USA: Inland communities bear the brunt of hurricane-related flood damage

When most people think of hurricanes, they think of mighty storm surges pounding coastal communities. But a team of researchers who studied 28 hurricanes from 2001 to 2014 found that freshwater flooding, sometimes thousands of miles inland, can cause even greater destruction.

“The number of insured residential losses from freshwater flooding is twice as high as that from storm surge,” says Gabriele Villarini, associate professor of civil and environmental engineering and associate director of IIHR–Hydroscience and Engineering at the University of Iowa. “Hurricanes are not just a coastal problem,” he adds. “In fact, the impact is felt far more inland than expected.”

Villarini is part of a team of five researchers that looked at freshwater flood risk from North Atlantic tropical hurricanes as part of a groundbreaking study linking hurricanes to flood insurance claims. The study also is the first to analyze future flood impacts due to climate change and urbanization.

The study’s findings, published in Scientific Reports, could influence the way policy makers think about risk management, emergency services, flood insurance, and urban development.

“This research sheds new light on the overall damage triggered by tropical cyclones along their entire path inland, not just on the coast,” says Jeffrey Czajkowski, director of sponsored research at the Wharton Risk Management and Decision Processes Center at the University of Pennsylvania. “Highlighting the areas susceptible to extensive inland flood losses from tropical cyclones is a critical step in improving community preparation.”

The left panel illustrates the location of all the communities with freshwater flood claims, and the right panel shows the number of times communities have experienced a major tropical-cyclone flood. Forty-five percent of freshwater claims were from states not traditionally associated with hurricane flood risk, including Illinois, Missouri, and Ohio. Freshwater flooding in areas such as these is often caused by heavy rainfall, including rainfall associated with hurricanes, researchers say.

Until now, research into freshwater flood risk due to hurricanes has been limited. Villarini, Czajkowski, and their team set out to change that. They analyzed all significant flood events associated with U.S.–landfalling hurricanes from 2001 to 2014, including a review of related discharge measurements from 3,035 U.S. Geological Survey stream gages in 38 states and flood claims from the National Flood Insurance Program (NFIP), the main provider of flood insurance in the U.S.

They found that though one-third of total residential flood insurance claims were related to storm surge, the impact of freshwater flooding from hurricanes was much more significant, affecting 21,705 communities, or an average of 775 communities per event, between 2001 and 2014.

In fact, 45 percent of freshwater claims were from states not traditionally associated with hurricane flood risk, including Illinois, Missouri, and Ohio. Freshwater flooding in areas such as these is often caused by heavy rainfall, including rainfall associated with hurricanes, researchers say.

When researchers looked more closely at the geographic distribution of flood claims, they found that 26 percent were located outside of Federal Emergency Management Agency (FEMA)–designated high-risk flood hazard areas. In general, homeowners who live outside these areas are less inclined to purchase flood insurance and therefore less likely to be financially prepared to recover from a flood.

The fiscal impact of flood damage is significant. Researchers estimate that the average flood-claim payout for the 28 hurricanes was about $530 million per event.

Looking ahead, researchers say that climate change and increased urbanization could result in even greater hurricane-related damage.

“Different modeling studies point to an increase in hurricane rainfall up to 20 percent in a warmer climate,” says Villarini. “Moreover, if urbanization continues at its current pace, we expect to see a roughly 3 percent increase in the number if insurance claims associated with flooding from tropical hurricanes.”

The timing of the research is important because national policies and ideas related to flood protection and resilience could be changing.

FEMA is in the process of remapping areas for flood risk, and a number of U.S. inland flood catastrophe models are in development in the private sector. In addition, the NFIP is set for reauthorization later this year. Policy debates surrounding its renewal likely will focus on the use of more sophisticated catastrophe models to provide more accurate maps and risk-based pricing, as well as expanded insurance coverage, especially in inland areas.

“The risk management industry is in need of additional risk assessment tools, and our study provides an added level of insight,” says Villarini. “We now have a better understanding of the areas that are most vulnerable.”

The U.S. Geological Survey and the Willis Research Network provided financial support for the hurricane study.

IHR–Hydroscience and Engineering is a world-renowned center for education, research, and public service focusing on hydraulic engineering and fluid mechanics. Based in the C. Maxwell Stanley Hydraulics Laboratory, a five-story red-brick building on the banks of the Iowa River, IIHR is a unit of the UI College of Engineering. At IIHR, students, faculty, and research engineers work together to understand and manage one of the world’s greatest resources—water. Students from around the world benefit from IIHR’s comprehensive multidisciplinary approach, which includes basic fluid mechanics, laboratory experimentation, and computational approaches.