing Risk of Compound Flooding from Storm Surge and Rainfall for Major U.S. Cities." A multi-institutional team from the private climate research firm Atmospheric and Environmental Research (AER) in Lexington, Massachusetts, as well as Columbia, Princeton, and Rutgers universities and the University of Iowa, examined the combination of sea level rise and tropical storm surge in the paper "Joint Projections of U.S. East Coast Sea Level and Storm Surge."

The research team at USF began by examining tidal gauge records, which in some areas of the country go back to the 19 th century. They selected the longest records available and then compared those with precipitation records from nearby weather stations, according to Thomas Wahl, Dr.Ing., a postdoctoral researcher at the College of Marine Science at USF, who was the lead author of the paper.

Although the team expected to find that periods of high storm surge were accompanied by periods of extreme precipitation, they were surprised to find that this correlation has been growing stronger over the past decades in many large cities, especially along the East and Gulf coasts. "It is more likely now that these two important flood drivers occur at the same time than it used to be 50 or 60 years ago," says Wahl, who notes that some types of flooding events in New York City are now twice as likely as they were in the mid-20 th century. "Not because of an increase in storm surges. Not because of an increase in precipitation. Just because they happen to occur more often simultaneously."

This linkage is important because storm surge impedes the runoff of heavy precipitation in low-lying areas, exacerbating flooding. Why this compound flooding is becoming worse is one of the team's next research topics. An early theory is that the warming of global sea surfaces may play a role. "We need to understand what has driven the changes that we observed," Wahl says. "That is absolutely important if we think about climate adaptation. We need to understand the drivers for these changes in the past in order to be able to project them into the future."

In the meantime, Wahl, whose doctoral degree is in civil and hydraulic engineering, notes that engineers working in coastal areas should consider a multivariate approach to examining flood risk. "When we do a flood-risk analysis for a certain area, we often run a storm-surge model and obtain the associated floodplain, then we use rainfall runoff and river models to develop a freshwater floodplain, and then we overlay those two," Wahl explains. "This ignores the dependency. If we were to use models that account for storm surge and rainfall, and use a multivariate approach to generate scenarios of those two, I would assume the risk maps would be affected by that."

The multi-institutional research team behind the other study was led by Christopher M. Little, Ph.D., a staff scientist at AER. The team also began by examining tide gauge records of storm surges, focusing on cities "that had a long tide-gauge record, at least to before 1950," says Radley Horton, Ph.D., a climate scientist at the Center for Climate Systems Research at  Columbia University's Earth Institute. "We wanted to have a rich historical record of floods so that we could really sample the frequency of occurrence and put our projections on top of that."

The team then utilized a suite of climate models to see if there was a correlation between projected sea level rise and the intensity of tropical cyclones. And, in fact, the models that project more extreme sea level rise along the Gulf and East coasts also tend to point to stronger storms. "We saw the biggest signal in the outlier models—the models that can give us a lot of sea-level rise along the East Coast and in the Gulf. Those models also point to a change in ocean temperatures that could make the strongest storms stronger," Horton says.

The research indicates that even moderate levels of sea level increases will dramatically increase the frequency of instances in which tropical cyclones and storm surge combine to create coastal flooding. In areas of New York City, for instance, a flooding incident with a 100 year return period will be five times more likely by 2100—even in moderate sea-level increase scenarios.

"For parts of the coastline now, it could be very common to get, hypothetically, a four-foot storm surge, but it could be pretty rare to get a six-foot storm surge. So if you have two feet of sea level rise, now the event that used to be rare...can become much more common," Horton says. "Sea-level rise is like raising the floor on a basketball court—you can get a lot more slam dunks."

In the worst-case scenario models, coastal flooding events will be several orders of magnitude more likely. "I call them outliers, but there's nothing to say that these models are less accurate than other climate models," Horton says. "It's a risk-management question, but there may be an argument for considering the worst-case scenarios, even if we believe they are less likely.

"For [infrastructure] that we really need to have last for a long period of time, our research points to the idea that the worst-case scenarios may actually be worse than you would have concluded if you had considered each of these hazards separately," he adds.

Although the research studies were conducted separately, they point to an emerging area of climate change examination in which factors such as sea level rise, storm surge, or storm intensity—which had once been considered separately—are examined together. And both studies conclude that these factors will have a significant compounding effect.



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