People walk through floodwater  in Houston on Sunday. (Thomas B. Shea/AFP/Getty Images)

In the aftermath of Hurricane Harvey, an event deemed “unprecedented” by the National Weather Service, catastrophic floodwater has swept the Houston metropolitan area.

The flooding can be attributed to a combination of long-lasting rains, which pumped extra water into the coastal waterways, and storm surge, which prevented the excess water from draining back into the ocean — a devastating set of simultaneous effects.

Hurricane expert Jeff Masters, co-founder of meteorology website Weather Underground, called the phenomenon “compound flooding” in a recent interview with Scientific American. “The water piles up from both sides,” Masters told the publication.

It’s a pretty typical effect when a major storm makes landfall.

“Always when there is a hurricane, you have compounding effects of ocean flooding — surge — and terrestrial flooding,” said Amir AghaKouchak, an associate professor of hydrology and remote sensing at the University of California at Irvine. Harvey, he said, is a prime example of how these two factors work together to create the perfect storm, producing catastrophic coastal flooding when they occur at once. And now, he said, we need to pay more attention to the way these factors work together when we’re estimating flood risks for coastal regions — before disaster actually strikes.

Scientists tend to focus on one flood driver or another when conducting flood hazard assessments for any given area — evaluating either the risk of terrestrial flooding, which occurs inland as a result of excess precipitation and overflowing rivers, or of surging ocean waters. But in many coastal areas, where rivers run out to meet the sea, both factors play a major role in the risk of regional flooding. Focusing on only one or the other can run the risk of underestimating the likelihood of a major flood.

In a new study, a group of scientists from the University of California at Irvine, including AghaKouchak, and from the University of Salento in Lecce, Italy, have pointed out the importance of addressing both factors at once — and they’ve come up with a method that allows scientists to do so. Their findings were published Monday in the journal Proceedings of the National Academy of Sciences.

“We developed a model that addresses the compounding effects between these two drivers, the interaction between the two drivers,” said AghaKouchak. “So it allows you to account for ocean flooding and terrestrial flooding, or [river] flooding, at the same time.”

The researchers used their model to evaluate flood risks for three coastal cities in the United States — San Francisco, Philadelphia and Washington, D.C. They chose these cities because coastal monitoring data suggested a particularly strong relationship between ocean and river flooding, AghaKouchak noted.

For each of the three case studies, the researchers used data on both coastal water levels and the daily flow of fresh water out to the sea, assuming that the fresh water flow came mainly from the major river system in each case (for instance, the Potomac River in Washington). But the main idea is to account for the compounding effects of both fresh water accumulating on land and seawater pushing in from offshore — the same effect that’s causing water to pile up in Houston right now.

In all three cities, they found that hazard assessments indicated a higher flood risk when both drivers were accounted for. In Washington, for instance, the level of flooding a typical hazard assessment might suggest would occur only once every 20 years was predicted to occur once every 16 years instead under the researchers’ new model.

The new study develops “a really nice framework, statistically, to actually account for these types of [compounding] effects,” said Thomas Wahl, an expert on coastal risks at the University of Central Florida, who was not involved with the study but has conducted similar research of his own.

It’s a topic that’s drawing increased attention among flood experts, he added. “Scientists have started to show how we really underestimate coastal risks, especially when it comes to flooding, if we concentrate only on one of the drivers,” he told The Washington Post.

The new study’s authors suggest that updating the methods typically used to evaluate coastal risks could help produce more accurate flood hazard assessments in the future. And they’ve also pointed out that the risk of coastal flooding will only increase as sea levels continue to rise under the influence of climate change. They conducted a separate assessment, looking at the risks under both a moderate and a severe future climate change scenario, and found that rising sea levels are associated with a significant increase in the risk of severe floods.

The study suggests that this increase in flood risk would occur even if no change in river flooding takes place. But AghaKouchak also noted that climate change may increase the probability of severe storms and heavy precipitation in many parts of the world — meaning river flooding may become more severe in some coastal areas as well. The combined influence of both increasing river floods and rising sea levels would create an even greater risk of severe floods in the future.

In the meantime, Hurricane Harvey is a prime example of how all of these factors come together to produce a devastating flood.

“What we are seeing right now in Houston is actually a compound flooding event,” Wahl said. “A lot of the rain that’s coming down right now can’t really drain into the open ocean as it’s supposed to do under normal sea level conditions because of the elevated water conditions.”  

Read more about Hurricane Harvey: 

Where Harvey is hitting hardest, 80 percent lack flood insurance

Houston is experiencing its third ‘500-year’ flood in 3 years. How is that possible?

ExxonMobil refineries damaged in Hurricane Harvey, releasing hazardous pollutants